## 3 The Train-to-Ground Wireless Communication Needs
### The Needs for Operational Services  
The operational services, also called critical services, can be divided into two types: those related to the safety of the train itself (operation control system and train traction data) and public safety ones (including OVC, TOSM, CCTV, and PIS). Similar to the HSR, all safety-related services of vactrain need the highest safety level (SIL4 [18]), low bandwidth (less than 1 Mbps per train), extremely significant delay constraints (milliseconds, less than 1 ms in the worst case), and the traffic pattern is usually Real-Time Variable Bit Rate (RT-VBR). 1. _Operation control systems_: Through computer control, computer network, communication and information processing, and other advanced technologies [36; 37], the operation control system is connected with the vehicle, traction [38], line, and turnout equipment or system of the maglev transportation system, and ultimately completes the tasks of train operation control, safety protection, automatic operation, and dispatch management. Similar to Shanghai maglev [7], the train operation systems for vactrain mainly consist of three layers: the central control system located in the control center, zone control systems corresponding to the traction section, and on-board control systems located on the train. As a critical service to train operation safety, OCS needs highest priority, end-to-end delay lower than 40 ms, and BER in the order of \(10^{-6}\) to ensure the reliable and robust communication. The bidirectional data transmission between zone control systems and train control systems could be provided by the train-to-ground communication system. These data mainly includes mobile authorization, temporary speed limit information, clock synchronization status, train identification number, etc., which requires the relatively short information coding length and small data size (most of the time less than 100 bytes), resulting in a low bandwidth requirement. Table 2 shows that the OCS bandwidth requirement of current railway transit does not exceed 500 Kbps per train. Thus, considering the higher safety requirements of vactrain, the minimum bandwidth of vactrain OCS could be set to 1 Mbps (up/down). 2. _Traction Control System_: Typically, TCS is integrated with OCS and deployed in the zone control systems which correspond to the traction sections. The TCS and OCS complement each other to ensure the safe operation of vactrain. To fully control the speed of vactrain, the train operational command data, such as velocity-position curve, should be transmitted to TCS from zone control systems through ethernet network. In addition, the magnetic pole phase angle data, which contains the train positioning information, needs to be transmitted from on-board control systems to TCS directly through the train-to-ground wireless links. This real-time train location information is the basis of automatic train driving, but is also an indispensable part of synchronous linear motor control and train interval control. Due to the vactrain's ultra-high velocity, the  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Data Type**} & \multicolumn{4}{c}{**End-to-end Transmission Delay/ms**} & \multicolumn{4}{c}{**Data Rate / Kbps**} & \multicolumn{4}{c}{**Bit Error Rate**} \\ \cline{2-13}  & **Urban Rail** & **HSR** & **Mglev** & **Vactrain** & **Urban Rail** & **HSR** & **Mglev** & **Vactrain** & **Urban Rail** & **HSR** & **Mglev** & **Vactrain** \\ \hline \multirow{2}{*}{OCS} & \multirow{2}{*}{100} & \multirow{2}{*}{50} & \multirow{2}{*}{40} & \multirow{2}{*}{40} & \multirow{2}{*}{200/ train} & \multirow{2}{*}{200/ train} & \multirow{2}{*}{200/ train} & \multirow{2}{*  
transmission of train positioning information needs to meet the requirement of extreme low end-to-end delay [39]. As shown in Table 2, the TCS of Shanghai maglev train (430 km/h) needs the transmission delay of train-to-ground wireless communication less than 5 ms [14].