EUROPEAN ORGANISATION
FOR THE SAFETY OF AIR NAVIGATION
EUROCONTROL
L-DACS2 System Definition
Proposal:
Deliverable D2
Edition Number
:
v1.0
th
Edition Date
:
11 May 2009
Status
:
Draft
Intended for
:
General Public
EUROPEAN AIR TRAFFIC MANAGEMENT
L-DACS2 System Definition proposal: Deliverable D2
DOCUMENT CHARACTERISTICS
TITLE
LDACS2 System Definition Proposal: Deliverable D2
Publications Reference:
ISBN Number:
Edition Number: 1.0
Document Identifier
Edition Date: 11/05/2009
Abstract
Keywords
LDACS2
Data link
L band
AMACS
GSM
LDL
FCI
Authors
Contact(s) Person
Tel
Nikos Fistas
+322 729 4777
Unit
CND/CoE/CNS/COM
STATUS, AUDIENCE AND ACCESSIBILITY
Status
Intended for
Accessible via
Working Draft
General Public
Intranet
Draft
EATM Stakeholders
Extranet
Proposed Issue
Restricted Audience
Internet (www.eurocontrol.int)
Released Issue
Electronic
copies
of
this
document
can
be
downloaded
from
http://www.eurocontrol.int/communications/public/standard_page/
LBANDLIB.html
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L-DACS2 System Definition proposal: Deliverable D2
DOCUMENT APPROVAL
The following table identifies all management authorities who have successively approved
the present issue of this document.
AUTHORITY
Edition: 1.0
NAME AND SIGNATURE
Draft
DATE
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L-DACS2 System Definition proposal: Deliverable D2
DOCUMENT CHANGE RECORD
The following table records the complete history of the successive editions of the present
document.
EDITION
NUMBER
EDITION
DATE
REASON FOR CHANGE
PAGES
AFFECTED
Publications
EUROCONTROL Headquarters
96 Rue de la Fusée
B-1130 BRUSSELS
Edition: 1.0
Tel:
+32 (0)2 729 4715
Fax:
+32 (0)2 729 5149
E-mail:
[email protected]
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Contents
CHAPTER 1 – Introduction .....................................................................................14
1.1
1.2
1.3
1.4
Background..............................................................................................................14
Objective and scope of the document .....................................................................15
Project Team ...........................................................................................................17
L-DACS2 design ......................................................................................................18
1.4.1 L-DACS2 overall description.......................................................................18
1.4.2
Outline of the specification..........................................................................19
CHAPTER 2 – Functional Architecture ..................................................................20
2.1
2.2
2.3
Introduction ..............................................................................................................20
Infrastructure............................................................................................................21
Communication link .................................................................................................21
CHAPTER 3 – Air interface (physical layer) ..........................................................22
3.1
3.2
3.3
Introduction ..............................................................................................................22
3.1.1 Transmitter/receiver frequency control .......................................................22
3.1.2
Digital reception by the receiver .................................................................22
3.1.3
Digital transmission.....................................................................................22
Modulation ...............................................................................................................23
3.2.1 Physical layer ..............................................................................................23
3.2.2
Standards....................................................................................................23
3.2.3
Modulation scheme.....................................................................................23
3.2.4
Modulation rate ...........................................................................................23
3.2.5
Time/amplitude profile of L-DACS2 transmission.......................................23
3.2.6
Ambiguity resolution and data transmission ...............................................24
3.2.7
Receiver - transmitter turnaround time .......................................................25
3.2.8
Frequency change during transmission......................................................25
Air interface..............................................................................................................25
3.3.1 Radio frequencies .......................................................................................25
3.3.2
Channel bandwidth .....................................................................................26
3.3.3
Polarization .................................................................................................28
3.3.4
Ground Frequency stability.........................................................................28
3.3.5
Aircraft Frequency stability .........................................................................28
3.3.6
Spurious emissions.....................................................................................28
3.3.7
Error phase specification ............................................................................29
3.3.8
Broad band noise........................................................................................29
3.3.9
Connection management............................................................................29
3.3.10 Transmission...............................................................................................29
3.3.11 Channel.......................................................................................................29
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3.3.12 Broadcast ....................................................................................................29
3.3.13 Air-to-air point-to-point communication.......................................................29
3.3.14 System performance...................................................................................30
3.3.15 Link Budget .................................................................................................30
3.4
Pulse blanking .........................................................................................................32
3.4.1 Protection of L-DACS2 transmissions ........................................................33
3.4.2
3.5
Protection of adjacent systems transmissions............................................33
Power control...........................................................................................................35
3.5.1 Received signals – ground .........................................................................36
3.5.2
Received signals – aircraft..........................................................................36
3.5.3
Single Antenna Interference Cancellation ..................................................36
3.6
Interference immunity ..............................................................................................36
3.6.1 Transmitting function...................................................................................36
3.6.1.1
Adjacent channel emissions..................................................................................36
3.6.2
Receiving function.......................................................................................37
3.6.2.1
Specified error rate................................................................................................37
3.6.2.2
Reference sensitivity level.....................................................................................37
3.6.2.3
Adjacent band immunity performance and co-channel immunity
performance ..........................................................................................................37
3.6.2.4
Out-of-band immunity performance ......................................................................38
3.6.2.5
Interference immunity performance ......................................................................38
3.7
FEC mechanism ......................................................................................................38
3.7.1 Inner code: convolutive punctured code .....................................................38
3.7.2
Interleaver ...................................................................................................39
3.7.3
Outer code: Reed Solomon code ...............................................................39
CHAPTER 4 – MAC sublayer ..................................................................................40
4.1
Introduction ..............................................................................................................40
4.1.1 Provision .....................................................................................................40
4.1.2
4.2
4.3
4.4
4.5
Edition: 1.0
MAC Layer for point-to-point and broadcast communication .....................41
Framing....................................................................................................................41
Synchronization .......................................................................................................42
4.3.1 Specified time reference .............................................................................42
4.3.2
Primary time synchronization mode............................................................42
4.3.3
Secondary synchronization mode...............................................................42
4.3.4
Derived synchronization mode ...................................................................42
4.3.5
Synchronisation of slots within a frame ......................................................43
4.3.6
Reversion ....................................................................................................43
Burst format .............................................................................................................43
4.4.1 Burst composition .......................................................................................43
4.4.2
Bursts occupying multiple slots...................................................................44
4.4.3
Ground station bursts..................................................................................46
Framing structure.....................................................................................................46
4.5.1 Frame Structure ..........................................................................................46
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4.5.2
Login and response ....................................................................................48
4.5.3
Frame section length alteration ..................................................................49
4.5.4
Framing structure default parameter values and ranges............................49
4.5.5
Adaptive burst alignment ............................................................................50
4.5.6
Burst transmission in relation to slot and frame structure...........................52
4.5.6.1
UP1 and UP2 sections ..........................................................................................52
4.5.6.2
LoG2 insertion section ..........................................................................................53
4.5.6.3
CoS1 and CoS2 sections ......................................................................................53
4.5.6.4
CoS1 section .........................................................................................................53
4.5.6.5
CoS2 section .........................................................................................................54
4.5.6.6
Summary of burst guard times ..............................................................................55
4.6
Quality of Service (QoS) management....................................................................56
4.6.1 Efficiency.....................................................................................................56
4.6.2
L-DACS2 QoS management ......................................................................56
4.6.2.1
Parameters............................................................................................................56
4.6.2.2
Priority information ................................................................................................57
4.6.2.3
End-to-end QoS management ..............................................................................57
4.7
4.8
4.9
Processing ...............................................................................................................57
Power Control management ....................................................................................58
MAC layer for Broadcast Service ............................................................................58
4.9.1 Frame..........................................................................................................58
4.9.2
Synchronisation ..........................................................................................58
4.9.3
Burst format ................................................................................................58
4.9.4
Frame structure...........................................................................................58
4.10 MAC layer for Air-to-Air Point-to-Point Service .......................................................58
4.10.1 Frame..........................................................................................................58
4.10.2 Synchronisation ..........................................................................................59
4.10.3 Burst format ................................................................................................59
4.10.4 Frame structure...........................................................................................59
CHAPTER 5 – Data link sublayer (DLS) .................................................................60
5.1
5.2
Introduction ..............................................................................................................60
Transmission procedure ..........................................................................................60
5.2.1 Uplink transmission procedures..................................................................61
5.2.2
5.3
5.4
5.5
5.6
Reception procedure ...............................................................................................62
Segmentation...........................................................................................................62
Reserved access protocol specification ..................................................................62
Random access protocol specification for transmission in CoS2............................63
5.6.1 Random access parameters.......................................................................63
5.6.1.1
Parameter p1 (Persistence CoS2) ........................................................................63
5.6.1.2
Counter VS3 (maximum number of access attempts) ..........................................63
5.6.2
5.7
Edition: 1.0
Downlink transmission procedures .............................................................61
Random access procedures .......................................................................64
Random access protocol specification for transmission in LoG2 ............................64
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5.7.1
Random access parameters.......................................................................64
5.7.1.1
Parameter p2 (Persistence LoG2) ........................................................................64
5.7.1.2
Counter VS4 (maximum number of access attempts) ..........................................64
5.7.2
Random access procedures .......................................................................65
CHAPTER 6 – L-DACS2 Link Management layer (LML)........................................66
6.1
6.2
6.3
Introduction ..............................................................................................................66
Login mechanism.....................................................................................................67
Hand-over mechanism.............................................................................................67
6.3.1 Controlled hand-over ..................................................................................68
6.3.1.1
Air-initiated controlled hand-over ..........................................................................68
6.3.1.2
Ground-requested air-initiated controlled hand-over ............................................71
6.3.2
ANNEX 1 –
A1.1
6.4
6.5
6.6
ANNEX 2 –
A2.1
A2.2
A2.3
A2.4
ANNEX 3 –
Uncontrolled hand-over...............................................................................77
Example of burst transmission.....................................................78
Burst transmission in UP1 or UP2 ...........................................................................78
Burst transmission in a single slot in CoS2 .............................................................80
Burst transmission in CoS1 slot...............................................................................82
Burst transmission in a LoG2 slot............................................................................84
Message structure..........................................................................87
Message type codes................................................................................................87
Message type codes................................................................................................87
Priority field ..............................................................................................................88
Messages ................................................................................................................88
System operations .........................................................................99
A3.1 Downlink ..................................................................................................................99
A3.1.1 A/C login .....................................................................................................99
A3.1.2 Aircraft has data to send ...........................................................................100
A3.1.3 Aircraft has no data to send ......................................................................100
A3.1.4 CoS2 random access................................................................................100
A3.1.5 Aircraft-initiated cell exit ............................................................................100
A3.1.6 GS request for aircraft cell exit with no recommendation .........................101
A3.1.7 GS request for aircraft cell exit with recommendation ..............................102
A3.2 Uplink .....................................................................................................................103
A3.2.1 GS has data to send .................................................................................103
A3.2.1.1
Data size is ≤2,048 octets, if transmitted in UP1 ................................................103
A3.2.1.2
Data size is >2,048 octets ...................................................................................103
A3.3 Acknowledgement messages ................................................................................103
ANNEX 4 –
A4.1
A4.2
A4.3
A4.4
ANNEX 5 –
Edition: 1.0
Coding and interleaving ..............................................................105
GMSK and convolutional coding: rate and expected performances .....................105
Considerations regarding practical C/N.................................................................108
Accounting for burst interference: interleaving and RS coding .............................109
Equalization ...........................................................................................................110
Impact of intra-system interference............................................111
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A5.1
A5.2
A5.3
A5.4
Intra system interference robustness ....................................................................111
Adjacent channel frequency guard time ................................................................113
Co-channel spacing guard band............................................................................113
Multiple channels operating in one cell..................................................................114
A5.4.1 Frequency Band........................................................................................114
A5.4.2 Spacing separation between stations .......................................................115
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List of Figures
Figure 1: Selection process of L-DACS system ....................................................................15
Figure 2: Proposed Approach...............................................................................................16
Figure 3: Time/amplitude profile of LDACS2 transmission....................................................24
Figure 4 – L-DACS2 Link Budget .........................................................................................31
Figure 5 – Adjacent band and Co-channel band reference interference ratio .......................37
Figure 6 : L-DACS/2 frame ...................................................................................................41
Figure 7: L-DACS/2 burst structure for downlink bursts ........................................................44
Figure 8: L-DACS/2 merged slot structure for downlink bursts..............................................45
Figure 9: L-DACS/2 merged slot structure structure for downlink bursts...............................45
Figure 10: L-DACS/2 merged slot structure for uplink bursts ................................................46
Figure 11: L-DACS2 frame structure (point-to-point) ............................................................47
Figure 12: Successful air-initiated controlled hand-over........................................................69
Figure 13: Air-initiated controlled handover: retransmit cell login ..........................................69
Figure 14: Air-initiated controlled handover: retransmit GS_ALLOC .....................................70
Figure 15: Air-initiated controlled handover: retransmit AC_CELL_EXIT ..............................70
Figure 16: Air-initiated controlled handover: retransmit GS_EXIT_ACK................................71
Figure 17: Successful ground-requested air-initiated controlled hand-over...........................73
Figure 18: Ground-requested air-initiated controlled hand-over: GS_CELL_EXIT retransmit73
Figure 19: Ground-requested air-initiated controlled hand-over: AC_EXIT_ACK retransmit .74
Figure 20: Ground-requested air-initiated controlled hand-over: GS_FRAME not received ..74
Figure 21: Ground-requested air-initiated controlled hand-over: unsuccessful login .............75
Figure 22: Successful ground-requested air-initiated controlled hand-over with
recommendation...................................................................................................................75
Figure 23: Ground-requested air-initiated controlled hand-over with recommendation:
alternative ground station .....................................................................................................76
Figure 24: Ground-requested air-initiated controlled hand-over with recommendation: no
successful logins ..................................................................................................................76
Figure 25: Uncontrolled handover ........................................................................................77
Figure 26: Illustration of burst format for a typical burst in UP1 or UP2.................................80
Figure 27: Illustration of burst format for a single slot burst in CoS2 .....................................82
Figure 28: Illustration of burst format for a burst in a CoS1 slot ............................................84
Figure 29: Illustration of burst format for a burst in a LoG2 slot.............................................86
Figure 30: GMSK theoretical performance..........................................................................106
Figure 31: Convolutional code performance .......................................................................107
Figure 32: Convolutional code (5,7), constraint length 3 .....................................................108
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Figure 33: Convolutional code (133,171), constraint length 7 .............................................108
Figure 34 – Separation distances between aircraft and nearest ground station ..................116
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List of Tables
Table 1 – Total channel bandwidth for various cell sizes ......................................................26
Table 2 – Latencies for TMA Large scenario (52 aircraft) .....................................................27
Table 3 – Latencies for En route Medium scenario (62 aircraft)............................................27
Table 4 – Latencies for En route large scenario (102 aircraft per channel) ...........................27
Table 5 – Throughput achieved ............................................................................................27
Table 6 – Spectrum mask (dB) .............................................................................................28
Table 7 – Spurious domain emissions ITU regulation...........................................................29
Table 8 – Effect of imposing burst length limits in the Reverse Link......................................34
Table 9 – FLIPINT and COTRAC messages sizes (downlink) ..............................................34
Table 10: Parameter definition for framing parameters.........................................................41
Table 11: Parameter definition for framing parameters.........................................................47
Table 12: Parameter definition for S1 parameter ..................................................................48
Table 13: Parameter definition for framing parameters.........................................................50
Table 14: Parameter definition for timing advance parameter T5..........................................50
Table 15: Allocation of T5 parameter to transmission delay..................................................52
Table 16: End guard period for UP1 and UP2 burst as a function of the number of slots
occupied by the burst ...........................................................................................................52
Table 17: Parameter definition for maximum burst length in slots in CoS2 ...........................54
Table 18: Guard period for CoS2 slot as a function of slot length .........................................55
Table 19: Summary of burst guard periods per frame section...............................................56
Table 20: Summary of burst guard periods at the start of frame sections .............................56
Table 21: Definition of parameter Priority Q22......................................................................56
Table 22: Mapping between message category, ATN priority and Q22 priority classification 57
Table 23: Random access parameters for CoS2 ..................................................................63
Table 24: Random access parameters for LoG2 ..................................................................64
Table 25: Available payload for user data in burst occupying two slots in UP1 or UP2 .........79
Table 26: Available payload for user data in a single slot in CoS2........................................81
Table 27: Available payload for user data in a CoS1 slot......................................................83
Table 28: Available payload for user data in a LoG2 slot ......................................................85
Table 29 – Link budget including interference contributions ...............................................112
Table 30 – Adjacent channel frequency guard time ............................................................113
Table 31 – Co-channel spacing guard band .......................................................................114
Table 32 – Multiple channels operating in one cell scenario ...............................................115
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EXECUTIVE SUMMARY
This document is the first of the deliverables of the study funded by EUROCONTROL to
provide the specifications for the L-DACS2 system. L-DACS2 is one of the two candidate
systems identified in ICAO and the SESAR Definition Phase to support the future
aeronautical communications infrastructure (FCI) in the continental enroute and TMA
environments.
L-DCAS2 is considered for operation in the L band, and has been developed from material
for AMACS, UAT, DME and VDL Mode 4. GSM technical elements have also been used as
basic technical background.
This document will be distributed for external review by the interested parties including the
SJU WP9 and WP15 partners.
Following the review the document will be updated to consider the received comments and
the new document (Deliverable D2) will provide the basis for the development of detailed Tx
and Rx prototype equipment specifications. These prototypes will be used to demonstrate
the spectrum compatibility of the candidate systems with the existing systems operating in
the L band and the suitability of its performance in the presence of interference from the
existing systems. These activities will facilitate the eventual selection of one system (LDACS) for the FCI
The deliverables of this study will be an input to the SJU relevant projects (in particular
P15.2.4).
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L-DACS2 System Definition proposal: Deliverable D2
CHAPTER 1 –
Introduction
1.1
Background
In support of the future SJU activities, EUROCONTROL is leading the investigations for the
future communications infrastructure (FCI) which is required to support the future
aeronautical communications.
One of the key recommendations concerning the development of a new data link, issued in
the AP17 Final Report and in SESAR Definition Phase Deliverables, is to finalise the
selection of a an L-Band data link (LDACS) that supports the continental airspace
environment. The development of the L-band data link is identified in the development
activities for the SESAR Implementation Package 3 (IP3) in the post 2020 timeframe.
Therefore, the outcome of this activity will be used as input to the SESAR JU activities.
Under the AP17 activities, various candidate technologies were considered and evaluated to
operate in the L band and supporting the requirements. However, it was found that none of
the considered technologies could be fully recommended primarily due to concerns about the
operational compatibility (spectrum interference) with existing systems in the L band.
Nevertheless, the assessment of the candidate technologies led to the identification of
desirable technology features that could be used as a basis for the development of an Lband data link solution that would be spectrally compatible.
Considering these features and the most promising candidates, two options for the L-band
Digital Aeronautical Communication System (LDACS) were identified. These options need
further consideration before final selection of a single data link technology.
The first option for LDACS is a frequency division duplex (FDD) configuration utilizing OFDM
modulation techniques, reservation based access control and advanced network protocols.
This solution is a derivative of the B-AMC and TIA-902 (P34) technologies.
The second LDACS option is a time division duplex (TDD) configuration utilizing a binary
modulation derivative of the implemented UAT system (CPFSK family) and of existing
commercial (e.g. GSM) systems and custom protocols for lower layers providing high qualityof-service management capability. This solution is a derivative of the LDL and AMACS
technologies.
AP17 and SESAR proposed follow on activities in order to further specify the proposed
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LDACS option, validate their performance, aiming at a final decision (single technology
recommendation for L-Band) by 2010.
Based on the information given above, in order to complete the selection of the LDACS, it is
required to:
•
Develop detailed specifications for LDACS1 and LDACS2
•
Develop and test LDACS1 and LDACS2 prototypes
•
Assess the overall performance of LDACS1 and LDACS2 systems.
The figure bellow explains the complete process applied to select the final L-DACS system.
The activity conducted under this contract (LDACS2 study) only concern the shaded box and
the LDACS2 system. A separate Eurocontrol contract (LDACS1 study) is addressing in a
similar manner the LDACS1 system.
Figure 1: Selection process of L-DACS system
1.2
Objective and scope of the document
The L-DACS2 Study objective is to support the realisation of the recommendation for the
selection of the L band data link which is expected to be progressed in the frame of the SJU
WP15 activities.
Under these activities, it is expected that industry will develop LDACS1/2 prototypes for
testing in order to evaluate the spectrum compatibility of LDACS1/2 with the existing users of
the L band and the overall capabilities of the LDACS1/2 system and eventually facilitate the
LDACS system selection. In this context, the LDACS2 study will provide input to SJU
activities (Project 15.2.4) by developing initial specifications for the L-DACS2 system in
sufficient detail so as to enable the subsequent development (outside the scope of this
project) of L-DACS2 prototype(s) for testing (aiming to confirm the spectrum compatibility of
L-DACS2 with the existing users of the L band and to demonstrate and validate the
capabilities of the system).
The LDACS2 system specification trade-off aims to achieve the following high level
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L-DACS2 System Definition proposal: Deliverable D2
objectives:
•
L-DACS2 can operate in the L band without interfering with the existing users of the
band.
•
The performance of the L-DACS2 is meeting the expected requirements in the
expected operational environment.
•
L-DACS2 development is facilitated and expedited through the choice of appropriate
components and/or mature standards.
•
The proposed specifications should be sufficiently complete and unambiguous to be
able to initiate the system design validation that will lead (with an iterative refinement
cycle if required) to proposals for international standards (ICAO, EUROCAE/RTCA).
There will be two main outputs from the L-DACS2 study. The first one will be the L-DACS2
system detailed characteristics. For this there will be an initial version for external review and
comments (Deliverable D1) and the final one taking into account the external comments
(Deliverable D2). The second outcome will be the detailed design specifications (Deliverable
D3) for the L-DACS2 Receiver and Transmitter prototype equipment to be used in the
testing.
This document represents Deliverable D2. For the external review, EUROCONTROL is
soliciting comments from the SJU WP15 and WP9 partners, as well as from the US and all
other interested parties.
Based on the outcome of the AP17 investigations, L-DACS2 draws from the features of the
AMCAS and LDL systems that have been considered in the AP17 investigations.
In order to develop the L-DACS2 system specifications, the L-DACS2 study partners have
used the AMACS system design as the baseline for the L-DACS2 system specification
considering other features form LDL (UAT) and GSM as appropriately. For this the AMACS
Description v1.0 (see bellow section 1.5.1) was analysed by the LDACS2 study partners,
identifying constraints, and defining a solution during an internal workshop with industry
participation.
These solutions were further analysed and updated after the workshop. The product of this
activity was an L-DACS2 Detailed Design, as illustrated below:
Figure 2: Proposed Approach
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L-DACS2 System Definition proposal: Deliverable D2
1.3
Project Team
The LDCAS2 study has been performed by the following team:
Edition: 1.0
•
EGIS AVIA has gained an important experience in international and multipartner project management. EGIS AVIA was also involved in several
communication system projects including a strong involvement in E-TDMA
definition as well as AMACS concept design.
•
Helios have supported the development of the future communications activities
over a number of years, by providing impartial technical advice from its pool
of communications expertise and supporting EUROCONTROL develop and
maintain the future deployment strategy for aeronautical mobile
communications through the Action Plan 17 activities. Helios has also
undertaken considerable independent technical evaluation work by simulating
and validating the leading candidate technologies including AMACS B-AMC
and P34.
•
SWEDAVIA, being a subsidiary company to LFV, has been deeply involved
in development and standardisation of VDL Mode 4. LFV initiated the XDL
activities, i.e. VDL Mode 4 implemented in the L-band, which is one of the
foundations of AMACS. LFV has also been very active in the ICAO ACP
activities regarding Future Communication System.
•
DSNA/DTI has initiated the E-TDMA activities in 1998, in cooperation with
EGIS AVIA. This initial work has been one of the foundations of the AMACS
concept. DTI has also been very active in the ICAO ACP activities regarding
the Future Communication System.
•
Telerad is France's supplier of Ground to Air Aeronautical
Telecommunication Systems for both Civil and Military areas. Telerad
equipment is in operation in more than 60 different Countries.
•
CNSS has gained experience in AIS and VDL Mode 4, acquired through its
contracts with LFV, Malmo Aviation, Skyways and Arlanda airport.
•
Selex is an important VHF/UHF Base Stations provider for Ground-AirGround communications, with a strong presence in Europe, Asia and South
America.
•
Rockwell-Collins has developed an airborne S-TDMA (VDL4) radio to
support the NUP II+ project and is currently developing a High Capacity Data
Radio in partnership with Thales. Rockwell Collins France has a large panel of
skills in digital signal processing and communication protocols, as well as a
strong experience in rapid design and prototyping. RCF are involved in
numerous studies and development of aeronautical communication solutions
with EUROCONTROL for several years.
•
AVTECH has participated in standardization committees for VDL Mode 4 as
a solution for ADS-B both on behalf of ICAO and within RTCA and
EUROCAE. AVTECH has also participated in projects for On-Board GSM
technology.
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1.4
L-DACS2 design
A general description of the L-DACS2 system is proposed bellow following by the outlines of
the specification.
1.4.1
L-DACS2 overall description
The L-DACS2 system is a multipurpose communication system based on the AMACS
concept and architecture. Its key design drivers are geared towards achieving a balance
between flexibility, scalability, determinism and capitalising on available standards. The
overall goal is to provide a cost-effective solution that provides the capabilities that meet the
demands of the future aeronautical communications environment, while providing and viable
and feasible solution in the intended time frame.
The air interface is based on modulation scheme derived from GSM and UAT. This is
complimented by an error recovery system based upon a strong data coding to ensure the
robustness of the link for the highest Quality of Service (QoS) in terms of latency.
The pool of radio frequencies intended for L-DACS2 is in the lower L-band, from 960 - 975
MHz. The lowest assignable frequency shall be 960.5 MHz (for spectrum compatibility) and
the highest assignable frequency shall be 975 MHz. The L-DACS2 bandwidth is set at 200
kHz offering a gross data rate of 270.833 kbit/s per channel. GMSK modulation scheme with
a modulation index h of 0.5 and a BT product of 0.3 has been retained in order to gain some
experience from GSM standard. The L-DACS2 system has the capability to provide airground communication service in a 200 NM radius. The channel bandwidth and data rate
allow the L-DACS2 system to achieve the stringent requirement in term of latency for the
stringent operational scenario in term of capacity and demands.
The L-DACS2 specifications also provide a link budget, a spectrum mask and the receiver
immunity performance in order to have the requirements for the design of a prototype. Those
physical specifications, based on the GSM and UAT standards, are in line with ITU regulation
in term of spectral compatibility. The waveform is design to minimise the spectral footprint
while minimising customisation of already available waveforms. The ramp up and down is
defined to minimise the impact of the L-DACS2 system in the L-Band environment. Power
control specifications and Pulse blanking has also been studied and guidelines are provided.
