# Post-Flood UAV-Based Free Space Optics Recovery Communications with Spatial Mode Diversity
## 5 Results and Discussion  
To evaluate the effectiveness of spatial mode diversity using linearly polarized modes for maintaining the signal quality of a UAV-based free space optical system, the time-varying SINR of the primary, backup, and composite channels were measured under various weather conditions: (i) light rain, (ii) medium rain, (iii) heavy rain, (iv) clear day. The rain level was classified as 'light','medium', and 'heavy' for precipitation rates of accumulation of about 0 to 2 mm per h, 2 to 10 mm per h, and 10 to 50 mm per h, respectively [63]. For each weather condition, the minimum and maximum \(C_{n}^{2}\) values were computed to determine the range of turbulence strengths. Although the signals were received at 20 Gbps, the acquisition and evaluation of the channel matrix and interfering modes at high data rates were computationally intensive and required time. Thus, the SINR was sampled at 1 Gbps. In addition, the hovering time of the UAV was 45 min, so a 30 min interval was selected to give the UAV sufficient time to return to the ground station. For each case, the time-varying SINR is plotted for the primary channel, backup channel, and the composite channel. The composite channel comprises various temporal segments of data from the primary channel and the backup channel such that when the SINR is equal to or more than the threshold value of 25 dB, the received data from the primary channel are forwarded to the composite channel. On the other hand, when the SINR on the primary channel drops below 25 dB, the received data from backup channel are forwarded to the composite channel instead. Figure 2 shows typical plots of the SINR versus time under light rain. The average SINR was 28.5 dB, with a standard deviation of 5.6 dB. Channel 1 was the primary channel (first graph) and Channel 2 was the backup channel (second graph), utilized when the signal in Channel 1 declined below 25 dB. The third graph in Figure 2 shows that the utilization of Channel 2 as a backup channel successfully prevented the SINR from dropping below 25 dB, especially between 1157 s and 1172 s.  
Figure 3 shows a typical plot of the SINR versus time under medium rain. The average SINR was 27.5 dB, with a standard deviation of 6.0 dB, underlining deeper variations in the SINR as compared to those under light rain. Channel 3 was the primary channel (first graph) and Channel 4 was the backup channel (second graph), utilized when the SINR in Channel 3 decreased below 25 dB. The third graph in Figure 3 illustrates that employing Channel 4 as a backup channel successfully prevented the composite signal from depreciating below an SINR of 25 dB, preventing link failures to the cloud, especially between 670 s and 701 s, and 1352 s and 1396 s.  
Figure 2: SINR versus time under light rain. Green line shows \(SINR\gamma\) value of 25 dB. Figure 4 shows typical plots of the SINR versus time under heavy rain. The average SINR was 25.9 dB, with a standard deviation of 6.5 dB, showing more pronounced fluctuations in the SINR as compared to those under medium rain. Channel 7 was the primary channel (first graph) whilst Channel 8 was the backup channel (second graph). The number of regions with compromised SINR in heavy rain was higher than that under medium rain. The SINR for the composite channel, the third plot in Figure 4, shows that leveraging Channel 7 as a backup channel successfully alleviated a large portion of the composite signal from deteriorating below 25 dB, including during a predominant dip between 667 s and 710 s.  
Figure 4: SINR versus time under heavy rain. Green line shows \(\textit{SINR}\gamma\) value of 25 dB. Figure 3: SINR versus time under medium rain. Green line shows \(\textit{SINR}\gamma\) value of 25 dB. On a clear day, the average SINR was 30.4 dB. Thus, the backup channel was not required, as the SINR remained above 25 dB, as shown in Figure 5.  
Uplink and downlink transmissions demonstrated similar SINR characteristics.