ACP WGC6/WP4
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
Working Group C – 6th meeting
Toulouse, France
20 – 24 October 2003
Agenda items 6: Evaluation of potential technologies
Summary of EUROCONTROL Activities On Wide Band Terrestrial Systems
Presented by EUROCONTROL
Prepared by G Bailey, D van Roosbroek, M Adnams, L Lommeart
SUMMARY
This paper presents the Eurocontrol investigations on mobile communication terrestrial
system based on wide band FDD and TDD/CDMA technologies.
1.
Introduction
Based on the need to provide additional capacity and capabilities to the mobile communication
infrastructure (as introduced in ACP WGC6 Working Paper entitled “Need to investigate
alternative technologies for mobile communications”) Eurocontrol is investigating the feasibility
of using, for the Aviation usage, wide band technologies as developed by the telecommunication
industry. This paper presents the investigations undertaken on FDD and TDD CDMA
technologies.
Note: The paper refers to different reports (identified with a .pdf extension). These reports can
be found in a companion CD-ROM (which can be requested from the Eurocontrol
representative or please send an E-mail to the EUROCONTROL CSM secretariat:
[email protected]).
2.
General
The 3rd Generation mobile communications systems are being implemented world wide. These
systems represent the future trend in mobile communications and are currently being
implemented by leading companies. A high-level feasibility study of the possible adequacy of
these standard to meet the ATC requirements is contained in the document
"Feasibility_wideband for ATCv10.pdf".
Wideband Flight Trials
The Agency has contracted the use of the QinetiQ BAC1-11 aircraft for a number of years in
support of datalink trials including PETAL II in the past and LINK 2000+ now.
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More recently the aircraft has been used to host Wideband equipment for a series of trials to
assess the feasibility of Wideband communications for Aviation use.
Wideband Interference Testing with VOR/ILS/Glidepath Frequencies
Knowing that VHF/UHF frequencies are an ideal candidate for hosting the Wideband system
and that finding free spectrum for its operation is almost impossible, investigation into the effect
of Wideband signals on the operation of existing aviation systems operating at those frequencies
is underway.
An initial laboratory exercise has shown encouraging results, it may be possible for the systems
to co-exist. In order to assess the real effects of Wideband on existing aircraft systems, flight
trials are planned with an ILS calibration aircraft. Both airborne and ground Wideband signals
will be generated and the effects measured.
Initial conclusions from the laboratory will be compared with Flight Trials results planned in the
end of the 1st quarter 2003.
Wideband/Radar Co-existence Study
An initial study has been conducted into the operation of Wideband technology in the radar band
and a number of issues are highlighted in the document.
Avionics Architecture for Co-existence
There is a great deal of investment in Avionics for ATS and Airline Operation (AOC) purposes
in PETAL and LINK 2000+ using low data rate data link such as VDL Mode 2. In order to
maximise this investment and minimise future investment a proposed architecture has been
defined such that multiple users including the passenger could share a high bandwidth datalink.
High priority traffic will always interrupt and access the link in preference to low priority
passenger traffic.
A prototype system has been built and shown to work in the laboratory. An overview is available
in the document "ATC_AOC_and_APC_Coexistence_Architetcure.pdf". Further work is
required to produce an Avionics form and fit compatible version.
3.
TDD systems (2 GHz Siemens system)
The Siemens 3G Testbed is a Time-Division Duplex (TDD) wideband CDMA system, based on
the 3rd Generation Partnership Project (3GPP) standard as it stood in February 1999, e.g. with a
chip rate of 4.096 megachip/sec (Mcps) and with 16 timeslots per 10ms frame. The standard has
evolved but the testbed is still representative of the UMTS Terrestial Radio Access (UTRA)
TDD mode. Appendix E of the document "Study into Modifying the Siemens 3G Testbed for
Airborne Trials – August 2002.pdf” contains tables showing the differences between the testbed
and the 3GPP Release 99 TDD standard.
For this trial, the off the shelf Siemens equipment operating Wideband Time Division Duplex at
2Ghz was installed on the BAC1-11 and flown against Siemens base stations deployed at
Boscombe Down in UK. The trials were conducted in 2 phases, Phase 1 in December 2001 and
Phase 2 in July 2002.