Finally, a complete study on the coding scheme is conducted that lead to the implementation
of a Reed Salomon plus convolutional coding to assure a corrected BER of 10-7. An
interleaver is also implemented in order to reduce the impact of pulsed and bursted
interfering on the L-DACS2 avionics communication.
The AMACS MAC layer has been updated to improve the delivery of time-critical messages.
The frame length has been set to 1 second to accommodate the most stringent
requirements. The L-DACS2 system is a half duplex system based on time division providing
specific sections for uplink and downlink transmission. The frame consists of two uplink
sections, UP1 and UP2, and two downlink sections CoS1 and CoS2, and a login section
LoG2. The length of each section can be change dynamically to accommodate the
operational need in one cell. The CoS1 section is composed of a number of slots which are
allocated deterministically to the mobiles logged in the cell. A mobile could send specifics
messages (Keep-Alive, Acknowledgement or Request to Send) in its unique CoS1 slots. A
synchronisation scheme between the ground station and its mobiles based on adaptive burst
alignment has been integrated in order to minimise the spectral resource loss in the guard
band. The UP1 and UP2 sections are used by the ground station to communicate with the
mobiles providing also synchronisation information, acknowledgment and allocated slots to
the mobiles. A priority management scheme has been included to increase the efficiency of
the L-DACS2 performance in term of latency for the highest QoS services. The CoS2 section
is reserved for downlink reserved transmission or random transmission. The protocol
applicable to each section is provided.
The L-DACS2 system is designed to handle performance regimes spanning several
operational scenarios from low level airport operations to TMA and En-route in high-density
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airspace, while covering a range of demand profiles for handling of ATS and AOC services.
The system has a built-in efficient air-initiated cell handover mechanism, which uses aircraft
knowledge of cell locations and characteristics – through on-board databases, Electronic
Flight Bags (EFB) or a Common Signalling Channel (CSC). There is a specific section in the
frame, (LoG2), designed for the insertion in the cell. The login mechanism and hand over are
described and illustrated in this document.
1.4.2
Outline of the specification
The specification presented herein focuses on elements of the L-DACS2 system design that
are relevant for the subsequent development of the ICAO standard. This specification may
require further iterations after completion of this ongoing EUROCONTROL activity. These
are expected to be carried out with the framework of the SESAR JU development activities
(WP 15).
This document is structure as follows:
•
Chapter 1 – provides general overview of the L-DACS concept and an overview of
the L-DACS2 main characteristics. It explains the scope of the L-DACS2 study and
presents the team behind this activity.
•
Chapter 2 – provides the general architecture of the L-DACS2 system.
•
Chapter 3 – provides the physical layer specification.
•
Chapter 4 – provides the Medium Access sub layer specification.
•
Chapter 5 – provides the Data Link sub layer specification.
•
Chapter 6 – provides the Link Management Layer specification.
•
Annex 1 – provides the burst structure in each frame section.
•
Annex 2 – provides the L-DACS2 message structure.
•
Annex 3 – provides example of system operations.
•
Annex 4 – provides first analysis of the coding scheme performance.
•
Annex 5 – provides an analysis of the impact of co-channel and adjacent channel
interference.
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CHAPTER 2 –
Functional Architecture
2.1
Introduction
L-DACS2 shall be able to simultaneously handle at least 2001 aircraft per cell in high-density
airspace.
L-DACS2 shall have an efficient air-initiated cell hand-over mechanism, which uses aircraft
knowledge of ground station cell locations and characteristics to ensure a handover step
without affecting the actual Quality of Service (QoS).
L-DACS2 shall also have a ground-requested air-initiated cell hand-over mechanism for use
when needed.
Note. – The information about cell parameters, ground station cell locations and the
frequencies necessary for the aircraft to initiate and maintain contact may be available
through an on-board database (uploaded/updated before the start of the flight).
Note. – The deployment of L-DACS2 is considered in the lower of ARNS frequency band (i.e.
960 – 975 MHz) in support of new ATM point-to-point services requiring a high QoS, giving
support to SESAR or NEXTGEN future concept. The VHF band constitutes also a potential
frequency band for the L-DACS2 extension.
L-DACS2 is designed to be flexible and configurable, for use for point-to-point and broadcast
communications. The aircraft can use L-DACS2 to communicate with aircraft (air/air point to
point) as well as with the ground station (using the appropriate channels), and the ground
station can selectively communicate with individual or all aircraft (multicast capability).
As far as possible, in order to reduce the time to market and some validation aspects,
existing GSM radio technology is proposed to be used.
Broadcast services could be provided in a segregated channel using specific system
configuration parameters. This service is not addressed in this document.
Air-to-air data communication could be provided in other segregated channels using specific
1
Simulations have shown that a single cell can support 204 aircraft. In most cases this is possible on a single channel. For the
densest scenarios with combined ATS and AOC traffic, two channels will be required with 102 aircraft served on each
channel (see section Error! Reference source not found.).
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system configuration parameters. This service is not addressed in this document.
Note. – L-DACS2 could support in a seamless manner AOC data communications if the
necessary extra spectrum is available.
2.2
Infrastructure
The ground L-DACS2 infrastructure shall comprise a number of L-DACS2 Ground Radio
Stations, which are organized into clusters.
Note. – Typically, the Ground Radio Stations in a cluster will be geographically adjacent,
providing overlaps of coverage (using different frequencies) in order to achieve the cell
handover in a transparent manner (“make” before “break” concept).
Each Ground Radio Station in a cluster shall be connected to some redundant concentrator,
the Ground Network Interface (GNI), which interfaces it to the transport network.
Note. – This interface may be via an ATN Air/Ground Router.
The Air/Ground Routers supporting each cluster shall be interconnected by a ground
transport network. This network shall also support Ground/Ground Routers for
interconnection with end-users.
Note. – From this description, the ATN A/G Routers are ground-based users of the L-DACS2
sub-network service and the airborne ATN routers are mobile users of the L-DACS2 subnetwork service.
2.3
Communication link
The aircraft communications system support air – ground communication in point to point.
The L-DACS2 system also supports broadcast air – ground communication in dedicated
channels. The aircraft communication system exchanges information with the station in
charge of the cell in which the aircraft is located. This ground station shall provide all the
communication services required to maintain the communication over the whole logged
mobile and the station.
Note. – The L-DACS2 system could be adapted to handle future air-air point to point and
broadcast services but those services are not developed in this document. This document
only addresses air – ground point to point services.
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CHAPTER 3 –
Air interface (physical layer)
3.1
Introduction
The physical layer shall provide the following functions:
transmitter and receiver frequency control;
digital signal reception and demodulation and decoding by the receiver;
digital signal coding modulation and transmission by the transmitter; and
notification services.
3.1.1
Transmitter/receiver frequency control
The L-DACS2 physical layer shall set the transmitter or receiver frequency as commanded
by the LME.
3.1.2
Digital reception by the receiver
The receiver shall demodulate and decode input signals and forward them to the higher
layers for processing.
3.1.3
Digital transmission
The physical layer shall appropriately encode, modulate and transmit information received
from higher layers over the RF channel.
The L-DACS2 system shall make use of specific channels for air/ground point-to-point
communications in a given cell. The channel shall allow air stations to have an exclusive slot
per frame for regular or high-QoS transmissions, with more slots available in the same frame
on request.
Note. – The frame is fully defined in Chapter 4. Each time the word ‘frame’ is used refers to
the frame described in Chapter 4.
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3.2
3.2.1
Modulation
Physical layer
For L-DACS2, a robust physical layer shall be used, based on the GMSK modulation scheme
used in commercial implementations like GSM. The physical layer will implement strong data
coding for achieving the highest QoS in terms of latency by minimising the need for reemission of messages.
3.2.2
Standards
The L-DACS2 Physical Layer uses features and characteristics of GSM, for which
international standards are available ([4], [5], [6], [7], [8], [9]).
3.2.3
Modulation scheme
The modulation scheme shall be GMSK. Binary ones and binary zeros shall be generated
with a modulation index h of 0.5 and a BT product of 0.3.
Note. – The modulated signal is passed through a band-limited linear filter. The type of the
filter used has a Gaussian impulse response in the time domain.
Note. – GMSK is a pre-filtered variant of the CPFSK which is used by UAT. It is popular in
the commercial mass market and is used with different characteristics in GSM, DECT and
TETRAPOL. Hence it offers significant advantage in cost reductions for the development and
manufacture of avionics and ground-based equipment.
Note. – L-DACS2 will use GMSK as the baseline modulation with the same GMSK
parameters as defined in [9]. Provisions are made to allow migration to higher order
modulation schemes such as those used by GSM evolutions such as GPRS and EDGE,
when the operational needs and traffic densities foreseen for Europe in the future warrant
such evolution.
3.2.4
Modulation rate
The modulating symbol rate shall be 1/TS = 1625/6 ksymb/s (i.e. approximately 270.833
ksymb/s), which corresponds to 1625/6 kbit/s (i.e. approximately 270.833 kbit/s), where T is
the symbol period.
Note. – In GMSK, 1 symbol is equivalent to 1 bit.
Note. – One symbol period or 1 bit has a duration of T = 6/1625000 s or approximately
3.6923 microseconds.
Note. – A logical 1 causes the carrier phase to increase by 90o over a bit period and a logical
0 causes the carrier phase to decrease by 90o. This phase change is produced by
instantaneously switching the carrier frequency between two different values f1 and f2:
f1 = fc + Rb / 4
(1)
f2 = fc − Rb / 4
(2)
where Rb is the modulation rate (1625/6 kb/s) and fc is the nominal carrier frequency.
3.2.5
Time/amplitude profile of L-DACS2 transmission
The Reference Time shall be defined as the beginning of the first bit of the synchronisation
sequence (the start of the Active part of the burst) appearing at the output port of the
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equipment.
Total burst duration
Start flag
8 bits
29.54 µs
Inactive
transmitter
Ramp down
Sync
sequence
26 bits
96.00 µs
End flag
Ramp up
8 bits
29.54 µs
User data, FEC, CRC
8 bits
8 bits
29.54 µs 29.54 µs
Active part of the burst
(bit transmission)
Inactive
transmitter
Reference Time
Figure 3: Time/amplitude profile of LDACS2 transmission
Between the inactive state of the transmitter and the transmission of the first bit of the
synchronisation sequence of a burst (the Reference Time), a ramp-up period of 8 symbol
periods, equal to 8 bits or 48/1625000 s (approximately 29.54 microseconds) shall be
respected.
Note. – The ramp up and ramp down period in currently available GMSK modulators are
each 28 microseconds per [9]. In order to reduce transient spectral emissions during the
ramp up and ramp down periods it may be necessary to increase the ramp up and ramp
down times to limit the impact of the transients on the spectral footprint. It is anticipated that
this may be one of the outcomes of the prototyping stage. For now, until prototype tests
prove otherwise, it is recommended to use a ramp period at least as long as per [9], as this is
known to be achievable in the GSM system.
Prior to 8 bit periods before the Reference Time, the RF output power at the point at which
the cable connects with the transmit antenna shall not exceed –80 dBm.
Within 8 bit periods after the end of the Active part of the burst, the RF output power at the
point at which the cable connects with the transmit antenna shall fall to a level not exceeding
–80 dBm.
3.2.6
Ambiguity resolution and data transmission
A synchronisation sequence consisting of 26 bits shall be transmitted following the ramp-up
time. The synchronisation specifications are described in section 3.3.
A start flag consisting of 8 bits shall be transmitted following the synchronisation sequence.
The transmission of the first bit of user data shall start 42 bit intervals (approximately 155.08
µs) after the nominal start of transmission.
Note. – Between the nominal start of transmission and the first bit of user data the 3-bit ramp
up time, a 26-bit synchronisation sequence, and an 8-bit flag are transmitted.
Note. – The nominal start of transmission is the start of the ramp-up period of a burst. The
start of the ramp-up period of a burst can in some sections of the frame coincide with the
start of a slot; in other sections it can be delayed relative to the start of a slot due to a guard
time being required to be maintained at the start of the slot; and in some sections of the
frame the burst can be started ahead in time of the start of a slot in order that the burst will
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coincide with the start of a slot upon reception, after propagation delay.
A Cyclic Redundancy Code (CRC) field shall be transmitted after the user data field and
used to provide detection of user data message errors.
A Forward Error Correction (FEC) field shall be transmitted and used to provide correction of
user data message errors.
An end flag consisting of 8 bits shall be transmitted after the FEC, immediately prior to the
ramp down period.
Note. – Some provisions for the CRC are made in the L-DACS\2 system in order to increase
detection and correction capability. This CRC should be design at the transport layer in order
to ask retransmission of the packet instead of the overall applicable message. This is out of
the scope of the specification and should coordinate with cross layer QoS management even
if provisions are made to implement the CRC. For the lower layer, the FEC gives sufficient
protection to ensure the robustness of the link.
3.2.7
Receiver - transmitter turnaround time
An L-DACS2 station shall be capable of beginning the transmission of the transmitter power
stabilization sequence within 2 milliseconds after terminating the receiver function.
An L-DACS2 station shall be capable of receiving and demodulating with nominal
performance an incoming signal within 2 milliseconds after completing a transmission.
Note. – It is likely that a L-DACS2 ground station will be organised with a transmitter/receiver
separated architecture
Note. – It is likely that an L-DACS2 airborne station will be organised with a
transmitter/receiver separated architecture including two receivers to allow transparent cell
handovers.
3.2.8
Frequency change during transmission
The phase acceleration of the carrier from the start of the synchronization sequence to the
data end flag shall be less than 300 Hz per second.
3.3
3.3.1
Air interface
Radio frequencies
The radio frequencies used shall be selected from the radio frequencies in the band 960 –
975 MHz. The lowest assignable frequency shall be 960.5 MHz (for spectrum compatibility)
and the highest assignable frequency shall be 975 MHz.
Note – The 500 KHz guard band in the lowest part of 960 – 975 MHz band aims to protect
the telecommunication systems localized bellow 960 MHz against the L-DACS2 emission.
The available spectrum shall be partitioned into a number of channels, each 200 KHz wide.
Each of these bands shall be occupied by a GMSK modulated RF carrier supporting a
number of TDMA time slots.
The RF carriers may be aggregated in two or more paired combinations to accommodate
different operational load in one cell. The separation between assignable frequencies that
guarantees non interference between frequencies assigned to the same cell (channel
spacing) shall be at least 600 kHz.
Note – However, the separation between assignable frequencies can be reduced by
imposing minimum clearance distance between the ground based antennas and the closest
operating aircraft. The conditions and constraints under which this can be made possible are
described in the Annex 5.
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3.3.2
Channel bandwidth
The L-DACS/2 channel shall have a nominal bandwidth of 200 kHz for a single channel.
For dense cells requiring high levels of capacity, the L-DACS2 channel may be combined in
aggregations of two or more 200 kHz channels.
Note – The process of channel aggregation may place practical constraints on the allowable
distance between the ground station antenna and the nearest operating aircraft if no guard
bands are used. The constraint exists in order for the ground station to be able to detect
weak signals coming from aircraft far a field in the adjacent channel. Further information on
these constraints is furnished in the Annex.
Note – The only constraint that exists whether adjacent channels are used or not, is the need
to synchronise the two frequencies so as one is not trying to receive whilst the other
transmits. In the absence of synchronisation, the transmissions from one channel may
completely wipe out reception on the other channel.
It has been found through simulation that a single 200 kHz channel offers sufficient capacity
to serve most operational scenarios. For scenarios requiring more than 132 kbit/s of pure
user demand, two 200 kHz channels will be required.
Note – This can be made possible by aggregating two (or more) channels as required, by
sharing the loading of the cell across these channels. In practise, there are a number of ways
to achieve this. One way is to perform segmentation of the cell using sectored antennas as is
done in the GSM system to enhance the spectral efficiency of the system and simplify
planning (as opposed to sharing a single omni-directional antenna).
The following table summarises the channel bandwidth to support a single L-DACS2 cell
under various loading conditions. These figures were obtained from the results of a
campaign of simulations considering the various scenarios defined in the COCR. These
scenarios are listed in the left column of the table.
Scenario
Services
Number of User
aircraft
demand
Channels
required
Total
bandwidth
TMA (large)
ATS and
AOC
52
47 kbps
1
200 kHz
Enroute
(medium)
ATS and
AOC
64
55 kbps
1
200 kHz
Enroute (large including AOC
services2)
ATS and
AOC
2043
188 kbps
2
400 kHz4
Table 1 – Total channel bandwidth for various cell sizes
All the simulations have been carried out using 200 kHz channels. The scenarios presenting
the highest user demand were considered.5 This includes the en route large scenario, the
loading of which was split between two 200 kHz channels with 102 aircraft assigned to each
respective channel. The simulations results have thus shown a single 200 kHz channel in the
basic configuration to support all but the heaviest loaded scenario. In the latter case, two
channels will be required.
2
3
4
5
This includes WXGRAPH service.
102 aircraft per channel.
Two 200 kHz channels.
TMA large (52 aircraft), En route Medium (62 aircraft) and En route large (204 aircraft distributed across two 200 kHz
channels).
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A summary of the latency and throughput performance offered in these scenarios is
furnished in the following tables. These tables provide simulation statistics generate indicated
evidenced in the caption of each table. The tables are divided into a number of columns
headed by five latency requirements derived from [9]. These latencies are 95th percentile
figures and are comparable to the 95th percentile figures achieved in the L-DACS2
simulations (figures in bold in the last row).
Required Latency
(s) (TT95)
Max latency (s)
Average (s)
90th percentile (s)
95th percentile (s)
1.4
2.02
0.57
0.96
1.04
2.4
2.93
0.57
1.00
1.11
4.7
1.97
0.66
1.12
1.17
13.6
6.44
1.18
2.22
2.64
26.5
6.85
0.87
1.65
1.94
Table 2 – Latencies for TMA Large scenario (52 aircraft)
Required Latency
(s) (TT95)
Max latency (s)
Average (s)
90th percentile (s)
95th percentile (s)
1.4
1.46
0.54
0.91
1.01
2.4
1.59
0.57
0.96
1.01
4.7
0.94
0.53
0.93
0.94
13.6
2.46
0.89
1.42
1.52
26.5
2.27
0.71
1.18
1.41
Table 3 – Latencies for En route Medium scenario (62 aircraft)
Required Latency
(s) (TT95)
Max latency (s)
Average (s)
90th percentile (s)
95th percentile (s)
1.4
2.08
0.68
1.10
1.21
2.4
7.67
0.85
1.58
2.14
4.7
5.52
0.80
1.33
1.64
13.6
7.48
1.36
2.49
2.91
26.5
11.90
1.20
2.24
2.97
Table 4 – Latencies for En route large scenario (102 aircraft per channel)
Scenario
Channel
Number of Achieved
bandwidth channels
throughput
TMA
LRG
aircraft)
(52 200 kHz
1
99.99%
ENR MED
aircraft)
(62 200 Khz
1
99.89%
ENR LRG
aircraft6)
(204 200 kHz
2
99.63%
Table 5 – Throughput achieved
6
102 aircraft on each channel
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3.3.3
Polarization
The design polarization of emissions shall be vertical.
3.3.4
Ground Frequency stability
The radio frequency of L-DACS2 ground station equipment shall not vary more than
±0.00001% (1 parts per million) from the assigned frequency.
3.3.5
Aircraft Frequency stability
The radio frequency of L-DACS2 aircraft equipment shall not vary more than ±0.00005% (5
parts per million) from the assigned frequency.
3.3.6
Spurious emissions
Spurious emissions shall be kept at the lowest value which the state of the technique and the
nature of the service permit.
The spectrum mask is presented in the following table considering that the power output of LDACS2 is higher than Power 43 dBm. For frequency offset below 1.8 MHz from the central
frequency, the reference bandwidth is 30 kHz, and for offset higher than 1.8 MHz from the
central frequency, the reference bandwidth is 100 kHz:
Frequency
offset from the
carrier (kHz)
100
200
300
500
600 to
1200
1200
to
1800
1800
to
6000
6000
to
10000
Attenuation
threshold (dBc)
– On board
-8.2
-35.2
-38.5
-75.2
-85.2
-85.2
-90
-90
Attenuation
threshold (dBc)
– Ground
-8.2
-35.2
-38.5
-75.2
-85.2
-92.2
-97
-97
Table 6 – Spectrum mask (dB)
Note. – This mask is achievable using a Surface Acoustic Wave (SAW) filter and a preselector filter before the amplificatory stage. A control of the output of the amplificatory stage
shall guarantee the spectral quality of the emission. (Ongoing study evaluates the impact of
this mask on the Eb/N0 needed to achieve a 10-3 BER, roughly estimated around 1 dB).
Note. – L-DACS2 out-of-band emissions are expected to comply with ITU-R SM. 329-10: the
spurious domain consists of frequencies separated from the centre frequency of the emission
by 250% of the necessary bandwidth of the emission. A reference bandwidth is a bandwidth
in which spurious domain emission levels are specified. The following reference bandwidths
are used:
•
100 kHz between 30 MHz and 1 GHz from the carrier,
•
1 MHz above 1 GHz from the carrier.
According to ITU-R SM. 329-10, the maximum permitted spurious domain emission power in
the relevant reference bandwidth is -70 dBc. The spectrum emission mask is described
above is in line with the ITU regulation. The following table presents the ITU regulation on
spurious emission:
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Frequency offset from the central
frequency
f > f0 +0.5 MHz or f < f0 – 0.5 MHz
Permitted spurious
domain emission,
dBm
-70 dBc
Reference
bandwidth, kHz
100
Table 7 – Spurious domain emissions ITU regulation
3.3.7
Error phase specification
For any 148-bits subsequence of the 511-bits pseudo-random sequence, defined in CCITT
Recommendation O.153 fascicle IV.4, the phase error trajectory on the useful part of the
burst (including tail bits), shall be measured by computing the difference between the phase
of the transmitted waveform and the phase of the expected one. The RMS phase error
(difference between the phase error trajectory and its linear regression on the active part of
the time slot) shall not be greater than 5° with a maximum peak deviation during the useful
part of the burst less than 20°.
3.3.8
Broad band noise
The Broad band noise shall be equal to -150 dBc/Hz (relative to carrier) at frequency offset
from the carrier > +/- 10 MHz.
Note. – This figure is sufficient for compatibility with GSM receiver. The minimum required
distance between L-DACS2 Tx and a mobile GSM receiver is roughly estimated below 2 m.
3.3.9
Connection management
The L-DACS2 system shall establish and maintain a reliable communications path between
the aircraft and the ground system while allowing but not requiring manual intervention.
3.3.10
Transmission
The physical layer shall encode the data received from the data link layer and transmit it over
the RF channel. RF transmission shall only take place when it is permitted by the MAC.
3.3.11
Channel
L-DACS2 channel shall be associated with one cell. L-DACS2 is a cellular system; frequency
reuse pattern shall be implemented in order to optimise the coverage and the frequency
band used.
Note. – First considerations toward the channel reuse and deployment is addressed in Annex
5.
3.3.12
Broadcast
L-DACS2 should use specific channels for broadcast communications.
Note. – The L-DACS2 broadcast channels use a VDL Mode 4 modified MAC structure and
frame structure (see Section 4.8). The requirements for Broadcast communication are not
further discussed in this document.
3.3.13
Air-to-air point-to-point communication
L-DACS/2 should use specific channels for air-to-air point-to-point communications.
Note. – The L-DACS/2 Air-to-air point-to-point channels use a VDL Mode 4 modified MAC
structure and frame structure (see Section 4.9). The requirements for air to air point to point
communication are not further discussed in this document.
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3.3.14
System performance
The system performance is designed to provide a required residual corrected Bit Error Rate
(BER) of 10-7 on the basis of a Physical Bit Error Rate (BER) of 10-3.
3.3.15
Link Budget
The system parameters retained for the link budget calculation are listed below:
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L-DACS2 System Definition proposal: Deliverable D2
TX Parameters
L-DACS2 TX ouput Power
Unit
dBm
Maximum TX Antenna Gain
Tx Cable loss
TX EiRP
dBi
dB
dBm
Propagation Parameters
Transmit upper Frequency
Tx-Rx Distance
MHz
Nm
Path Loss
Miscellaneous Margins
Interference Margin
Implementation Margin
Safety Margin
Banking Loss Margin
UL
DL
ENR
ENR
ENR
TMA
APT
ENR
ENR
ENR
TMA
APT
Governing Equation
55,44068 55,44068 55,44068 55,44068 55,44068 46,9897 46,9897 46,9897 46,9897 46,9897 a
8
8
8
8
8
2,5
2,5
2,5
2,5
2,5
60,94068 60,94068 60,94068 60,94068 60,94068
0
3
43,9897
0
3
43,9897
0
3
43,9897
0
3
43,9897
0b
3c
43,9897 d =a + b - c
977
200
977
120
977
60
977
40
977
10
977
200
977
120
977
60
977
40
977 e
10 f
g = 37,8 + 20log(f) +
117,60 20log(e)
dB
143,62
139,18
133,16
129,64
117,60
143,62
139,18
133,16
129,64
dB
dB
dB
dB
0
0
6
0
0
0
6
0
0
0
6
0
0
0
6
7
0
0
6
7
0
0
6
0
0
0
6
0
0
0
6
0
0
0
6
7
Maximum RX Antenna Gain
Rx Cable loss (incl. Duplexer)
dBi
dB
0
3
0
3
0
3
0
3
0
3
8
2,5
8
2,5
8
2,5
8
2,5
L-DACS2 RX receive signal
Thermal Noise Density@290K
Bandwidth
Thermal Noise Power
Receiver Noise Figure
Composite Noise Figure
dBm
dBm/Hz
Hz
dBm
dB
dB
-82,68
-174
200000
-120,99
10
13
-78,24
-174
200000
-120,99
10
13
-72,22
-174
200000
-120,99
10
13
-68,70
-174
200000
-120,99
10
13
-56,66
-174
200000
-120,99
10
13
-91,63
-174
200000
-120,99
7
9,5
-87,19
-174
200000
-120,99
7
9,5
-81,17
-174
200000
-120,99
7
9,5
-77,65
-174
200000
-120,99
7
9,5
-65,61
-174
200000
-120,99
7
9,5
n=d-g+l-m
o
p
q = o + 10log(p)
r
z
Total Rx Noise Power
Eb/No @ BER=10-3
L-DACS2 bit rate
Required C/N
L-DACS2 Rx Sensitivity
L-DACS2 C/N available
L-DACS2 net margin
@BER=10-3
dBm
dB
bps
dB
dBm
dB
-107,99
10
270833
11,32
-96,67
25,31
-107,99
10
270833
11,32
-96,67
29,75
-107,99
10
270833
11,32
-96,67
35,77
-107,99
10
270833
11,32
-96,67
39,29
-107,99
10
270833
11,32
-96,67
51,33
-111,49
10
270833
11,32
-100,17
19,86
-111,49
10
270833
11,32
-100,17
24,30
-111,49
10
270833
11,32
-100,17
30,32
-111,49
10
270833
11,32
-100,17
33,84
-111,49
10
270833
11,32
-100,17
45,88
s = q + z + i + [h + j + k]
t
u
v = t + 10log(u/p)
w=v+s
n-s
8,00
12,43
18,45
14,97
27,02
2,54
6,98
13,00
9,52
0
0
6
7
h
i
j
k
Notes
Tx_Pout - (UL: 350W) - (DL: 50W)
TX_AntGain - (UL: DME antenna reference) - (DL:
Antenna Gain = 3dB - polarization loss = 3dB)
TX_CableLoss - (UL: 50*0.04 + 2*0.25)
TX EiRP = TX_Pout + TX_AntGain - TX_CableLoss
Free Space model (using nm unit)
InterfMargin(TBD)
ImpMargin
SafetyMargin(TBD)
Banking(TBD)
RX Parameters
dB
Figure 4 – L-DACS2 Link Budget
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8l
2,5 m
21,56 n-s-v
RX_AntGain - (UL: Antenna Gain = 3dB - polarization loss
= 3dB) - (DL: DME antenna reference)
RX_CableLoss - (UL: 50*0.04 + 2*0.25)
RxPower = TX_EiRP - PathLoss + Rx_AntGain Rx_CableLoss
10log(kT )
BW
10log(k.T) +10log(BW )
Rx_NF
Composite noise including Rx noise and cable loss
N = Rx_NF + 10log(k.T.BW)
Eb/No
R
C/N = Eb/No + 10log(R/BW)
Cmin = C/N + N
Considering ImpMargin and [other margins above]
L-DACS2 System Definition proposal: Deliverable D2
The Tx output power is set at 350 W in the ground station and 50 W in the airborne
equipment. This ground output power is achievable with a power amplifier and a power
supply of 50 V.
