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Challenge H: For an even safer and more secure railway

Reliability of the GSM-R CommunicationSystem against Railway Electromagnetic

InterferencesS. Dudoyer, N. Ben Slimen, V. Deniau and M. Berbineau

Univ Lille Nord de France- F-59000 Lille, IFSTTAR, LEOST, F-59650 Villeneuve d’Ascq, France

E-mails: {name}@ifsttar.fr

AbstractIn this article, we study the reliability of the GSM-R system against the electromagnetic (EM)disturbances present in its railway operating environment.We thus first present the results of electromagnetic environment characterization campaigns on boardmoving trains in EM conditions representative of those met during a train journey. Then, we describe anew experimental protocol that has been developed in laboratory for immunity testing of GSM-Rmobiles with the aim of highlighting the most influential elements on the operational quality of service ofthe GSM-R system.

Introduction

ContextThe GSM-R (Global System for Mobilecommunications - Railways) is a keycomponent of the new ERTMS/ETCS(European Rail Traffic ManagementSystem/European train Control System)standard. It is a digital wirelesscommunication system between trainsand control centers deployed in Europein order to ensure the interoperability oftrain movements on the Europeanterritory. This system carries thesignalling information directly to thecabin and then to the train driver,enabling higher train speeds and traffic density with a high level of safety. It will ensure voice and datatransmissions between trains and control centers and between trains for control-commandapplications and other specific railway applications in order to enhance train operations.The GSM-R radio layer is based on the GSM phase2+ standard. It employs two elements: fixed BaseTransceiver Stations (BTS) installed along the railway tracks and GSM-R mobile stations embeddedon board trains and connected to GSM-R antennas fixed on the roof.

ChallengesBecause of their proximity to the catenary, the GSM-R antennas can receive electromagnetic (EM)transient disturbances induced by defects in the sliding contact between the pantograph and thecatenary. Moreover, the GSM-R coexists with other communication systems such as the public GSMand UMTS (Universal Mobile Telephone System) which employs frequency bands (E-GSM) adjacentto the GSM-R ones and whose antennas are sometimes fixed on the same poles as those for GSM-R. Since the GSM-R will have to ensure the transmission of voice and data (mainly data signalling)essential for safer and secure railway, it is necessary to guarantee its immunity against the EMdisturbances provided by the railway environment.

Trains controlcenter

Embedded GSM-Rmobile

Base TransceiverStation

Data andvoice

transmissions

mailto:@ifsttar.fr


The GSM-R system

As previously mentioned, the GSM-R is based on the GSM standard but it has its own frequencychannels. Two specific frequency bands are allocated to the GSM-R: 876-880 MHz for the up-link(from the trains to the base stations) and 921-925 MHz for the down-link (from the base stations to thetrains). Thus the GSM-R frequency bands are directly adjacent to the E-GSM band, as can be seen infigure 1, which is also allocated to UMTS.

Figure 1. Frequency bands of the coexistent communication systems

The GSM-R provides advanced functions specifically developed for the railway domain to meet railwayrequirements and called ASCI (Advance Speech Call Items). In particular, it offers applications such asfunctional addressing, location dependant addressing, voice group call services (VGCS), voicebroadcast services (VBS), call priority and call preemption in case of emergency.As in the case of the GSM, the frequency spacing between each physical channel is 200 kHz. It is alsoa Time Division Multiple Access (TDMA) system: for each carrier frequency, data transmission is madeof periodical TDMA frames. Each frame is divided in 8 time slots (TS) of 577 µs and has a period of4.615 ms. Each time slot is attributed to one user, thus 8 different users can employ the samefrequency communication channel. Each user has then access to the channel in turn and during 577 µs(= time slot duration), as shown in figure 2. One time slot contains a burst composed of 156 bitsincluding 148 of information, which give us a one bit transmission duration of about 3.7 µs.GSM is a circuit mode system. GPRS is deployed based on the existing infrastructure. In this paper weonly consider the circuit mode system.

