# Free Space Optics Communication for Ultra-High-Speed Train Running in Evacuated Tube
## 3 Handover Mechanism  
First of all, in the system's initial state, there are two controllers in the proposed system (method) to do the handover mechanism to keep the train connected to the internet. The first controller manages the networking of the train's track, such as the communication of the BSs, sensors, and the backbone network [5; 27]. The second controller is located inside the train and used for managing the networking and the communication inside the train vehicles. Furthermore, all base stations are in standby mode for power consumption. Therefore, they stand fully closed (the barrier that carries the transceiver is 0 degrees with the horizontal and 90 degrees with the train receiver), indicating that the bars (the BS's holder) are still not moving to wait for the signal, see Figure 3. In addition, sensors are placed along the track to assess train position for network purposes and other objectives. Figure 3: The initial state of the handover mechanism. Moreover, each transceiver communicates with the outside network by considering that the transceiver of each BS might be connected to a fiber-optic backbone, where a wavelength between 1530 and 1565 nm (i.e., C-band) is usually employed [13; 14]. For an ultra-speed train traveling at 1000 km/h, the distance between the barrier (BS) and the closest point (C) that the BS can cover and communicate with is 0.55 km (the train's speed is 1000 km/h which means 277 m/s, and the barrier needs 2 s to close, the distance reached by the train will be 277 m + 277 m = 0.55 km), and the bar may be entirely closed without being disturbed by the train; the barrier requires 2 s to fully close [28; 29], and if the train is traveling at 2700 km/h, the nearest point will be 1.5 km (the train's speed is 2700 km/h which means 750 m/s, and the barrier needs 2 s to close, the distance reached by the train will be 750 m + 750 m = 1.5 km). Moreover, these distance measurements impact the divergence angle that should be employed, transmitted power, and the data rate and BS coverage range. BS1 maintains communication with the front train until the train reaches the C point, at which point the controller sends a control signal to BS2 to open the barrier fully. Once the bar (BS's holder) opens communication with the transceiver at the back of the train, when controller 2 receives a signal from the transceiver at the back of the train, it sends a control signal to the front transceiver. However, at a distance of 0.55 km or 1.5 km (depending on train speed), this space allows barrier 1 to open and connect with the train's transceiver fully, see Figure 4. Figure 5 provides an overview of the handover process. Figure 4: BS2 handover mechanism. Figure 5: Overview of the handover mechanism. When BS2 does not receive any laser beam from the front train transceiver, the barrier of BS2 does close and turn to standby mode waiting for the next train coming. These procedures continue for the next BSs all along the track until the train reaches the destination. Moreover, Figure 6 shows a flowchart of the process of the system to do the handover. For a train speed of 1000 km/h, the coverage distance of BS1 will be as follows: uncovered area BS1C is 550 m, this is the nearest point to the BS which is (C), when the train front transceiver is at that point; the same distance will not be covered by BS2 when BS1 needs to be closed or at the nearest point (C, 2 s closing procedure). In addition, the train length should be considered as it affects the final length, which is assumed to be a 100 m train. Therefore, uncovered area will be \(|\)BS1C\(|\) + \(|\)BS2C\(|\) + train length, and equals 1200 m. This distance should be subtracted from the total area between BS1 and the start point \(|\)BS1T\(|\), which is 14.2 km, so, the BS for speed 1000 km/h actually covers a distance of 13 km.