The ground cable loss has been fixed to 2.5 dB using 50 meters of a 7/8’ type cable (loss =
0.04 dB/m) and 2 m of a RG214U (loss = 0.25 dB/m) and connectors. The aircraft cable loss
is set to 3 dB in line with industrial input and L-DACS\1 assumptions.
The L-DACS2 system is based on half duplex communication architecture (see Chapter 4).
No diplexer either duplexer is needed to support the Rx and Tx function. An RF isolated
switch shall be used to commute between Rx and Tx function. The Rx – Tx turn around time
is specified in section 3.2.7.
The antenna gain for the ground is 8 dB, DME reference has been considered. The aircraft
antenna gain is set to 0 dB in line with the technology involved in UAT.
The sensitivity considered in this link budget concerns the whole receiver block, receiver plus
cable.
The Banking loss is in line with L-DACS\1 assumptions.
The safety margin has been set to 6 dB.
Note. – This link budget doesn’t take into account the antenna pattern for the different
operational scenario presented above.
Note. – The impact of adjacent channel interference and co-channel interference has also
been studied in order to confirm that the L-DACS2 system presents sufficient margin toward
the impact of intra-system interference. This evaluation has been done in a statistical way
and gives first ideas concerning the deployment of the system. The prototyping phase should
confirm this analysis provided in Annex 4.
Note. – The link budget includes implementation losses. The prototype phase should show
that extract net margin is sufficient to cope with fading losses and excess propagation losses.
3.4
Pulse blanking
Current pulsed interference mitigation techniques fall under two categories:
•
Time-domain approach, and
•
Frequency domain approach.
Pulse blanking is the time-domain method. It nulls out the portion where the amplitude of the
signal exceeds a certain threshold level with respect to the noise. Pulse blanking has a
number of advantages. It is simple to implement, it can be executed in real time without extra
delay and can be activated only when the interference exists. However, when blanking the
interference pulses, it also blanks out any other signals over that time slot. If the pulses are
very dense in time, all received signals including DME/TACAN pulses and LDACS signals
will be blanked, and data may be consequently lost due to unavailability of the signal.
Furthermore, because of the Gaussian pulse tailing effect, pulse blanking cannot completely
suppress the unwanted pulses and some residual interference will remain.
Notch filtering is an alternative means of mitigating pulse interference in the frequency
domain, where the pulsed signals e.g. DME and TACAN appear as narrow-band frequency
tones. If the signal spectral density at certain frequencies is above the noise spectral density,
these frequency components will be filtered out. Notch filtering can suppress DME/TACAN
pulsed interference, including the central part of the Gaussian pulse and the tails. It also
preserves the energy of the signal superposed with the interference pulses in the time
domain. However, it not only filters interference, but also removes the signal energy at the
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DME/TACAN frequencies. Even during the time period when there are no DME/TACAN
pulses, the LDACS signal at these frequencies is still suppressed. If there are multiple
DME/TACAN transponders nearby, the filter design may be excessively complicated due to
multiple notches in the filter.
Note. – Hybrid blanking is an alternative technique developed for operation of GNSS in the LBand that exploits the advantages of both pulse blanking and notch filtering. When an
interference pulse is detected (in the time domain), it triggers the notch filtering of a brief slice
(few microseconds) of data cantered at the estimated pulse position. Therefore, filtering is
only implemented when DME/TACAN pulses exist. In doing so, it overcomes the
disadvantage of regular notch filtering, which always filters out the corresponding frequency
components of the signal even when there is no interference. For the portions of data that
are covered by DME/TACAN pulses, hybrid blanking preserves most of the signal energy,
and thus overcomes the disadvantage of time-domain pulse blanking. The filter design is
simple, because there is only one notch in the filter. Although this technique has merits, it is
more complex to implement than simple pulse blanking and the narrow band nature of the LDACS2 (just 200 kHz) makes this solution less viable than for wideband signals.
Note. – One of the key problems for a future communication component operating in the Lband (960 -1215 MHz) is co-siting with other radio transceivers that operate in the same
frequency band. Even if a frequency separation is implemented, providing some decoupling
with other spurious emissions, the robustness of the new communication link will be
drastically affected by the proximity of other pulse transmitters on the same aircraft.
3.4.1
Protection of L-DACS2 transmissions
L-DACS/2 shall make use of Pulse Blanking Techniques, as necessary, to reduce the effect
of strong interference sources (that is, the case on board aircraft due to very small system
isolation) against L-DACS2 receiver damage.
Note. – Such a pulse blanking mechanism is defined in the UAT standards and has a
common suppression bus interconnecting the avionics elements that could benefit from the
information provided (pulse blanking signal whenever a transmitter is on). This primarily
immunizes the receivers from co-site transmissions which would otherwise damage them or
render inoperative for prolonged periods.
Taking into account that the main sources of interference for the new communication system
will be high powered, short pulsing transmitters (DME, UAT and SSR/Mode S), the duration
of the jamming pulses will be equivalent to or shorter than the L-DACS2 symbol duration for
which FEC data coding will be the appropriate answer to mitigate the impact of the
interference on the frame error rate.
Note. – The typical suppression pulse for a UAT burst is 510 microseconds. The impact of
the interference in his case will be therefore limited to a few bits in the frame for which FEC
data coding will be the appropriate answer to mitigate the impact of the interference on the
frame error rate. This means that the pulse blanking techniques are not required for limiting
the interference on L-DACS2 but are useful to protect the L-DACS2 receiver from physical
damages.
The use of the suppression bus is one of a number of methods to implement pulse blanking.
Therefore there is no hard requirement to define an interface to the suppression bus for the
L-DACS2 receiving function.
3.4.2
Protection of adjacent systems transmissions
L-DACS2 operates a power control scheme in order to limit interference on adjacent
systems. Consequently, when operating in the vicinity of a ground station, the power emitted
by the airborne transmitter is limited, and therefore unlikely to emit signals at damaging
levels for other equipment on board the aircraft. Conversely, when operating close to the
edge of the cell, the airborne radio radiates at maximum power, and the use of pulse
blanking should be required to protect other L band receiving devices on board.
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When irradiating at maximum power, the L-DACS2 transmitter outputs an estimated 47 dBm
(50W). This is of sufficient strength to cause saturation of other L-band receivers, and
potential damage to their front end circuitry. When operating at such power levels, it is
therefore necessary that the L-DACS2 transmitter provides a pulse to activate the
suppression bus during the transmission interval. Given the similarities in the power budget
to that of UAT, it is considered feasible to use the same threshold cut-off of -20dBm for
activation of the suppression pulses7.
The suppression interval required for L-DACS2 is thus derived on a similar basis to the UAT
requirement for pulsed suppression. The L-DACS2 symbol period is approximately 3.69
microseconds (for 200 kHz channels), which is of similar length to the pulses emitted by the
DME and SSR systems. However, during the transmission of a burst, the L-DACS2
transmitter is active for a period of time during which the amplitude is stable and relatively
constant. The amplitude stability means that over the course of the active transmission, the
GMSK symbols in the payload part of the burst are transmitted within a constant power
envelope. The length of the transmission itself is several orders larger than that of the pulsed
DME and SSR systems and therefore unable to generate unsolicited SSR squitters, or false
DME returns.
The following table illustrates the impact of imposing a limitation to the transmission burst
length on the latency performance offered for the various service levels in the reverse link.
The results were obtained by simulating the heaviest available scenario (204 aircraft across
two channels). A variety of burst lengths are considered ranging from a single slot up until
eleven slots.
Note – The equivalent duty cycle is evaluated on a frame basis (one second), and provides a
means of assessing transmitter active time as a percentage of the frame length. The overall
channel loading due to user demand across a flight hour is considerably less (of order 0.3%).
Latency requirements (s)
Burst
length
limitation
(slots8)
Burst
payload
(bytes)
Frame
duty cycle
1.4
(%)
1
157
0.67
1.29
2.78
2.93
3.41
3.55
5
853
3.33
1.19
2.66
1.92
2.88
2.68
11
1897
7.33
1.17
1.86
1.39
2.75
2.53
2.4
4.7
13.5
26.6
Table 8 – Effect of imposing burst length limits in the Reverse Link
The above figures show that for most services, a burst length limitation of 1 slot will
accommodate the majority of services within the latency requirement. However, the burst
length is dimensioned by the largest transmitted messages of the FLIPINT and COTRAC
services, as summarised in the table below.
Service
Size
(Downlink)
Latency Requirement
FLIPINT
2763 bytes
2.4 s
COTRAC
1380 bytes
2.4 s
Table 9 – FLIPINT and COTRAC messages sizes (downlink)
7
8
Per the UAT Manual, the -20 dBm requirement was derived from the maximum power allowable without SSR transponders
generating unsolicited replies. These unsolicited replies occur when signal levels from the interferer signal envelope that lies
within the SSR transponder receiver band is above the transponder receiver threshold.
Slots in the COS2 segment.
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In these two cases, the minimum burst length possible is 9 slots and 5 slots respectively.
For example, the 2763-byte FLIPINT message will be transmitted in 2 seconds
(mod[2763/1549]) when using a 5 slot limit. However, since a fragment may occasionally be
missed and thereby retransmitted, an 11 slot limitation may be necessary to accommodate
the longest down linked messages within the 95th percentile latency requirement.
Note. – Data from simulations conducted on the basis of a large en - route scenario
considering an L-DACS2 cell serving 204 aircraft were used to estimate the expected worst
case duty cycle for the pulse blanking scheme. The simulation considered default L-DACS2
parameters.
Given the rise time of 3 tail bits for the GMSK signal (equivalent to approximately 11
microseconds), it is considered feasible to use the same suppression tolerances of 5
microseconds required for UAT. The worst case maximum suppression interval allowed at
any given time shall be 6.01 milliseconds including 5 microseconds before the start of the LDACS2 transmission Interval, and 5 microseconds after the end of the same transmission.
This suppression pulse shall be triggered when the L-DACS2 transmission exceeds -20dBm.
Note. – The validity of this threshold figure should be validated through trials.
3.5
Power control
In order to reduce the level of interference for point-to-point, the L-DACS/2 transmitter shall
have the capability of reducing the power of the transmitted signal using a power control
function. This shall be done by using a small capacity in the signalling channels in the uplink
frame section (afforded by the high capacity offered by the GMSK modulation option).
Note. – Power control may only be necessary for operation of L-DACS/2 in specific
environments; however provisions are made in the signalling message to implement such a
function.
Airborne installation – The effective radiated power shall be such as to provide an EIRP
power output between 0.95 and 15.02 V/m (200 mW to 50 W for the output power of the
transmitter) on the basis of free space propagation, at ranges and altitudes appropriate to the
conditions pertaining to the areas over which the aircraft is operating at 10 meters from the
emission. This range of EIRP output is calculated to obtain the same level of Rx signal input
from cell range of 10 Nm to 200 Nm.
The airborne transmitter minimum power is 200 mW.
The airborne transmitter maximum power is 50 W.
Ground installation – The effective radiated power shall be such as to provide an EIRP power
output between 7.99 and 105.72 V/m (2 W to 350 W for the output power of the transmitter)
within the defined operational coverage of the facility, on the basis of free space propagation
at 10 meters from the emission. This range of EIRP output is calculated to obtain the same
level of Rx signal input from cell range of 10 Nm to 200 Nm. For the ground installation, the
effective radiated power shall not change dynamically (see the note below).
The ground station transmitter minimum power is 2 W.
The ground station transmitter maximum power is 350 W.
Note. – The Ground station is supposed to reach all the mobiles in its operational range.
Power control is not useful for the Uplink transmission. But if several stations are deployed to
cover different airspaces (En-route, TMA, APT) the ground output power could be tuned to
cope with the operational needs.
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The power control management is described in section 4.8.
3.5.1
Received signals – ground
The ground base station shall perform a continuous measurement of the received signals
from each aircraft and then return appropriate update power adjustment information to each
aircraft ensuring the performance of link while optimising the power level at airborne side.
3.5.2
Received signals – aircraft
On reception of this information, the aircraft terminal shall use it to activate a power control
function.
Note. – Advantage can be taken from algorithms developed and validated for GSM.
3.5.3
Single Antenna Interference Cancellation
SAIC is a well known technology implemented in GSM to improve the interference mitigation
and more precisely to improve the frequency reuse factor and the mobile load in each cell. A
gain from 3 to 5 dB for the co-channel interference ratio is foreseen in certain context using
SAIC compliant mobile in GSM. Furthermore, this co-channel and adjacent channel
interference mitigation technique has an efficiency improvement if the global network is
synchronised.
To cope with the L-band environment and to enforce the global capacity of L-DACS2 system,
it is recommended that a SAIC feature is implemented in the L-DACS2 receiver equipment.
Note. – Different SAIC family algorithms exists Joint Demodulation (JD), also called Joint
Detection and Blind Interference Cancellation (BIC). Both types of algorithm are well suited
for GMSK modulation. It is observed from GSM simulation that the JD algorithm has better
performance in interference mitigation than BIC algorithms for some scenarios but as a
counter part JD algorithm complexities the receiver by 5 whereas the BIC algorithms induces
a multiplication by 3 of the receiver complexity.
Note. – SAIC techniques have not been taken into account in L-DACS2 design. Anyway
considering the potential benefit of SAIC, the prototyping stage should make provision to
implement such techniques to evaluate or improve L-DACS2 spectral compatibility.
3.6
3.6.1
Interference immunity
Transmitting function
3.6.1.1
Adjacent channel emissions
The amount of power from an L-DACS2 ground transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the first adjacent channel
shall not exceed -28.2 dBc.
The amount of power from an L-DACS2 ground transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the second adjacent channel
shall be less than -33 dBc.
The amount of power from an L-DACS2 ground transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the third adjacent channel
shall be less than -77 dBc and from thereon it shall monotonically decrease at the minimum
rate of 5dB per octave to a maximum value of -41.6 dBm.
The amount of power from an L-DACS2 airborne transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the first adjacent channel
shall not exceed -28.2 dBc.
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The amount of power from an L-DACA2 airborne transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the second adjacent channel
shall be less than -33 dBc.
The amount of power from an L-DACS2 airborne transmitter under all operating conditions
when measured over a measurement bandwidth of 200 kHz in the third adjacent channel
shall be less than -77 dBc and from thereon it shall monotonically decrease at the minimum
rate of 5dB per octave to a maximum value of -50 dBm.
3.6.2
Receiving function
3.6.2.1
Specified error rate
The specified error rate for L-DACS2 operation shall be the maximum uncorrected Bit Error
Rate (BER) of 1 in 103 (0.1%).
3.6.2.2
Reference sensitivity level
The aircraft receiving function shall satisfy the specified error rate with desired signal
strength of not more than -96.67 dBm. The above specification shall be met by the aircraft
receiving function with the two adjacent timeslots 20 dB above the own timeslot.
The ground receiving function shall satisfy the specified error rate with desired signal
strength of not more than -100.17 dBm. The above specification for BS shall be met when
the two adjacent timeslots to the wanted are occupied with signals at 50 dB above the power
on the wanted timeslot.
Note. – The sensitivity figures specified in this chapter concern the whole receiver block,
receiver plus Rx cable.
Note. – The above physical layer requirement is derived from [9] recommendations.
Note. – This considers static sensitivity in the absence of fading. Once fading conditions are
included, although baseband dependant, typically 3 dB extra signal-to-noise is required.
Blocking tests carried out using GSM equipment have demonstrated good BER margin over
the GSM specification of 2% even with a blocker at 2 dB over the required level.
3.6.2.3
Adjacent band immunity performance and co-channel immunity
performance
The receiving function shall satisfy the specified error rate in section 3.6.2.1 with a desired
signal input level of 10 dB above the reference sensitivity level, and for a random,
continuous, GMSK-modulated interfering signal.
The reference interference ratio for the ground and airborne receiving function shall be as
defined in the following table.
For co-channel interference
for adjacent (200 kHz) interference
for adjacent (400 kHz) interference
for adjacent (600 kHz) interference
9 dB
-9 dB
-41 dB
-49 dB
Figure 5 – Adjacent band and Co-channel band reference interference ratio
The interference ratio defined above lead to the evaluation of the adjacent frequency band
and co-channel frequency band for frequency reuse and the use of multiple channels in one
cell (see Annex 5).
The minimum frequency guard band between 2 adjacent frequencies assigned to 2 adjacent
cells shall be 400 kHz.
The minimum frequency guard band between 2 frequencies assigned in the same
operational cell shall be 600 kHz.
Note. – The actual interference ratio is defined as the interference ratio for which the BER of
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L-DACS2 System Definition proposal: Deliverable D2
10-3 is met.
3.6.2.4
Out-of-band immunity performance
The airborne receiving function shall satisfy the specified error rate with a desired signal field
strength of not more than -82.68 dBm and with an undesired signal at least 59 dB higher
than the desired signal on any assignable channel 800 kHz away from the assigned channel
of the desired signal.
The ground receiving function shall satisfy the specified error rate with a desired signal field
strength of not more than -91.63 dBm and with an undesired signal at least 59 dB higher
than the desired signal on any assignable channel 800 kHz away from the assigned channel
of the desired signal.
Note. –The range assumption used is 200 Nm.
3.6.2.5
Interference immunity performance
The receiving function shall satisfy the specified error rate with a desired signal field strength
of not more than -131 dBV/m, and with one or more out-of-band signals (except for VHF FM
broadcast signals) having a total level at the receiver input of -33 dBm.
3.7
FEC mechanism
The error coding scheme shall be based on concatenated code based upon convolutional
coding (inner code) and RS coding (outer code).
Note. – The use of concatenated error coding is also considered to make the objective of a
C/I of 9 dB in co-channel interference attainable.
Assuming a BER without coding at 10-3, the BER at the output of the RS coder shall be at
least at 10-7.
Note. – An interleaver schemes should be added. Two specific types of interleavers
applicable to AMACS are block interleaving and diagonal interleaving. Further evaluation
should be performed by running software simulations on a simplified transmission chain (no
synchronization process, framing), with the appropriate transmission channel model, which
could be the Ricean channel.
Note. – First discussion shown that the use of turbocode is not appropriated for short
messages. Indeed the coding scheme is too heavy compared to the benefits. Another
possibility is to code the whole uplink section as this section could be concatenated to form
one long message. But this technique implies that one aircraft must receive the whole
message before decoding. The decoding process is iterative and could have a non-negligible
processing time. Therefore this idea was also rejected. A last idea to investigate could be to
gather in the beginning of the uplink section all the sensitive signalisation information to be
sent by the aircraft (framing message, Clear-To-Send (CTS) message and synchronisation
parameters) and to protect it with a stronger coding scheme like turbocode. This method
needs further investigation.
Note. – A discussion is provided in Annex 4 regarding coding and interleaving issues.
3.7.1
Inner code: convolutive punctured code
The role of the convolutive code is to remove efficiently isolated errors. The well-known
convolution code with a constraint length (LC) of (LC = 7, 171,133), already used with
several puncturing scheme in DVB, will provide a range of performance regarding correction
and coding rate.
To be able to match the expected performances, the inner code coding rate shall be set to ¾
or 4/5.
Decoding shall be soft-decision Viterbi decoder.
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3.7.2
Interleaver
The interleaver depth shall be equal to one slot. This will ensure total independence of each
communication.
Note. – The use of bloc interleaver or diagonal interleaver shall be assessed through proper
simulations.
3.7.3
Outer code: Reed Solomon code
Different RS codes have been considered, but because of:
•
the relative length of a burst of error,
•
the interleaver depth
•
the length of a codeword
RS(31,X,5) do not seem appropriate. RS(15,11,4) should provide better performance,
because as the codeword is shorter, the errors will be spread over more codeword, leading
to a relative smaller number of errors per codeword. A complete discussion is proposed in
Annex 4.
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CHAPTER 4 –
MAC sublayer
4.1
Introduction
The MAC sublayer shall be the sublayer that acquires the data path and controls the
movement of bits over this data path.
The MAC sublayer shall acquire the shared communication path in order to provide the
services defined in this chapter.
Note. – The functions performed by the MAC sub-layer should be “transparent” to higher
functional layers.
Note. – For air/ground point-to-point communication L-DACS/2 has a high-integrity
deterministic MAC sublayer, employing deterministic slot scheduling. Future extension to airair and broadcast functionality may employ self-organising TDMA principles in addition to
deterministic slot scheduling.
Note. – The L-DACS2 frame length is designed for fast delivery of time-critical messages and
has been set at 1 second. Simulations have shown the validity of this choice to achieve the
highest performance requirements.
Note. – A QoS system is proposed to permit the use of channel resources according to the
message transmit priority required. Specific channel slots are reserved for Request to Send
(RTS) messages. This system allows the ground station, upon receipt of requests to send
data corresponding to different priority categories, to prioritise the transmission of data with
high priority.
4.1.1
Provision
The MAC sub-layer shall provide:
•
TDMA media access;
•
time synchronization of the start of uplink and downlink slots in the channel;
•
transmission processing;
•
received transmission processing.
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The MAC sub-layer shall receive from the L-DACS2 services sub-layer (LSS) a burst number
for transmission, and the time of transmission. The MAC sub-layer shall provide the LSS with
the received burst data, slot busy/idle status, slot occupancy status, signal level, and the
status of the bursts sent for transmission.
4.1.2
MAC Layer for point-to-point and broadcast communication
The MAC layer structure and usage for point to point communication shall be as specified in
Sections 4.1 to 4.7.
The MAC layer structure and usage for broadcast communication shall be as specified in
Section 4.8.
The MAC layer structure and usage for air to air communication shall be as specified in
Section 4.9.
4.2
Framing
L-DACS2 shall support a frame of duration M1 seconds, repeating every M1 seconds.
A frame shall be equal in duration to M2 equally-spaced basic time slots. A frame shall
consist of a number of frame sections. Each frame section shall be equal in duration to an
integral number of basic slots, unless defined otherwise.
A basic time slot shall have duration M5 = 1/M2 seconds.
Note. – A basic time slot may also be termed as a slot or a full slot.
Note. – A frame section may contain slots of shorter length than the basic time slot, provided
that a frame section or a group of frame sections where so defined, has duration equal to an
integral number of basic slots.
The framing parameters shall have the default values as shown in Table 3.
Parameter description
Parameter name
Value
Frame duration (in seconds)
M1
1
Number of basic slots per frame
M2
150
Duration of 1 basic slot (in seconds)
M5
1/150
Table 10: Parameter definition for framing parameters
Note. – The frame cycle is illustrated below.
Note. – The L-DACS2 frame length is designed for fast delivery of time-critical messages and
has been set at 1 second. Simulations have shown the validity of this choice to achieve the
highest performance.
Frame n - 1
Frame n
Frame n + 1
Frame cycle
Frame cycle
Frame cycle
Figure 6 : L-DACS/2 frame
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4.3
Synchronization
Note. – For air-ground communications time synchronisation by mobiles based on
measurement of the transmissions from ground station is sufficient. However the
synchronisation specification for LDACS2 is designed to be compatible with future definition
of an air-air service provided by the L-DACS2 system with system operation being possible
air-to-air even if contact with the ground station is lost.
Note. – The L-DACS2 system shall be based on accurate time synchronisation between the
ground station and aircrafts in range, in order to minimise the inefficient use of channel
resources by the provision of a large guard time between each downlink slot.
Note. – The L-DACS2 synchronisation scheme operates in different modes to ensure the
robustness of the synchronisation scheme. One of the synchronisation modes, the derived
mode, is designed to be independent of external on-board time sources and is provided by
the L-DACS2 system itself to avoid common mode failure between the L-DACS2
communication system and another avionics system.
Note. – In GSM, 6 bits are used for the timing correction and 26 bits for Timing Advance –
These 26 bits limit the coverage of 35 km in GSM. In the case of the GSM Timing Advance
mechanism, the number of bits to code the Timing Advance and the correction is required to
be in line with the maximum distance between the coverage edge of the cell and the ground
station.
4.3.1
Specified time reference
UTC time shall be specified as the time reference for station synchronization.
Note. – Each frame shall start at the beginning of a UTC second (see section 4.5).
4.3.2
Primary time synchronization mode
Under normal operating conditions, a mobile station shall maintain time synchronization such
that the start of each successive group of M2 slots (a frame) is synchronized with the start of
the specified time reference (see Section 4.3.1) second to within a 2-sigma value of 1
microsecond.
Note. – This is also defined as the primary time source.
4.3.3
Secondary synchronization mode
When primary time is unavailable, a mobile station shall be capable of maintaining time
synchronization such that the start of each successive group of M2 slots (a frame) is
synchronized with the start of the specified time reference (see Section 4.3.1) second to
within a 2-sigma value of 5 microseconds.
Note. – This is also defined as the secondary time source.
Note. – Secondary time is used only when the primary source has failed.
A mobile station using secondary time shall however revert to using primary time whenever
primary time is available.
