## 3 Communication Technologies for Improved Energy Efficiency and Safety in Railways
### State of the Art in Communication Systems in Railways
These features may become highly attractive in future high-speed railways, which should feature increased transportation safety levels. For that to happen, augmented communication capabilities are needed [112] due to inherent problems with high-velocity train localisation (positioning) using the existing railway communication networks [15]. Radio channel modelling was proposed in [112] to facilitate the design of a novel communication system that is suitable for high-speed railways and to assess its performance under realistic operating conditions. Some recent advances in wireless communication technologies that could be applicable in future smart railways include (i) intelligent self-powered sensor nodes [113] and related communication systems for the condition monitoring of freight wagons [114], (ii) stochastic modelling [115] and the application of radio-over-optical fibre communications [116] aimed at improving wideband communications in high-speed railways, (iii) energy-efficient communication protocols [117] and wide-area networks (WANs) in intelligent transportation [118] includ \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Technology** & **Robustness** & \begin{tabular}{c} **Real-Time** \\ **Performance** \\ \end{tabular} & **Range** & **Throughput** & \
\begin{tabular}{c} **Network** \\ **Scalability** \\ \end{tabular} &
\begin{tabular}{c} **Power-Saving** \\ **Awareness** \\ \end{tabular} \\ \hline IEEE 802.11 & Not compliant & Not compliant & Full & Full & Partial & Not compliant \\ \hline IEEE 802.15.4 & Partial & Not compliant & Partial & Not compliant & Partial & Full \\ \hline Zigbee & Partial & Partial & Partial & Not compliant & Full & Partial \\ \hline Zigbee Pro & Partial & Partial & Full & Not compliant & Full & Partial \\ \hline IEEE 802.15.1 & Partial & Full & Not compliant & Partial & Not compliant & Partial \\ \hline Bluetooth & Partial & Full & Not compliant & Partial & Not compliant & Partial \\ \hline Wireless HART & Full & Full & Partial & Not compliant & Partial & Full \\ \hline ISA 100.11a & Full & Full & Partial & Not compliant & Partial & Full \\ \hline WISA & Full & Full & Not compliant & Partial & Partial & Full \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of different wireless sensor networks technologies in terms of compliance with railway communication requirements [108]. ing the IoT paradigm [119], and (iv) the development of effective high-throughput wireless networks for railways using 5G communication technologies [120]. V2V communications have not yet been deployed in railways, but there is a clear research trend in exploring the possibilities of the future incorporation of direct and indirect (V2I-based) V2V communication systems into smart railway systems. V2I communications can be viewed as keys for rail transport management, such as by using virtual coupling between autonomous trains running in platoons and communicating over a centralised radio link [121]. According to [121], this concept can also be used for the dynamic coupling and decoupling of individual freight wagons and should notably improve the railway system capacity and throughput under varying freight transport demand. A comparative analysis of direct and indirect V2V communication between trains using fifth generation radio networks for railways (5G-R) for the purpose of platooning was presented in [122], wherein direct communication, although faster, requires additional means of synchronisation due to the absence of a central clock on the fixed communications node (repeater). The results in [122] also indicate that using advanced communications characterised by low latency can facilitate the high-precision synchronisation of train speeds and mutual distance within a platooning configuration.