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Figure 2. Sharing of the users time access to the GSM-R channels

Concerning the coverage level of the GSM-R signals, we give in figure 3 one result of a measurementcampaign carried out on board trains on railway lines equipped with GSM-R [1]. We will give moredetails on this campaign (localisation, measurement configuration and results) in next section.This figure shows the evolution of the maximal reception level (down-link signal) measured in dBmwith a spectrum analyzer over the last GSM-R channel and during one travel from Schaerbeek toHerent in Belgium. As can be seen, the reception level varies from -20 dBm when the GSM-Rantenna is very close to the BTS to -90 dBm when the train (thus the GSM-R antenna fixed on itsroof) is halfway between two successive BTS.


Figure 3. Coverage level measured over the last GSM-R channel (924.8 MHz) along the travel with aspectrum analyzer

Characterization of the EM disturbances received by the GSM-R antenna

EM disturbances which can affect the GSM-R communication systemTwo types of EM disturbances can be received by the GSM-R antenna: transient signals andpermanent noise [1].The transient signals are produced by defects in the sliding contact between the pantograph and thecatenary. Indeed, when losses of contact occur between the pantograph and the catenary, transientsignals are produced because of the created potential difference between these two elements. Thosesignals are then conducted by the different metallic elements that constitute the pantograph and thecatenary, which result in transient emissions covering large frequency bands including the frequencychannels of the GSM-R.The permanent noise comes from the other communication systems the GSM-R has to coexist with:public GSM and UMTS. As previously shown in figure 1, the GSM-R frequency bands are directlyadjacent to the E-GSM band. Consequently, public GSM and UMTS signals can cover some of theGSM-R channels at the end of the GSM-R frequency bands and interfere with GSM-R communicationsignals if the mandatory modulation characteristics are not respected.Two measurement campaigns were carried out in order to characterize the EM operating environmentof the GSM-R system: one for permanent disturbances and one for transient signals.

Measurement campaign to characterize the permanent disturbancesThe measurement campaign to characterize permanent noise was performed in Belgium on boardmoving train from SNCB on railway lines equipped with GSM-R. Following figure 4 shows the railroute for which measurement results are presented in this section [1].

Figure 4. Rail route of the measurement campaign

During this run, a spectrum analyser was connected to a GSM-R antenna fixed on the roof of the trainto measure the maximal signal levels received by the antenna on the 850 MHz - 1 GHz frequency


band. We then obtained the coverage levels of GSM as well as GSM-R signals. Figure 5 makes acomparison between coverage measurements over the last GSM-R frequency channel (924.8 MHz),the first public GSM one (925.2 MHz) and the intermediate channel (925 MHz) which is supposed tobe unused [1].

zz

Figure 5. Measurements of coverage level for the last GSM-R channel (924.8 MHz), unusedintermediate channel (925 MHz) and first public GSM one (925.2 MHz)

These measurements show that the public GSM or UMTS signals on channel 925.2 MHz overlap theadjacent (925 MHz) and alternate (924.8 MHz) channels. For example, we observe that the first peakon public GSM channel is also visible on the two other channels with lower power levels. Indeed, overthe 925.2 MHz channel, the level of this peak is -44 dBm and over the others it decreases untilreaching a level of -75 dBm on the GSM-R channel.Due to the fact that (according to the measurement results presented in figure 3), the lowest level thatthe GSM-R signals can reach, when the train is halfway between two BTS, is about -90 dBm, the levelof -75 dBm for public GSM or UMTS signals on the 924.8 MHz channel could be sufficient to disturbthe GSM-R communications taking place on this channel.

Measurement campaign to characterize the transient disturbancesThe second measurement campaign to collect transient disturbances was carried out in France withSNCF on the railway line Blois - Saint Pierre des Corps - Nantes. The results we present in this articlecome from the measurements performed between Nantes and Saint Pierre des Corps on board atrain whose cruising speed was about 160 km/h and maximal speed was 200 km/h and whichoperated with a 25 000 V AC (50 Hz) supply voltage. The followed rail route is presented in figure 6.

Saumur

Blois

Nantes

25000 V AC

160 - 200 km/h Saint Pierredes Corps

Figure 6. Rail route during the measurement campaign to collect the transient disturbances receivedby the GSM-R antenna on board train


The measures were performed using a digital oscilloscope connected to a GSM-R antenna fixed onthe roof of the train at approximately 80 m from the pantograph, as illustrated in figure 7 [2].

