## 2 System and Channel Models
### Outage Probability
can also be corroborated from Figure 3. Similar to Figure 3, outage curves in Figure 5 decrease heavily when transmission powers are larger than a threshold, which are different for different weather conditions. Figure 6 depicts the relationship between outage probability and the transmitted-transmission power of UAV-assisted hybrid systems and hybrid FSO/RF direct links for different RF fading parameters \(m_{x,y}m\),weather conditions, relay-assisted structures, and aperture diameters \(D_{x,y}D\) when \(\gamma_{th}=1\) dB\(r_{th}=1\) dB and a light rain condition is applied. Similar to Figure 4, compared to the hybrid direct link, we have the following observations. (1) The UAV-assisted hybrid system can significantly improve system outage performance; (2) the aperture averaging effect can further improve outage performance; (3) the RF fading parameter \(m_{x,y}m\) of the UAV-assisted hybrid system is also an important factor for the outage performance; (4) more transmission paths will reduce the outage probability, while more hops in each path will increase the outage probability. Figure 4: The relationship between the BER performance and transmittedtransmission power of UAV-assisted hybrid systems and the hybrid direct link without PE. (**a**) \(m_{x,y}=1m=1\). (**b**) \(D_{x,y}=10\) cm. Figure 5: The relationship between outage probability and transmittedtransmission power of hybrid FSO/RF direct links without PE. (**a**) \(m_{x,y}=1m=1\). (**b**) \(m_{x,y}=2m=2\). Based on the experimental results in Figures 2-6 and considering pointing errors, further analysis is conducted on the impact of the proposed system on mitigating the atmospheric environment. Figure 7 shows the relationship between the bit error rate and the transmitted power of UAV-assisted hybrid systems with (5, 3) relay structure and hybrid FSO/RF direct links for different parameters, such as weather, RF fading parameters \(m_{x,y}\), receiver apertures \(D_{x,y}\), and the standard deviation of the pointing errors on the receiver plane \(\sigma_{x,y}^{s}\). According to Figure 7, it can be seen that under the consideration of pointing errors, compared to the hybrid FSO/RF direct link, the UAV-assisted hybrid system significantly improves the BER performance under any conditions. As \(m_{x,y}\) increases, \(D_{x,y}\) increases, or \(\sigma_{x,y}^{s}\) decreases, it can further improve the error rate performance of UAV-assisted hybrid systems and hybrid FSO/RF direct links. Furthermore, from Figure 7a, we can observe that when the transmitted power is low, the bit error rate performance of UAV-assisted hybrid systems and hybrid FSO/RF direct links under light rain conditions is better than that under clear air, while when the transmitted power is high, this situation is exactly the opposite. Similarly, we also found in Figure 7d that when the transmitted power is low, the bit error rate performance of UAV-assisted hybrid systems and hybrid FSO/RF direct links with \(\sigma_{x,y}^{s}=100\) cm is better than that under \(\sigma_{x,y}^{s}=50\) cm, and when the transmitted power is high, this situation is also opposite. This is because when the transmitted power is low and the standard deviation of the PE is large or the atmospheric attenuation is large, the RF sublink replaces the FSO sublink as the main method of information transmission, and the error rate performance is actually better. As the transmitted power increases, the SNR of the FSO sublink is higher and the bit error rate performance is rapidly improved. When \(m_{x,y}=1\), \(\gamma_{th}=10\) dB, and light rain, Figure 8 describes the relationship between the outage probability and the transmitted power of UAV-assisted hybrid systems with a (5, 3) relay structure and hybrid FSO/RF direct links for different parameters, such as receiver apertures \(D_{x,y}\) and the standard deviation of the pointing error on the receiver plane \(\sigma_{x,y}^{s}\).