3.1
December 2001 Flight Trial
The result of these trials are described in document “Airborne Trials with the Siemens 3G
Testbed – December 2001.pdf”.
The main objective of the initial trials carried out on December 12th and 13th 2001 with the
Siemens 3G Testbed was to show that high data rate communication between air and ground is
possible using prototype 3G mobile telecommunications technology.
The mobile station was fitted into a BAC1-11 aircraft operated by QinetiQ and the ground
station was located on top of a building at Boscombe Down, UK.
The demonstrations of live air-to-ground video and still photographs and of simultaneous
ground-to-air data transfer (e.g. web-browsing and streaming video) worked satisfactorily at the
maximum distances attempted, around 13km. Total data transfer rates exceeding 1200
kilobits/sec were experienced. In general, negligible loss of picture quality occurred. The videos
received at the ground station have been edited to provide a Quicktime movie.
The trials also yielded valuable information about signal strength level and variability that will
be used to update the link budget calculations. The measurements have validated the assumption
of near free-space path loss and suggest that given some additional gain at the base station
antenna, a range of 25km should be achievable with the Siemens 3G Testbed, operating at
2GHz.
Data from the BAC1-11 flight recorder has been analysed and synchronised with the logging
performed by the Siemens 3G Testbed. The desired near-circular orbits, to minimise the Doppler
effect on the received radio frequency, were mostly achieved. At all other times, the automatic
frequency control facility at the mobile station was activated to enable its RF unit to follow the
Doppler shifted frequency of the broadcast channel burst that was received from the base station.
3.2
July 2002 Flight Trial
The result of these trials are described in document “Second Airborne Trials with the Siemens
Testbed – September 2002.pdf”.
The main objective of the flight trials carried out on June 27th and July 3rd 2002 was to validate
various improvements made to the Siemens 3G Testbed since the first airborne demonstrations
in December 2001. The improvements were:
 Doppler compensation was implemented to enable full communication to be maintained
regardless of the aircraft’s speed and heading. The maximum radial velocity encountered
was about 300 knots, which equates to a Doppler shift of 1000Hz at the transmission
frequency of 2GHz).
 Three co-located base stations were set up, each with an antenna having 120º azimuth
beamwidth, in order to demonstrate handover of applications as the aircraft moved from the
coverage area of one base station to the next.
 Enhancement of the mean transmitted RF power using a baseband clipping technique, to
maximise the operating range. The target operating range was 25km.
 The prototype changes to the timeslot structure and the timing advance process, to cope with
propagation over 25km, were consolidated.
 By using better codecs (compression software), live air-to-ground video and audio were
transmitted using less bandwidth with no significant reduction in quality.
 New recording software at the ground station was written to handle the new codecs and to
enable QuickTime compatible files to be generated from the captured data.
 Introduction of a push-to-talk facility to resemble an air traffic controller’s ability to contact
a pilot quickly, rather than having to dial a telephone number and wait several seconds.
The mobile station was fitted into BAC1-11 aircraft operated by QinetiQ and the ground station
was located in the Old Fire Station at Boscombe Down, UK. Live internet access was possible
via an external Ethernet connection to the ground station.
Problems were experienced on the first day, due to a combination of low elevation angle, aircraft
banking and the height of the base station antennas being restricted.
On the second day, following some software modifications designed to improve system
resilience and with the aircraft flying higher (16000ft), the demonstrations of live air-to-ground
video and photographs and of simultaneous ground-to-air data transfer (e.g. web-browsing and
streaming video) worked well, out to the maximum distances attempted. Only 4 losses of signal
were experienced, when the aircraft banked sharply, and the system recovered within a few
seconds on every occasion. Over 40 handovers were logged and Doppler compensation was
effective at all times.
Data from the BAC1-11 flight recorder (only available for the second flight) has been analysed
and synchronised with the logging performed by the Siemens 3G Testbed. The results are
reflected in this report via a variety of graphical plots.
3.3
Study into modifying the Siemens 3G testbed for airborne trials
The study report is contained in the document “Study into Modifying the Siemens 3G Testbed
for Airborne Trials – August 2002.pdf”.