GSM-R antenna pantograph

locomotive

80 m

Figure 7. Position of the GSM-R antenna used to collect the transient disturbances on board trainduring the measurement campaign

The used oscilloscope sampling frequency was 20 GHz which permitted us to record each transientevent, detected during the train path, in a 200 ns time window. An example of one transient EMdisturbance collected at the output of the GSM-R antenna is given in figure 8 [3].

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Figure 8. Measurement configuration and example of one collected transient

The great number of transients collected during this measurement campaign allowed us to perform astatistical analysis on the recorded events to study two of their time characteristics: duration and risetime. The definitions of duration and rise time considered for this study are those given in theEuropean Standard EN 61000-4-4 [4]. This standard aims at defining a common and reproduciblebasis for the evaluation of the performances of electrical and electronic equipment facing electricalfast transients on its different inputs. It gives the following definitions for duration and rise-time:

- the duration corresponds to the time duration when the EM noise level of the transient eventis higher than 50 % of its maximal amplitude.- the rise time is the time between 10 % and 90 % of the maximal amplitude of the transientevent.

Figure 9 illustrates these two definitions.

Duration (D) Rise Time (RT)

Figure 9. Definition of duration and rise time of transients


The distributions of durations and rise times are given in figure 10. These graphs show that first, therise times take values between 0.2 and 1 ns and their most common value is 0.4 ns. Then, thedurations are systematically inferior to 20 ns with mean time duration of 5 ns [2]. In a first step, thesetypical values for transient time characteristics, extracted from our statistical analysis, will be used tomake a comparison between the values for rise time and duration defined in standards and thoseobserved on board trains. Then, we will employ them to build the test signals for immunity testing inlaboratory, as explained in following sections.

Experimental PDFEmperical PDFExperimental PDFEmperical PDF




Figure 10. Distributions of durations and rise times of transients collected on board moving trains

Existing standardized immunity test methodologies

Existing standardsStandardized immunity tests exist for communication equipments and are described in commonlyapplied standards such as the ETSI EN 301 489 series [5]. However these standards, which deal with"electromagnetic compatibility (EMC) for radio equipment and services", are not specific to the railwaydomain. Then, they mainly give general descriptions about the way to perform tests and refer to EN61000-4 series [6] for configurations and methodologies of test on such radio equipments, but they donot define precise criteria to apply in order to guarantee their immunity to the interferences ofexposure. Indeed, if functional criteria are well defined, regarding the impact on the integrity of data,the definition of the immunity criteria is not clear. For example, a way to decide if the EquipmentUnder Test (EUT) has passed the test is to answer simple questions such as: "does the EUT continueto operate as intended during and/or after the exposure to disturbances?" or "was thecommunications link lost during the test?". The only not-functional parameter (i.e. with a measurablevalue) they refer to is the Rxqual. The Rxqual is a quality parameter given by the GSM-R mobilewhich evaluates the received signal quality before the decoding process and expressed in terms oflevels from 0 to 7, each value corresponding to a given BER (Bit Error Rate) interval. A high RXQUALimplies a poor received signal quality. Nevertheless, this parameter is measured by the GSM-Rmobile, which does not constitute a measuring apparatus [3]. Moreover, in the particular case of theGSM-R system, the immunity criteria for data transmissions should be well defined and evaluated toensure that this system complies with the reliability required by the railway domain and notablydefined via Key Performance Indicators (KPIs).

Proposed laboratory testsFor immunity testing the reference standards are the EN 61000-4 series and in particular the EN61000-4-4 for evaluating the immunity of a system against EM transient disturbances. The majorproblem with this standard is that it is not specific to the telecommunications field and thus theproposed test methodology is not well-adapted to perform tests on communication equipments.Indeed, it consists in injecting disturbances on all the ports of the EUT (including communicationones) but we can not just do so for the evaluation of the performances of the communication channel.Moreover the sequences of transients are injected on the different ports thanks to a capacitivecoupling clamp. The major problem with such a clamp is that the level really injected at the clampextremity depends considerably on the coupling and load impedance conditions [7]. It is thus verydifficult to control the level of interferences really suffered by the EUT and even to reproduce exactly


the same EM disturbance conditions for each test. As for illustrate, figure 11 shows an example ofone immunity test set-up when using the capacitive coupling clamp to inject the transient disturbancesignals on the cable between a GSM-R mobile and its antenna or network simulator.