4.3.4
Derived synchronization mode
A mobile station shall be capable of deriving time synchronization from the framing message
of the ground station with which it is in or wishes to make contact, such that the start of each
successive group of M2 slots (a frame) is synchronized to within a 2-sigma value of 20
microseconds.
Note. – This is defined as the derived time source.
Note. – A mobile station will only be able to derive time from the framing message
transmission of the ground station if the mobile station has knowledge of the distance
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between the ground station and itself. To achieve this, the mobile station may have
knowledge of its position from inputs from onboard position sources and may have a
database containing ground station positions. Future message definitions for transmissions
by mobile stations and ground stations may also include a requirement for the station
positions to be transmitted on a regular basis (in the framing message), which may be
recommended to aid timing functions and to be beneficial for other applications.
Note. – Derived time is used only when both the primary and secondary sources have failed.
4.3.5
Synchronisation of slots within a frame
All slots within the frame shall be synchronised according to the frame structure provided by
the ground station in the framing message (see section 4.5).
4.3.6
Reversion
A mobile station operating in the derived synchronisation mode shall revert to primary time
whenever primary time is available. A mobile station operating in the derived synchronisation
mode shall revert to secondary time whenever secondary time is available and primary time
is not available.
4.4
Burst format
4.4.1
Burst composition
A burst shall be composed of:
•
•
•
•
•
•
•
transmitter ramp-up period;
synchronization sequence, start flag;
user data (includes source and destination address if required);
FEC, CRC code bits;
end flag;
transmitter ramp-down period;
propagation guard time.
A guard time of the specified duration shall be maintained between the end of the ramp-down
period and the end of the slot or group of slots occupied by the burst.
Note. – All stations in the system maintain their own time reference, and maintain the frame
structure and timing advertised by the ground station synchronised to this time reference.
However all transmitted bursts suffer propagation delay in transit (unless the transmitter and
receiver are co-located), such that all bursts arrive with some time delay after the slot start
time defined by the framing structure. If a burst is transmitted at the start of a slot, the burst
has to be shorter in time than the duration of a time-slot, to prevent the received burst
overlapping a burst in the following slot (the following burst could be close to the start of the
slot if the transmitter and receiver for that burst are closely located). The amount of time by
which the transmitted burst is shorter than the duration of a slot is the guard time. The
required duration of the guard time in this case is equal to the propagation delay that would
occur for a mobile transmitting from the edge of the cell to the ground station (at the cell
centre), which for this system is defined as 200 NM. The necessary inclusion of a guard time
reduces the maximum length of bursts, and therefore restricts the net data rate that can be
supported by the system. Combining multiple 1-slot messages into a single multi-slot burst
helps to mitigate the loss of data caused by guard time, as only one guard time is required
for a multi-slot burst. In selecting the slot duration to be used by the system, the required
guard time is taken into account. Where a mobile station is making regular transmissions to a
ground station, an alternative method is possible whereby:
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a) The ground station measures the propagation delay of the first transmission by the
mobile;
b) The ground station indicates the propagation delay to the mobile in a dedicated
message;
c) The mobile uses the propagation delay information from the ground station to
transmit its next burst to the ground station ahead of the scheduled slot start time by
the amount of the expected propagation delay;
d) The transmission from the mobile then arrives at the ground station at the slot start
time, within a given uncertainty or error.
e) The ground station continues to monitor changes in the burst arrival time from the
mobile and continues to provide the correction information to the mobile.
This method is referred to as adaptive burst alignment and the propagation delay information
provided by the ground station to the mobile is referred to as the Timing Advance parameter
(see Section 4.5.5). This method still also requires a guard time, but it allows the duration of
the guard time to be reduced, thus permitting bursts to be longer than if the method is not
used. The framing structure that is proposed for LDACS-2 used adaptive burst alignment in
the parts of the frame where it is feasible to do so, and in the other parts the full guard time
based on the size of the cell has to be used. An additional guard time is required between
certain frame sections, and this is achieved with a guard time at the start of the uplink
sections of the frame as well as at the end of the uplink sections. Details of the duration of
the guard time required in different parts of the frame and of the dependence in this system
of the guard time on the number of slots occupied by a burst are provided in Section 4.5.6.
For uplink bursts, a guard time of the specified duration shall additionally be maintained
between the start of the slot or the first slot of a group of slots occupied by the burst and the
start of the transmitter ramp-up period.
Note. – The burst structure for downlink bursts is illustrated in the figure below. The
equivalent uplink burst is preceded by an additional guard period before the transmitter
ramp-up period.
Guard time
Start flag
8 bits
29.54 µs
Ramp down
Sync
sequence
26 bits
96.00 µs
End flag
Ramp up
8 bits
29.54 µs
FEC,
CRC
User data
Next slot
8 bits
8 bits
29.54 µs 29.54 µs
Active burst duration
Total burst duration
Total slot duration
Figure 7: L-DACS/2 burst structure for downlink bursts
4.4.2
Bursts occupying multiple slots
If a ground station or a mobile station requires several concurrent slots for one transmission,
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then the transmitted ramp-up time and synchronization interval shall only be present at the
start of the initial slot, and the transmitter ramp-down time and propagation guard time shall
only be present at the end of the last slot.
Note. – A single burst spanning two concurrent slots (and containing one message) is
illustrated in the figure below for a downlink burst. The equivalent uplink burst is preceded by
an additional guard period before the transmitter ramp-up period.
Guard time
Start flag
8 bits
29.54 µs
Ramp down
Sync
sequence
26 bits
96.00 µs
End flag
Ramp up
8 bits
29.54 µs
FEC,
CRC
User data
Next slot
8 bits
8 bits
29.54 µs 29.54 µs
Active multi-slot burst duration
Total multi-slot burst duration
Total 2-slot duration
Figure 8: L-DACS/2 merged slot structure for downlink bursts
Where more than one message is contained in one burst, each pair of messages shall be
separated by a one-octet flag. In this case, each message shall contain its own FEC error
correction section of the message, within the flags bounding that message.
Note. – The number and position of the FEC code bits will be dependent on the size of the
transmission.
Note. – A single burst containing two messages, with a flag separating the messages, is
illustrated in the figure below for a downlink burst. The equivalent uplink burst is preceded by
an additional guard period before the transmitter ramp-up period.
Guard time
8 bits
29.54 µs
FEC,
CRC
Ramp down
Start flag
8 bits
29.54 µs
User data
End flag
Sync
sequence
26 bits
96.00 µs
FEC,
CRC
Flag
Ramp up
8 bits
29.54 µs
User data
Next slot
8 bits
8 bits
29.54 µs 29.54 µs
Active multi-slot burst duration
Total multi-slot burst duration
Total 2-slot duration
Figure 9: L-DACS/2 merged slot structure structure for downlink bursts
Note. – The number of user data octets that can be accommodated in a single burst
spanning slots 1 to n is higher than the total number of user octets that can be
accommodated if separate bursts are use to transmit the same data in slots 1 to n.
Note. – For a channel that is lightly loaded, a mobile station should aim to transmit data using
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multiple (evenly spaced) single bursts to transmit data to the ground station, in order to
minimise the interference effect on other aircraft systems that could be caused by the
transmission of long bursts.
4.4.3
Ground station bursts
The uplink sections are entirely organised by the ground station, and since the ground station
has sole us of this portion of the channel, it transmits continuously in the UP1 and UP2
sections. Therefore the burst transmitted in a given uplink section could be (and most often
is) several slots long. The least length an uplink burst could be is two slots, as illustrated in
the figure below. The upper limit of the length is determined by the size of the UP1 or UP2
section.9As with multiple messages sent in downlink bursts, each message in the burst is
accompanied by an FEC field.
Figure 10: L-DACS/2 merged slot structure for uplink bursts
4.5
4.5.1
Framing structure
Frame Structure
A frame shall consist of two uplink sections, UP1 and UP2, and two downlink sections CoS1
and CoS2, and a login section LoG2.
Note. – The frame structure is illustrated in the figure below:
9
See further detail in Annex 1.
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Start of UTC
second
1 Frame
UP1
CoS1
UP2
CoS2
Framing
message
Uplink
section
Downlink
section
Uplink
section
Downlink
section
LoG2
Login (Downlink)
section
Figure 11: L-DACS2 frame structure (point-to-point)
In the LoG2 frame section, M3 LoG2 slots will exist in place of 1 slot.
In the CoS1 frame section, M4 CoS1 slots will exist in place of 1 slot.
A LoG2 slot shall have duration M6 = 1/(M2*M3) seconds.
A CoS1 slot shall have duration M7 = 1/(M2*M4) seconds.
The above framing parameters shall have the default values as shown in Table 3.
Parameter description
Parameter name
Value
Number of LoG2 slots per basic slot
M3
2
Number of CoS1 slots per basic slot
M4
6
Duration of a LoG2 slot (in seconds)
M6
1/300
Duration of a CoS1 slot (in seconds)
M7
1/900
Table 11: Parameter definition for framing parameters
The UP1 and UP2 sections of a frame shall only be used by a ground station.
The UP1 and UP2 sections of a frame shall be used by a ground station to send uplink
messages to mobile stations in range, to send acknowledgement messages to mobile
stations for data successfully received, to send Clear-To-Send (CTS) messages to mobile
stations in response to Request-To-Send (RTS) messages received, and/or to send framing
messages.
The ground station shall concatenate all its messages into a single continuous burst, one for
each UPx section (i.e. two per frame). Each of these bursts therefore spans a number of
basic slots, with one burst to be transmitted in UP1 and one to be transmitted in UP2. The
upper limit of the burst length is determined by the length of the UPx section. Each message
in a ground station burst shall contain its own FEC section and shall be separated from a
following message by one 8-bit flag.
The CoS1, CoS2 and LoG2 sections of a frame shall only be used by mobile stations to
transmit to a ground station.
The length of each section of the frame shall be configured dynamically by each ground
station. A ground station shall not be required to coordinate the lengths of the frame sections
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(UP1, CoS1, LoG2, UP2, CoS2) with other ground stations. A mobile station shall listen to
the framing message broadcast by a ground station in order to acquire knowledge of the
framing lengths applicable to a particular ground station at a specific time.
The UP1 section of the frame shall consist of M11 basic slots.
The UP2 section of the frame shall consist of M12 basic slots.
The CoS2 section of the frame shall consist of M14 basic slots.
The CoS1 section of the frame shall consist of M13 CoS1 slots.
The login section LoG2 of the frame shall consist of M15 LoG2 slots.
Note. – These parameters are defined in Table 6.
The length of the CoS1 section plus the LoG2 section shall be equivalent to an integer
number of basic slots.
The CoS1 section of the frame shall be used by mobile stations to transmit in its guaranteed
CoS1 slot (one slot per frame allocated by the ground station after the mobile’s login
insertion transmission, see Section 4.5.2).
When a mobile station has data to transmit to the ground station, it shall transmit an RTS to
the ground station to request slots to be reserved for data transmission in CoS2.
If a mobile station has received data from the ground station it shall transmit an
acknowledgement to the ground station in the next available CoS1 slot, which
acknowledgement may be combined with an RTS.
Each mobile allocated a CoS1 slot shall transmit a message in its CoS1 slot at least once per
S1 frames. If the mobile does not have an RTS or acknowledgment to transmit to the ground
station then it shall transmit a Keep-Alive message in its CoS1 slot at least once per S1
seconds in order to maintain transmissions in its CoS1 slot.
Parameter description
Maximum number of frames
between transmissions by a
mobile station in its allocated
CoS1 slot
Parameter
name
Default value
Increment
Range
S1
10
1
1 to 30
Table 12: Parameter definition for S1 parameter
The CoS2 section of the frame shall be used by mobile stations to transmit data to a ground
station, either in slots that have been reserved for it by the ground station (reserved access),
or in slots selected by the random access procedure defined in Section 5.6.
Note. – The number of slots assigned by random access and the number assigned by
reserved access is variable. The number of slots assigned by reserved access should greatly
exceed the number assigned by reserved access.
4.5.2
Login and response
The LoG2 section of the frame shall be used by mobile stations to transmit a login message,
using one LoG2 slot per frame, in one of the LoG2 slots made available by the ground
station.
A mobile station intending to transmit in the LoG2 section of the frame shall randomly select
one of the available LoG2 slots using the slot selection algorithm defined in Section 5.7.
The ground station shall allocate a CoS1 slot in the CoS1 section of the frame to each mobile
station that has logged in to the ground station, for exclusive use by that mobile station while
it remains within range of the ground station. The ground station shall provide to the mobile
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station a unique local address both for itself and for the mobile station, to be used for all
subsequent message exchanges between the ground station and mobile during the period
that the mobile is logged in to the ground station.
A login uplink response message shall be transmitted by the ground station in the UP1
section providing the slot allocation for each mobile station that has transmitted a login
message in the login section LoG2 in the previous frame.
Note. – The login slots are used for initial contact when entering the cell and during the handover process. Cell exit exchanges are used during hand-overs (see section 6.3.1.1). Login
messages are used during hand-overs and during initial contact.
4.5.3
Frame section length alteration
The length of each section of the frame shall be configured dynamically by the ground station
according to the number of aircraft in range and the local demand for uplink and downlink
capacity.
The lengths of each section of the frame shall be broadcast in a framing message by the
ground station, transmitted at the start of the UP1 section, at a rate of M16 times per minute.
The ground station shall indicate a change to the frame section lengths, and provide the new
section lengths, in the framing message, M17 frames in advance of the change to the frame
section lengths.
The ground station shall indicate in the framing message which frame number will be the one
with the new section sizes by decrementing counter L1 in each framing message.
Note. – Parameters M16, M17 and L1 are defined in Table 13.
Note. – When the ground station intends to change the frame section sizes, mobiles with
which it is in contact need to be notified multiple times in advance to ensure that all aircraft
receive the frame change notification with a high probability. This implies notification several
frames in advance.
4.5.4
Framing structure default parameter values and ranges
The framing structure parameters shall have the default values and ranges as shown in
Table 13.
The default values shall only be used in the case of a ground station starting up having been
previously switched off.
Parameter description
Parameter
name
Default value (use
only for starting
from switched off
state)
Increment/
decrement
Range
Number of slots in UP1 (basic
slots)
M11
30
1
2 to 129
Number of slots in UP2 (basic
slots)
M12
30
1
2 to 129
Number of slots in CoS1
(CoS1 slots)
M13
80
2
0 to 510
Number of slots in CoS2
(basic slots)
M14
66
1
5 to 145
Number of slots in LoG2
(LoG2 slots)
M15
8
1
4 to 33
Number of times per minute
that framing message is sent
M16
60
1
1 to 60
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Number of frames in advance
of a change to the frame
lengths that the ground
station is required to provide
notification in the framing
message
M17
10
1
1 to 60
L1
10
1
0 to 31
Counter to be decremented
prior to a change to frame
section size
Table 13: Parameter definition for framing parameters
M13/M4 + M15/M3 shall be an integer.
The number of slots in each frame shall equal M11 + M12 + M13/M4 + M14 + M15/M3.
Note. – The default size of the login section (LoG2) is based on the following calculation: for
a typical ground station with a range of 200 NM and the maximum possible number of aircraft
(204) distributed across the cell, there is calculated to be an average of 12 aircraft about to
enter the cell in any one minute. If there are 4 LoG2 slots in each frame then over one minute
there will be 240 slots available. The login sub-section of the frame shall always contain at
least 4 LoG2 slots. This is to reduce the likelihood of different aircraft attempting to use the
same slot if several handovers and/or initial logins are taking place simultaneously.
Note. – The number of slots in the CoS1 section should closely follow the number of loggedin aircraft.
Note. – Each aircraft is allocated one exclusive downlink slot to transmit an RTS in CoS1,
allowing the request of slots in the CoS2 section, in order to guarantee fast access to the
channel for messages requiring high QoS. This use of deterministic slot assignments is
essential to achieve a high QoS performance.
4.5.5
Adaptive burst alignment
A mobile station shall time the start of its login transmission with the slot start times indicated
by the ground station for the LoG2 section of the frame.
Note. – The mobile maintains its own time, primary, secondary or tertiary. The ground station
will indicate in its framing message a timeslot structure relative to the start of a UTC second.
Note. – The mobile does not use adaptive burst alignment (timing advance) for its login
transmission in a LoG2 slot. Transmissions in the LoG2 section by the mobile will be initiated
at the start of the LoG2 slot boundary, but will have suffered propagation delay by the time
they reach the ground station.
A ground station in receipt of a login message from a mobile station in one of the login slots
shall measure the time difference, TD1 between the start of the slot as measured by the
ground station and the time that the transmission from the mobile arrived.
The ground station shall convert the time difference TD1 into a timing advance parameter T5
for that mobile according to the ranges specified in Table 15.
The ground station shall include the value of the timing advance parameter T5 for each
mobile in a timing advance message that is transmitted to the mobile in response to its login
message.
Parameter description
Timing advance parameter
Parameter
name
Default value
Increment
Range
T5
0
1
1 to 96
Table 14: Parameter definition for timing advance parameter T5
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For each mobile, once the ground station has transmitted the first timing advance message
to the mobile, the ground station shall monitor the timing of all the transmissions from the
mobile in CoS1 and CoS2 for the purpose of assessing if the T5 parameter provided to the
mobile needs to be changed.
Note. – If a mobile does not have any data or acknowledgment to transmit to transmit to the
ground station, a Keep-Alive synchronisation message must be sent to the ground station at
least once per S1 frames (see Section 4.5.1).
A ground station in receipt of a burst from a mobile station in CoS1 or CoS2 shall measure
the time difference, TD2 between the start of the slot for the ground station’s own framing
and the time that the transmission from the mobile arrived. The ground station shall convert
the time difference TD2 into a timing advance parameter T5 for that mobile according to the
ranges specified in Table 15.
Using the measurement of TD2, the ground station shall, at least every S1 seconds, for each
mobile, indicate whether the mobile must increase or decrease its timing advance parameter
T5, and by what amount, or whether it must leave its T5 parameter unchanged.
Note. – In LDACS2, a one-way measure of time difference due to propagation delay is used,
as opposed to a two-way measure, since the LDACS2 mobile stations are required to
maintain their own time, synchronised to UTC. In systems where mobiles derive time from
the ground station, a mobile’s transmissions are delayed by the time they reach the ground
station by an amount equivalent to twice the propagation distance (the mobile would
measure the start time of a frame from the transmissions by the ground station – the mobile’s
frame structure is delayed by an amount due to the one-way delay from the ground station).
Note. – The timing advance parameter has been specified assuming that aircraft speeds for
aircraft using this communication system are no higher than 600 knots (Mach 1 at 25,000 ft
under standard conditions). With this assumption, a message needs to be transmitted at
least once per 30 seconds from each mobile to the ground station to allow the ground station
to assess, based on variations in the timing of the mobile station’s burst start time with
respect to the timing expected (from the current T5 value allocated to the mobile), whether a
modified T5 value needs to be provided to the mobile. A maximum 30 second update period
for T5, accommodated by the maximum setting for S1 of one transmission in CoS1 per 30
frames, provides an appropriate balance between changes in the synchronisation of the
mobile bursts with the ground station slot structure and the load caused by the regular
update of T5 in the ground station to mobile communication exchanges. A default setting for
S1 of one transmission in CoS1 per 10 frames provides increased assurance that aircraft are
provided with the correct T5 value corresponding with their distance from the ground station,
considering that some transmissions may be lost in either direction between ground station
and mobile.
Note. – A mobile station may be able to verify the correctness of the T5 value provided to it
by the ground station if it has knowledge of the distance between itself and the ground
station. The mobile station may be able to use this information to filter out isolated T5 values
that are obviously incorrect. However it should not use this information to disregard more
than one T5 value at a time, as the assumption should be made that the ground station
always has an accurate measure of the times of arrival of mobile transmissions with respect
to the slot start times measured by the ground station.
Range of TD1/TD2
Corresponding range
between ground
station and mobile
Allocated T5 value
Time by which
mobile advances
transmission
0 < Tdiff <= 0.01931 ms
0 < R <= 3.125 NM
0
0
0.01931 ms < Tdiff <=
0.03861 ms
3.125 NM < R <= 6.25
NM
1
0.01931 ms
0.03861 ms < Tdiff <=
6.25 NM < R <= 9.375
2
0.03861 ms
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0.05792 ms
NM
0.05792 ms < Tdiff <=
0.07722 ms
9.375 NM < R <= 12.5
NM
3
0.05792 ms
continued until
continued until
continued until
continued until
1.81467 ms < Tdiff <=
1.83398 ms
293.75 NM <= R
94
1.81467 ms
1.83398 ms < Tdiff <=
1.85328 ms
296.875 NM <= 300
95
1.83398 ms
1.85328 ms < Tdiff
300 NM < R
96
1.85328 ms
Table 15: Allocation of T5 parameter to transmission delay
4.5.6
Burst transmission in relation to slot and frame structure
4.5.6.1
UP1 and UP2 sections
For bursts transmitted by a ground station in UP1 or UP2, the start of the transmitter ramp-up
period of a burst shall begin after a start guard period of 408 bits (approximately 1.50646 ms)
from the start of the UP1 or UP2 frame section.
Note. – The guard period at the start of the UP1 and UP2 frame sections is needed to
provide a 1.5 ms transmit to receive turnaround time for the mobiles and a 1.5 ms receive to
transmit turnaround time for the ground station.
For bursts transmitted by a ground station in UP1 or UP2, a guard period of length indicated
in Table 7, in which no transmission is made by the ground station, shall be reserved
between the end of the ramp down period and the end of the slot.
Number of slots
occupied by burst
in UP1 or UP2 (n =
integer greater
than zero)
Length of concurrent
slot group in bits
Guard period to be
provided at the end
of a burst in UP1 or
UP2 in bits
Approximate
duration of guard
period to be provided
at the end of a burst
in UP1 or UP2
9n – 8
(9n – 8)*16250/9
755 + 5/9
2.78974 ms
9n – 7
(9n – 7)*16250/9
755 + 1/9
2.78810 ms
9n – 6
(9n – 6)*16250/9
755 + 2/3
2.79015 ms
9n – 5
(9n – 5)*16250/9
755 + 2/9
2.78851 ms
9n – 4
(9n – 4)*16250/9
755 + 7/9
2.79056 ms
9n – 3
(9n – 3)*16250/9
755 + 3/9
2.78892 ms
9n – 2
(9n – 2)*16250/9
755 + 8/9
2.79097 ms
9n – 1
(9n – 1)*16250/9
755 + 4/9
2.78933 ms
9n
(9n)*16250/9
755
2.78769 ms
Table 16: End guard period for UP1 and UP2 burst as a function of the number of slots
occupied by the burst
Note. – The guard time required is based upon the following considerations:
a) Following transmission of a burst by a ground station in UP1 or UP2, upon reception at the
mobile station the burst will be delayed with respect to the start of the slot by the time taken
for the burst to travel from the ground station to the mobile station. The guard period
therefore needs to provide protection from burst overlap for ground station to mobile station
distances that are likely with a 200 NM radius cell. To provide 200 NM protection, the guard
period must exceed 1.23552 milliseconds.
b) At the end of UP1 or UP2, the ground station needs a transmit to receive turnaround time
of 1.5 ms, and mobiles transmitting at the start of CoS1 or CoS2 (which follow UP1 and UP2
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respectively) need a receive to transmit turnaround time of 1.5 ms, which leads to a need to
extend the guard time by 1.5 ms.
c) Within the 30 s timing advance parameter update period, a mobile transmitting in the first
slot of CoS1 or CoS2 could travel closer to the ground station, causing a burst to overlap
either the UP1 or UP2 sections by up to 0.03089 ms, and could have a clock that is ahead of
that of the ground station by up to 0.020 ms, leading to a need to extend the guard time by a
further 0.05089 ms.
d) The guard time at the end of an UP1 or UP2 slot thus needs to be in excess of 1.23552 +
1.5 + 0.05089 = 2.78639 ms
e) The guard period is then adjusted above this value depending on the number of slots
occupied by the ground station burst, in order to keep the useable part of any burst equal in
length to a whole number of bits.
4.5.6.2
LoG2 insertion section
For login bursts transmitted by a mobile station in the LoG2 section of the frame, the start of
the transmitter ramp-up period of a burst shall be aligned with the start of a time slot at the
time of transmission.
For login bursts transmitted by a mobile station in LoG2, a guard period of 345 + 7/9 bits
(approximately 345.777 bits or 1.27672 ms) in which no transmission is made by the mobile
station shall be reserved between the end of the ramp down period and the end of the slot.
Note. – The guard time required is based upon the following considerations:
a) Following transmission of a burst by a mobile station in LoG2, upon reception at the
ground station the burst will be delayed with respect to the start of the slot by the time taken
for the burst to travel from the mobile station to the ground station. Therefore in the LoG2
sections of the frame, a guard time sufficient to provide protection from burst overlap for
mobile ranges at least out to the cell radius of 200 NM needs to be provided, for which a
guard time above 1.23552 milliseconds is required.
b) Two mobiles transmitting in adjacent slots in the LoG2 section could each have a clock
running in error by up to 0.02 ms, and thus 0.04 ms needs to be added to the guard period.
c) The above results in a requirement for a guard time of at least 1.27552 ms.
d) The guard period is then adjusted above this value, in order to keep the useable part of
any burst equal in length to a whole number of bits.
4.5.6.3
CoS1 and CoS2 sections
For bursts transmitted by a mobile station in CoS2 and in CoS1, the start of the transmitter
ramp-up period of a burst shall begin in advance of the scheduled slot start time at the
ground station by an amount indicated by the ground station to the mobile in the Timing
Advance Parameter T5 (see Section 4.5.5).
Note. – In the CoS1 and CoS2 frame sections, the mobiles transmit using a feature called
Timing Advance (used in GSM), in order to obtain a synchronised frame at the ground station
while at the same time minimising the required length of guard time to be allowed for at the
end of a burst. Timing Advance cannot be used by the mobiles in the LoG2 section because
prior to logging in to the ground station the mobiles do not have knowledge of the mobile to
ground station propagation delay.
4.5.6.4
CoS1 section
For bursts transmitted by a mobile station in CoS1, a guard period of 33 + 25/27 bits
(approximately 33.926 bits or 0.12526 ms) in which no transmission is made by the mobile
station shall be reserved between the end of the ramp down period and the end of the slot.