Eurocontrol investigates the potential use of 3G technology for air-ground communication. The
purpose of the present study carried out by Roke Manor Research Ltd (RMR) was to investigate
the modifications needed to the existing Siemens 3G Testbed, which was designed and built by
RMR (Roke Manor), to enable it to be used in airborne trials.
The study has addressed the following issues and has generated solutions:
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Transmission at 5.1GHz rather than at the normal 3G frequency of around 2GHz involves
modification of RF units. The design of a new antenna switch was particularly challenging
since significantly higher transmit power was required, but a working prototype has been
achieved. As a result of various ideas to combat the adverse link budget at 5.1GHz, it is
thought that it may be possible to achieve 25km operating range in an airborne environment.
The desired range of 25km requires alterations to the timing advance mechanism that
enables transmissions from a mobile station to arrive at a base station at the right time. A
solution has been demonstrated that does not sacrifice data throughput capacity.
Aircraft speeds require specific compensation to be added to the signal processing to handle
Doppler frequency shift. The effect is proportionally worse at 5.1GHz than at 2GHz.
Simulations have been carried out, resulting in new algorithms but efficient implementation
will be required in view of the current loading on the digital signal processors.
The 3G Testbed already provides seamless handover of a voice or data call as a mobile
station moves from the domain of one base station to another but the distances and speeds
involved in an airborne application required the handover algorithm to be reconsidered.
A scenario has been defined for the proposed future airborne trials that will enable handover
between base stations to be demonstrated without requiring the creation of a high risk, high
cost ground network.
An air traffic controller is currently able to call a pilot in less than 0.1 second. This time
could not be matched with the 3G Testbed but it was deemed worthwhile to investigate the
changes that could be made beneficially to the call set up mechanism, given that a
permanently open channel was undesirable. It is anticipated that the call set up time with the
3G Testbed could be reduced to about 0.5 second. Some of the wider issues associated with
controller-pilot communications in a real 3G environment have also been discussed.
Hardware and software prototyping and simulation have been carried as far as possible, resulting
in high confidence in the proposed solutions. In addition, airborne trials were carried out with
the standard 2GHz Siemens 3G Testbed in December 2001 ( see the press release at
http://www.eurocontrol.int/dgs/press/2002/020205_third_en.pdf). This initial trial confirmed
expectations and made it easier to prepare for further trials, which were completed successfully
in early July 2002 and included all the modifications discussed in this report, except for the
conversion of RF units to operate at 5.1 GHz.
4.
FDD systems
These trials used a Wideband Code Division Multiple Access (WCDMA) prototype produced by
AGILENT based on the standard UMTS 3G TS25.101 V3 release 99. The equipment
implemented Frequency Division Duplex (FDD) mode where separate uplink and downlink
channels are used for receive and transmit, the separation criteria between receive and transmit
channels is laid down in the standard.
The trials were conducted in 2 different spectral regions, C Band and VHF. The same baseband
equipment was used in both cases but with different front-end conversion. Whilst the base
standard was UMTS at both C Band and VHF, the configuration at C Band maintained
compliance with the UMTS chip rate but the configuration at VHF used a chip rate identical to
that of the IS-95 standard. (see technical reports).
4.1
C Band
During October 2002, Wideband Frequency Division Duplex equipment was installed on the
BAC1-11 modified to operate at 5GHz aviation frequency. On the ground similar equipment
was installed at Boscombe Down UK acting as base stations.
Two types of ground antenna were tested:
1) Omni-directional,
2) High Gain antenna specially developed for the trial.
The trials were successful but highlight challenges to be resolved in operating at these
frequencies.
The Technical Summary of C Band flight trials is available in the file Tech Summary C Band
Flight Trials.pdf
4.1.1
Executive Overview C Band Trial
The main objective of the C band flight trials is to prove that safety-of-life services can be
supported by radios based on third generation (3G) concept.
All these radios operate following a code division multiple access (CDMA) principle. CDMA
frequency division duplex (FDD) uses - for each direction (up and down link) - a separate
frequency carrier. As all users transmit on the same frequency each individual CDMA user (or
channel) is distinguished by assigning him one or more unique orthogonal codes.
Data rates investigated are varying between 9,6 kbps and 320 kbps ( in full duplex).
