# Towards the Internet of Smart Trains:
## 2 Communication Systems in Railway Scenarios
### Intra-Car Communication Networks  
Since the early 1980s, on-board communication networks were installed on trains to reduce the wiring used to transfer information between distinct devices like Human-Machine Interface (HMI) or Heating, Ventilation and Air Conditioning (HVAC). Multiplexing digital information techniques over a serial cable have tried to replace most of the classical point-to-point copper lines or so-called train lines. In 1999, wired communication networks were standardized for on-board railway applications (the standard was superseded in 2010 [16]) by defining Wire Train Bus (WTB) and Multifunction Vehicle Bus (MVB) networks for Train Control and Management System (TCMS) application. Standards like CANOpen, LonWorks, Profibus, WorldFIP, Leaky Coaxial Cable (LCX) or Train Communication Network (TCN) are deployed either for metro or trains. Since the 2000s, manufacturers considered the Real-Time Ethernet (RTE) technologies by adding new standards to IEC 61375 standard series [17], such as Ethernet Train Backbone (ETB) or Ethernet Consist Network (ECN). Besides the control-command functions provided by classical field bus technologies, RTE provides Internet Protocol (IP) traffic. For example, a wired Ethernet network could be taken in consideration, but it implies high installation costs. In recent years, Power Line Communication (PLC) technology has experienced significant developments. A review on railway embedded network solutions is presented in [18]. In this scenario the links created by the APs provide wireless access to the passengers and to the sensors inside the car. Such a scenario is prone to backscattering, which results in attenuations [13]. Three wireless access modes can be enabled to provide good coverage inside the cars:  
* Direct transmission from the Base Station (BS). The problem in this mode is that the signal from the BS has to penetrate into the car, what derives in a loss of up to 24 dB that needs to be compensated by incrementing the transmission power and the receiver sensitivity. * Use of in-car repeaters. The signals from the BS are received by an on-vehicle transceiver, which forwards them to a micro-base or to a Wi-Fi signal repeater. Note that this scheme increases the signal power through repeaters, but these additional devices increase the communications delay significantly. For this reason, a topic under research is the design and implementation of transmission schemes that offer good coverage for repeaters at high speeds. * Two-hop access mode. In this mode the transmission requires first to travel from the BS to the antennas located on top of the train, and then to the receiver placed inside the train. This approach usually avoids the penetration losses related to a direct transmission from the BS. Nevertheless, it is worth noting that, since high frequency bands have large attenuations and path losses, its use may derive in a limited coverage. When the second and third access modes are used, it is necessary that the signal penetrates the vehicle, what usually causes interferences. Furthermore, the selection of a proper communication technology depends on the bandwidth requirements, which are mainly conditioned by the services provided and by the number of simultaneous users. Assuming around 130-180 passengers per car, a bandwidth of up to 3.6 GHz would be needed if half of the passengers demand real-time HD video. If the streaming service has to provide bidirectional HD video (i.e., video conference), the bandwidth requirement may double. These bandwidth requirements cannot be fulfilled by LTE, which only makes use of 20 MHz instead of the 7.2 GHz that would be demanded. The solution might be provided by mmWave/sub-mmWave bands at 28 and 300 GHz, and the 5G communication systems, which offer larger bandwidths and higher data rates. For instance, massive Multiple-Input Multiple-Output (MIMO) and 3D beamforming may be used with many users to enhance the system capacity [3]. For wide area coverage, signals at frequency bands below 6 GHz are needed.