Note. – The guard time required is based upon an estimated maximum burst overlap at the
ground station for two mobile stations, A and B, transmitting in adjacent slots using timing
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advance, plus maximum errors in clock synchronisation for these two mobiles, as follows:
a) Mobile A transmits in the first of the two slots; mobile B transmits in the following slot;
b) Maximum additional propagation delay of mobile A’s burst due to mobile A having the
longest propagation delay compatible with the allocated T5 value: 0.01931 milliseconds;
c) Maximum additional propagation delay of mobile A’s burst due to mobile A having
travelled away from the ground station at 600 knots in the 30 seconds that have elapsed
since the T5 value was allocated to the mobile (600 knots is Mach 1 at 25,000 ft under
standard conditions): 0.03089 milliseconds;
d) Maximum additional delay of mobile A’s burst due to the clock of Mobile A being slow and
at the limit of the allowed synchronisation tolerance: 0.020 milliseconds;
e) Maximum additional delay for burst of Mobile A: 0.07020 milliseconds;
f) Maximum reduction in propagation delay of mobile B’s burst due to mobile B having the
shortest propagation delay compatible with the allocated T5 value: 0.0 milliseconds;
e) Maximum reduction in propagation delay of mobile B’s burst due to mobile B having
travelled towards the ground station at 600 knots in the 30 seconds that have elapsed since
the T5 value was allocated to the mobile (600 knots is Mach 1 at 25,000 ft under standard
conditions): 0.03089 milliseconds;
f) Maximum reduction in delay of mobile B’s burst due to clock of Mobile B being fast and at
the limit of the allowed synchronisation tolerance: 0.020 milliseconds;
g) Maximum reduction in delay for burst of Mobile B: 0.05089 milliseconds;
h) The maximum burst overlap for two mobiles using adjacent slots if there was no guard
time would be: 0.07020 + 0.05089 = 0.12109 milliseconds.
The guard period is then adjusted above this value, in order to keep the useable part of any
burst equal in length to a whole number of bits.
4.5.6.5
CoS2 section
A mobile station intending to transmit in the CoS2 section of the frame shall transmit bursts
occupying between 1 and M21 slots, where the value of M21 is defined in the Table 17.
The value of M21 shall be broadcast by the ground station in its framing message.
Parameter description
Maximum length of burst to
be transmitted by a mobile
station in CoS2 (in slots)
Parameter
name
Default value
Increment
Range
M21
5
1
1 to 10
Table 17: Parameter definition for maximum burst length in slots in CoS2
For bursts transmitted by a mobile station in CoS2, a guard period of length indicated in
Table 18, in which no transmission is made by the mobile station, shall be reserved between
the end of the ramp down period and the end of the slot.
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Number of slots
occupied by burst
in CoS2 (n =
integer greater
than zero)
Length of concurrent
slot group in bits
Guard period to be
provided at the end
of a burst in CoS2 in
bits
Approximate
duration of guard
period to be provided
at the end of a burst
in CoS2
1
16250/9
33 + 5/9
0.12390 ms
2
2*16250/9
33 + 1/9
0.12226 ms
3
3*16250/9
33 + 6/9
0.12431 ms
4
4*16250/9
33 + 2/9
0.12267 ms
5
5*16250/9
33 + 7/9
0.12472 ms
6
6*16250/9
33 + 3/9
0.12308 ms
7
7*16250/9
33 + 8/9
0.12513 ms
8
8*16250/9
33 + 4/9
0.12349 ms
9
9*16250/9
33
0.12185 ms
10
10*16250/9
33 + 5/9
0.12390 ms
Table 18: Guard period for CoS2 slot as a function of slot length
Note. – The guard time required is based upon an estimated maximum burst overlap at the
ground station for two mobile stations, A and B, transmitting in adjacent slots using timing
advance, plus maximum errors in clock synchronisation for these two mobiles. The derivation
in Section 4.5.6.4 showed that the maximum burst overlap for two such mobiles using
adjacent slots if there was no guard time would be: 0.07020 + 0.05089 = 0.12109
milliseconds. The guard period is then adjusted above this value, in order to keep the
useable part of any burst equal in length to a whole number of bits.
4.5.6.6
Summary of burst guard times
Note. – Table 10 summarises the guard times applicable to each section of the frame, for
guard periods to be maintained after the end of each burst (after the 3 bit ramp down period).
Section Section of Tx to Rx / Guard time
Burst
of frame
frame
Rx to Tx
to protect
overlap
following
to 200 NM from timing
time
needed at
advance
end of
and/or
section
mobile
clock error
Minimum
duration
guard
period
must
satisfy
Allocated
Approximate
actual guard
actual guard
period in bits period duration
UP1
CoS1
1.5 ms
1.23552 ms
0.05089 ms
2.78639 ms See Table 5 in
Section
3.5.6.1
CoS1
LoG2
not
required
(see Note)
not
applicable
0.12109 ms
0.12109 ms
33 + 7/18 bits
(See Section
3.5.6.4)
0.12328 ms
LoG2
UP2
0.04 ms
1.27552 ms
345 + 7/9 bits
(See Section
3.5.6.2)
1.27672 ms
UP2
CoS2
1.5 ms
1.23552 ms
0.05089 ms
2.78639 ms See Table 5 in
Section
3.5.6.1
CoS2
UP1
not
required
(see Note)
not
applicable
0.12109 ms
0.12109 ms
1.23552 ms
not
required
(see Note)
See Table 5 in
Section 3.5.6.1
See Table 5 in
Section 3.5.6.1
See Section
3.5.6.5
Note. – In the case of the Log2 and CoS2 frame sections, there is no requirement for an additional
guard time at the end of these sections, since the Tx to Rx / Rx to Tx turnaround time is catered for by
a 1.50646 ms guard time at the start of the UP1 and UP2 frame sections (see following table). In the
case of the CoS1 section, this additional guard time is not required because the CoS1 section is
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followed by a further downlink section.
Table 19: Summary of burst guard periods per frame section
Note. – Table 20 summarises the guard times applicable to each section of the frame, for
additional guard periods to be maintained at the start of a frame section.
Section of frame
Section of frame
following
Tx to Rx / Rx to Tx
time needed at
start of frame
section
Allocated actual
guard period at
start of frame
section in bits
Approximate actual
guard period
duration at start of
frame section
UP1
CoS1
1.5 ms
408 bits
1.50646 ms
CoS1
LoG2
not required – Tx to
Rx / Rx to Tx is
catered for by guard
time at end of UP1
-
-
LoG2
UP2
not required
-
-
UP2
CoS2
1.5 ms
408 bits
1.50646 ms
CoS2
UP1
not required – Tx to
Rx / Rx to Tx is
catered for by guard
time at end of UP1
-
-
Table 20: Summary of burst guard periods at the start of frame sections
4.6
Quality of Service (QoS) management
4.6.1
Efficiency
Note. – The efficient handling of QoS is based on the TDMA structured MAC layer and
provides transmission based on data priority.
4.6.2
L-DACS2 QoS management
4.6.2.1
Parameters
The L-DACS2 system shall permit handling of QoS based on the Priority parameter Q22.
Parameter description
Parameter name
Default value
Range
Q22
3
0 to 3
Priority
Table 21: Definition of parameter Priority Q22
Message category
ATN
Priority
Q22
value
Network/systems management
14
3
Distress communications
13
3
Urgent communications
12
3
High priority flight safety messages
11
3
Normal priority flight safety messages
10
2
Meteorological communications
9
2
Flight regularity communications
8
2
Aeronautical information service messages
7
2
Network/systems administration
6
2
Aeronautical administrative messages
5
1
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Unassigned
4
1
Urgent priority administrative and UN charter communications
3
1
High priority administrative and state/government communications
2
1
Normal priority administrative
1
0
Low priority administrative
0
0
Table 22: Mapping between message category, ATN priority and Q22 priority classification
Note. – This mapping is an initial proposition. It could evolve with the definition of the QoS
management with end to end perspective (e.g. revision of the ATN priority table). The LDACS2 resource allocation mechanism is design to meet this future QoS management
requirement (deterministic/best effort resource allocation).
4.6.2.2
Priority information
Note. – The priority flag Q22 shall be used to distinguish the relative importance of the
exchanged data within a given QoS (best effort or guaranteed) with respect to gaining
access to communications resources and to maintaining the requested QoS.
Note. – The priority of different message categories has been specified by ICAO in terms of
the ATN priority (a subset of those categories belongs to the guaranteed QoS). The definition
of the Q22 priority parameter is based on the ATN priority.
On sending data to be queued for transmission the Priority Q22 shall be set to the
appropriate Q22 value for the message content as indicated in Table 7.
A station shall prioritise transmission of data with higher Priority Q22 value over data with
lower Priority Q22 value.
When L-DACS2 has multiple messages queued to send with different Q22 priorities, then it
shall take account of the Q22 priority in deciding which messages to send first.
When a capacity request has been done for a message and a new message arrives in the
sending queue with a higher Q22 priority it shall be transmitted first (if possible according to
reservation size).
4.6.2.3
End-to-end QoS management
Note. – End-to-end communication will involve heterogeneous networks, including mainly an
air-ground link (typically L-DACS2 radio link or another equivalent radio link) and a ground
transport network. Management of QoS on the L-DACS2 link has been addressed in Section
4.6.1. In order to be able to provide end-to-end QoS management between the airborne
system and the ground controller system, two alternatives are envisaged:
•
Implementation of QoS management mechanisms on the ground network
infrastructure. Solutions based on IP based infrastructure, using IntServ or DiffServ
model, are envisaged.
• Implementation of QoS management mechanisms at the transport level. This
transport protocol shall be designed to be used over a network layer that provides
best-effort service differentiation (called EDS – Equivalent Differentiated Services).
This solution has the advantage of providing this information directly to end users in
order to decide whether the communication infrastructure is capable of providing the
expected QoS.
These options should be investigated within the SESAR development phase within the
project 15.2.4.
4.7
Processing
Bursts received from the MAC sub-layer shall be forwarded to the physical layer, along with
the time for transmission.
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4.8
Power Control management
The L-DACS2 airborne power control mechanism shall be supported by a power control
parameter send by the ground station to the mobile logged in its service area. This
parameter encoded on 5 bits shall code the range of possible airborne power control output
as described in section 3.5.
From section 3.5, the airborne output power could be adapted from 23 dBm to 47 dBm. The
power control parameter encodes the 25 possible states of the output power using a 1 dB
step among the 32 available states allowed with 5 bits. If the desirable output power is 23
dBm, the ground station set the power control field to 00001. If the desirable output power is
47 dBm the ground station set the power control field to 11001. 00000 corresponds to not
monitor.
The ground station shall send a message to update the airborne output power as soon as the
ground station detect that an evolution in the ground received power sufficiently significant to
change the state of the transmitted power. If the received power is bellow -90 dBm, the
ground shall increase the power control parameter from one state. If the receiver power is
above -89 dBm during 5 seconds, the ground station shall decrease the power control
parameter from one state.
The ground station shall use the power control field available in the uplink message if one
message will be sent to the mobile or use the power control field available in the uplink
power control message (see Annex 2).
4.9
4.9.1
MAC layer for Broadcast Service
Frame
The broadcast service framing shall be as defined in Section 4.2.
4.9.2
Synchronisation
The broadcast service synchronisation shall be as defined in Section 4.3.
4.9.3
Burst format
The broadcast service burst format shall be as defined in Section 4.4.
4.9.4
Frame structure
The broadcast service frame structure shall start with a ground-quarantine section of length
B1 slots for fixed access by ground stations. The remaining slots in the frame shall be
available to all stations (mobile and ground) by reserved and random access.
The fixed access, reserved access, and random access protocols shall be based on modified
VDL Mode 4 fixed access, reserved access, and random access protocols.
Note. – Most VDL Mode 4 broadcast protocols will be used, but no point-to-point
transmissions will be permitted on the broadcast channel. Therefore some modifications will
be required.
Note. – Further definition of the broadcast service will take place at a later stage.
4.10
4.10.1
MAC layer for Air-to-Air Point-to-Point Service
Frame
The air-to-air point-to-point service framing shall be as defined in Section 4.2.
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4.10.2
Synchronisation
The air-to-air point-to-point service synchronisation shall be as defined in Section 4.3.
4.10.3
Burst format
The air-to-air point-to-point service burst format shall be as defined in Section 4.4.
4.10.4
Frame structure
The air-to-air point-to-point service frame structure shall start with a ground-quarantine
section of length B1 slots for fixed access by ground stations. The remaining slots in the
frame shall be available to all stations (mobile and ground) by reserved and random access.
The fixed access, reserved access, and random access protocols shall be based on modified
VDL Mode 4 fixed access, reserved access, and random access protocols.
Note. – Most VDL Mode 4 air-to-air point-to-point protocols will be used, but some
modifications will be required.
Note. – Further definition of the air-to-air point-to-point service will take place at a later stage.
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CHAPTER 5 –
Data link sublayer (DLS)
5.1
Introduction
The data link service (DLS) sublayer shall be the sublayer that manages the transmit queue,
creates and destroys Data Link Entities (DLEs) for connection-orientated communications,
provides facilities for the LME to manage the DLS, and provides facilities for connectionless
communications.
The data link sublayer (DLS) shall support communications on a shared communications
channel as described in this chapter.
The DLS shall provide the following services
•
transmission of user data,
•
indication that user data has been sent,
•
reception of user data,
•
indication that the DLS link has been established, and
•
indication that the DLS link has been broken.
The DLS shall rely on the MAC layer to ensure that messages corrupted during transmission
are detected and discarded.
5.2
Transmission procedure
The user data shall be split up into separate segments if the message size exceeds the
maximum that can be accommodated into the first available slot or slots. The first segment
will be transmitted in the first available slot or slots.
Note. – The maximum message size for one slot in CoS2 corresponds approximately to 158
octets of user application data (see Section 3.4).
Note. – For each additional consecutive slot that is available over and above a single slot,
approximately an additional 173 octets of user data can be transmitted in the combined slot
window. This increment in user data greater than the number of octets that can fit in a single
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slot as a result of savings from not having to include additional flags, addresses or guard
time.
Each segment shall carry a sequencing marker to indicate both the total number of segments
and each segment’s number in the sequence.
A message identifier field shall be included in each message in addition to the message type
field.
Note. – The message identifier field ensures that stations can be certain as to which of their
transmissions have been acknowledged.
Note. – For example if a long data message is transmitted in several segments and one
segment is not correctly received it would be inefficient to have to retransmit the whole
message.
The message identifier shall be a rolling sequence number, with values in the range 1 to 63.
Note. – It is not necessary for every message to have a unique ID, merely for the messages
from a station to be distinguishable within a period of time.
The receiving station shall include the message identifier of the received message in its
acknowledgement, in order to indicate which message is being acknowledged.
5.2.1
Uplink transmission procedures
Note. – The UP1 and UP2 sections of the frame are reserved for uplink transmissions. These
blocks are to be used by the ground station for normal data transmissions to aircraft,
acknowledgements to downlink messages, CTS messages, framing messages and login and
“cell exit” exchanges. The slots in the uplink sections shall be concatenated and shall not
require separate ramp-up and ramp-down times nor guard-times in between messages.
If, at the start of the UP1 section, the ground station has a data message or multiple data
messages to send, then, provided there is space available in the UP1 section, it shall
concatenate the data message or messages with any framing message or login response
message or combined ACK/CTS message and transmit the concatenated set of messages in
one single burst in UP1, using an 8-bit flag to separate each message.
If, at the start of the UP2 section, the ground station has a data message to send or multiple
data messages, then, provided there is space available in the UP2 section, it shall
concatenate the data message or messages with any combined ACK/CTS message and
transmit the concatenated set of messages in one single burst in UP2, using an 8-bit flag to
separate each message.
5.2.2
Downlink transmission procedures
If, at the time the aircraft has the opportunity to transmit in its allocated CoS1 slot, the mobile
station has data in its queue for downlink, the aircraft shall transmit a Request To Send
(RTS) reservation request for CoS2 slots in its CoS1 slot indicating the number of slots
required, and the priority of the messages to be transmitted.
On reception of the RTS, a Clear To Send (CTS) shall be transmitted by the ground station in
the UP2 section.
If slots are available in the CoS2 section, the CTS shall acknowledge the request for time
slots and shall indicate which slots have been allocated in CoS2.
The ground station shall reserve slots in CoS2 starting with the earliest time slots in CoS2
being used first.
If CoS2 slots have been reserved, the mobile station shall transmit the data in its allocated
CoS2 slots.
If no slots are available in the CoS2 section, the CTS shall acknowledge the request for time
slots and shall indicate that no slots are available.
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If some, but insufficient slots, are available in the CoS2 section, the CTS shall indicate those
slots that have been reserved and acknowledge that the request remains in place for further
time slots to complete the data transfer.
If no or insufficient slots are available but the CTS has acknowledged that the request for
slots remains in place then the mobile station shall issue a further RTS in its CoS1 slot in the
next frame, adding in any additional requests that it may now have as a result of a demand
for transmission of further messages. If no CTS is received by the aircraft, or if the ground
station replies with a NACK, then the aircraft shall attempt to transmit the data by random
access in accessible slots in CoS2.
If a CoS2 data transmission from the aircraft is not acknowledged by the ground station in
the next frame, or if the ground station replies with a NACK, then the aircraft shall issue a
further RTS to the ground station using the CoS1 slot in the next frame.
If a mobile station receives queued data to send after the start of its allocated CoS1 slot and
before the end of the CoS2 section, and it knows that there are available slots in CoS2, , it
shall attempt to transmit the data by selecting slots for random access transmission using the
random access transmission algorithm specified in Section 5.6.
In order to prevent conflicting transmissions, all aircraft shall listen to all CTS transmissions
to record in their reservation tables which slots have been reserved (in all sections of the
frame).
5.3
Reception procedure
On reception of user data blocks from the MAC layer, the DLS shall determine (from the
sequencing numbers) how many blocks to expect.
The re-assembly of the blocks shall be done by using the sequencing numbers.
5.4
Segmentation
The DLS shall handle the segmentation of user data queued for transmission by higher
layers into appropriate blocks for the MAC layer.
The DLS shall handle the de-segmentation (re-assembly) of received blocks from the MAC
layer into a single user data packet for the upper layer.
5.5
Reserved access protocol specification
Every station (air and ground) shall keep a table of all known stations. For each station, the
table shall include:
the type of the station;
the station’s local address;
a copy of the last type of transmission;
the time of the last transmission.
Note. – The details and processing of this table are dependent upon the implementation
adopted by the radio manufacturers.
A station shall maintain a table of all reservations in the current frame.
For each reserved slot, the reservation table entry shall consist of the local address and
(when available) 27-bit address of the intended transmitter, the local address and (when
available) 27-bit address of the destination (if any) and the type of reservation made.
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An aircraft’s reservation table shall be updated during each frame after the receipt of the
ground station’s uplink messages.
An aircraft shall also update its reservation table on receipt of reservation messages from
other aircraft or from the ground station.
When a station has a data to transmit for which it has a reservation, it shall transmit the
scheduled data in the reserved slots.
Note. – The reserved slots which are used shall depend on the identity of the station and the
amount of data to be sent.
Note. – The details and processing of this table are dependent upon the implementation.
5.6
Random
access
protocol
transmission in CoS2
specification
for
When there is sufficient demand for slots in CoS2, the ground station shall, where possible,
satisfy the demand for slots by allocating reserved slots to mobiles using the reserved
access protocol.
Note. – The ground station may allocate all the slots available in COS2 using the reserved
access protocol. Use of the reserved access protocol is preferential to use of the random
access protocol, since the reserved access protocol uses the channel resources with greater
efficiency.
When the mobile station has one or more bursts to transmit in CoS2 for which it does not
have a reservation, it shall transmit according to the random access procedure defined in
Section 5.6.2, using the random access parameters defined in Section 5.6.1.
Note. – The random access procedure uses a non-adaptive p-persistent algorithm.
5.6.1
Random access parameters
The random access protocol shall implement the system parameters defined in Table 20.
Symbol
p1
VS3
Parameter name
Persistence CoS2
Maximum number
of access attempts
Minimum
Maximum
Recommended default
Increment
1/64
1
16/64
1/64
1
100
10
1
Table 23: Random access parameters for CoS2
5.6.1.1
Parameter p1 (Persistence CoS2)
Parameter p1 (Persistence CoS2) shall be the probability that the station will transmit any
random access attempt in CoS2.
5.6.1.2
Counter VS3 (maximum number of access attempts)
Counter VS3 shall be used to limit the maximum number of random access attempts (VS3)
that a station will make for any transmission request in CoS2.
The VS3 counter shall be cleared upon system initialization, reaching the end of the CoS2
section, or a successful access attempt.
The VS3 counter shall be incremented after every unsuccessful random access attempt in
CoS2.
When the VS3 counter reaches the maximum number of random access attempts,
authorization to transmit shall be granted as soon as the channel is available.
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5.6.2
Random access procedures
When the station has one or more bursts to transmit for which it does not have a reservation,
it shall use the p-persistent algorithm defined as follows:
a) The station shall select a slot, or block of slots as required, for the data in its random
access queue by choosing a slot or block of slots that have not been reserved by the ground
station for other mobile stations.
b) If the station is able to select a slot or block of slots, then the station shall transmit in the
slot, or block of slots with probability p1 (defined in 4.6.1.2)
c) The station shall clear the VS3 counter (VS3 counter cancelled) if it was able to transmit.
d) If the mobile station is unable to transmit at the first attempt, and it has not reached the
end of the current CoS2 section, and the VS3 timer has not expired, it shall increment the
VS3 counter, and then make a further random access attempt, restarting the process at a).
e) If the mobile station is unable to select a slot at the first attempt, and it has not reached the
end of the current CoS2 section, and the VS3 timer has expired, it shall select slots as in a)
and then transmit in those slots with probability p1=1.
If the mobile station is unable to select a slot within the current CoS2 section, this shall be
regarded as an unsuccessful random access attempt, and the mobile station shall issue an
RTS request for slots to send the data in its next CoS1 slot, and clear the VS3 counter (VS3
counter cancelled).
5.7
Random
access
protocol
transmission in LoG2
specification
for
When the mobile station needs to login to a GS, it shall transmit in the LoG2 section of the
frame according to the random access procedure defined in Section 5.7.2, using the random
access parameters defined in Section 4.7.1.
Note. – The random access procedure uses a non-adaptive p-persistent algorithm.
5.7.1
Random access parameters
The random access protocol shall implement the system parameters defined in Table 21.
Symbol
p2
VS4
Parameter name
Persistence LoG2
Maximum number
of access attempts
Minimum
Maximum
Recommended default
Increment
1/64
1
32/64
1/64
1
6
3
1
Table 24: Random access parameters for LoG2
5.7.1.1
Parameter p2 (Persistence LoG2)
Parameter p2 (Persistence LoG2) shall be the probability that the station will transmit any
random access attempt in LoG2.
5.7.1.2
Counter VS4 (maximum number of access attempts)
Counter VS4 shall be used to limit the maximum number of random access attempts (VS4)
that a station will make for any transmission request in LoG2.
The VS4 counter shall be cleared upon system initialization, reaching the end of the LoG2
section, or a successful access attempt.
The VS4 counter shall be incremented after every unsuccessful random access attempt in
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LoG2.
When the VS4 counter reaches the maximum number of random access attempts,
authorization to transmit shall be granted as soon as the channel is available.
5.7.2
Random access procedures
When the station has a login bursts to transmit, it shall use the p-persistent algorithm defined
as follows:
a) The station shall select a slot at random from the available slots in the LoG2 section, such
that it is equally likely to select any of the available slots.
b) If the station is able to select a slot, then the station shall transmit in the slot with
probability p2 (defined in 5.7.1.1).
c) The station shall clear the VS4 counter (VS4 counter cancelled) if it was able to transmit.
d) If the mobile station is unable to transmit at the first attempt, and it has not reached the
end of the current LoG2 section, and the VS4 timer has not expired, it shall increment the
VS4 counter, and then make a further random access attempt, restarting the process at a).
e) If the mobile station is unable to select a slot at the first attempt, and it has not reached the
end of the current LoG2 section, and the VS4 timer has expired, it shall select slots as in a)
and then transmit in those slots with probability p=1.
If the mobile station is unable to select a slot within the current LoG2 section, this shall be
regarded as an unsuccessful random access attempt, and the mobile station shall clear the
VS4 counter (VS4 counter cancelled) and restart the process for the LoG2 section in the
following frame.
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CHAPTER 6 –
L-DACS2 Link Management layer
(LML)
6.1
Introduction
Each ground station and each aircraft mobile station supporting air/ground point-to-point
communication services shall include the functionality of an L-DACS2 Link Management
Entity (LLM). An LLM shall be responsible for the data link management policy of the system.
In a mobile system, the LLM shall be responsible for determining with which ground
station(s) the aircraft system should maintain data link communication at any given time.
In a ground station, the LLM shall be responsible for determining which aircraft mobile
system(s) should be provided with data link communications.
An LLM shall have a Link Management Entity (LME) for each peer LME. Hence, a ground
station LLM shall have an LME for each aircraft mobile system and an aircraft mobile LLM
shall have an LME for each ground station with which it is communicating.
Note. – If an aircraft’s mobile system receives a burst from a ground station, only one LME
will process and react to that burst.
The L-DACS2 link management layer (LML) is divided into four sub-layers:
•
Media Access Control (MAC) sublayer that requires the use of Time Division Multiple
Access (TDMA);
•
an L-DACS2 Services sub-layer (LSS) that provides communication by using a
flexible burst format and associated transmission and reservation protocols over the
MAC sub-layer;
•
a Data Link Services sublayer (DLS) that provides connection-oriented and broadcast
services over the LSS;
•
a Link Management Entity (LME) that establishes and maintains connections.
The LML shall provide a reliable point-to-point service by using a connection-orientated DLS.
The LML shall provide an unacknowledged broadcast service by using a connectionless
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DLS.
Note. – There is one LSS entity for each L-DACS2 channel that is accessed by the station
(airborne or ground). The LSS provides service to the LLM as well as to the LME associated
with other L-DACS2 peer systems, their associated Data Link Entities (DLEs) and the DLS.
The LSS is served by the MAC that is associated with its particular L-DACS2 channel.
6.2
Login mechanism
An aircraft entering a cell shall know (from on-board information) the frequency of the
corresponding ground station.
The aircraft shall listen on this frequency for the framing message transmitted by the ground
station.
Note. – This message contains information about the slot structure.
The aircraft shall randomly select one of the login slots in the LoG2 section of the frame. It
shall then announce its presence to the ground station by transmitting a message in the
chosen login slot.
Note. – Randomly selecting a login slot reduces the likelihood of the aircraft attempting to
login at the same time in the same slot as another aircraft.
Note. – The aircraft is expected to transmit this login message in the same frame as the
framing message which it received (i.e. less than 1 second later).
The ground station shall reply in UP1 or UP2 informing the aircraft of the position of its
allocated slot in the CoS1 section.
The ground station shall give the aircraft a local 9-bit address.
The ground station shall inform the aircraft of the ground station’s own local (7-bit) address.
Note. – These short addresses are used within the cell for identification instead of the longer
27-bit ICAO address.
The aircraft shall then be able to transmit RTSs in its allocated slot in the CoS1 section of the
same frame.
Note. – It is expected that the aircraft will be able to transmit in an allocated CoS1 slot very
soon after reaching the new cell (as a framing message is transmitted by the ground station
every 1 second in the default setting).
6.3
Hand-over mechanism
There shall be two possible means of hand-over: controlled and uncontrolled.
