Railway Operators Needs in Terms of Wireless Communications
This chapter is dedicated to the railway operators needs in terms of wireless communications, through railway communication standards, safety aspects and classifications of the applications. Then, the chapter details the operational needs for communication and signaling systems or monitoring systems. Finally, the chapter highlights the needs for services to passengers, though the Internet access on board especially.
Introduction to Wireless Communications for Railway Applications
This section introduces the context of wireless communications between train and ground for railway applications, the railway communication standards and the safety aspects. Then, different classifications are performed to distinguish between the applications, depending on intended applications, required throughputs and railway sectors. Finally, the basic architectures for train-to-ground applications are introduced.
Required railway services lead to the development of railway communication systems. The need to deploy a communication network on board trains appeared in the late seventies, at first for presenting diagnostic information to the driver and maintenance staff to obtain the status of the whole control system with synthetic messages . In the past, applications for signaling and data communications in the railway domain were assured by robust wires that carried information with significant current load and voltage level from 24 to 110 V . Since several years, wireless systems replaced the wired ones . With the development of electronics,
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E. Masson and M. Berbineau, Broadband Wireless Communications for Railway Applications, Studies in Systems, Decision and Control 82,
information and telecommunication technologies, the needs for transmission in the railway domain have increased in order to increase travel safety, optimize the use of existing infrastructure by increasing the frequency of trains, reduce operating costs and maintenance, thus reducing the impact of transport on the environment. The needs of transmissions related to the operation and maintenance of trains and tracks also add the needs of information and services to staff and customers at any time. With the increase of transmission needs, we talk about Intelligent Transportation Systems (ITS) to reflect the introduction of electronic and computer devices contributing to the automation of functions. Systems are called “informed” and vehicles “connected” .
Furthermore, the European Rail Research Advisory Council (ERRAC), an advisory body to the European Union (EU) Commissions representing Member States and all stakeholders in this sector, targeted for the year 2020 to double passenger and freight traffic by rail . The objective, presented in the Multi-Annual Action Plan of the Shift2Rail Joint Undertaking, is to provide a seamless, integrated and safe high speed passenger service, door-to-door freight service and an efficient metropolitan and urban mass transport . Such a goal should be achieved by reducing costs, enhancing environmental sustainability and offering new services to passengers.
The objectives will then be reached by increasing the number of information exchanges systems between stakeholders. Each functional entity of the global transport system or each application requires information exchanges more or less frequent and more or less consuming in terms of radio spectrum resources. As a consequence, more and more wireless communication devices operating at different frequencies will be deployed inside the trains and along European Railway lines in the next years, to meet all these communication needs. The particular case of High Speed Train (HST) and the fast increase of train speed lead to more and more attention on the issue of train operation safety. From an operational point of view, the track infrastructure, the rolling-stock and the signaling system are the three main parts contributing to safety operation. The signaling system represents the key part and can be seen as the nerve center of the system . Concerning the services to passengers, such as the Internet on board, needs are growing drastically due to the current public telecommunication services, that increase the needs of mobility services.
The specificity of railway environments implies several complex factors to establish a wireless communication from ground to wayside [6, 7]. First of all, the radio propagation channel characteristics depend on the type of geographic environment encountered. The case of a railway environment is characterized by specific scenarios, such as tunnels, cuttings, crossing bridges or railway stations. The widespread of tunnels is especially a barrier to seamless communication due to the need for radio coverage inside the tunnels. Moreover, the high speeds of HST context lead to problems that are not encountered in highway context for instance. Until 200 km/h, wireless channel can be assumed to be Wide-Sense Stationary Uncorrelated Scattering (WSSUS). This WSSUS assumption is no more valid because of the rapid time-varying and non-stationary HST context. Furthermore, the LOS component of the signal exists in a majority of railway scenarios. The spatial correlation between different multipath is then quite important .
The other complicating factors are summarized below:
- • The possible metallic structure of the train behaves as a Faraday cage causing important losses on transmission signals (one can note that the development of trains in composite materials will modify this);
- • Railway environment can be defined as a “high vibration” and “high interferences” environment, which can lead to a need of isolation of communication devices;
- • Large temperature variations are observed;
- • Railway environment suffers of high electrical stresses: cohabitation between high power (traction) and low power systems (electronic), strong magnetic fields as for the MAGLEV trains and trains not designed to provide a stabilized power supply;
- • Railway companies constantly add or remove rail cars from trains. It is necessary that the communication networks discover this automatically.
The major prerequisite condition to guarantee a reliable communication is the knowledge of the wireless channel and its propagation characteristics . Indeed, works and models were largely developed for public land mobile communications but some of them are not suitable for the specific case of railway environments. For instance, it was shown that the conventional Hata model prediction may reach an error of 20 dB in a HST context with cuttings and viaducts . This domain is an extensive domain of publications these recent years. Channel modeling can deal with different kinds of communication: 60 GHz communications , V2V communications , MIMO communications  and the channel models can rely on different techniques such as modeling from channel sounding .
Finally, research works are currently on going for the case of Train Control and Monitoring System (TCMS) in the European project Roll2Rail . The aim of the Work Package dedicated to TCMS is to develop a new generation of train communication systems relying on wireless technologies, thus reducing on board communication cables and simplifying train coupling procedures.