## 3 The Train-to-Ground Wireless Communication Needs  
The wireless communication systems of HSR, intercity railway, subway, light rail, and other rail traffic are deployed with various wireless standards. To ensure safe and reliable operation of railways, these wireless standards have to be reliable and robust. Applications such as on-board video surveillance, train control operations, signaling, diagnostics, and monitoring are delay-intensive and complex. In addition, passengers onboard also expect to access Internet for business and entertainment such as on-vehicle video conference, online games, chatting, live broadcast, etc. Train control operations require a lower data rate than passengers' entertainment needs, but are of significantly higher priority.  
Although various wireless communication technologies are adopted in the railway traffic systems, the data transmitted between train and ground are basically identical. These data could be divided into the following categories depending on intended applications, required throughput and railway sectors [18].  
1. The operational services include * the safety-related applications for operation of trains and signaling, also called \"Operation Control System\" (OCS), which typically integrated with \"Traction Control System\" (TCS); * the robust Operational Voice Communication systems (OVC); * Train Operation Status Monitoring data (TOSM); * the image transmission for video surveillance: real-time HD video transmissions supporting automatic driving, security Closed Circuit Television (CCTV) in the train; and * Passenger Information Service (PIS).
2. The services to passengers: it comprises mainly the Internet access on board trains, including business and entertainment such as on-vehicle video conference, online games, chatting, live broadcast, etc.  
The classification of services of train-to-ground communication for vactrain is basically identical to that of current railway systems. However, due to the distinguishing characteristics of vactrain, such as vacuum tube wall and ultra-high speed, more stringent transmission requirements for train-to-ground communication are put forward in terms of end-to-end delay and bandwidth. For example, environmental real-time monitoring data (such as air pressure and temperature) of tube and train needs to be transmitted to the ground control center. Besides, due to the ultra-high operation speed, extremely low end-to-end delay and handover delay are required for operational services transmission, such as train location information. Furthermore, real-time High-Definition (HD) video surveillance for the area in front and rear of trains and along the tubes is needed in the virtue of sealed vacuum metal tube. Thus, more HD cameras would be installed inside and outside the train, resulting a higher requirement for throughput.  
The amount of data to be transmitted on uplink/downlink and the Key Performance Indicators (KPI), such as end-to-end transmission delay, transmission periodicity, packet loss, or BER, can be used to describe the performance of railway wireless applications. Based on this, Table 2 summarizes the train-to-ground wireless communication performance requirements for urban rail transit (CBTC), HSR, Shanghai magley trains [14; 34; 35], as well as the vactrain. The bandwidth and KPI requirements of vactrain's train-to-ground communication services are analyzed based on the reference to current railway transportation systems, but also the consideration of the extremely high velocity, special operation scenarios and passenger capacity of vactrain.