Note. – Hand-over principles are very similar in various types of systems10.
Note. – The L-DACS2 hand-over procedure differs from the GSM hand-over procedure
because it is expected that most hand-overs will be mobile-initiated. Ground stations may
request hand-overs but cannot initiate them. This is to avoid “hang-ups”.
Passive scanning at the mobile is used to detect nearby ground stations according to the
local link management policy.
Note. – With current technology this may require separate receivers since the aircraft needs
to be monitoring the UP segment in the current channel all the time. However there are
recent trends in development of software radio capability that allow software functions to
scan other parts of the band that the radio is not “tuned” into.
10
http://www.ieee802.org/21/archived_docs/Documents/OtherDocuments/Handoff_Freedman.pdf
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6.3.1
Controlled hand-over
Controlled hand-overs can be air-initiated or ground-requested air-initiated.
Note. – A controlled hand-over is a make-before-break (“soft”) hand-over.
6.3.1.1
Air-initiated controlled hand-over
An air-initiated controlled hand-over shall be triggered if any of the following events occur:
According to the local link management policy, the signal quality on the current link is
deemed to be insufficient for maintaining reliable communications and the signal quality
of another ground station is significantly better, for more than the channel timeout period.
The channel busy timer expires. In this case, the mobile LME shall only initiate the handover procedure if the signal from another ground station is of sufficient signal quality.
The mobile LME is at a position which, according to the local link management policy,
requires that a connection with a new ground station is established.
Note. – An aircraft shall know, from on-board information, when it is nearing the edge of the
coverage area of the current cell.
Note. – If an aircraft has commenced approach to its destination airport and its current link is
with a ground station that does not offer data link service at that airport, then the aircraft
should hand-over as soon as possible to a ground station that does offer data link service at
that airport.
At an appropriate time, the aircraft shall transmit a “cell exit” message to the ground station in
CoS1.
Note. – This appropriate time shall be implementation-dependent. It may be influenced by
factors such as received signal power (SQP) and bit error rate (BER).
As the “cell exit” message is being transmitted, the aircraft shall also commence the login
procedure described in section 6.2.
Note. – The aircraft will know which frequency to search from its on-board information. The
assumption is that the aircraft knows its own position and the positions of all ground stations
and their frequencies. Prediction of which ground stations should be connected to base on
aircraft route tracing and prediction is not assumed to be required, but may improve
handover reliability.
The current ground station shall reply to the aircraft’s “cell exit” message in an UP1 or UP2
slot, to confirm that the aircraft is leaving the cell
When the aircraft receives the “exit confirmation” message from the current ground station in
the UP1 or UP2 slot, it shall send an ACK message to the ground station, indicating that the
“exit confirmation” message is being acknowledged.
Note. – This will be the last message that the aircraft sends to the current ground station.
Note. – The aircraft will not do this until is has correctly logged-on to the next ground station.
If the connection with the current ground station is lost before the aircraft has acknowledged
the “exit confirmation” message, then the slot will be de-allocated according to the procedure
in section 6.3.2.
When the current ground station receives the ACK message from the aircraft, it shall deallocate the aircraft’s CoS1 slot and shall consider the link to be terminated.
See Figure 12 for the diagram of a successful handover.
Although the aircraft may simultaneously be in communication with both the current and next
ground stations, no data messages shall be transmitted whilst the hand-over process is
occurring. Data messages shall only be sent once the hand-over is completed or failed.
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Previous
ground
station
Next
ground
station
Aircraft
Listen on appropriate
frequency
GS_FRAME
AC Cell exit
[COS1 slot]
[UP1 or UP2]
GS_EXIT_ACK
CELL_LOGIN
Ack = 1
[UP1 or UP2]
[LoG2 slot]
Address allocated
Slot allocated
GS_ALLOC
[UP1 or UP2]
*AC_ACK
Ack = 1, ID = CELL_EXIT
[COS2]
Slot de-allocated
Hand-over complete
*Only performed if still in range.
If AC_ACK is not transmitted, the link will time-out
Figure 12: Successful air-initiated controlled hand-over
Previous
ground
station
Next
ground
station
Aircraft
Listen on appropriate
frequency
GS_FRAME
AC Cell exit
[COS1 slot]
[UP1 or UP2]
GS_EXIT_ACK
CELL_LOGIN
Ack = 1
[UP1 or UP2]
[LoG2 slot]
Next frame
retransmit
Not received
correctly
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
*AC_ACK
Slot de-allocated
Hand-over complete
Ack = 1, ID = CELL_EXIT
[COS2]
*Only performed if still in range.
If AC_ACK is not transmitted, the link will time-out
Figure 13: Air-initiated controlled handover: retransmit cell login
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Previous
ground
station
Next
ground
station
Aircraft
Listen on appropriate
frequency
GS_FRAME
AC Cell exit
[COS1 slot]
[UP1 or UP2]
GS_EXIT_ACK
CELL_LOGIN
Ack = 1
[UP1 or UP2]
[LoG2 slot]
Address allocated
Slot allocated
GS_ALLOC
Not received
correctly
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
New slot allocated
(Allocated address
is re-used)
GS_ALLOC
[UP1 or UP2]
*AC_ACK
Slot de-allocated
Hand-over complete
Ack = 1, ID = CELL_EXIT
[COS2]
*Only performed if still in range.
If AC_ACK is not transmitted, the link will time-out
Figure 14: Air-initiated controlled handover: retransmit GS_ALLOC
Previous
ground
station
Next
ground
station
Aircraft
Listen on appropriate
frequency
GS_FRAME
AC Cell exit
[COS1 slot]
Not received
correctly
GS_EXIT_ACK
CELL_LOGIN
Ack = 0
[UP1 or UP2]
*Next frame
retransmit
AC Cell exit
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[COS1 slot]
[UP1 or UP2]
GS_EXIT_ACK
Ack = 1
[UP1 or UP2]
*AC_ACK
Slot de-allocated
Hand-over complete
Ack = 1, ID = CELL_EXIT
[COS2]
*Only performed if still in range, otherwise link will time-out
If AC_ACK is not transmitted, the link will time-out
Figure 15: Air-initiated controlled handover: retransmit AC_CELL_EXIT
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Previous
ground
station
Next
ground
station
Aircraft
Listen on appropriate
frequency
GS_FRAME
AC Cell exit
[COS1 slot]
GS_EXIT_ACK
CELL_LOGIN
Ack = 1
[UP1 or UP2]
Not received
correctly
AC_ACK
[LoG2 slot]
Address allocated
Slot allocated
GS_ALLOC
Ack = 0, ID = CELL_EXIT
[COS2]
[UP1 or UP2]
AC Cell exit
*Next frame
retransmit
[COS1 slot]
GS_EXIT_ACK
Ack = 1
[UP1 or UP2]
*AC_ACK
Slot de-allocated
Hand-over complete
Ack = 1, ID = CELL_EXIT
[COS2]
*Only performed if still in range, otherwise link will time-out
If AC_ACK is not transmitted, the link will time-out
Figure 16: Air-initiated controlled handover: retransmit GS_EXIT_ACK
6.3.1.2
Ground-requested air-initiated controlled hand-over
The ground station may be able to determine, from the location information in an aircraft’s
ADS-B transmissions, when the aircraft is nearing the edge of a cell.
A ground-requested air-initiated controlled hand-over may be triggered if any of the following
events occurs:
According to the local link management policy, the current ground station determines that
the aircraft is leaving the cell and is sufficiently close to a neighbouring ground station for
the hand-over process to commence.
According to the local link management policy, the current ground station determines that
the aircraft is sufficiently close to a neighbouring ground station for the hand-over process
to commence and the current ground station data load has exceeded the overload
setting.
Note. – The overload setting for a ground station is implementation-dependent.
Note. – The overload hand-over process attempts to share channel loading between
adjacent ground stations.
The signal quality from the aircraft may not be sampled to trigger a ground-requested airinitiated hand-over. If the ground station receives garbled data it shall only reply with a
NACK.
Note. –If the signal quality has degraded to an unacceptable level then the aircraft will initiate
the hand-over process.
The current ground station shall have knowledge of the cell loadings of the other ground
stations in its cluster from the GNI. If it is possible for the aircraft to hand-over to a selection
of ground stations then the current ground station shall compare the cell loadings of these
ground stations.
If the ground station determines that a hand-over is appropriate, it shall transmit a “cell exit”
message to the aircraft. If the current ground station determines that the aircraft should handover to a particular neighbouring ground station then this shall be indicated in the “cell exit”
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message.
Note. – The criteria for determining whether or not a hand-over is appropriate are
implementation specific.
Note. – The indication of which neighbouring ground station the aircraft should contact is
optional, is only possible within a GNI cluster, and is dependent on knowledge of adjacent
ground stations’ loadings.
The aircraft, on receiving a “cell exit” message from the current ground station, shall attempt
communication with the ground station in the next cell.
If the “cell exit” message indicates a particular future ground station then the aircraft shall
preferentially attempt to communicate with it (see Figure 22). If the signal quality from the
suggested future ground station is too low then the aircraft shall attempt communication with
any other possible future ground station (see Figure 23).
Note. – The aircraft will know which frequency to search for from its on-board information.
Because both the aircraft and the current ground station know the location and frequency
channel of the ground station in the next cell, it is not necessary for the current ground
station to transmit any information other than a “cell exit” trigger. This differs from the typical
GSM/GPRS procedure.
If the communication with the next ground station is successful and the aircraft is allocated a
CoS1 slot in the next cell, it shall reply to the current ground station with an “exit
confirmation” message indicating that the current link can be terminated (see Figure 17).
If the communication with the next ground station is not successful, the aircraft shall reply to
the current ground station with an “exit confirmation” message indicating that the current link
cannot be terminated (see Figure 20)..
When the current ground station receives an “exit confirmation” message showing that the
current link can be terminated, it shall transmit an ACK to the aircraft, shall de-allocate the
aircraft’s CoS1 slot and shall consider the link to be terminated (see Figure 17).
If the current ground station receives an “exit confirmation” message from the aircraft
showing that the current link cannot be terminated, and it still desires that the hand-over
is carried out, then it shall re-transmit the “cell exit” message to the aircraft in the next frame
(see Figure 21).
Otherwise the aircraft shall maintain its communication with the current ground station.
The ground-requested air-initiated controlled hand-over from the current ground station shall
not be completed before the aircraft has successfully made contact with the next cell.
Note. – If contact between the aircraft and the current ground station is lost before the
ground-requested air-initiated controlled hand-over has been completed, then the
uncontrolled hand-over process will be initiated.
Note. – Unlike Figure 13 to Figure 16, not all possible login error conditions are assessed in
the following diagrams; the process in Figure 21 will take place if a login is unsuccessful for
any reason.
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Previous
ground
station
Next
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
AC_EXIT_ACK
Ack = 1 [COS1 slot]
Slot de-allocated
GS_ACK Ack = 1, ID = EXIT_ACK
[UP1 or UP2]
If not received, then
aircraft still considers
exit to be complete
Hand-over complete
Figure 17: Successful ground-requested air-initiated controlled hand-over
Previous
ground
station
Next
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Not received
correctly
AC_EXIT_ACK
Ack = 0, Ack slot number = 0
[COS1 slot]
Next frame retransmit
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
GS_FRAME
[UP1 or UP2]
[Repeat as Figure 15]
Figure 18: Ground-requested air-initiated controlled hand-over: GS_CELL_EXIT retransmit
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Previous
ground
station
Next
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
AC_EXIT_ACK
Ack = 1 [COS1 slot]
Not received
correctly
GS_ACK Ack = 0, ID = EXIT_ACK
[UP1 or UP2]
AC_EXIT_ACK
Ack = 1 [COS1 slot]
GS_ACK
Hand-over complete
Ack = 1
ID = EXIT_ACK
[UP1 or UP2]
Figure 19: Ground-requested air-initiated controlled hand-over: AC_EXIT_ACK retransmit
Previous
ground
station
Aircraft
Next
ground
station
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
GS_FRAME not received.
Aircraft notes login as unsuccessful
AC_EXIT_ACK
Ack = 0, Ack slot number = 0
[COS1 slot]
*Next frame
retransmit
GS_CELL_EXIT
[UP1 or UP2]
[Repeat as Figure ]
*Only performed if aircraft is still in range, otherwise link will time-out
Figure 20: Ground-requested air-initiated controlled hand-over: GS_FRAME not received
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Previous
ground
station
Next
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
Slot not allocated
(for any reason)
AC_EXIT_ACK
Aircraft notes login
as unsuccessful
Ack = 0, Ack slot number = 0
[COS1 slot]
*Next frame
retransmit
GS_CELL_EXIT
[UP1 or UP2]
[Repeat as Figure 15]
*Only performed if aircraft is still in range, otherwise link will time-out
Figure 21: Ground-requested air-initiated controlled hand-over: unsuccessful login
Previous
ground
station
Recommended
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
recommended frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
AC_EXIT_ACK
Ack = 1 [COS1 slot]
Slot de-allocated
GS_ACK Ack = 1, ID = EXIT_ACK
[UP1 or UP2]
If not received, the
aircraft still considers
exit to be complete
Hand-over complete
Figure 22: Successful ground-requested air-initiated controlled hand-over with
recommendation
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Previous
ground
station
Next
ground
station
Aircraft
GS_CELL_EXIT
[UP1 or UP2]
Aircraft does not recognize the
recommended GS, or the login
procedure is unsuccessful
Aircraft determines another
possible GS
Aircraft starts to listen on
appropriate frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
AC_EXIT_ACK
Ack = 1 [COS1 slot]
Slot de-allocated
GS_ACK Ack = 1, ID = EXIT_ACK
[UP1 or UP2]
If not received, the
aircraft still considers
exit to be complete
Hand-over complete
Figure 23: Ground-requested air-initiated controlled hand-over with recommendation:
alternative ground station
Previous
ground
station
Aircraft
Next
ground
station
GS_CELL_EXIT
[UP1 or UP2]
Aircraft starts to listen on
appropriate frequency
Login attempts with recommended
GS and other possible GSs are all
unsuccessful
Aircraft notes login
as unsuccessful
AC_EXIT_ACK
Ack = 0, Ack slot number = 0
[COS1 slot]
*Next frame
retransmit
GS_CELL_EXIT
[UP1 or UP2]
[Repeat as Figure 20]
*Only performed if aircraft is still in range, otherwise link will time-out
Figure 24: Ground-requested air-initiated controlled hand-over with recommendation: no
successful logins
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6.3.2
Uncontrolled hand-over
If communication between the aircraft and the ground station is lost, for more than a predetermined time period, then both the aircraft and the ground station shall consider their link
to be terminated. This process could be assured by the monitoring of the aircraft traffic.
Indeed as described in section 4.5.1, the mobile shall send at least a keep alive message in
a specific time frame defined by S1 parameter.
Note. – The purpose of the time-out period is to avoid brief signal fluctuations causing
unnecessary hand-over procedures. If the signal is not detected for this “significant” period of
time then it is likely that the aircraft has moved out of the relevant ground station’s coverage.
Note. – An uncontrolled hand-over may be a break-before-make (“hard”) hand-over if the
aircraft does not successfully login with another ground station before considering the
connection with the previous one to be lost.
Note. – The appropriate time-out period shall be implementation-dependent. For this process
to occur correctly, the appropriate value for the time-out period must be chosen (as it may be
affected by local factors).
The ground station shall assume that the aircraft’s dedicated CoS1 slot is no longer required
by the aircraft and shall de-allocate it.
The aircraft shall determine the appropriate (new) ground station to contact and shall begin
the “login” procedure on the relevant frequency.
Note. – This procedure means that de-allocation of the old link may occur before the new link
has been established. This is a “hard” hand-over.
The process is illustrated in Figure 25; note that it is assumed in this diagram that the
receipts of the GS_FRAME, CELL_LOGIN and GS_ALLOC messages are successful. For
alternative situations during login see Figure 13 to Figure 16.
Previous
ground
station
Next
ground
station
Aircraft
[Link time-out occurs]
Slot de-allocated
Aircraft considers link to be lost
Aircraft determines possible GS to
contact and starts to listen on the
appropriate frequency
GS_FRAME
[UP1 or UP2]
CELL_LOGIN
[LoG2 slot]
GS_ALLOC
Address allocated
Slot allocated
[UP1 or UP2]
[Repeat as for Figures 10-14]
Figure 25: Uncontrolled handover
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ANNEX 1 –
Example of burst transmission
A1.1
Burst transmission in UP1 or UP2
Table 12 derives the number of user octets in a burst designed to fit in two consecutive slots
in UP1 or UP2. This is just an example and on average it expected that a burst in UP1 or
UP2 will occupy several or tens of slots, in which case the guard time (which is larger than
for e.g. CoS2) will be proportionally smaller when compared to the amount of user data
transmitted per unit of slot length.
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Parameter description
Derivation
Value
Duration of 1 symbol/bit
(TS)
6/1625000 s
or
~ 0.0036923 ms
Number of symbol
durations (TS) per frame
1625000/6 bps
or
~ 270833.333 bps
Duration of UP1/UP2 slot
(M5)
1/150 s
or
~ 6.666 ms
Bits per 1 slot
1625000/(6 * 150) bits
1805 + 5/9 bits
or
~ 1805.555 bits
Bits per 2 slots
2 * 1625000/(6 * 150) bits
3611 + 1/9 bits
or
~ 3611.111 bits
Guard time prior to start of
burst
See Section 3.5.6.1
408 bits
or
~ 1.50646 ms
Guard time at end of burst
See Table 5 in Section 3.5.6.1
755 + 1/9 bits
or
755.111 bits
~ 2.78810 ms
Bits per slot – start guard time bits –
end guard time bits
2448 bits
Ramp up time + ramp down time
16 bits
Bits per slot – (ramp up bits + ramp
down bits) – guard time bits
2432 bits
Based on GSM (16 +10 bits)
26 bits
2 x 8 bits
16 bits
16 bits
16 bits
Bits available for FEC,
addresses and user data
Bits per active slot – 58 bits
2374 bits
Bits available after
deduction of bits required
for FEC
30% FEC coding: divide bits available
by 1.3, round down to nearest octet
1824
Octets available after
deduction of FEC overhead
Bits available/8
228
Approximate number of
octets required for: version
number, source address,
message type, message
ID, destination address,
data length
27 bit address (includes 24 bit ICAO
address see Note 1); 2-bit version
number
4 octets
Active slot duration
Transmitter stabilisation
Usable slot duration
Synchronisation (training)
sequence
Start and end flags
CRC
Approximate number of
octets available for user
data
224 octets
Use of ICAO 24-bit address is worst case. Mobile stations and ground stations will be allocated local
addresses by the ground station, which will save on bits.
Table 25: Available payload for user data in burst occupying two slots in UP1 or UP2
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The following figure illustrates the number of bits allocated to each section of a burst
transmitted in the UP1 or UP2 section. For the purposes of illustration, this burst occupies
two basic (full) slots, each of 6.666 ms duration. In practice, the uplink transmissions can be
as long as the entire UPx section, depending on how much data is scheduled for
transmission.
Start guard time
Burst
Occupying two
6.666 ms slots
(3611.111 bits)
408 bits
Ramp up
8 bits
Sync sequence
26 bits
Start flag
8 bits
User data – Addresses, msg IDs
32 bits (4 octets)
User data – Application message
1792 bits (224 octets)
FEC
550 bits
CRC
16 bits
End flag
8 bits
Ramp down
8 bits
End guard time
755.111 bits
Figure 26: Illustration of burst format for a typical burst in UP1 or UP2
6.4
11
Burst transmission in a single slot in CoS2
Table 13 derives the number of user octets in a burst designed to fit in a single slot in CoS2.
11
Note that in this illustrative example, there is only one message in this uplink burst, hence only one FEC field.
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Parameter description
Derivation
Value
Duration of 1 symbol/bit
(TS)
6/1625000 s
or
~ 0.0036923 ms
Number of symbol
durations (TS) per frame
1625000/6 bps
or
~ 270833.333 bps
Duration of CoS2 slot (M5)
1/150 s
or
~ 6.666 ms
Bits per slot
1625000/(6 * 150) bits
1805 + 5/9 bits
or
~ 1805.555 bits
Guard time
Reduced guard time based on use of
Timing Advance (see Section 3.5.6.5)
33 + 5/9 bits
or
33.555 bits
~ 0.12390 ms
Bits per slot – guard time bits
1772 bits
Ramp up time + ramp down time
16 bits
Bits per slot – (ramp up bits + ramp
down bits) – guard time bits
1756 bits
Based on GSM (16 +10 bits)
26 bits
2 x 8 bits
16 bits
16 bits
16 bits
Bits available for FEC,
addresses and user data
Bits per active slot – 58 bits
1698 bits
Bits available after
deduction of bits required
for FEC
30% FEC coding: divide bits available
by 1.3, round down to nearest octet
1304
Octets available after
deduction of FEC overhead
Bits available/8
163
Approximate number of
octets required for: version
number, source address,
message type, message
ID, ground station local
address, acknowledgement
of uplinked data, data
length, Request-To-Send
(RTS)
27 bit address (includes 24 bit ICAO
address see Note 1); 2-bit version
number
8 octets
Active slot duration
Transmitter stabilisation
Usable slot duration
Synchronisation (training)
sequence
Start and end flags
CRC
Approximate number of
octets available for user
data
155 octets
Use of ICAO 24-bit address is worst case. Mobile stations and ground stations will be allocated local
addresses by the ground station, which will save on bits.
Table 26: Available payload for user data in a single slot in CoS2
The following figure illustrates the number of bits allocated to each section of a single slot
burst in CoS2.
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Full size
slot length
6.666 ms
(1805.555 bits)
Ramp up
8 bits
Sync sequence
26 bits
Start flag
8 bits
User data – Addresses, msg IDs
64 bits (8 octets)
User data – Application message
1240 bits (155 octets)
FEC
394 bits
CRC
16 bits
End flag
8 bits
Ramp down
8 bits
Guard time
33.555 bits
Figure 27: Illustration of burst format for a single slot burst in CoS2
6.5
Burst transmission in CoS1 slot
Table 14 derives the number of user octets in a burst designed to fit in a single CoS1 slot.
It is currently not intended that a CoS1 slot carries data messages, apart from ACK and RTS.
Therefore this table calculation serves as a check that the CoS1 slot size is sufficient to
accommodate those message fields that are required.
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Parameter description
Derivation
Value
Duration of 1 symbol/bit
(TS)
6/1625000 s
or
~ 0.0036923 ms
Number of symbol
durations (TS) per frame
1625000/6 bps
or
~ 270833.333 bps
Duration of CoS1 slot (M7)
Bits per CoS1 slot
1/900 s
or
~ 1.111 ms
1625000/(6 * 900) bits
300 + 25/27 bits
or
~ 300.926 bits
Reduced guard time based on use of
Timing Advance (see Section 3.5.6.4)
33 + 25/27 bits
or
33.926 bits
~ 0.12526 ms
Bits per slot – guard time bits
267 bits
Ramp up time + ramp down time
16 bits
Bits per slot – (ramp up bits + ramp
down bits) – guard time bits
251 bits
Based on GSM (16 +10 bits)
26 bits
2 x 8 bits
16 bits
16 bits
16 bits
Bits available for FEC,
addresses and user data
Bits per active slot – 58 bits
193 bits
Bits available after
deduction of bits required
for FEC
30% FEC coding: divide bits available
by 1.3, round down to nearest octet
144
Octets available after
deduction of FEC overhead
Bits available/8
18
Approximate number of
octets required for: version
number, source address,
message type, message
ID, ground station local
address, acknowledgement
of uplinked data, data
length, Request-To-Send
(RTS)
27 bit address (includes 24 bit ICAO
address see Note 1); 2-bit version
number
7 octets
Guard time
Active slot duration
Transmitter stabilisation
Usable slot duration
Synchronisation (training)
sequence
Start and end flags
CRC
Approximate number of
octets available for user
data
11 octets
Use of ICAO 24-bit address is worst case. Mobile stations and ground stations will be allocated local
addresses by the ground station, which will save on bits.
Table 27: Available payload for user data in a CoS1 slot
As an option for future message definition, the 11 octets available for user data could be
used to transmit the position of the aircraft. This information could be used for
independent/inherent security functions, surveillance functions, communication system
management etc. This would assume that position data is available from an onboard
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navigation system. Further analysis should determine if the inclusion of position information
would provide a benefit.
The following figure illustrates the number of bits allocated to each section of a burst in
CoS1.
CoS1 slot
length
1.111 ms
(300.926 bits)
Ramp up
8 bits
Sync sequence
26 bits
Start flag
8 bits
User data – Addresses, msg IDs
56 bits (7 octets)
User data – Application message
88 bits (11 octets)
FEC
49 bits
CRC
16 bits
End flag
8 bits
Ramp down
8 bits
Guard time
33.926 bits
Figure 28: Illustration of burst format for a burst in a CoS1 slot
6.6
Burst transmission in a LoG2 slot
Table 15 derives the number of user octets in a burst designed to fit in a LoG2 slot.
It is currently not intended that a LoG2 slot caries data messages. Therefore this table
calculation serves as a check that the LoG2 slot size is sufficient to accommodate those
message fields that are required.
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Parameter description
Derivation
Value
Duration of 1 symbol/bit
(TS)
6/1625000 s
or
~ 0.0036923 ms
Number of symbol
durations (TS) per frame
1625000/6 bps
or
~ 270833.333 bps
Duration of LoG2 slot (M6)
Bits per LoG2 slot
1/300 s
or
~ 3.333 ms
1625000/(6 * 300) bits
902 + 7/9 bits
or
~ 902.777 bits
See Section 3.5.6.2
345 + 7/9 bits
or
345.777 bits
~ 1.27672 ms
Bits per slot – guard time bits
557 bits
Ramp up time + ramp down time
16 bits
Bits per slot – (ramp up bits + ramp
down bits) – guard time bits
541 bits
Based on GSM (16 +10 bits)
26 bits
2 x 8 bits
16 bits
16 bits
16 bits
Bits available for FEC,
addresses and user data
Bits per active slot – 58 bits
483 bits
Bits available after
deduction of bits required
for FEC
30% FEC coding: divide bits available
by 1.3, round down to nearest octet
368
Octets available after
deduction of FEC overhead
Bits available/8
46
Approximate number of
octets required for: version
number, ICAO 27-bit
source address, message
type, message ID, ground
station 27-bit ICAO
address, authentication
27 bit address (includes 24 bit ICAO
address); 2-bit version number
13 octets
Guard time
Active slot duration
Transmitter stabilisation
Usable slot duration
Synchronisation (training)
sequence
Start and end flags
CRC
Approximate number of
octets available for
additional user data (if
required)
33 octets
Table 28: Available payload for user data in a LoG2 slot
The following figure illustrates the number of bits allocated to each section of a burst in a
LoG2 slot.
