CEPT
Doc. ?_?_SE24
ECC
Electronic Communications Committee
To:
SE24
Cc:
Date issued:
July 9, 2014
Source:
European Association of Operators of Toll Road Infrastructures
(ASECAP, www.asecap.com)
Status:
For consideration
Subject:
5 GHz RLAN studies, TTT
Password protection required? (Y/N)
N
Summary:
This document presents basic conditions for mitigation techniques to protect the existing
TTT against interference from new proposed RLAN at 5.8 GHz. A summary of TTT
communication properties and calculations of TTT detection power levels are presented.
Proposal:
SE24 is invited to review these analyses and include the results in the 5 GHz RLAN report.
Background:
Studies so far showed that coexistence is difficult to achieve between RLAN and TTT
without the use of mitigation techniques. This report does not present a mitigation method
but describes some basic conditions which have to be fulfilled by future mitigation
techniques to protect TTT against interference from RLAN at 5.8 GHz.
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1
1.1
SUMMARY OF TTT COMMUNICATION PROPERTIES AT 5,8 GHZ
Standards
The TTT standard was developed in Europe but is used in many countries all over the word except in
North America. A list of relevant standards can be found in the chapter references. Italy uses a similar
system for road tolling, but the parameters of this system are slightly different compared to the
European standard. If detailed studies will be performed, it will be important to also study the
coexistence with the Italian system. However results from such studies are expected to be more or less
the same as from studies for the European TTT standard. In the remaining text only the European TTT
system is described.
1.2
Basic properties
The TTT standard allows DL (Down Link) communication from the RSU (Road Side Unit) to the
OBU (On Board Unit) based on ASK modulation and UL (Up Link) communication from the OBU to
the RSU based on BPSK modulation. The system is a half-duplex system meaning that either the UL
or the DL is active, but not both at the same time. A backscatter method is used by the OBU, and
because of this it does not need its own transmitter. An unmodulated signal is transmitted to the OBU
which is modulated and reflected. Benefits of the backscatter system are low OBU production costs
and well defined communication zones; a drawback is the need for high sensitivity in the RSU.
Because the OBU receiver has much less sensitivity than the RSU receiver, the studies of in-band
interference to the OBU receiver can be omitted. For out-of-band studies also interference to the OBU
receiver must be studied because the OBU reception bandwidth is wider compared to the RSU which
uses narrow band filters to reject adjacent channel interference.
Most OBUs use an internal battery. To save battery consumption, the OBU is usually in a “sleep”
mode. When the OBU receives signals from an RSU it immediately wakes up to “active” mode and
after the communication is finished it returns to “sleep” mode again.
As with all modern modulation techniques the first part of a frame contains a sequence of pulses to
synchronize the transmitter and receiver clock. Additionally the first part of the frame contains a fixed
preamble. By reading this information it is easy to identify that this communication is based on TTT.
1.3
Protocol
The protocol differs depending on the application. A road toll protocol always starts with broadcast
messages from the RSU. The passing OBU reads the broadcast message and if it decides to answer, it
sends a reply, requesting to open up a communicating session. A complete session including several
messages is needed to achieve an approved transaction.
1.4
Redundancy
Each frame contains a checksum of 16 bits. If the received message does not fit the checksum, a
retransmission is requested. The amount of retransmissions is limited because of the limited time the
vehicle is within the communication zone.
The normal failure rate of lost transactions for a free flow system is less than 0,1% and the most
common reason for this is a missing or not correctly mounted OBU. Even with a missed transaction
the operator can achieve that the toll is paid, e.g. by using enforcement pictures etc. However the
manual handling for this is very costly and therefore an increased failure rate will be very costly to
handle. If the number of lost transactions is increasing to several percent the tolling system will be
useless because the cost of handling lost transactions will become higher than the income. But usually
toll operators will not accept a total failure rate of more than 0.5%. Therefore in practice an additional
failure rate of 0.1% can be seen as upper interference limit.
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A toll plaza system normally uses barriers or traffic lights and the consequence if the TTT
communication link does not work is that the barrier or traffic light will not open for the vehicle. If the
number of unsuccessful transactions becomes high, the toll plaza will become blocked.
Typical free-flow installation with three lanes
Typical toll plaza with an automatic barrier (left) and automatic lanes (right)
1.5
RSU antennas
Because of the backscatter OBU technology, the RSU needs to transmit an unmodulated carrier while
receiving at the same time. For this reason, the RSU is always equipped with separate antennas for the
transmitter and the receiver. In road toll applications it is important to define a communication zone
and a non-communication zone, ensuring that the RSU is communicating with the correct vehicle.
This is achieved by down tilting the RSU antenna by typically 45 degree and use well defined antenna
lobes.
The RSU receiver antenna side lobe attenuation varies in horizontal direction. The worst case is
typically -15 dB.
The RSU transmitter antenna side lobe attenuation in the horizontal direction varies between typically
-20 and -30 dB and it is recommended to use a side lobe of -25 dB in the studies.
