## 7 Conclusions and Future Work  
The vacuum tube high-speed flight train, whose speed exceeds 1000 km/h, is an novel rail transportation technology. Therefore, a reliable bidirectional communication link between the train and the ground plays a key role in guaranteeing the safe operation of this system. Currently, there is almost no research on train-to-ground communication technology of vactrain. Against this background, this paper summarized several key challenges of train-to-ground wireless communication for the vactrain, and explored the feasibility of current wireless communication technologies for speed over 1000 km/h. In addition, the bandwidth and QoS requirements of vactrain's train-to-ground communication service are analyzed quantitatively. In terms of wireless access inside the tube, we proposed a leaky waveguide with simple architecture but high performance. The simulation of the leaky waveguide is conducted, and the results show the uniform phase distribution along the horizontal direction of the tube, but also the smooth field distribution at the point far away from the leaky waveguide, which can suppress Doppler frequency shift, indicating that the time-varying frequency-selective fading channel could be approximated as a stationary channel. As the wireless channel can be approximated as a stationary channel, an electromagnetic lens system is proposed to achieve direct wireless coverage for train. Moreover, moving cell scheme based on the C-RAN architecture is adopted to deal with the extremely frequent handoff.  
In future work, more research is need to test and simulate the communication performance of the architecture based on leaky waveguide at system level. In the meantime, further studies should investigate the multi-band leaky waveguide and the propagation characteristics of leaky wave inside the full-sealed metal tube. In addition, further research is needed on the near-field convergence technology, of which the basic idea is to form a couple of leaky waveguides into a certain array, and thus converge the radiated leaky wave on the lens through beam-forming. As for the moving cell scheme, the results comparing train speed vs number of switches per second would be considered for a better perspective about the importance of moving cell. In addition, the simulation of moving cell based on C-RAN would be conducted to test the handover performance.  
Conceptualization, C.Q. and L.L.; methodology, L.L.; software, Z.L.; validation, J.Z., C.Q. and T.Z.; formal analysis, C.Q.; investigation, L.L.; resources, B.H.; data curation, T.Z.; writing--Original draft preparation, C.Q.; writing--Review and editing, L.L.; visualization, C.Q. and Z.L.; supervision, L.L.; project administration, L.L.; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript.  
The research was supported by the Fundamental Research Funds for the Central Universities under grant 2018 JBZ102, Beijing Nova Programme Interdisciplinary Cooperation Project(Z191100001119016), and the Center of National Railway Intelligent Transportation System Engineering and Technology (Contract No. RITS2019 KF01), China Academy of Railway Sciences.  
The authors thank the anonymous reviewers for their constructive comments that helped us improve the quality and presentation of this paper.  
The authors declare no conflicts of interest.