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LoG2 slot
length
3.333 ms
(902.777 bits)
Ramp up
8 bits
Sync sequence
26 bits
Start flag
8 bits
User data – Addresses, msg IDs
104 bits (13 octets)
User data – Application message
264 bits (33 octets)
FEC
115 bits
CRC
16 bits
End flag
8 bits
Ramp down
8 bits
Guard time
345.777 bits
Figure 29: Illustration of burst format for a burst in a LoG2 slot
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ANNEX 2 –
Message structure
A2.1
Message type codes
This section shows diagrams for all of the message types, showing the fields and the
numbers of bits required for each one.
The following assumptions have been made:
•
•
•
•
•
All messages shall start with the ISO/IEC 13239 flag
Single-bit flags are used for ACK, RTS and CTS
ICAO 27-bit addresses are only used when necessary
Aircraft local addresses shall be 9-bit addresses
° 0 0000 0000 is not permitted
° 1 1111 1111 means ‘broadcast’ to all aircraft and is only used by the GS
Message type is followed by the destination address – the station will analyse the
source address and the message type to see if the message is expected
A2.2
Message type codes
6 bits are used for message types to allow space for future codes. ’00 0000’ is not used. All
binary codes which are not shown are unallocated.
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Message type
Message name
Binary code
RTS_COS1
00 0001
Mobile station CoS1 Keep alive
KEEP_ALIVE
00 0010
Mobile station CoS2 data downlink
DATA_COS2
00 0011
DATA_RA_COS2
00 0100
Mobile station cell exit
AC_CELL_EXIT
00 0101
Ground station cell exit
GS_CELL_EXIT
00 0110
Mobile station cell exit ACK
AC_EXIT_ACK
00 0111
Ground station cell exit ACK
GS_EXIT_ACK
00 1000
GS_DATA
00 1001
GS_FRAME
00 1010
Ground station ACK
GS_ACK
00 1011
Mobile station CoS2 ACK
AC_ACK
00 1100
CTS_ACK_ALL
00 1101
CELL_LOGIN
00 1110
GS_ALLOC
00 1111
GS_TADVANCE
01 0000
Mobile station CoS1 RTS/ACK downlink
Mobile station CoS2 random access data
downlink
Ground station data uplink
Ground station framing message
Ground station ACK/CTS ALL message
Mobile station LoG2 login
Ground station login accept
Grand station timing advance
Table A-1 : Message type codes
A2.3
Priority field
The priority field follows the ATN numbering scheme and can have values from 0 to 14.
A2.4
Messages
Note. – Power control fields have to be developed in the uplink message structure. This is
under development.
Note. – The CRC has been estimated to be sufficient at 2 octets per message. This will be
validated in a next version of the document.
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Login message: CELL_LOGIN
This message shall be transmitted by a mobile station in a LoG2 slot.
Note. – 384 bits are available for user data in a LoG2 slot.
ISO flag
Version number
Address length flag
A/C ICAO address
Message type
Message identifier
GS ICAO address
Authentication
FEC
CRC
ISO flag
8
2
1
27
6
6
27
32
31
16
8
164
Binary 0111 1110
Binary 00
Binary 1 for 27-bit ICAO address
Binary 00 1110
1 to 64 (00 0001 to 11 1111)
Destination ground station
Size not fixed
30% FEC coding
Binary 0111 1110
bits
The mobile station will listen for the framing message to identify the position in the frame of
the LoG2 slots.
The ground station reply to the login message will be transmitted in the UP1 section of the
following frame.
Login reply message: GS_ALLOC
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS ICAO address
Message type
Login message identifier
A/C ICAO address
A/C local address
GS local address
CoS1 slot number
Power Control Field
FEC
CRC
ISO flag
8
2
1
27
6
6
27
9
7
9
5
29
16
8
160
Binary 0111 1110
Binary 00
Binary 1
Binary 00 1111
1 to 64 (00 0001 to 11 1111)
510 possible addresses (all 0’s and all 1’s invalid)
126 possible addresses (all 0’s and all 1’s invalid)
Measured from start of frame, range 100 – 459
00000 for not monitor, range 1 -25
30% FEC coding
Binary 0111 1110
bits
The ground station will transmit a reply to the mobile station in UP1 after reception of a login
message from that mobile station.
The allocated CoS1 slot will be exclusively assigned to the mobile station until that mobile
station leaves the cell.
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Cell exit message: AC_CELL_EXIT
This message shall be transmitted by a mobile station in CoS1.
Note. – With M4 = 6, 235 bits are available for this message in a CoS1 slot.
ISO flag
Version number
Address length flag
ICAO address
Message type
Message identifier
ICAO address
Next station flag
Next station address
Authentication
FEC
CRC
ISO flag
8
2
1
27
6
6
27
1
27
32
39
16
8
200
Binary 00
Binary 1
Transmitting A/C
Binary 00 0101
1 to 64 (00 0001 to 11 1111)
Current ground station
Binary 0 or 1
Suggested future ground station
Size not fixed
30% FEC coding
Binary 0111 1110
bits
If the next station flag is 0 then the next station address field shall be all 0's.
Cell exit message: GS_CELL_EXIT
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
ICAO address
Message type
Message identifier
ICAO address
Next station flag
Next station address
Authentication
FEC
CRC
ISO flag
8
2
1
27
6
6
27
1
27
32
39
16
8
200
Binary 00
Binary 1
Transmitting GS
Binary 00 0110
1 to 64 (00 0001 to 11 1111)
Destination A/C
Binary 0 or 1
Suggested future ground station
Size not fixed
30% FEC coding
Binary 0111 1110
bits
If the next station flag is 0 then the next station address field shall be all 0's.
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Cell exit reply message: AC_EXIT_ACK
This message shall be transmitted by a mobile station in CoS2.
ISO flag
Version number
Address length flag
ICAO address
Message type
Exit message identifier
ICAO address
ACK flag
ACK slot number
FEC
CRC
ISO flag
8
2
1
27
6
6
27
1
Binary 00
Binary 1
Transmitting A/C
Binary 00 0111
1 to 64 (00 0001 to 11 1111)
Destination GS
CoS2 slot for the A/C ACK, range 350 - 499.
Set to 0 for an A/C reply to a GS CELL_EXIT
24 30% FEC coding
16
8 Binary 0111 1110
135 bits
9
Cell exit reply message: GS_EXIT_ACK
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
ICAO address
Message type
Exit message identifier
ICAO address
ACK flag
ACK slot number
FEC
CRC
ISO flag
Edition: 1.0
8
2
1
27
6
6
27
1
Binary 00
Binary 1
Transmitting GS
Binary 00 0111
1 to 64 (00 0001 to 11 1111)
Destination A/C
CoS2 slot for the A/C ACK, range 350 - 499.
Set to 0 for an A/C reply to a GS CELL_EXIT
24 30% FEC coding
16
8 Binary 0111 1110
135 bits
9
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Framing message: GS_FRAME
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS ICAO address
Message type
Message identifier
A/C local address
UTC time and date
Active UP1 length
Active CoS1 length
Active UP2 length
Active CoS2 length
Active LoG2 length
New frame number
New UP1 length
New CoS1 length
New UP2 length
New CoS2 length
New LoG2 length
FEC
CRC
ISO flag
8
2 Binary 00
1 Binary 1
Necessary for aircraft entering the cell to
27
register this message
6 Binary 00 1010
6 1 to 64 (00 0001 to 11 1111)
9 Binary 1 1111 1111 to indicate broadcast
28 Used for synchronization and time stamping
7 Current number of slots, 2 - 129
9 Current number of slots, 0 - 510
7 Current number of slots, 2 - 129
8 Current number of slots, 5 - 145
4 Current number of slots, 4 - 33
Frame which will change to new section sizes,
8
0 if section sizes are not changing.
8 New number of slots in UP1, 2 - 129
8 New number of slots in CoS1, 0 - 510
8 New number of slots in UP2, 2 - 129
8 New number of slots in CoS2, 5 - 145
4 New number of slots in LoG2, 4 - 33
48 30% FEC coding
16
8 Binary 0111 1110
238 bits
The following fields:
New UP1 length
New CoS1 length
New UP2 length
New CoS2 length
New LoG2 length
shall be ignored by the mobile station if the ‘New frame number’ field is set to 0.
If the mobile station receives a GS_FRAME message in which some or all of the section
sizes are out-of-range, then it shall transmit a NACK to the GS and use the existing frame
section sizes.
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GS data uplink message: GS_DATA
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS local address
Message type
Message identifier
A/C local address
Power Control Field
Data length (octets)
Data
FEC
CRC
ISO flag
8
2
1
7
6
6
9
5
11
8 - 16384
16 - 4928
16
8
103 21386
Binary 00
Binary 0
Binary 00 1001
1 to 64 (00 0001 to 11 1111)
00000 for not monitor, range 1 -25
Range 1 - 2,048 octets
Variable length
30% FEC coding
Binary 0111 1110
bits
CoS1 keep-alive message: KEEP_ALIVE
This message shall be transmitted by a mobile station in CoS1.
ISO flag
Version number
Address length flag
A/C local address
Message type
Message identifier
GS local address
FEC
CRC
ISO flag
8
2
1
9
6
6
7
10
16
8
73
Binary 00
Binary 0
Binary 00 0010
1 to 64 (00 0001 to 11 1111)
30% FEC coding
Binary 0111 1110
bits
This message is transmitted by the mobile station when it has no data and no
acknowledgement to send.
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CoS1 RTS downlink message: RTS_COS1
This message shall be transmitted in CoS1 by a mobile station that has a requirement to
transmit an ACK and/or an RTS to the ground station.
ISO flag
Version number
Address length flag
A/C local address
Message type
Message identifier
GS local address
ACK flag
ACK of message type
8
2
1
9
6
6
7
1
6
ACK of message ID
6
RTS flag
Priority (0 to 14)
Reservation length
FEC
CRC
ISO flag
Binary 00
Binary 0
Binary 00 0001
1 to 64 (00 0001 to 11 1111)
Reply to UP1 uplink
Message type acknowledged (all 0's if unknown)
Message identifier of the message being ACKed
(all 0’s if unknown)
Request for CoS2 slot(s)
Unassigned if RTS flag is 0
Number of slots required (1 - 20)
30% FEC coding
1
4
5
17
16
8 Binary 0111 1110
103 bits
The reservation length field shall be set to 0 if the RTS flag is 0.
ACK/CTS message to all aircraft: CTS_ACK_ALL
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS local address
Message type
A/C local address
CTS flag
RTS message identifier
Priority (0 to 14)
Reserved slot number
Reservation length
Power Control Field
ACK flag
ACK of message type
ACK of message ID
FEC
CRC
ISO flag
Edition: 1.0
8
2
1
7
6
9
1
6
4
9
5
5
1
Binary 00
Binary 0
Binary 00 1101
Binary 1 indicates CTS
1 to 64 (00 0001 to 11 1111)
Unassigned if RTS flag is 0
Binary 0 0000 0000 if no CTS, 350 - 499
Max value 20 slots, 0 if no CTS
00000 for not monitor, range 1 -25
Binary 0 for NACK, binary 1 for ACK
Message type acknowledged (all 0’s if
6 unknown)
ID of message being ACKed (all 0’s if
6 unknown)
0.3 *(16+52n) 30% FEC coding
16
8 Binary 0111 1110
48+52n+0.3
*(16+52n) bits
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The highlighted box is repeated according to the number of aircraft (n) to which an ACK or
CTS is being sent.
GS ACK uplink message: GS_ACK
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS local address
Message type
A/C local address
Power Control Field
ACK flag
ACK of message type
ACK of message ID
FEC
CRC
ISO flag
8
2
1
7
6
9
5
1
6
6
15
16
8
90
Binary 00
Binary 0
Binary 00 1011
00000 for not monitor, range 1 -25
Reply to CoS2 downlink
Message type acknowledged (all 0’s if unknown)
ID of message being ACKed (all 0’s if unknown)
30% FEC coding
Binary 0111 1110
bits
A NACK to a transmitted message shall trigger a re-send by the mobile station.
A mobile station shall ignore an ACK/NACK for an unrecognized message (type or ID).
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CoS2 downlink message: DATA_COS2
This message shall be transmitted by a mobile station in CoS2 that has a requirement to
send data, or an ACK and data, or and ACK and RTS and data, to the ground station.
ISO flag
Version number
Address length flag
A/C local address
Message type
Message identifier
GS local address
ACK flag
8
2
1
9
6
6
7
1
ACK of message type
6
ACK of message ID
RTS flag
Priority (0 to 14)
Reservation length
Data length (octets)
Data
FEC
CRC
ISO flag
Binary 00
Binary 0
Binary 00 0011
1 to 64 (00 0001 to 11 1111)
Reply to UP2 uplink
Message type acknowledged (all 0’s if
unknown)
ID of message being ACKed (all 0’s if
unknown)
Request for CoS2 slot(s)
Unassigned if RTS flag is 0
Number of slots required (1 - 20)
Range 1 - 2,048
Variable length
30% FEC coding
6
1
4
5
11
8 - 16384
22 - 4935
16
8 Binary 0111 1110
127 –
21416 bits
The ACK fields shall not be included in the message if the ACK flag is set to 0.
The RTS fields shall not be included in the message if the RTS flag is set to 0.
A/C ACK downlink message: AC_ACK
This message shall be transmitted by a mobile station in CoS2 that has a requirement to
send an ACK to the ground station.
ISO flag
Version number
Address length flag
A/C local address
Message type
GS local address
ACK flag
ACK of message type
ACK of message ID
FEC
CRC
ISO flag
8
2
1
9
6
7
1
6
6
12
16
8
82
Binary 00
Binary 0
Binary 00 1100
Reply to UP2/CoS2 uplink
Message type acknowledged (all 0’s if unknown)
ID of message being ACKed (all 0’s if unknown)
30% FEC coding
Binary 0111 1110
bits
A NACK to a transmitted message shall trigger a re-send by the ground station.
The ground station shall ignore an ACK/NACK for an unrecognized message ID.
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CoS2 random access long data message: DATA_RA_COS2
This message shall be transmitted by a mobile station in CoS2 that has a requirement to
send data to the ground station using the random access procedure.
ISO flag
Version number
Address length flag
A/C local address
Message type
Message identifier
GS local address
Data length (octets)
Data
FEC
CRC
ISO flag
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8
2
1
9
6
6
7
11
8 - 16384
15 - 4928
16
8
97 - 21386
Binary 00
Binary 0
Binary 00 0100
1 to 64 (00 0001 to 11 1111)
Range 1 - 2,048. Length 0 is invalid.
Variable length
30% FEC coding
Binary 0111 1110
bits
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Framing message: GS_TADVANCE
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS local address
Message type
A/C local address
T5 Parameter
FEC
CRC
ISO flag
8
2
1
7
6
9
5
0.3 *(16+14n)
16
8
48+14n+0.3
*(16+14n)
Binary 00
Binary 0
Binary 00 1101
Range 0 to 96
30% FEC coding
Binary 0111 1110
bits
The A/C local address and corresponding T5 parameter are repeated for n aircraft logged in
to the GS cell.
GS Power Control message: GS_ACK
This message shall be transmitted by a ground station in UP1 or UP2.
ISO flag
Version number
Address length flag
GS local address
Message type
A/C local address
Power Control Field
FEC
CRC
ISO flag
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8
2
1
7
6
9
5
11
16
8
73
Binary 00
Binary 0
Binary 00 1011
00000 for not monitor, range 1 -25
30% FEC coding
Binary 0111 1110
bits
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ANNEX 3 –
System operations
The following text indicates the possible sequences of events.
A3.1
Downlink
Each aircraft has an allocated CoS1 slot.
A3.1.1
A/C login
Note. – This represents the communication initiation when an aircraft contacts a new ground
station under any circumstances.
The aircraft shall use its knowledge of the area to identify the frequency of the ground station
whose cell it is about to enter.
The aircraft shall listen on this frequency for a minimum of 2 seconds to hear the framing
message (GS_FRAME) transmitted by the ground station.
Note. – This framing message contains the ICAO address of the ground station, the current
sizes of sections within the frame and the UTC time and date.
When the aircraft has heard the GS_FRAME message, it transmits a CELL_LOGIN message
in one of the dedicated LoG2 slots.
The CELL_LOGIN message contains the ICAO address of the aircraft.
If the GS correctly receives the CELL_LOGIN message, it shall transmit a GS_ALLOC
message to the aircraft in UP1 or UP2.
Note. – The GS_ALLOC message contains the new local 9-bit address for the aircraft, the
local 7-bit address of the ground station and the location of the exclusive high-QoS CoS1 slot
for the aircraft.
If the aircraft does not receive the GS_ALLOC message correctly then it shall re-transmit the
CELL_LOGIN message in one of the dedicated login slots
If the GS does not receive the CELL_LOGIN message correctly, it shall take no action except
wait for the aircraft to re-transmit.
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A3.1.2
Aircraft has data to send
If the aircraft has data to send, the aircraft shall transmit an RTS_COS1 message with RTS
flag set to 1.
Note. – This requests reservation of a specified number of slots in CoS2.
If the RTS_COS1 transmission is correctly received by the GS, the GS responds in the
combined CTS_ACK_ALL message in UP1 or UP2, setting the ACK flag to 1, indicating
which message is being acknowledged, setting the CTS flag to 1 and indicating the allocated
slot(s) for the data in CoS2.
If the CTS in the CTS_ACK_ALL message is not correctly received by the aircraft then it
shall retransmit an RTS for the data in CoS1 or using a DATA_CoS2 message with RTS field
set to 1.
If the CTS in the CTS_ACK_ALL message is correctly received by the aircraft then it shall
transmit the data in the allocated slot(s) in CoS2.
If the aircraft does not receive an ACK from the GS for the data transmitted in the next
CTS_ACK_ALL message then it shall re-transmit an RTS for the data in its CoS1 slot or
using a DATA_CoS2 message with RTS field set to 1
If the DATA_COS1 transmission is not correctly received by the GS, the GS sets the ACK
flag to 0 in the CTS_ACK_ALL message in UP1 or UP2 and indicates which message is
being NACKed.
Upon receipt of a NACK from the GS, the aircraft shall re-transmit an RTS for the data in
CoS1 or using a DATA_CoS2 message with RTS field set to 1.
A3.1.3
Aircraft has no data to send
If the aircraft has no data to send and wishes to maintain the link with the GS, it shall transmit
a KEEP_ALIVE message in its CoS1 slot if required to maintain the aircraft’s mandatory rate
for CoS1 transmissions of at least one transmission per S1 frames.
A3.1.4
CoS2 random access
If an aircraft has one or more messages in its transmit queue at the start of the CoS2 section
of the frame, the aircraft shall identify from its previous reception of GS transmissions all
CoS2 slots in the current frame that are free and select a slot or slots from the available free
slots for random access according to the random access procedure specified in Section 4.6.
If slots are successfully selected according to the random access procedure, the aircraft shall
transmit in the selected slots in CoS2 according to the random access procedure using the
DATA_RA_COS2 message, and remove the data transmitted from its transmit queue.
A GS in receipt of data transmitted by an aircraft by random access in CoS2 shall transmit an
ACK in the UP1 or UP2 section of the frame.
An aircraft receiving an ACK from the GS for data transmitted by random access shall
remove the data transmitted from its transmit queue.
An aircraft receiving an NACK from the GS, or which receives no ACK from the GS for data
transmitted by random access in the following UP1 or UP2 frame sections, shall maintain the
data in its transmit queue for subsequent transmission.
A3.1.5
Aircraft-initiated cell exit
If the aircraft is about to leave the cell of the GS it shall transmit an AC_CELL_EXIT
message in its dedicated CoS1 slot.
If the GS correctly receives the CELL_EXIT message it shall transmit a GS_EXIT_ACK
message in UP1 or UP2, with the ACK flag set to 1, indicating an ACK slot in CoS2 for the
aircraft to confirm.
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If the aircraft correctly receives the GS_EXIT_ACK message, the aircraft shall transmit an
AC_ACK in the allocated CoS2 slot, with the ACK flag set to 1, the message ID field set to
CELL_EXIT and the repeat number field set to 000.
If the aircraft does not (correctly) receive the GS_EXIT_ACK message and can still contact
the GS, the aircraft shall transmit an AC_ACK in the allocated CoS2 slot, with the ACK flag
set to 0, the ACK of message ID field set to CELL_EXIT and the repeat number field set to
000.
The aircraft shall then attempt to re-transmit the AC_CELL_EXIT message in its dedicated
CoS1 slot in the next frame.
If the aircraft correctly receives the GS_EXIT_ACK message and is out-of-range of the GS,
the aircraft shall do nothing, and its dedicated CoS1 slot at the old GS will time-out.
If the GS receives an AC_ACK message from the aircraft with the ACK flag set to 1, the
aircraft's dedicated CoS1 slot shall be de-allocated and the hand-over will be complete.
If the GS does not receive an AC_ACK, or receives an AC_ACK with the ACK flag set to 0,
the GS shall do nothing. If the aircraft is still in range then the CELL_EXIT shall be retransmitted in the next frame, otherwise the link will time-out.
If the GS does not correctly receive the AC_CELL_EXIT message, the GS shall do nothing. If
the aircraft is still in range then it shall retransmit the CELL_EXIT in the next frame, otherwise
the link will time-out.
A3.1.6
GS request for aircraft cell exit with no recommendation
If the aircraft is about to leave the cell of the current GS, or the current GS is overloaded, and
the GS wishes to trigger a hand-over procedure, the current GS shall transmit a
GS_CELL_EXIT message in UP1 or UP2.
If the aircraft correctly receives the GS_CELL_EXIT message from the current GS, then the
aircraft will determine the frequency of the new GS to contact in the next cell.
The aircraft shall attempt to hear the new GS on the expected frequency.
If the aircraft receives a framing message from the new GS then it shall commence the cell
login procedure (see Section A3.1.1).
If the cell login procedure is successful, the aircraft shall transmit an AC_EXIT_ACK
message to the current GS in its dedicated CoS1 slot, with the ACK flag set to 1 and the
ACK slot number field set to 0.
If the current GS receives the AC_EXIT_ACK message correctly, it shall reply with a
GS_ACK message with the ACK flag set to 1 and the message ID field set to EXIT_ACK. It
shall then de-allocate the aircraft's CoS1 slot and consider the link to be terminated.
If the aircraft receives the GS_ACK with the ACK flag set to 1 it shall consider the link to be
terminated.
If the aircraft receives the GS_ACK with the ACK flag set to 0 it shall re-transmit the
AC_EXIT_ACK.
If the aircraft does not receive a GS_ACK message it shall still consider the "exit" process to
be complete.
Note. – This is to avoid the situation where a GS_ACK could be transmitted but not received
and the aircraft thinks that its old CoS1 slot is still available when in fact it had been reallocated by the GS.
If the cell login procedure with the new GS is not successful or the aircraft cannot
successfully communicate with the new GS on the new frequency then the aircraft will
transmit an AC_EXIT_ACK message to the current GS in its dedicated CoS1 slot, with the
ACK flag set to 0 and the ACK slot number field set to 0.
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If the current GS receives an AC_EXIT_ACK message with the ACK flag set to 0, or does not
(correctly) receive an AC_EXIT_ACK at all, then it shall re-transmit the GS_CELL_EXIT
message in the next frame, unless the aircraft transmits an AC_CELL_EXIT message
beforehand.
If the aircraft does not correctly receive the GS_CELL_EXIT message then it shall transmit
an AC_EXIT_ACK message to the current GS in its dedicated CoS1 slot, with the ACK flag
set to 0 and the ACK slot number field set to 0.
Note. – The re-transmission of CELL_EXIT messages by the current GS will only continue
while the communications link is still open. If the link times-out then the current GS will
consider the aircraft to have left the cell and will de-allocate the aircraft's CoS1 slot.
A3.1.7
GS request for aircraft cell exit with recommendation
Note. – This procedure can only be followed when the aircraft is located within the coverage
of several ground stations which are all part of the same cluster.
If the aircraft is about to leave the cell of the current GS, or the current GS is overloaded, and
the GS wishes to trigger a hand-over procedure, and from data received from the GNI the
current GS believes that the aircraft should preferentially hand-over to a specific GS, then the
current GS shall transmit a GS_CELL_EXIT message in UP1 or UP2, indicating the GS to
which the aircraft should hand-over.
If the aircraft correctly receives the CELL_EXIT message from the current GS, and
recognizes the suggested GS, then the aircraft shall determine the frequency of the
suggested GS to contact.
The aircraft will attempt to hear the suggested GS on this frequency.
If the aircraft receives a framing message from the suggested GS then it shall commence the
cell login procedure (see Section A3.1.1).
If the cell login procedure is successful, the aircraft will transmit an EXIT_ACK message to
the current GS in its dedicated CoS1 slot, with the ACK flag set to 1 and the ACK slot
number field set to 0.
If the current GS receives the AC_EXIT_ACK message correctly, it shall reply with a
GS_ACK message with the ACK flag set to 1 and the message ID field set to EXIT_ACK. It
shall then de-allocate the aircraft's CoS1 slot and consider the link to be terminated.
If the aircraft receives the GS_ACK with the ACK flag set to 1 it shall consider the link to be
terminated.
If the aircraft receives the GS_ACK with the ACK flag set to 0 it shall re-transmit the
AC_EXIT_ACK.
If the aircraft does not receive a GS_ACK message it will still consider the "exit" process to
be complete.
Note. – This is to avoid the situation where a GS_ACK could be transmitted but not received
and the aircraft thinks that its old CoS1 slot is still available when in fact it had been reallocated by the GS.
If the aircraft does not recognize the address of the suggested GS, or the cell login
procedure with the suggested GS is not successful, then the aircraft shall determine the
frequency of another possible GS (determined from location).
The aircraft shall attempt to hear the GS on this frequency.
If the aircraft receives a framing message from the GS, then it shall commence the normal
login procedure for this new GS (as in A3.1.1).
If the login procedure is successful then the hand-over shall be considered to be complete.
If the login procedure is not successful then the aircraft will attempt to contact another
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possible GS.
If the cell login procedure with all possible future GS's is not successful then the aircraft shall
transmit an AC_EXIT_ACK message in its dedicated CoS1 slot, with the ACK flag set to 0
and the ACK slot number field set to 0.
If the GS receives an AC_EXIT_ACK message with the ACK flag set to 0, or does not
(correctly) receive an AC_EXIT_ACK at all then it shall re-transmit the GS_CELL_EXIT
message in the next frame, unless the aircraft transmits a AC_CELL_EXIT message
beforehand.
If the aircraft does not correctly receive the GS_CELL_EXIT message then it shall transmit
an AC_EXIT_ACK message in its dedicated CoS1 slot, with the ACK flag set to 0 and the
ACK slot number field set to 0.
If the GS receives an AC_EXIT_ACK message with the ACK flag set to 0, or does not
(correctly) receive an AC_EXIT_ACK at all then it shall re-transmit the GS_CELL_EXIT
message in the next frame, unless the aircraft transmits a AC_CELL_EXIT message
beforehand.
