## 3 The Train-to-Ground Wireless Communication Needs
### The Needs for Operational Services
Assuming that the speed of vactrain exceeds 1000 km/h, and the transmission delay is inversely proportional to the speed, the transmission delay requirement for TCS of vactrain is less than 1 ms.
3. _The robust operational voice communication systems:_ These systems allow not only trains to communicate with the rail traffic control centers, but also trains drivers, rail traffic, and maintenance agents to communicate with each others in conference mode (group calls). Besides, the train broadcasting function needs to be supported by OVC to realize the emergency call and passengers broadcasting. Thus, when emergencies occur, passengers can set up a call with the central disaster prevention dispatcher in time. The amount of information exchanged of the voice communication applications that involve safety can be low, but are demanding in terms of robustness and availability. Each route voice call needs a data rate of 32 Kbps. The service priority of OVC is only second to OCS. Thus, its communication requirements are similar to those of OCS, and include BER in the order of 10\({}^{-5}\) and transmission delay below 40 ms.
4. _The image transmission for video surveillance:_ Image processing allows ensuring safety and security of railway systems. When an incident related to security occurs, the monitoring system supports the operator in making the good decision and provide all required information. In the HSR systems, the drivers can only obtain the images of the cabin through the cameras disposed inside the train. Note that vactrain operate at the mode of fully automatic driving, thus the traditional drivers are replaced by the Rail Traffic Control Centers (RTCC) to perform train dispatch, velocity control, and other operations. In this situation, the images of the train's front, interior area, and that along the vacuum tube line also need to be transmitted to RTCC dispatchers in real-time. Especially in the case of malfunctions, the train needs to perform special operations under the control of RTCC, such as running in peristaltic mode and entering the evasion line. For these specific operations, the assistance of video surveillance system is indispensable. In addition, the number of CCTV cameras inside the cabin should also increase to improve the emergency disposal efficiency. Thus, when emergencies detection devices are activated, such as passenger emergency phones and smoke detection device, the images of accident scene could be transmitted to RTCC in time. Video surveillance services capture live 720p or 1080p true HD video images from High-Definition Television (HDTV) IP cameras and High-Definition Serial Digital Interface (HD-SDI) cameras located on the train's body (for train operation) and the interior of the pod (for passengers condition monitoring). Typically, there is only one capsule/pod for vactrain, whose length is \(\sim\)30 m [2]. The surveillance of train needs six cameras in total, including 2 HDTV cameras inside the train and four HD-SDI cameras outside the pod. Considering the transmission of the flow of six cameras, with 25 images per second, and a rate of compression of 1:50 (using H.264 compression algorithm for instance), the uplink throughput could be evaluated at \(\sim\)15 Mbps. As for the video applications that involve security services (emergencies scenes and crime evidences), less capacity could be needed, but some QoS criteria are required, such as delay less than 300 ms, BER of about 10\({}^{-3}\), cohabitation with other wireless systems without interference, etc. 5. _Train monitoring systems:_ The constant increase of trains' speeds, together with the vacuum scenarios, make the knowledge of the state of the train much important. Typically, Wireless Sensor Networks (WSNs) can be adopted to monitor the railway infrastructure and tube environment. WSNs allow the rise of large-scale information for everything related to the vactrain, the vacuum tube systems, and the infrastructure or the equipment diagnosis.