Reliability of the GSM-R Communication System against Railway Electromagnetic Interferences

In 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 board moving trains in EM conditions representative of those met during a train journey. Then, we describe a new experimental protocol that has been developed in laboratory for immunity testing of GSM-R mobiles with the aim of highlighting the most influential elements on the operational quality of service of the GSM-R system. Introduction Context The GSM-R (Global System for Mobile communications Railways) is a key component of the new ERTMS/ETCS (European Rail Traffic Management System/European train Control System) standard. It is a digital wireless communication system between trains and control centers deployed in Europe in order to ensure the interoperability of train movements on the European territory. This system carries the signalling information directly to the cabin and then to the train driver, enabling higher train speeds and traffic density with a high level of safety. It will ensure voice and data transmissions between trains and control centers and between trains for control-command applications 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 Base Transceiver Stations (BTS) installed along the railway tracks and GSM-R mobile stations embedded on board trains and connected to GSM-R antennas fixed on the roof. Challenges Because 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 the catenary. Moreover, the GSM-R coexists with other communication systems such as the public GSM and UMTS (Universal Mobile Telephone System) which employs frequency bands (E-GSM) adjacent to the GSM-R ones and whose antennas are sometimes fixed on the same poles as those for GSMR. 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 EM disturbances provided by the railway environment. Trains control center Embedded GSM-R mobile Base Transceiver Station Data and voice transmissions Challenge H: For an even safer and more secure railway The GSM-R system As previously mentioned, the GSM-R is based on the GSM standard but it has its own frequency channels. 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 the trains). Thus the GSM-R frequency bands are directly adjacent to the E-GSM band, as can be seen in figure 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 railway requirements and called ASCI (Advance Speech Call Items). In particular, it offers applications such as functional addressing, location dependant addressing, voice group call services (VGCS), voice broadcast 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 also a Time Division Multiple Access (TDMA) system: for each carrier frequency, data transmission is made of periodical TDMA frames. Each frame is divided in 8 time slots (TS) of 577 μs and has a period of 4.615 ms. Each time slot is attributed to one user, thus 8 different users can employ the same frequency 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 bits including 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 we only consider the circuit mode system. burst 3.7 μs bit time Time us er 2 us er 1 us er 4 us er 3 us er 7 us er 8 us er 6 us er 5 TDMA frame= 4.6 ms us er 2 us er 1 us er 4 us er 3 us er 7 us er 8 us er 6 us er 5 8 logical channels 924.8 MHz