## 1 Introduction
The 38 GHz MMW system could provide a capacity reach up to 100 Mbps, but also ensure the delay less than 5 ms and the Bit Error Rate (BER) of safe-related data transmission in the order of \(10^{-5}\)[14]. The mmW communication system is also a candidate for the system offering high-speed communication services in HSTs. [PERSON] and [PERSON] [15] proposed the DAS-based 28 GHz millimeter-wave mobile communication system for HSTs, and analyzed the potential of the system through the computer simulations, showing that the enormous throughput exceeding 2 Gbps can be achieved in the HST moving at a speed of 400 km/h. Table 1 summarizes an overall performance comparison of the mentioned wireless technologies in high-speed scenario. However, the maximum supporting speed of these technologies does not exceed 500 km/h, whereas the vactrain operates at a speed exceeding 1000 km/h. In terms of 5G system, Section 6.1.5 of the standardized manuscript 3 GPP TR 38.913 [16] clearly indicates that 5G NR supports a maximum speed of 500 km/h. Thus, the current railways communication system based on LTE cannot support the train-to-ground communication for vactrain. Same as the current HSR [17] and maglev train [7], the train-to-ground wireless communication is crucial for the safe operation of vactrain. The communication system is responsible for providing bidirectional and reliable data transmission channels between the train and ground to meet the specific needs of integrated services transmission [18]. However, due to the two unique features of vactrain--closed metal vacuum tube and ultra-high velocity--the train-to-ground wireless communication for vactrains will be more challenging than that of the current HSR [19] and maglev train. Therefore, a wireless communication system of high reliability and high performance dedicated to scenario of speed exceeding 1000 km/h needs to be investigated, catering for the strong train-to-ground communication demand of vactrain. The goal of this article is to study wireless communication systems that aim to make wireless access available between the vactrain and ground. The main contributions are listed as follows. (1) We summarized a list of the unique challenges and opportunities associated with the wireless communication for vactrain. (2) The bandwidth and QoS requirements of vactrain's train-to-ground communication service are quantitatively analyzed and presented. (3) A leaky waveguide is proposed to suppress the severe Doppler frequency shift in high-speed scenario. (4) An electromagnetic lens system is proposed to achieve direct wireless coverage for user equipments inside the train. (5) The moving cell scheme based on the C-RAN architecture is adopted to deal with the issue of extremely frequent handoff. The structure of this article is as follows. We first reviews wireless communication technologies researched or deployed in the railway systems, then analyze their feasibility for vactrain's train-to-ground communication. The rest of this paper is laid out as follows. Section 2 summarizes several key challenges of train-to-ground wireless communication for the vactrain. In Section 3, the bandwidth and QoS requirements of vactrain's train-to-ground communication service are analyzed quantitatively. In Section 4, a leaky waveguide with simple architecture is proposed for the wireless access inside the tube, of which the simulation results show that the severe Doppler frequency shift is restrained largely. Section 5 introduces some solutions to establish wireless link between the train user equipments and leaky waveguide. In Section 6, the moving cell scheme based on the C-RAN architecture is adopted to deal with the issue of extremely frequent handoff. Finally, in Section 7, we provide concluding remarks.