GSM-Rantenna

ornetworksimulator

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Figure 11. Example of set up for immunity test of communication port using the capacitive couplingclamp

Characteristics of the test signalsAnother reason why the test methodology described in EN 61000-4-4 is not applicable to the GSM-Rsystem, is the choice of time characteristics for the applied test signals. In fact, the characteristics ofthe transient defined in that standard differ significantly from those of real transients collected onboard moving trains and previously presented. However, the noise level produce on the GSM-Rfrequency bands by a transient signal depends on its time characteristics. To illustrate this point, wehave built two transient models using the values of duration and rise time previously given and thosedefined in EN 61000-4-4 and we have calculated their FFT with a Scilab program in order to comparethe noise level produced by each transient model depending on its time characteristics. For eachtransient, the peak amplitude was set to 1 V in order to study only the effect of the duration and risetime. The result of this comparison appears in figure 12 which gathers the time characteristics valuesused to build the two signals, the time representations of transients and their FFT graphs.

Up-link & down-linkGSM-R frequency bands

Time characteristics extracted from 61000-4-4Time characteristics from measurement campaign

FFT

Figure 12. Comparison between the time characteristics of the test signals described in EMCstandards and those of transient signals collected on board trains


When comparing the levels produced by the two transients on the GSM-R frequency bands, weobserve that the level produced by the transient with duration and rise time extracted from EN 61000-4-4 (-61 dBm on average) is much lower than the one for the transient with characteristics frommeasurement campaign (-32 dBm). Consequently if performing immunity tests with signals asdescribed in that standard, we could underestimate the real impact of on board trains transientdisturbances on the GSM-R system. We thus have preferred to use the typical values of duration andrise time extracted from our statistical analysis to build our specific transient test signalsrepresentative of the railway environment.

Immunity tests

Employed test signalThe used transient test signal was a double exponential (duration=5ns, rise time=0.4 ns) modulatedby a sinus at the frequency 923 MHz which corresponds to the center frequency of the GSM-R down-link frequency band. We remind that we have employed for rise time and duration the values resultingfrom the statistical analysis performed on transients collected on board trains. Figure 13 gives thetime representation of our test signal whose mathematical expression is given by (1):

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(1)

Figure 13. Used transient test signal

Methodology of testThe methodology we have employed to perform immunity tests on the GSM-R system is illustrated infigure 14. Basically, the principle is to establish a communication between a GSM-R mobile and onenetwork either simulated by a specific piece of equipment or coming from a base station installed atproximity. Then EM disturbances signals (permanent noise and transient signals) are generated andtheir impact on the GSM-R communication is evaluated thanks to criteria we are going to introduce inthe following paragraph.

GSM-Rmobile

Networksimulator

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Figure 14. Principle of the tests

Employed immunity criteriaWe can employ two criteria: Rxqual (which was defined in a previous part) and Bit Error Rate (BER).The BER corresponds to the percentage of erroneous bits in a given transmission length [8]:

%100bitsofnumberTotal

bitserroneousofNumberBER (2)


A relationship exists between BER and Rxqual: each value of Rxqual from 0 to 7 is associated with arange of values of BER [9]. As can be seen in figure 15, a high Rxqual implies a high BER. To providea good quality of communication, standards such as [10] require that the Rxqual is inferior or equal to3.

Figure 15. Correspondence table between BER and Rxqual

Immunity test benchWe now present in figure 16 the test bench employed to perform immunity tests in laboratory with theaim of reproducing the EM conditions that the GSM-R system is susceptible to meet on board trains.It is composed of three main parts:

- the communication system which consists in a GSM-R mobile connected to a networksimulator called CMU 200 from Rohde & Schwarz.- the noise generation which permits us, thanks to the two signal generators, to simulate thepresence of permanent and EM transient noises simultaneously or separately.- the area "analysis in frequency domain" is used to control the power of the exchanged

signals at the input of the GSM- R mobile.

