# Modeling and Spatial Diversity-Based Receiving Improvement of In-Flight UAV FSO Communication Links
## 3 Performance Analysis of the In-Flight UAV FSO Communication Link
### Transmission Analysis of the FSO Communication for UAV in Straight Flight  
In this section, the simulation of the FSO communication process when the receiving-end UAV is in the state of straight flight is carried out. The influence of the fuselage tilt angle of the UAV in horizontal and straight flights on the communication quality is analyzed, considering the angle pointing deviation and position deviation caused by the UAV jitter. In the simulation, setting \(\theta_{FOV}\) = 40 mrad, \(\sigma_{pp}^{2}=\sigma_{pp}^{2}=\)1 cm, \(\sigma_{ro}^{2}=\sigma_{lo}^{2}=\)1 mrad, and select the tilt angles of the fuselage as 0, 5 mrad, 10 mrad, and 20 mrad, respectively.  
Figure 4 shows the distribution of the optical communication channel coefficient under different fuselage tilt angles when the UAV flies in a straight line on the same horizontal plane. As the tilt angle of the fuselage increases, the value of the channel coefficient \(h\) gradually becomes smaller. Therefore, the FSO communication quality between UAVs has also deteriorated.  
As shown in Figure 5, when the UAV flies in a straight line on the same horizontal plane, the fuselage tilt will greatly influence the communication quality. As shown in Figure 5a, the outage probability of the UAV optical communication link increases as the  
Figure 4: Normalized distribution of the channel coefficient at different tilt angles when the UAV flies straight at the same altitude.  
tilt angle increases. When \(\varphi=0\), that is the states of UAV are similar to that with hovering, the outage probability is close to \(10^{-4}\) when the SNR is 20 dB. However, increasing the tilt angle of the UAV to 20 mrad, the outage probability becomes greater than \(10^{-2}\) under the same SNR. In order to make the outage probability reach \(10^{-3}\), the SNR should be increased from 16 dB to more than 20 dB when the tilt angle of the UAVs increases from 0 to 10 mrad.  
As shown in Figure 5b, the BER will increase as the tilt angle of the UAV increases. For example, when the SNR equals 20 dB, the BER will increase from 2 \(\times\)\(10^{-6}\) to \(10^{-2}\) when the tilt angle of the UAV increases from 0 to 20 mrad. To make the average BER reach \(10^{-5}\), the SNR should be raised from 13 dB to 20 dB when the tilt angle of the UAVs increases from 0 to 5 mrad. When the tilt angle is limited (less than 10 mrad), the UAV can achieve reliable communication just by increasing the SNR. However, when the tilt angle continues to increase, the communication quality will be seriously disturbed. When the tilt angle is greater than 10 mrad, the average BER is still higher than \(10^{-4}\) even if the SNR reaches 20 dB. When the tilt angle is too large, only a small part of the light can reach the detection area after passing through the lens, and the caused communication interruption and symbol loss seriously limit the upper limit of the performance of the receiving system. Therefore, even if the SNR increases, the BER does not decrease significantly. As a result, the optical communication link between UAVs cannot be established at this time.