## 7 Conclusions  
The Hyperloop adopting the magnetic levitation and vacuum tube technologies can achieve an ultra-high-velocity of over 1000 km/h, which brings great challenges to the existing communication systems. In this paper, we propose a train-to-ground wireless communication system solution based on the prevailing 5G system, involving wireless channel, network architecture, resource management. Our contributions together with future works can be summarized as follows:  
1. The feasibilities of two wireless access methods, i.e., the DAS and LWS, are analyzed. Specifically, the Doppler power spectrums for the DAS at different positions are characterized. Then, we analyze the radio coverage from an aspect of the handover. In future, the accurate radio propagation characterization of the LWS in the near field will be investigated, especially, the Doppler effect of the LWS should be analyzed in detail.
2. As for the network structure, C-RAN is utilized to integrate several nearby AAUs into a logical cell, achieving a free-handover effect inside this cell. To deal with the inevitable group handover when traveling across different macro cells, a novel access network structure is investigated to reduce the resource migration cost. However, such proposal can alleviate the cost evidently in the mesh Hyperloop lines, but exerts little impacts on the single sparse Hyperloop line, which will be solved in future.
3. In terms of the coexistence of eMBB and uRLLC traffic, we propose a novel PRB multiplexing scheme considering the latency margin of mission-critical services, which aims to maximize the network throughput subject to the stringent requirements of different types of uRLLC traffic. Though we proposed a solution based on the PSO algorithm, an optimization solution with a closed-form expression will be much helpful especially in terms of the low-latency traffic.
4. To enhance the QoE of passengers' Internet access, a cache-based mechanism of \"staggering the peak\" of data transmission (including pre-fetching and post-uploading schemes) is proposed to boost the transmission performance. In the simulation, we only consider the coordination of two  
Figure 12: The diagram of the post-uploading scheme.  
adjacent AAU. However, it can be inferred that the joint of more AAUs will definitely yield a better throughput performance, which will be investigated in the future.  
In summary, we make our endeavors to investigate on the train-to-ground communication system for Hyperloop from the aspect of the whole system architecture. We wish our works could enlighten the future research of the Hyperloop.  
Conceptualization, J.Z., L.L., T.Z., and B.A.; investigation, J.Z., Z.L., and D.W.; methodology, J.Z., B.H., Z.L., K.W., and B.A.; project administration, L.L.; supervision, L.L.; writing--original draft, J.Z. and L.L.; and writing--review and editing, J.Z. and L.L. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Fundamental Research Funds for the Central Universities under grant 2018J BZ102, and the Beijing Nova Programme Interdisciplinary Cooperation Project (Z19110001119016).  
The authors thank the anonymous reviewers for their constructive comments which are much helpful for enhancing the quality and presentation of this paper.  
The authors declare no conflict of interest.