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2
2.1
DETECTION POWER LEVELS
General
One possible mitigation method is that the interferer pays attention to the transmissions of the victim,
and when recognizing this transmission, takes necessary protecting actions, such as no transmission
for a limited time on the particular channel. It is essential that the victim is already recognized when
the path loss isolation given by the distance to the victim is not sufficient to protect the victim from
harmful interference.
Calculations were made to study the required detection power levels. Two scenarios were simulated,
the urban environment scenario A1 shows in the MCL calculations the shortest required separation
distance, while the rural environment scenario A2 shows the longest required separation distance.
2.2
Formulas and parameters
The interfering signal strength at the victim was calculated by:
Interferer EIRP
- Wall loss
- Path loss
- Victim receiver side lobe
The victim's power level at the interferer's receiver was calculated by:
Victim EIRP
- Victim transmitter side lobe
- Polarization loss
- Path loss
- Wall loss
- Interferer receiver antenna gain
The following parameters were used:
Interferer EIRP
10 dBm/MHz (scenario A1)
17 dBm/MHz (scenario A2)
Wall loss
15 dB (scenario A1)
0 dB (scenario A2)
Victim receiver side lobe
15 dB
Victim EIRP
33 dBm/MHz
Victim transmitter side lobe
25 dB
Polarization loss
3 dB
Interferer receiver antenna gain 0 dB
Interference limit including polarization loss and antenna gain -128 dBm/MHz
Exactly the same parameters were used with the MCL calculations.
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2.3
Results
The graphs below shows the results from the calculations.
Scenario A1 in urban environment results in a required separation distance of 252 m. At this distance
between the victim and the interferer, the total received power level from the victim's transmission
is -118 dBm (1 MHz bandwidth).
Scenario A2 in rural environment results in a separation distance of 3399 m. At this distance between
the victim and the interferer, the total received power level from the victims transmission is -125 dBm
(1 MHz bandwidth).
The physical thermal noise at 1 MHz bandwidth is -114 dBm/MHz. The studied scenario shows a 4 dB
and an 11 dB lower power level of the victim signal at the interferer position than the equivalent
thermal noise.
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3
CONCLUSIONS
A summary of the TTT standard is presented.
Available power levels of the victim transmissions in the interferer receiver were studied with two
scenarios. Results show that the necessary detector sensitivity should be 4 dB, and for the other
scenario respectively 11 dB, lower than the equivalent thermal noise.
REFERENCES
The reference list is copied from report M77_Info3_SE24_FM(14)096_Road toll systems at 5800
MHz.
The CEN 5.8GHz DSRC communication stack is defined by the following set of standards:
 EN 12253
DSRC Physical layer using microwave at 5.8 GHz
 EN 12795
DSRC Data link layer: Medium Access and Logical Link Control
 EN 12834
DSRC Application layer
 EN 13372
DSRC profiles for RTTT applications
For electronic fee collection applications, an application interface standard defines addressing
mechanisms, basic functions and generic data attributes:
 EN ISO 14906
Electronic Fee Collection – Application interface definition for dedicated
short-range communication
Three application standards are currently available for electronic fee collection purposes:
A charging communication (i.e. pay at the passage of a gantry). For Germany this application is
important for European interoperability, i.e. to enable trucks equipped with German on-board
equipment to pay abroad:
 EN 15509
Road transport and traffic telematics. Electronic fee collection.
Interoperability application profile for DSRC
An enforcement application. This is the transaction first employed in the German system to enforce
users from other systems and after full migration for all users:
 CEN ISO/TS 12813
Electronic fee collection - Compliance check communication for
autonomous systems
An application to augment satellite navigation at difficult locations:
 CEN ISO/TS 13141
Electronic fee collection - Localisation augmentation communication for
autonomous systems
ETSI Standards
ETSI TS 102 486-2-3 V1.2.1 (2008-10)
Intelligent Transport Systems (ITS); Road Transport and Traffic Telematics (RTTT); Test
specifications for Dedicated Short Range Communication (DSRC) transmission equipment; Part 2:
DSRC application layer; Sub-Part 3: Abstract Test Suite (ATS) and partial PIXIT proforma
ETSI TS 102 486-2-2 V1.2.1 (2008-10)
Intelligent Transport Systems (ITS) Road Transport and Traffic Telematics (RTTT); Test
specifications for Dedicated Short Range Communication (DSRC) transmission equipment; Part 2:
DSRC application layer; Sub-Part 2: Test Suite Structure and Test Purposes (TSS&TP)
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ETSI TS 102 486-2-1 V1.2.1 (2008-10)
