## 1 Introduction  
The rise of High Speed Railway (HSR) around the world enables a faster, safer, and cheaper transportation system that caters for the demand of numerous passengers. However, the further development of HSR is mainly constrained by the three factors: resistance between wheel and rail, resistance of the air, and aerodynamic noise. The measured data reveals that when the velocity of the train reaches up to 400 km/h, the resistance of the air will account for more than 80% of the train traction, causing a large waste of energy. As a consequence of the above three constraints, the critical velocity of HSR is 600 km/h [1]. With the rapid development of HSR, the next generation of ultra-high-speed transportation technology--vactrain, has gradually come to people's attention. The vactrain is a magnetically levitated or magnetically powered train system, which operates inside the evacuated or partly evacuated (air-less) tubes. As mentioned above, aerodynamic drag is the dominant factor hindering speedup of HSRTherefore, the reduced air resistance could permit vactrain to travel at very high speeds with relatively little power up to 1000-4000 km/h, or 5-6 times the speed of sound (Mach 1) at standard conditions. The concept of vacuum tube transportation was first proposed by [PERSON], the father of modern rockets, in 1904 [1]. In 2013, [PERSON], the CEO of Tesla and SpaceX, published a white paper called \"Hyperloop Alpha\", proposing the hyperloop technology based on the vactrain concept [2]. At present, significant research efforts are underway to promote the practical application of vacuum tube transportation in China and the United States. On 12 May 2016, Hyperloop One, an American vacuum tube transportation company, first comprehensively tested the propulsion system of hyperloop [3]. In 2018, another US vacuum tube rail developer, Hyperloop Transportation Technologies (HTT), announced plans to build hyperloop lines in Toulouse, France and Tongren, Guizhou, China [4; 5]. In 2017, China Aerospace Science and Industry Corporation (CASIC), the country's state-run space contractor, announced that they has initiated research on project of vacuum tube high-speed flying train [6]. At present, almost no research is available on train-to-ground communication for vactrain according to the published results. Thus, the wireless communication technologies that have been studied or applied in the railway systems will be first reviewed. The current wireless communication technologies applied to railway systems mainly include GSM for Railway (GSM-R), LTE-R (LTE for railways), Wireless Local Area Networks (WLANs), LTE for Metros (LTE-M), 38G mm wave technology (Shanghai maglev [7]), etc. GSM-R aims at the characteristics of train dispatching, train control, and high-speed operation in railway communication [8]. Despite the overwhelming maturity and popularity, GSM-R is on the way out due to the hindrance of increasing interference from public networks and increasing bandwidth requirements. In the future, LTE-R, which is based on the LTE standard, is a likely candidate for GSM-R [9]. The world's first rail service to support LTE is China's heavy haul railway from Shenchi to Huanghua [10]. For urban transit rail systems, IEEE 802.11a/b/g/n/ac based on WLANs is currently a prevalent choice due to the available commercial-off-the-shelf equipments [11]. However, WLANs are not specifically designed for high mobility scenarios, and there are some problems for WLAN-based Communications-Based Train Control (CBTC) systems, such as handoff and Quality of Service (QoS) issues [12]. Recently, LTE-M, based on the TD-LTE standard, has been proposed as the communication method in urban transit rail systems. In China, LTE has been adopted as the communication method in the passenger information system of Zhengzhou metro Line 1, which operates in the 1795 to 1805 MHz frequency band [13]. The 38 GHz MilliMeter-Wave (MMW) wireless communication technology is the applied in Shanghai maglev system [7].