Note. – The re-transmission of GS_CELL_EXIT messages by the GS will only continue while
the communications link is still open. If the link times out then the GS will consider the aircraft
to have left the cell and will de-allocate the aircraft's CoS1 slot.
A3.2
Uplink
Each GS has dedicated uplink sections in the frame.
A3.2.1
GS has data to send
A3.2.1.1 Data size is ≤2,048 octets, if transmitted in UP1
If the data size is ≤2,048 octets the GS shall transmit a GS_DATA message to the aircraft
with the ACK slot number field set to 0 0000 0000, the repeat flag set to 0 and the repeat
number field set to 000.
If the UP1 transmission is correctly received by the aircraft, the aircraft shall send an ACK to
the ground station, either in an RTS_COS1 message or in a DATA_COS2 message, setting
the ACK flag to 1, and indicating which message is being acknowledged.
If the UP1 transmission is not correctly received by the aircraft, the aircraft shall send a
NACK to the ground station, either in an RTS_COS1 message or in a DATA_COS2 message
setting the ACK flag to 0, and indicating which message is being NACKed.
If the GS does not correctly receive an acknowledgement for the GS_DATA, or receives a
NACK, the GS shall re-transmit the GS_DATA message in UP1 or UP2, with the repeat flag
set to 1 and the repeat number incremented from the previous attempt.
A3.2.1.2 Data size is >2,048 octets
If the data size is >2,048 octets, the GS will split the data into several GS_DATA messages
(each one of which will contain up to 2,048 octets of data).
These GS_DATA messages will then be transmitted separately.
The method for this is implementation-dependent and is outside the scope of this document.
A3.3
Acknowledgement messages
An acknowledgement flag shall always be followed by a field indicating the message type.
If a station (GS or aircraft) is sending an ACK/NACK message then the message type field
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shall be set to the message type which is being acknowledged.
If a station (GS or aircraft) is sending an ACK/NACK message then the message identifier
field shall be set to the identifier of the message which is being acknowledged.
If an ACK or NACK is received by a station (GS or aircraft) and the message type field does
not match any of the messages for which an ACK or NACK is expected, then it shall be
ignored. The station shall then wait for another ACK/NACK message. If one is not received
within a given time12, this shall be treated as a NACK.
If an ACK or NACK is received by a station (GS or aircraft) and the message type field
matches one or more message for which an ACK or NACK is expected but the message
identifier field does not also match the identifier number of any message or messages for
which an ACK or NACK is expected, then it shall be ignored. The station shall then wait for
another ACK/NACK message. If one is not received within a given time13, this shall be
treated as a NACK.
12
This requires definition of a timer, which is considered to be outside the scope of this document.
13
This requires definition of a timer, which is considered to be outside the scope of this document.
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ANNEX 4 –
Coding and interleaving
This annex provides first results on the coding scheme performance and robustness.
Simulations and tests should be performed to validate the performance.
This evaluation has been done with the assumptions of a 1366 bits slot length. The
performance is even better considering longer slot lengths.
A4.1
GMSK and convolutional coding: rate and expected
performances
We propose here a quick evaluation of the performance for some punctured Convolutional
codes with a GMSK.
This discussion is based on existing results that can be found in the literature. These are
used to derive some performances which could be expected, but this approach requires
running complete simulation to assess definitely the performances of the system.
Uncoded GMSK is quite similar to MSK (a 0.5 dB degradation is observed, in theory, due to
ISI introduced by pre-filtering) when a coherent demodulator is applied (see [10] for a
complete discussion).
This curve below has been obtained by shifting the MSK (with coherent demodulation)
performance by 0.5 dB.
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Figure 30: GMSK theoretical performance
The convolutional code used on the next figure is the well known (LC = 3, (5,7), dfree = 5),
and is represented below (LC for Constraint Length). The simulated performances are for a
BPSK over AWGN channel. The figure above provides an idea of the BER which can be
expected at the output of the Viterbi decoder, for a given BER at the input. One can check
that the ratio is quite linear (with a log-log scale) for BER at the input smaller than 10-3.
These well-known results can be found in [11], chap. 8.
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Figure 31: Convolutional code performance
G2 = 5(8)
c2
bk
D
bk-1
D
bk-2
c1
G1 = 7(8)
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Figure 32: Convolutional code (5,7), constraint length 3
For the code LC = 7, (171,133), the free distance dfree = 5 corresponds to a rate ¾. As
performance of a code depends mainly on its free distance when the SNR increases, the two
codes can be considered equivalent in terms of performances, though mostly if SNR is “high
enough”. This code is represented below.
G2 = 133(8)
output 2
bk
D
bk-1
D
bk-2
bk-3
D
D
bk-4
bk-5
D
bk-6
D
input bit
output 1
G1 = 171(8)
Figure 33: Convolutional code (133,171), constraint length 7
The table below shows the relation between coding rate and corresponding free distance.for
punctured convolutional codes, based on (LC = 7, 171,133).
Rc
½
2/3
¾
5/6
7/8
dfree
10
6
5
4
3
Table 2: Free distance for some code rate
Based on these considerations, we can derive the BER at the input of the coder from Figure
30: GMSK theoretical performance, and then derive the BER after decoding from Figure 31:
Convolutional code performance.
Example of application to L-DACS2:
C/N = Eb/N0 + 10 log (se), where se is the spectral efficiency.
RC = ½, se = 270/200
If C/N = 10 dB then Eb/N0 = 8.7 dB
This leads to a BER (undecoded) equal to 5.10-3, and the BER (decoded, inner code) is then
6.10-4, which leads to less than 10-7 if either RS(31,23,5) or RS(15,11,4) are used.
A4.2
Considerations regarding practical C/N
The following discussion is based on information derived from the link budgets which have
been computed for L-DACS2.
Receiver threshold: - 96 dBm
N = FkTB, with F = 10 dB, T =288.4 K, B = 200 kHz, N = -108 dBm
At receiving threshold, C/N = 12 dB.
This is 2 dB more than used to derive performances in the previous paragraph, and
correspond to a significant engineering margin. Though it does not represent a proof of the
performances, the fact that a margin is still available strengthens this approach.
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A4.3
Accounting for burst interference: interleaving and
RS coding
In this discussion regarding interleaving and outer coding, we assume a punctured
convolutional code (LC = 7, 133,171), with a rate ¾ or 5/6, as inner code.
The objective is to evaluate the right parameters for coding and interleaving, in the presence
of strong bursts of errors, due to a very near (co-located) interfering transmitter.
To ensure independence between two different communications, we limit the depth of the
interleaver to a slot length (1366 coded bits).
RS notation: RS (N, N-2T,k), where N = 2^k – 1, and T is the number of corrected errors.
Burst length:
Symbol duration
3,69231E-06
Duration (ms)
Duration (bits)
Burst, type 1
275
75
Burst, type 2
420
114
The slot length is 1366 bits, equivalent to 1024 bits before convolutional encoding with a rate
¾., so one slot is at most 7 codeword long, if RS(31,X,5) is used.
For a burst of 114 bits, with bloc interleaving, it is likely that about 16 consecutive bits will be
strongly errored after de-interleaving. This leads to 4 errors in a codeword. RS(31,23,5)
should thus be considered rather than RS(31,29,5) or RS(31,27,5).
Another option is to use RS(15,11,4), for which a code word is 60 bits long, leading to about
18 codewords in a slot. After de-interleaving, 6 consecutive bits at most are errored, and
correspond to 2 to 3 errors in a codeword.
BER_in
RS(31,29,5) RS(31,27,5) RS(31,25,5) RS(31,23,5) RS(15,13,4) RS(15,11,4)
5,0E-03
2,2E-03
4,8E-04
7,9E-05
1,0E-05
6,0E-04
5,9E-05
1,0E-03
2,5E-05
1,2E-06
3,9E-08
1,0E-09
5,8E-06
1,2E-07
2,0E-04
2,0E-07
2,1E-09
1,4E-11
7,2E-14
4,8E-08
1,9E-10
1,0E-04
2,8E-08
1,3E-10
4,4E-13
1,1E-15
6,0E-09
1,2E-11
1,0E-05
2,8E-11
1,3E-14
4,4E-18
1,1E-21
6,1E-12
1,2E-15
155 bits
155 bits
155 bits
155 bits
60 bits
60 bits
0.935
0.870
0.806
0.742
0.866
0.733
Code word length
Rate
Table 3: Performances of various RS codes
The results in the table above are for uncorrelated errors, and cannot be applied
straightforwardly to burst of errors. The presence of an ideal interleaver would solve this
problem, but cannot be implemented in practice. Anyway we may consider these results as
providing an idea of the performances which may be reached, though dedicated simulations,
modelling both the interfering burst signal and the propagation channel are mandatory to
derive accurate results.
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A4.4
Equalization
Filtering the signal (with the Gaussian filter) introduces inter symbol interference (ISI), even
over a single path channel. This is the reason why GMSK is less efficient than MSK.
In the GSM system, equalization is used to fight ISI, but it is true that ISI in GSM is due to
both pre-filtering and multipaths propagation.
The influence of equalization should be considered for L-DACS2 as well, probably through
channel estimation and MLSE equalizer (Maximum Likelihood Sequence Estimate, Viterbi
algorithm using equivalent discrete channel estimation for weighting). This could improve
performance at the cost of introducing a training sequence (CAZAC sequence is used in
GSM, but PRBS can also be used).
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ANNEX 5 –
Impact of intra-system
interference
A5.1
Intra system interference robustness
An evaluation of the co-channel impact has been conducted in order to have statistic on the
impact of deployment of the system over the link budget.
The link budget presented with co-channel interference figure should be considered with
caution. It is related to the channel deployment of the system. Currently the attenuation of the
interfered signal coming from a cell using the same frequency is 9 dB based on a 12
frequency reused pattern and the performance of the GMSK modulation. The degradation
caused by a man-made signal, with the same characteristics (modulation, frequency,
bandwidth, etc.) on the signal of interest, is smaller than the degradation caused by additive
white Gaussian noise but for this exercise it was assimilated to additive white Gaussian
noise. Additional test on the prototype should be performed in order to validate the current
attenuation of a co-channel signal and the accurate distance needed to use frequency
reused channel. First analysis is conducted hereafter in order to give first input for the
frequency plan of the L-DACS2 system. Nevertheless those inputs need to be validated
during the prototype phase.
The overall impact of one channel interfering in the same frequency band could deeply
reduce the net margin considering the operational case where the interfering aircraft is close
to the station and still transmits with the maximum power. Power control should decrease this
impact for those cases.
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TX Parameters
L-DACS2 TX ouput Power
Unit
dBm
Maximum TX Antenna Gain
Tx Cable loss
TX EiRP
dBi
dB
dBm
Propagation Parameters
Transmit upper Frequency
Tx-Rx Distance
MHz
Nm
Path Loss
Miscellaneous Margins
Interference Margin
Implementation Margin
Safety Margin
Banking Loss Margin
UL
DL
ENR
ENR
ENR
TMA
APT
ENR
ENR
ENR
TMA
APT
Governing Equation
55,44068 55,44068 55,44068 55,44068 55,44068 46,9897 46,9897 46,9897 46,9897 46,9897 a
8
8
8
8
8
2,5
2,5
2,5
2,5
2,5
60,94068 60,94068 60,94068 60,94068 60,94068
0
3
43,9897
0
3
43,9897
0
3
43,9897
0
3
43,9897
0b
3c
43,9897 d =a + b - c
977 e
10 f
g = 37,8 + 20log(f) +
117,60 20log(e)
Notes
Tx_Pout - (UL: 350W) - (DL: 50W)
TX_AntGain - (UL: DME antenna reference) - (DL:
Antenna Gain = 3dB - polarization loss = 3dB)
TX_CableLoss - (UL: 50*0.04 + 2*0.25)
TX EiRP = TX_Pout + TX_AntGain - TX_CableLoss
977
200
977
120
977
60
977
40
977
10
977
200
977
120
977
60
977
40
dB
143,62
139,18
133,16
129,64
117,60
143,62
139,18
133,16
129,64
dB
dB
dB
dB
0
0
6
0
0
0
6
0
0
0
6
0
0
0
6
7
0
0
6
7
0
0
6
0
0
0
6
0
0
0
6
0
0
0
6
7
Maximum RX Antenna Gain
Rx Cable loss (incl. Duplexer)
dBi
dB
0
3
0
3
0
3
0
3
0
3
8
2,5
8
2,5
8
2,5
8
2,5
L-DACS2 RX receive signal
Thermal Noise Density@290K
Bandwidth
Thermal Noise Power
Receiver Noise Figure
Composite Noise Figure
dBm
dBm/Hz
Hz
dBm
dB
dB
-82,68
-174
200000
-120,99
10
13
-78,24
-174
200000
-120,99
10
13
-72,22
-174
200000
-120,99
10
13
-68,70
-174
200000
-120,99
10
13
-56,66
-174
200000
-120,99
10
13
-91,63
-174
200000
-120,99
7
9,5
-87,19
-174
200000
-120,99
7
9,5
-81,17
-174
200000
-120,99
7
9,5
-77,65
-174
200000
-120,99
7
9,5
-65,61
-174
200000
-120,99
7
9,5
n=d-g+l-m
o
p
q = o + 10log(p)
r
z
Total Rx Noise Power
Eb/No @ BER=10-3
L-DACS2 bit rate
Required C/N
L-DACS2 Rx Sensitivity
L-DACS2 C/N available
L-DACS2 net margin
@BER=10-3
dBm
dB
bps
dB
dBm
dB
-107,99
10
270833
11,32
-96,67
25,31
-107,99
10
270833
11,32
-96,67
29,75
-107,99
10
270833
11,32
-96,67
35,77
-107,99
10
270833
11,32
-96,67
39,29
-107,99
10
270833
11,32
-96,67
51,33
-111,49
10
270833
11,32
-100,17
19,86
-111,49
10
270833
11,32
-100,17
24,30
-111,49
10
270833
11,32
-100,17
30,32
-111,49
10
270833
11,32
-100,17
33,84
-111,49
10
270833
11,32
-100,17
45,88
s = q + z + i + [h + j + k] N = Rx_NF + 10log(k.T.BW)
t
Eb/No
u
R
v = t + 10log(u/p)
C/N = Eb/No + 10log(R/BW)
w=v+s
Cmin = C/N + N
n-s
dB
8,00
12,43
18,45
14,97
27,02
2,54
6,98
13,00
9,52
%
3,00
3,00
3,00
3,00
3,00
3,00
3,00
3,00
3,00
9
9
9
9
9
9
9
9
9
0
0
6
7
h
i
j
k
Free Space model (using nm unit)
InterfMargin(TBD)
ImpMargin
SafetyMargin(TBD)
Banking(TBD)
RX Parameters
RX Parameters with
interference
Duty Cycle
C/I: Frequency reuse
interference assumption
Interference I due to frequency
reuse
Total Rx Noise Power without
interference
Noise increase due to
interference
Total Rx Noise Power
Eb/No @ BER=10-3
L-DACS2 bit rate
Required C/N
L-DACS2 Rx Sensitivity
L-DACS2 C/N available
L-DACS2 net margin
@BER=10-3
dB
dBm
-106,91
-102,47
-96,45
-92,93
-80,89
-115,86
-111,42
-105,40
-101,88
dBm
-101,99
-101,99
-101,99
-94,99
-94,99
-105,49
-105,49
-105,49
-98,49
dBm
dBm
dB
bps
dB
dBm
dB
1,21
-100,78
10
270833
11,32
-89,46
18,10
2,78
-99,21
10
270833
11,32
-87,90
20,97
6,61
-95,38
10
270833
11,32
-84,06
23,16
4,16
-90,83
10
270833
11,32
-79,51
22,13
14,27
-80,72
10
270833
11,32
-69,40
24,06
0,38
-105,11
10
270833
11,32
-93,79
13,48
0,99
-104,50
10
270833
11,32
-93,19
17,31
3,06
-102,43
10
270833
11,32
-91,12
21,26
1,64
-96,85
10
270833
11,32
-85,53
19,20
6,78
9,66
11,84
10,81
12,75
2,16
5,99
9,95
7,89
dB
8l
2,5 m
21,56 n-s-v
RX_AntGain - (UL: Antenna Gain = 3dB - polarization loss
= 3dB) - (DL: DME antenna reference)
RX_CableLoss - (UL: 50*0.04 + 2*0.25)
RxPower = TX_EiRP - PathLoss + Rx_AntGain Rx_CableLoss
10log(kT )
BW
10log(k.T) +10log(BW )
Rx_NF
Composite noise including Rx noise and cable loss
Considering ImpMargin and [other margins above]
3,00 @
9z
-89,84 x
10log(10^((n-z)/10)*@/100)
N = Rx_NF + 10log(k.T.BW) + ImpMargin + [other margins
-98,49 s = q + z + i + [h + j + k] above]
9,21
-89,28
10
270833
11,32
-77,97
23,67
y
t
u
v = t + 10log(u/p)
w=v+s
n-y
10log(10^(s/10)+10^(x/10))-s
s+noise increase
Eb/No
R
C/N = Eb/No + 10log(R/BW)
Cmin = C/N + N
12,36 n-y-v
Table 29 – Link budget including interference contributions
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A5.2
Adjacent channel frequency guard time
Based on the adjacent band reference interference ration presented in section 3.6.2.3 and
using on operational scenario, an estimation of the frequency band needed for adjacent
channel was conducted.
The operational scenario considered is two airplanes logged in two adjacent cells and
located in the border of the cell (at the maximum range of the cells). One aircraft receives a
transmission from the ground (left column) and from the other airplane (right column)
The reference interference ratio C/Ia (Ia for Interference adjacent) for the ground and airborne
receiving function are listed in the following table.
for adjacent (200 kHz) interference
for adjacent (400 kHz) interference
for adjacent (600 kHz) interference
C/Ia
-9 dB
-41 dB
-49 dB
Two evaluation were conducted
•
Using 200 kHz adjacent frequency (left table)
•
Using 400 kHz adjacent frequency (right table)
In order to ease the frequency plan of the L-DACS2 system only multiple of 200 KHz (the
bandwidth of one channel) are studied.
Table 30 – Adjacent channel frequency guard time
In the 200 KHz scenario case, the distance between the two mobiles has to be 10 Nm in
order to respect the interference ratio. Considering the 400 KHz scenario, the receiver could
still demodulate the ground signal even if the distance between the two mobiles is 0.23 Nm.
This distance is far less than the required separation between two mobiles. Therefore the
frequency band for adjacent channel has to be fixed at 400 KHz at minimum.
A5.3
Co-channel spacing guard band
For Co-channel frequency band, the operational scenario takes into account two mobiles
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operating in the same channel but not in the same cell. One mobile is receiving a
transmission form the ground while the other one is transmitting. The objective is to get the
minimum distance between the two mobiles that leads to the demodulation of the signal
transmitted by the ground to one mobile while the other one transmits.
The figure below demonstrates that 80 Nm is required in order to respect the 9 dB of C/Ic (Ic
for interference co-channel)
Table 31 – Co-channel spacing guard band
A5.4
Multiple channels operating in one cell
A5.4.1
Frequency Band
The same kind of evaluation has been processed for co-channel cohabitation. The objective
of this evaluation is to fix the frequency guard time between two channels assigned in the
same cell. The same kind of operational scenario that above has been used for co-channel
frequency band considering than the two mobiles could be even closer. 1000 ft has been
selected as the minimum distance between two mobiles.
The figure below shows the level of received signal in this scenario.
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Table 32 – Multiple channels operating in one cell scenario
The C/I ratio obtained in this scenario is -44.88 dB. Therefore according the table presented
in section 3.6.2.3, a 600 KHz frequency band at minimum has to be set between two
operational channel operating in the same cell.
A5.4.2
Spacing separation between stations
Although the use of adjacent channels introduces the probability of a finite amount of
interference, undesired cross-talk effects across the two or more channels operating in the
cell can be minimised through the use of sitting contingencies. On the ground side,
appreciable isolation is possible by means of the directivity properties of sectored antennas
and the use of frequency planning rules (for example, a minimum of two channels separation
within the same ground site). There is no requirement for any specific treatment of a two (or
more) channel system on the aircraft, since this aircraft will simply tune to the single channel
to which it has been assigned by the ground system. Note however that the use of adjacent
channels in the same cell is not a nominal case, and that such case is a planning constraint
rather than design constraint.
The adjacent protection specifications allow for the signal in the adjacent channel to be 9 dB
greater than the desired signal for acceptable BER. The constraints imposed by the
operation of two adjacent channels are illustrated in the following figure. In this extreme
scenario, the ground station (operating across two channels) has active links with an aircraft
in the near vicinity on one channel (for example, on the airport surface), and with a second
aircraft in the far field on the second channel (for example, at the edge of the TMA or En
route sector).
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La
Lb
Figure 34 – Separation distances between aircraft and nearest ground station
La is path loss to far victim aircraft.
Lb is path loss to near interferer aircraft.
Considering the above scenario where both aircraft transmit simultaneously, the interference
calculation evaluated at the ground station is as follows.
To evaluate the required protection distance, it is assumed that:
aircraft maximum power output is 50 dBm
path loss La at 975MHz at 150 NM is 140 dB
the available power control range is at least 30 dB
For the first adjacent channel, the minimum required isolation is: (best case)
(Power of aircraft B – Path loss from aircraft B) - (Power of aircraft A – Path loss from aircraft
A) < 9dB, giving:
(20 - Lb) - (50 - La) < 9 dB
50 - La > 20 - Lb - 9
39 - La > - Lb
La - 39 < Lb
Giving Lb > 101 dB
Therefore for simultaneous of the first adjacent channel, the nearest allowable distance
between an aircraft and a ground station is 2500m.
In practise, the directivity pattern provided by a sectored antenna (e.g. cardioid) offers
approximately 6dB gain at the sector boundary. Therefore, for the near interfering aircraft on
the first adjacent channel, Lb is reduced to 95 dB. This is equivalent to a separation distance
of 1250 m.
To use contiguous channels therefore, there is a limitation of between 1250 and 2500 m for
the distance between the ground station and the nearest aircraft.
Note - If a 200 kHz guard band is implemented (the two channels being separated by 200
kHz) in the same cell, the resulting offered protection is increased by an additional 40dB. In
this case, the allowable separation is lowered to less than 150m.
This illustrates that:
use of adjacent channels is possible and facilitates frequency planning, but is not
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strictly necessary;
use of power control is a critical element.
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REFERENCES
[1]
"Future Communications Infrastructure - Technology Investigations. Description of
AMACS (Draft)", v1.0, EUROCONTROL, 2007.
[2]
"Study on the options for time synchronisation in the VDL Mode 4 datalink
system", version 1.2, EUROCONTROL, January 2002.
[3]
"VDL Mode 4 Timing Study", Helios Technology Ltd, 1999.
[4]
ICAO Doc 9816 (AN/448), Manual on VHF Digital Link (VDL) Mode 4, 1st ed.,
2004.
[5]
EUROCAE ED-108A — MOPS for the Very High Frequency (VHF) Digital Link
(VDL) Mode 4 Aircraft Transceiver
[6]
RTCA DO-282A — Minimum Operational Performance Standards for Universal
Access Transceiver (UAT) Automatic Dependent Surveillance – Broadcast
[7]
ETSI EN 301 842 — Electromagnetic compatibility and Radio spectrum Matters
(ERM); VHF air-ground Digital Link (VDL) Mode 4 radio equipment; Technical
characteristics and methods of measurement for ground-based equipment.
[8]
ETSI EN 302 842 — Electromagnetic compatibility and Radio spectrum Matters
(ERM); VHF air-ground and air-air Digital Link (VDL) Mode 4 radio equipment;
Technical characteristics and methods of measurement for aeronautical mobile
(airborne) equipment.
[9]
ETSI specification 3GPP TS 05.05 – Technical specification group GSM/EDGE;
Radio Access Network; Digital cellular telecommunications system (Phase 2+);
Modulation. Release 1999.
[10]
Kazuaki Murota and Kenkichi Hirade. GMSK modulation for digital mobile
radio, telephony. IEEE Transactions On Communications, COM-29(7), July
1981.
[11]
Communications numériques, A. Glavieux, M. Joindot, MASSON.
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ABBREVIATIONS
µs
microsecond
ACK
Acknowledge
ADS-B
Automatic Dependent Surveillance – Broadcast
AMACS
All-purpose Multi-channel Aviation Communication System
AOC
Airline Operational Communication
APT
Approach
ARNS
Aeronautical Radio Navigation Service
ATM
Air Traffic Management
ATN
Air Traffic Network
ATS
Air Traffic Service
AWGN
Additive White Gaussian Noise
BCH
Bose, Chaudhuri and Hocquenghem
BER
Bit Error Rate
BIC
Blind Interference Cancellation
BPSK
Binary Phase Shift Keying
BT
Bandwidth duration
C/I
Signal to Interference ratio
C/N
Signal to Noise ratio
CC
Convolutional Code
CoCr
Communications Operating Concept & Requirements
CoS
Class(es) of Service
CPFSK
Continuous-Phase Frequency-Shift Keying
CRC
Cyclic Redundancy Check
CSC
Common Signalling Channel
CTS
Clear To Send
dB
decibel
DECT
Digital Enhanced Cordless Telecommunications
DLE
Data Link Entity
DLS
Data Link Sub-layer
DME
Distance Measuring Equipment
EIRP
Equivalent Isotropically Radiated Power
EFB
Electronic Flight Bags
E-TDMA
Extended Time Division Multiple Access
FDD
Frequency Division Duplex
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FEC
Forward Error Correction
GMSK
Gaussian-filtered Minimum Shift Keying
GNI
Ground Network Interface
GS
Ground Station
GSM
Global System for Mobile communications
ICAO
International Civil Aviation Organization
ISI
Inter Symbol Interference
ISO
International Organization for Standardization
ITU
International Telecommunication Union
JD
Joint Demodulation
kbps
kilobits per second
kHz
kilohertz
LC
Constraint Length
LDACS
L-band Digital Aeronautical Communication System
LDL
L-band Data Link
LME
Link Management Entity
LML
Link Management Layer
LoG2
Login Section
LSS
L-DACS2 Services sub-layer
MAC
Media Access Control
MER
Message Error Rate
MHz
Megahertz
ms
millisecond
MS
Mobile Station
MSK
Minimum Shift Keying
mW
milli watt
NACK
Non Acknowledge
QoS
Quality of Service
RF
Radio Frequency
RS
Reed-Solomon
RTS
Request To Send
Rx
Receiver
SAW
Surface Acoustic Wave
SSR
Secondary Surveillance Radar
TDMA
Time Division Multiple Access
TDD
Time Division Duplex
TMA
Terminal Area
Tx
Transmitter
UAT
Universal Access Transceiver
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UP
Uplink
UTC
Coordinated Universal Time
VDL
VHF Digital Link
W
Watt
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