## 2 Key Challenges of Vactrain's Wireless Communications  
The distinctive scenario of enclosed metal tube and ultra-high terminals' speed impose numerous challenges on the modeling, design, analysis, and evaluations of the train-to-ground wireless communication systems of vactrain [20]. In the meantime, high mobility and the fully enclosed vacuum tube wall also provide unique opportunities that can be exploited to facilitate system designs and to improve system performance. The key challenges faced by the design of vactrain's wireless communication, as well as some opportunities that can benefit system designs, are listed as follows. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
\begin{tabular}{c} \textless{}500 km/h \\ \end{tabular} & Emerging \\ \hline
28G mmW & 2000 Mbps & \
\begin{tabular}{c} 27 GHz \\ \end{tabular} &
\begin{tabular}{c} \textless{}400 km/h \\ \end{tabular} & Emerging \\ \hline
38G mmW (Maglev) & \begin{tabular}{c} 8 Mbps/4 Mbps \\ \end{tabular} & \begin{tabular}{c} 37.1\(\sim\)38.5 GHz \\ \end{tabular} & \begin{tabular}{c} \textless{}500 km/h \\ \end{tabular} & \
\begin{tabular}{c} \textless{}400 km/h \\ \end{tabular} &
\begin{tabular}{c} \textless{}400 km/h \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Applied wireless technologies in railway systems. 1. Propagation characteristics in the tube: For the design of wireless communications network, the knowledge of the wireless channel, which has a close relationship with propagation scenarios, is the fundamental basis. Different from traditional tunnel scenarios [21; 22; 23; 24], which mainly consists of stone and clay, the vactrains are operating inside the fully enclosed metal tube wall, resulting in the unique propagation environment for wireless signal. From the aspect of waveguide, the concept of \"mode\" can be used to analyze the propagation characterization of radio waves in the tube [25; 26]. Based on this theory, the high-order mode gradually decays and disappears as the distance between transmitter and receiver increases, only leaving the low-order mode. Due to this reduction of modes, the signal subspace of the Multiple-Input, Multiple-Output (MIMO) channel matrix is decayed, resulting in a decrease in the capacity of the MIMO channel, that is, the keyhole effect [27; 28; 29]. In addition, if the leaky waveguide is adopted for wireless coverage, the leaky wave propagation coupling characteristics will be completely different. Due to the fully enclosed metal wall, the near-field radiation characteristics and the spatial distribution of the radiation power density will be greatly affected. Besides, the cross sectional shape and size of the metal tube will result in some unique characteristics of the mode distribution. 2. Fast and frequent handovers: Handover, or handoff, is the process by which a Mobile Station (MS) maintains its connection active while moving from one cell to another. Due to the ultra-high velocity, the handoff frequency of vactrain will be much higher than the current HSR under the same configuration of network. It is important to remark that the number of handovers on the systems must be minimized. However, the handover times of current railway communication system are in the order of 100 ms to 1 s, which are absolutely unacceptable for vactrain. For instance, a cell coverage radius of 300 m combined with a vactrain speed of 1000 km/h means a handover every 2.25 s, and in combination with a handover time of 1 s, the communication performance loss could reach up to 0.44. Same as HSR systems, the group handover is another problem when dozens of in-train MSs trying to handover at the same time, causing the so-called signaling storm, which greatly increases the handover load of network. Thus, the new handover strategies and algorithms need to be improved to meet the requirements of handovers in vactrain. 3. Serious Doppler Frequency shift: When the velocity of the train reaches 1000 km/h, the channel impulse response will change rapidly, experiencing fast fading characteristics [30].