## 2 System and Channel Models
### Outage Probability
The larger the receiver aperture \(D_{x,y}D\), the more significant improvement of the BER, and the more obvious aperture averaging effect. Besides, from Figure 3, it can be easily observed that the BER is prone to be affected by different weather conditions, where the impact of light fog is the greatest, while the impact of clean air is the smallest. The BERs rapidly decrease when transmittedtransmission powers are larger enough, for example the corresponding transmittedtransmission power at the turning point is also higher for weather with greater light attenuation. This is because when the transmittedtransmission  
Figure 2: The relationship between BER performance and transmitted powertransmission of the hybrid FSO/RF direct link without PE under different weather conditions and modulation schemes. (**a**) Clear air. (**b**) Haze. (**c**) Light fog. (**d**) Light rain. power is low, the main factor affecting the BER is the attenuation of light intensity. Therefore, a limited increase of the transmittedtransmission power will result in a stable decrease in the BER curve; when the transmittedtransmission power becomes larger enough, the main factor affecting the BER changes to the turbulence. Therefore, large transmittedtransmission power will lead to a sharp decrease in the BER. These results indicate that although the BER of the hybrid direct link is significantly affected by both the FSO and RF subchannels, the improvement in any subchannel can significantly improve the BER performance of the hybrid direct link. In practice, light rain weather condition can have an impact on both FSO and RF links; it would be mandatory to evaluate the BER performance of UAV-assisted hybrid systems under light rain. Figure 4 shows the relationship between the BER and the transmittedtransmission power with CBPSK modulation under light rain for different RF fading parameters \(m_{x,y}m\), relay-assisted structures, and receiver aperture diameters. From Figure 4, it can be seen that under the same system conditions, compared to the hybrid direct link, UAV-assisted hybrid systems can significantly improve the BER performance, and the aperture averaging effect can enhance the performance. In Figure 4, it can be seen that the RF fading parameter \(m_{x,y}m\) plays an important role on the BER performance of the UAV-assisted hybrid system. Specifically, the larger the value of \(m_{x,y}m\), the smaller the BER. In addition, Figure 4,b show that different relay-assisted structures have various effects on the UAV-assisted hybrid system. More specific, the structure of (5, 3) has the best BER performance, while (1, 3) is the worst. This is because the increase in the number of transmission path not only enlarges the coverage but also increases the diversity gain, which further reduces the system BER. The increase in the number of transmission hops not only enlarges the distance but also leads to the accumulation of bit error rates, which further increases the system BER. Figure 5 shows the relationship between the outage probability and transmittedtransmission power for different RF fading parameters \(m_{x,y}m\), receiver aperture diameter \(D_{x,y}D\), and weather conditions when the decision threshold is 1 dB. From Figure 5,b, it can be seen that the fading parameter \(m_{x,y}m\) of the RF sublink dominates the outage probability of the hybrid direct link. This is because (1) the larger the value of \(m_{x,y}m\), the smaller the outage probability. (2) The larger the receiver aperture \(D_{x,y}D\), the more improvement of the outage performance. Moreover, it can be easily observed from Figure 5 that the outage probability of hybrid direct links is easily affected by different weather conditions, which  
Figure 3: The relationship between the BER performance and transmittedtransmission power of hybrid FSO/RF direct links without PE. (**a**) \(m_{x,y}=1m=1\). (**b**) \(m_{x,y}=2m=2\).