# Free Space Optics Communication for Ultra-High-Speed Train Running in Evacuated Tube
## 2 System Model  
The proposed scheme that is used in this paper is illustrated in Figure 1. In this paper, the laser beam propagation model follows a Gaussian distribution [11]. A train vehicle has an FSO transceiver installed on the roof in our model, and each BS on the ground has an FSO transceiver. The model contains three stages constituting the FSO communications system: a transmitter to emit optical signals, a free-space transmission channel, and a receiver to receive the signals. To enable full-duplex FSO communications, each party (i.e., the transmitter and receiver) is often outfitted with a transceiver that serves as both a transmitter and a receiver simultaneously, following the same principle that is used in [3,4,8]. Laser diodes operating at wavelengths between 780 and 1600 nm are usually preferred as the light sources for this application because they may attain high data rates over long distances. For instance, FSO communication systems are expected to provide data rates in the range of Gigabits per second for high-speed trains [6]. In this study, we will concentrate on ground-to-train communications for simplicity. Furthermore, because the transmitter and receiver of a transceiver are mutually aligned, establishing a ground-to-train communications link also guarantees a train-to-ground link [12]. As a result, our research applies to both of them. In addition, according to the authors' investigations, the speeds for evacuated tube transportation exceed 1000 km/h [1]. Therefore, two different speeds, 1000 km/h and 2700 km/h, were considered as realistic examples addressed in the paper. The proposed technique is depicted in Figure 1. The base station is protected by a barrier on the tube's ceiling and makes a 90-degree angle with the horizontal at the maximum openness to send and receive data, while being perfectly aligned with the train's transceiver. When the train is too far away from the base station, it will fully close to make a 0-degree angle with the horizontal and enter standby mode to save power. This is the main advantage compared to other FSO techniques: The moving barrier allows perfect alignment with the transceiver on board the train, thus leading to higher data rates and longer contact time with the BS, while avoiding collision when the train reaches the BS (as it folds away while the train is at a safe distance away, after handover to another BS). This study assumes that the train's transceiver and the BSs along the tube utilize a wavelength of 1550 nm. The 1550-nm wavelength was chosen for its availability, reliability, high-performance capabilities, and reduced cost of the transmitter and detector, as well as its suitability for eye safety [13]. In addition, the paper also considers that each BS's transceiver may be connected to a fiber-optic backbone with a wavelength of 1530 to 1565 nm (i.e., C-band) [13,14]. Although this wavelength choice allows smooth interfacing with fiber optic networks, it should be noted that different wavelengths could be used at the FSO and fiber links, with appropriate conversion carried out at the BS to relay the data over fiber using the needed wavelength. The top view of the geometrical model of the ground-to-train FSO communications system is shown in Figure 2. In this diagram, we suppose that the train follows a track. BST is the farthest distance to the BS (BS and the train front transceiver); it is 14.2 km, considering the earth's curvature [15, 16, 17]. The vertical distance between the BS and the ground is set to 4 m (Figure 1), and it specifies the location of the shortest coverage point (C) of the beam on the track, which is 200 m away. Finally, \(\Theta\) is the laser beam's divergence angle. This angle impacts the beam radius \(w\) and the track coverage length, calculated in (1) and (2). Figure 1: The proposed method.