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Figure 16. Employed immunity test bench

Protocol of testThe aim was to test the immunity of the previously established GSM-R communication whenpermanent noise and EM transient disturbances were simultaneously present. That is the reason whywe have used the last GSM-R channel (924.8 MHz) that is adjacent to the first E-GSM one(925.2 MHz). The level of the GSM-R signals was set to -70 dBm and the one of public GSM or UMTSwas variable.As for the transient signals, the measurement campaign to characterize them had shown that,contrary to duration and rise time, it was not possible to determine a typical value for the recurrence of


transients since it is very variable and depends on several operating conditions (speed of the train,nature of the crossed areas…). For example, we have observed that very few transients appeared atlow speed whereas they could occur with a time interval of about 5 µs at about 200 km/h. Interestedreaders can find a detailed study of this parameter in [2]. Thus, we have decided to use therecurrence of transients as a parameter for our immunity tests: for each measurement, the transientdisturbances were generated with a constant time interval (TI) between two successive transients asillustrated in figure 17. Then, one other selected value (from 1700 to 10 µs) was applied for the timeinterval in order to generate one other sequence of transient disturbances and study the impact of therecurrence of transients on the GSM-R communication.

Time intervalTime

Figure 17. Illustration of the time interval (TI) between the successive transient disturbances

In fact, the same protocol was repeated for each series of measurements:

1. Start the GSM-R mobile and the other equipments.2. Configure the equipments (network simulator and signal generators).3. Connect the mobile to the network simulated by the CMU.4. Establish a communication between the mobile and the CMU.5. Send the disturbance signals thanks to the two signal generators.6. Measure the BER with the CMU.7. Note down the average of Rxqual calculated by the CMU from the values given by the mobile.8. Apply next value for time interval between two successive transients and go to step 5.

When all selected values of time interval had been used, we could vary the power of the public GSMsignals and restart one series of BER and Rxqual measurements (from step 5) for the same selectedvalues of time interval.

ResultsThe results we present in this section were obtained during immunity tests performed in presence ofthe two types of disturbances (public GSM and transients). The employed communication channelswere those previously mentioned. As for the generation of the disturbance signals, threeconfigurations were considered, as presented in figure 18: presence of permanent noise only(configuration 1) and presence of permanent noise and transients simultaneously with two arbitrarilychosen values for the transient time interval which are TI=150 µs (configuration 2) and TI=550 µs(configuration 3).

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Figure 18. Configurations considered for immunity tests in presence of the two types of disturbances


The following graph, on the right of figure 19, shows the results of the BER measurements in thesethree configurations of test. The vertical axis gives the value of the BER in % and the horizontal axisrepresents the power level, on the channel 925.2 MHz, of the E-GSM band signals which induce thepermanent noise on the GSM-R channel. The first curve (black one with points) corresponds to theevolution of the BER without transient and the two others (orange with squares and blue with trianglesones) with transients for the two considered values of time interval. These values were chosen so that3 transients can occur during the time duration of one GSM-R burst in the first case (TI=150 µs) andonly one in the second case (TI=550 µs), as can be seen in the illustration on the left of figure 19.

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Figure 19. Results of the BER measurements performed in presence of transient signals andpermanent noise with GSM-R signal power = -70 dBm

We observe that in the absence of transient signals (black curve with points), the power of the signalin the E-GSM band has to be superior to -20 dBm (a situation that can be met near a publicGSM/UMTS BTS) to start affecting the BER and thus disturbing the GSM-R communication. Knowingthat the GSM-R signal power was set to -70 dBm, this means that the interference signal over the400 kHz adjacent channel has to be 50 dBm higher than the wanted signal on the GSM-Rcommunication channel to deteriorate the quality of the transmission. Besides, that difference of50 dBm is in compliance with the standard EN 300 910 [11] which defines the adjacent interferencelevels that a mobile has to tolerate.Then, we notice that the public GSM signals have to reach a power level of -9 dBm to induce aRxqual equal to 3 whereas in the presence of transient disturbances with a time interval of 150 µs, alevel of -15 dBm is sufficient. In other words, the impact on the GSM-R communication of the transientdisturbances "adds" to the one of signals in the E-GSM band. We thus conclude that the susceptibilityof the GSM-R to permanent noise is higher in the presence of transient disturbances.Obviously, these results are linked to the GSM-R signal power used for the test (-70 dBm) and wewould have obtained a better level of immunity if setting up the GSM-R signal to a higher level ofpower. However, we are not going to develop this point in this article, we invite the interested readersto consult [3] in which they can find further studies on the immunity of the GSM-R system.

