# Free Space Optics Communication for Ultra-High-Speed Train Running in Evacuated Tube
## 1 Introduction  
Due to the general advancements in modern economy and technology, transportation has expanded dramatically since the 1950s, leading to significant demand for ultra-high-speed trains for ground transportation, with speeds exceeding 1000 km/h [1]. However, the researchers demonstrate that when the train speed approaches 400 km/h, the resistance will be more than 80% of the overall resistance. Furthermore, in a vehicle system, aerodynamic noise is proportional to one-eighth of the power speed [2]. As a result, the higher the speed, the worse the noise pollution. Furthermore, according to the European Environmental Agency, the transport sector produced 27%of total European greenhouse gas emissions in 2016. Consequently, a new magnetically levitated ultrafast train operating along a low-pressure tube is required to reduce CO\({}_{2}\) emissions [3]. Thus, many countries have researched evacuated tube transportation (ETT), such as China and the United States [1]. Since the demand for high-speed internet connectivity is becoming increasingly important in our daily lives, providing Internet access in EFTs is one of the key incentives for railway operators to attract more passengers. However, radiofrequency wireless technologies are utilized to provide Internet access to travelers. Existing infrastructure based on radio frequency technology, such as Wi-Fi/WiMAX, can theoretically provide peak data rates of 54/75 Mbps, but in practice, data rates drop to less than 10 Mbps [4; 5]; furthermore, the authors of [6] optimized a cellular access to the universal mobile telecommunications system in long-term evolution for 4G networks to cover the railways by the system cells; in [7], based on Saleh-Valenzuela and WINNER II model for 5G technology, they proposed a novel non-stationary mmWave MIMO theoretical model for high-speed train communication; also, to provide a high data rate for the train in 5G+/6G networks, in [8] the authorused unmanned aerial vehicles (UAVs)/drones for providing high data rates to mobile relays placed on top of high-speed train vehicles. However, the ultra-high-speed train based on ETT poses several communications issues. Initially, there are ultra-fast handovers and increasing latency. In addition, the ultra-high speed will cause higher Doppler spread which will cause interchannel interference for the high mobility devices [9]. Other previous works have demonstrated high speed by using Free Space Optics (FSO) rather than RF [10]. The train's transceivers must cooperate, and there is no redundant standby equipment to support reliable communication. In the FSO scenario, the operator has to erect several base stations (BSs) to maintain line of sight contact between the train and the ground and provide seamless coverage. In general, the large number of BSs incurs high costs and, in rural areas, raises operating expenses, making it a crucial concern for railway operators [4]. Therefore, it is an important target for the researchers to minimize the number of BSs. Furthermore, traveling away from a BS's coverage area and into the coverage area of the next BS initiates a handover process, which may result in communications interruption and a considerable handover time, particularly with the ultra-high-speed train. The time connectivity for each BS will be 0.72 s, or in some cases, 0.26 s if the train is traveling at 2700 km/h [1]. The train must undergo various handovers, influencing system performance (data rate). Furthermore, the large divergence angle covering a specified track distance reduces the data rate. So, addressing the communication issue at ultra-high speed remains a significant challenge. The authors of [1] proposed a strategy for employing FSO in ETT; the proposed model contains an optical access point fixed at the ceiling of the evacuated tube every 200 m, it works as a transceiver for optical signals communication; furthermore, two optical transceivers are installed on the train, which are located on both the front and back of the train.