The CDMA radio is operating in the unused part of the Microwave Landing System band (MLS:
5030-5090; CDMA 5090-5150 MHz). This particular spectrum allocation will be under
discussion during the world radio conference in 2003 (WRC 03). Therefore the outcome of the
flight trials is important for pointing out the appropriate strategy to be followed at spectrum
defence level.
A Base band (BB) CDMA radio, a C band RF front end and a monitoring and control PC were
installed into a BAC 1-11 aircraft - based at Boscombe Down UK - and operated by QinetiQ.
The Base Station was located at Boscombe Down airport and was fitted alternately with either
an omni-directional or high gain (12 dBi) antenna.
Link quality is measured by transmitting continuously a Pseudo Random Bit Sequence (PRBS),
a sequence which is known by the receiver. This allows the receive side to detect exactly all
errors in the sequence transmitted. No attention was paid to applications such as Internet,
video or voice as such applications do not provide any information on bit errors. (Errors within
the link are hidden by the higher layers of these applications due to the initiation of retransmissions. These re-transmissions are not noticeable to the user at application level).
The following conclusions can be drawn from the flight tests:
1. Cell ranges with an omni directional antenna of up to 25 NM have been reached for voice
rate or low data rate channels (Spreading Factor (SF) 128) for unloaded cells.
2. Cell ranges with and omni directional antenna of up to 12.9 NM have been reached for high
data rates of up to 384 kbps (SF 16, dual channel) for unloaded cells.
3. With a high gain antenna of 12 dBi Cell ranges of 90NM were obtained for low data rates
with SF 128 and in an unloaded cell (and dry weather conditions).
4. The following Frame error rates were measured (including errors when A/C enters or leaves
heavy banking angles):
 0.1% at 9,6 kbps
 0.1% at 320 kbps
 0.1% at 9,6 and 320 kbps
 0.1% at 57,6 and 320 kbps
 Note : when excluding errors at start or end frame error rate is often nearly error free.
Doppler effects such as Doppler shift (known to be huge at 5 GHz), Doppler sign change and
Doppler spreading has been fully compensated and no measurable impact was found on the Bit
error rate.
Several cell load tests have been performed. Very few frame errors are being generated when
the cell was 50% loaded compared to frame error measurement reference flights (without
interference). Also the 75% cell load tests are successful though some more interruptions are
seen.
The 12 dBi High Gain antenna performs according to the static test results for large spreading
factors (SF128). However the antenna does not perform as expected for high data rates ( small
spreading factors). Cell ranges obtained during these test were smaller than expected.
Therefore we can conclude that a Radio based on FDD is an excellent candidate for the future
support of safety-of-life services.
However the deployment of such a system needs to take the following issues into account:
1. Physical location of the C band antenna(s) on the A/C frame is of primary
importance for the later successful deployment of such a system within a TMA area.
2. Physical location of the radio should be close to the antenna seen the accompanying
large cable losses at these frequencies.
3. Duplexer design (weight and isolation requirements) should not be underestimated.
4. All tests have been taking place (except for sortie 4) under favourable weather
conditions. Heavy rain will decrease the range with 0,117 dB/km.
4.1.2
C Band High Gain Antenna
A phased array antenna was developed to support the Wideband Frequency Division Duplex
Trials at 5Ghz. The antenna gives a gain of 12 dbi.
Specifications and details for the antenna are available in the document,
C_Band_Target_Test_Spec.pdf
4.2
VHF band
During December 2002, Wideband Frequency Division Duplex equipment was installed on the
BAC1-11 modified to operate at VHF aviation frequency operating against similar equipment
installed on the ground.
The aircraft was detached to the Azores islands in the Atlantic for these trials in order to mitigate
the known interference issues associated with operating wideband at these frequencies (i.e.
interference with operational voice).
The trials were highly successful showing remarkable stability at long range, this is clearly a
good target frequency for the system. The challenge will be to find space in the crowded
spectrum to operate the system.
The Technical Summary of VFH Flight Trials is available in the file Tech Summary VHF Flight
Trials.pdf
4.3
Executive Overview VHF Trials
Since long the VHF band assigned to ATC communication and located between 118 and 137
MHz is heavily congested. As spectrum is scarce at a worldwide scale, it is important to make
sure that spectrum is used in its most efficient way. Previous studies done by EUROCONTROL
shows that by using Code Division Multiple Access (CDMA ) techniques spectrum efficiency
can be improved dramatically.
