# Free Space Optics Communication for Ultra-High-Speed Train Running in Evacuated Tube
## 4 Results and Discussion
### Simulation Results
#### 4.1.2 The Received Power  
The received power is undoubtedly affected by the transmitted power, impacting the data rate. Furthermore, according to Equations (4) and (5), the received power directly influences the required transmitted power; therefore, a fixed received power ensures the desired data rate, such as 1.25 Gbps for example. The received power will be \(-\)36 dBm and can employ a variety of transmitted power, which will affect the power consumption of each base station and transceiver on the train and the overall system power consumption. Furthermore, fluctuations in transmitted power with fixed needed received power (leading to a constant data rate) can vary the system design by employing an adaptive system based on the distance between the transmitter and the receiver. Figure 9 represents three places on the train track: 550 m is the last point covered by the BS for train speeds of 1000 km/h, 1500 m is the end covered position by the BS for train speeds of 2700 km/h, and 14,200 m is the first and farthest point from the BS. In addition, \begin{table}
\begin{tabular}{c c} \hline
**Distance (m)** & **Divergence Beam Diameter (m)** \\ \hline
550 & 0.0504 \\
1400 & 0.101 \\
1500 & 0.11 \\
14,200 & 1 \\ \hline \end{tabular}
\end{table} 
Table 2: Variable distance and beam divergence for fixed divergence angle 6.944 \(\times\) 10\({}^{-5}\). Figure 8: Gaussian beam diameter versus distance for a fixed divergence angle. \begin{table}
\begin{tabular}{c c} \hline
**Full Divergence Angle** & **Divergence Beam Diameter (m)** \\ \hline
6.944 \(\times\) 10\({}^{-5}\) & 0.047 \\
2.0944 \(\times\) 10\({}^{-4}\) & 0.11 \\
2.4944 \(\times\) 10\({}^{-4}\) & 0.13 \\ \hline \end{tabular}
\end{table} 
Table 3: Variable divergence angles and beam diameter for a fixed distance of 550 m.  
the figure depicts several divergence angles for each position to compare the influence of the divergence angle on the transmitted power for the data rate of 1.25 Gbps, the whole angle ranging from 6.944 \(\times\) 10\({}^{-5}\) to 0.0523 rad [30]. The divergence angle affects the transmitted power, which is less in narrow angles compared to wide divergence angles. Furthermore, the same angle varies with distance, and it is clear from the figure that the close points to the BS require less power than the far points to the BS, as expected, in order to achieve the target data rate. Figure 10 displays two divergence angles employed in the proposed system (6.944 \(\times\) 10\({}^{-5}\) and 2.0944 \(\times\) 10\({}^{-4}\) rad), with distance changes and effects on transmitted power (dBm). Furthermore, Table 4 reveals that at 550 m, the transmitted power is 1.7 \(\times\) 10\({}^{-9}\) W, while at 14,200 m, the transmitted power is 1.2 \(\times\) 10\({}^{-4}\) W for an angle of 6.944 \(\times\) 10\({}^{-5}\), yielding a fixed received power of \(-\)36 dBm and a data rate of 1.25 Gbps. According to the authors' investigations, the system that was used in ultra-high-speed trains in ETT [1] did not present results for the transmitted or received power. Hence, we found results for the transmitted power that was used in the FSO systems for a train to ground BSs only in high-speed trains; therefore, this paragraph describes and compares the paper results with works that used FSO in the high-speed train scenario, and that used  
Figure 10: Distance versus transmitted power required for fixed data rate 1.25 Gbps. Figure 9: Transmit power versus divergence angle for several fixed distances: (a) distance of 550 m, (b) distance of 1500 m, (c) distance of 14,200 m.  
transmitted power equal to 27 mW [10; 26]. Thus, Figure 11 shows the received power (8) for a fixed transmit power of 27 mW. The distance close to the BS has the highest received power, as expected, when the transmit power is fixed. The value decreases when the distance increases, but remains more than the \(-36\) dBm level (Table 1) needed to acquire 1.25 Gbps; so the results are good enough to maintain a high data rate for the passenger and make it possible to run live stream programs.