ConclusionsIn this article, we have presented our works dealing with the GSM-R and its susceptibility to the EMnoise conditions met in the railway environment. A precise characterization of this environment wasperformed during measurement campaigns on board moving trains on lines equipped or not withGSM-R. We had previously identified the two main EM disturbance sources that the GSM-R will have


to face: permanent noise such as GSM and UMTS signals in the E-GSM band adjacent to the GSM-Rband and transient signals coming from defects in the sliding contact between the pantograph and thecatenary. The measurements carried out on trains have permitted us to extract some of thecharacteristics of these interfering signals such as power levels of public GSM/UMTS and GSM-Rsignals and durations and rise times of transient signals. This information has been used to build atest bench and test signals for immunity testing in laboratory. The different tests we performed havefirst shown that the GSM-R system well complies with the specifications. Then, they have highlightedthat both public GSM/UMTS and transient disturbances can deteriorate the quality of GSM-Rcommunications. Moreover we have observed that the effects produced on the BER by one of theseinterfering signals add to those produced by the other one when both types of EM interferences aresimultaneously present during a GSM-R communication. Thus, it is not possible to separate the studyof the impact of permanent noise from the one of the impact of transient signals.

AcknowledgementsThe authors of this paper would like to thanks SNCB and SNCF to have given us access to theirtrains to perform measurements in real conditions and also ALSTOM which provided us with GSM-Rmobile and fixed equipments. This work was performed in the framework of the RAILCOM projectsupported by the PCRD 6 and CISIT projects supported by the North Region and the FEDER.

References[1] T. Hammi, N. Ben Slimen, V. Deniau, J. Rioult and S. Dudoyer, "Comparison between GSM-Rcoverage level and EM noise level in railway environment", Proceedings of the 9th InternationalConference on Intelligent Transport System-Telecommunications, ITST 2009, pp 123-128, October2009, Lille, France.[2] N. Ben Slimen, V. Deniau, J. Rioult, S. Dudoyer and S. Baranowski, "Statistical characterisation ofthe EM interferences acting on GSM-R antennas fixed above moving train", Eur. Phys. J. Appl. Phys.48, 21202 (2009) doi: 10.1051/epjap/2009155.[3] S. Dudoyer, V. Deniau, R. Adriano, M. N. Ben Slimen, J. Rioult, B. Meyniel and M. Berbineau,"Study of the Susceptibility of the GSM-R Communications to the Electromagnetic Interferences of theRail Environment", accepted for publication in IEEE Transactions on electromagnetic compatibility.[4] "Electromagnetic Compatibility - Part 4-4: Testing and Measurement techniques - Electrical fasttransient/burst immunity test - Basic EMC Publication", European Standard NF EN 61000-4-4, June1995.[5] ETSI EN 301 489 series: "Electromagnetic compatibility and Radio spectrum Matters (ERM);ElectroMagnetic Compatibility (EMC) standard for radio equipment and services".[6] EN 61000-4 series: "Electromagnetic Compatibility (EMC) - Part 4: Testing and measurementtechniques - Section 3, 4, 5 and 6".[7] F. Duchemin, "Experimental studies for the comparison between the industrial environmentalconstraints and the results obtained by the electromagnetic immunity and emission standard tests",Ph.D. dissertation, Dept. Electronics, Lille University of Science and Technology, Lille, 1999.[8] Gary Breed, "Bit Error Rate: Fundamental Concepts and Measurement Issues", High FrequencyElectronics, vol. 2, no. 1, pp. 46-48, January 2003.[9] Xavier Lagrange, Philippe Godlewski, Sami Tabbane, "GSM-DCS Networks" ("Réseaux GSM-DCS"), Hermès, Paris, 1995, 1996, pp. 207-208.[10] ITU-T Recommendation K.48, "EMC requirements for each telecommunication equipment -Product family Recommendation", July 2003.[11] "Digital cellular telecommunication system (Phase 2+); Radio transmission and reception",European Standard ETSI EN 300 910 (V8.5.1), November 2000.

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