## 1 Introduction  
From the development of China's high-speed railway (HSR), it can be learned that high velocity brings about a number of benefits and merits such as shortening the travel time and helping establish socioeconomically balanced societies [1; 2]. However, three inevitable factors primarily restrict the further acceleration, i.e., the mechanical resistance as well as the noise stemmed from the wheels and tracks, and the air resistance derived from air friction. According to reference [3], the actual measurement data demonstrates that the aerodynamic drag accounts for a majority of the resistance (over 80%) as the train proceeds at a velocity of 400 km/h, leading to a great waste of power energy obviously. Moreover, the generated mechanical noise will rise abruptly along with the acceleration (about seventh-order or eighth-order of speed) and become unbearable for passengers. Given above constraints, the maximum velocity of current HSR cannot exceed 600 km/h. Hyperloop is a novel designed transportation method, which refers to a magley train (vactrain) moving forward at near or supersonic speed (1000-4000 km/h) inside a vacuum metal tube [4]. The closed transportation scenario together with magley technology can yield a new way of transportation with the merits of low mechanical friction, low air resistance, and low noise mode regardless of all weather conditions, not to mention the great conveniences to customers and quality promotion of rail services due to the ultra-high-velocity. As such, maintaining a dependable and reliable communication link is a pivotal issue to guarantee the safe operation of the Hyperloop and provide real-time multimedia information. Rekindled by a white paper entitled \"Hyperloop Alpha\" published by [PERSON] in 2013 [5], the Hyperloop receives extensive attention both from academia and industry. In 2016, an American vactrain company, i.e., Hyperloop One, published the first study of the Hyperloop system, referring to a 28-min link connecting Helsinki and Stockholm [6]. Then, it first tested a full-scale Hyperloop with power propulsion system, braking system, etc., by using a test line in the northern desert of Las Vegas in 2017 [7]. In 2018, Hyperloop Transportation Technologies (HTT) company declared plans to establish Hyperloop lines in Toulouse, France, and Tongren, Guizhou, China [8]. In 2019, China underlined a series of key tasks for building national strength in transportation in a published official document, among which points out that efforts will also be made to develop the technologies of the low vacuum tube (tunnel) high-speed train [9]. Recently, TUV SUD published the first completed certification safety guidelines for Hyperloop applications which define the safety-related requirements relevant to this type of technology in 2020 [10]. The guidelines, which detail over 125 system requirements, are like a milestone on the road to maturity and commercialization of Hyperloop. As a new-born mode of transportation, the research on train-to-ground communication technology of the Hyperloop almost stays at its infancy stage currently. The primary communication challenge is the fast-fading channel along with the frequent handover effects caused by the ultra-high-speed. However, existing railway-dedicated communication including the widespread commercial used Global System for Mobile Communications-Railway (GSM-R) and Long Term Evolution-Railway (LTE-R) can only support a top speed of 500 km/h [11], which is far below the velocity of the Hyperloop. Currently, some military communication systems such as the common data link (CDL) can support a mobile station (MS) with a speed more than Mach 5 (over 6000 km/h) when providing a data rate of 200 kbps for uplink and 234 Mbps for downlink [12]. However, it is economically expensive and nation-security-endangered to deploy such a military communication system.