The main purpose of the VHF flight trials, carried out at Santa Maria (Azores) from the 28 of
November till the 1st of December, was to validate a “ narrow band “ CDMA based radio
operating within a 1,25 MHz bandwidth. This narrowband CDMA version was taken due to the
lack of spectrum availability in the VHF/UHF band.
The following criteria were investigated, and for each of them the results obtained are presented
:
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How can spectrum efficiency be improved in the VHF band: By using the Forward18
test sequence as defined in the IS95 standard EUROCONTROL has proved that the 19
MHz used for voice communication can be reduced to 5 MHz only by implementing a
narrowband CDMA system.
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Unloaded Cell ranges of 240 NM (FL 350) were obtained for data rates using a
spreading factor (SF) of 32, therefore cell ranges larger than 400 NM can be predicted
for SF128 ( the ultimate range will be limited by the radio horizon).
Impact on cell range for spreading factors 128, 64 and 32 have been validated,
Doppler effects generated by aircraft speeds of up to 400 knots were compensated
without any noticeable bit error degradation.
Closed loop power control has been successfully demonstrated during both auto-pilot
and manual flying.
The ability to set different Quality of Service levels of a data stream has been
demonstrated and validated.
Quality of Service levels for applications allowing bit error rates of 1.10-3 (voice
vocoder) and 1.10-6 (packet data) have been validated and corresponding input BER
settings have been obtained.
Inner cell interference (simulating other A/C) as well as outer cell interference
(simulating surrounding base stations) has been tested and results have been validated.
Robustness against self interference has been proven.
The flexibility of third generation CDMA systems in selecting different data rates and/or
setting up simultaneously different data channels has been proven.
Link stability and data link quality has been investigated by flying the aircraft manually
in weave and holding patterns and by generating banking angles of up to 48 degrees,
causing antenna fade levels of more than 14 dB.
The main conclusion is that a CDMA base radio using data rates with large spreading factors is
probably today’s best choice for supporting safety-of-life services. Radio connections are very
stable and links are nearly error free. Loaded Cell coverage for low data rates having large
spreading factors are expected to be between 180 and 200 NM.
When using orthogonal variable spreading factor ( OVSF ), higher data rates could be offered
but with a reduced probability coverage. Larger coverage probability could be obtained by using
multi-code transmissions but at the expense of a more complex receiver ( more RAKE fingers)
and transmitter (higher PAR) design.
The use of the short PN offset used in IS95A and CDMA2000 should be investigated with
respect to minimum and maximum cell boundaries while taking into account the radio horizon
existence.
5.
Spectrum issues
The ITU (International Telecommunication Union) WRC (World Radio Conference) is
responsible for allocating frequencies to new radio services, including aviation service. In
Europe, a group of 44 countries (CEPT) has been constituted to formulate common position for
WRCs.
The CEPT has recognised the aviation wide band spectrum need and has supported this position
at the WRC 03.
At the World Radio Conference in 2003 (WRC03), the WRC 03 adopted a resolution supporting
ICAO and CEPT request and introduced the following agenda item at the WRC 07 conference:
[WRC-07 considers additional allocations for the aeronautical mobile (R) service in parts of the
bands between 108 MHz and 6 GHz],
With the following particular recommendations to ITU-R:
[1
to investigate, as a first step, the bands currently available for use by aeronautical
systems in the frequency range between 108 MHz and 6 GHz in order to determine whether
additional allocations to the aeronautical mobile (R) service are required and can be
accommodated in these bands without placing undue constraints to services to which the
frequency bands are currently allocated;
2
to further investigate, in case the first step above would not lead to satisfactory results,
also the frequency bands currently not available for use by aeronautical systems, subject to not
constraining the existing and planned use of such bands, taking account of existing use and
future requirements in these bands;
3
to investigate how to accommodate the requirements for aeronautical systems in the
band 5 091-5 150 MHz,]
6.
Conclusions
The meeting is requested:
1. To take note of the Eurocontrol activities on Wide Band terrestrial technologies
2. To exchange similar available information
[END]