Intelligent Transport Systems (ITS); Road Transport and Traffic Telematics (RTTT); Test
specifications for Dedicated Short Range Communication (DSRC) transmission equipment; Part 2:
DSRC application layer; Sub-Part 1: Protocol Implementation Conformance Statement (PICS)
proforma specification
ETSI TS 102 486-1-3 V1.2.2 (2009-05)
Intelligent Transport Systems (ITS); Road Transport and Traffic Telematics (RTTT); Test
specifications for Dedicated Short Range Communication (DSRC) transmission equipment; Part 1:
DSRC data link layer: medium access and logical link control; Sub-Part 3: Abstract Test Suite (ATS)
and partial PIXIT proforma
ETSI TS 102 486-1-2 V1.2.1 (2008-10)
Intelligent Transport Systems (ITS); Road Transport and Traffic Telematics (RTTT); Test
specifications for Dedicated Short Range Communication (DSRC) transmission equipment; Part 1:
DSRC data link layer: medium access and logical link control; Sub-Part 2: Test Suite Structure and
Test Purposes (TSS&TP)
ETSI TS 102 486-1-1 V1.1.1 (2006-03)
Electromagnetic compatibility and Radio spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Test specifications for Dedicated Short Range Communication (DSRC)
transmission equipment; Part 1: DSRC data link layer: medium access and logical link control; SubPart 1: Protocol Implementation Conformance Statement (PICS) proforma specification
ETSI TS 102 916-3 V1.1.1 (2012-05)
Intelligent Transport Systems (ITS); Test specifications for the methods to ensure coexistence of
Cooperative ITS G5 with RTTT DSRC; Part 3: Abstract Test Suite (ATS) and partial Protocol
Implementation eXtra Information for Testing (PIXIT)
ETSI TS 102 916-2 V1.1.1 (2012-05)
Intelligent Transport Systems (ITS); Test specifications for the methods to ensure coexistence of
Cooperative ITS G5 with RTTT DSRC; Part 2: Test Suite Structure and Test Purposes (TSS&TP)
ETSI TS 102 916-1 V1.1.1 (2012-05)
Intelligent Transport Systems (ITS); Test specifications for the methods to ensure coexistence of
Cooperative ITS G5 with RTTT DSRC; Part 1: Protocol Implementation Conformance Statement
(PICS)
ETSI TS 102 792 V1.1.1 (2012-10)
Intelligent Transport Systems (ITS); Mitigation techniques to avoid interference between European
CEN Dedicated Short Range Communication (CEN DSRC) equipment and Intelligent Transport
Systems (ITS) operating in the 5 GHz frequency range
ETSI TR 102 960 V1.1.1 (2012-11)
Intelligent Transport Systems (ITS); Mitigation techniques to avoid interference between European
CEN Dedicated Short Range Communication (RTTT DSRC) equipment and Intelligent Transport
Systems (ITS) operating in the 5 GHz frequency range; Evaluation of mitigation methods and
techniques
ETSI TR 102 654 V1.1.1 (2009-01)
Electromagnetic compatibility and Radio spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Co-location and Co-existence Considerations regarding Dedicated Short Range
Communication (DSRC) transmission equipment and Intelligent Transport Systems (ITS) operating in
the 5 GHz frequency range and other potential sources of interference
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ETSI ES 200 674-1 V2.4.1 (2013-05) (only used in Italy)
Intelligent Transport Systems (ITS); Road Transport and Traffic Telematics (RTTT); Dedicated Short
Range Communications (DSRC); Part 1: Technical characteristics and test methods for High Data
Rate (HDR) data transmission equipment operating in the 5,8 GHz Industrial, Scientific and Medical
(ISM) band
ETSI EN 300 674-2-2 V1.1.1 (2004-08)
Electromagnetic compatibility and Radio spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Dedicated Short Range Communication (DSRC) transmission equipment (500
kbit/s / 250 kbit/s) operating in the 5,8 GHz Industrial, Scientific and Medical (ISM) band; Part 2:
Harmonized EN under article 3.2 of the R&TTE Directive; Sub-part 2: Requirements for the OnBoard Units (OBU)
ETSI EN 300 674-2-1 V1.1.1 (2004-08)
Electromagnetic compatibility and Radio spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Dedicated Short Range Communication (DSRC) transmission equipment (500
kbit/s / 250 kbit/s) operating in the 5,8 GHz Industrial, Scientific and Medical (ISM) band; Part 2:
Harmonized EN under article 3.2 of the R&TTE Directive; Sub-part 1: Requirements for the Road
Side Units (RSU)
ETSI EN 300 674-1 V1.2.1 (2004-08)
Electromagnetic compatibility and Radio spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Dedicated Short Range Communication (DSRC) transmission equipment (500
kbit/s / 250 kbit/s) operating in the 5,8 GHz Industrial, Scientific and Medical (ISM) band; Part 1:
General characteristics and test methods for Road Side Units (RSU) and On-Board Units (OBU)
ETSI EN 300 674 V1.1.1 (1999-02) (old revision, not valid any more)
ElectroMagnetic Compatibility and Radio Spectrum Matters (ERM); Road Transport and Traffic
Telematics (RTTT); Technical characteristics and test methods for Dedicated Short Range
Communication (DSRC) transmission equipment (500 kbit/s / 250 kbit/s) operating in the 5,8 GHz
Industrial, Scientific and Medical (ISM) band
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