## 2 System and Channel Models
### Outage Probability
From Figure 2, it can be seen that compared to the weather conditions of clear air and light rain weather, haze and light fog weather have a more severe impact on the BER of the hybrid direct link. The trend of the BER of hybrid direct links under haze and light fog weather conditions behave similarity, where for each single condition, CBPSK is the best, while NBFSK is the worst one in terms of their BER. Besides, based on Figure 2a, when the transmittedtransmission power is less than \(-15\) dBm\(-15\) dBm, the BER of the CBFSK is superior than that of the DBPSK scheme, while the performance of the CBFSK is gradually behind the DBPSK scheme when the transmittedtransmission power is larger than \(-15\) dBm. Based on Figure 2d, when the transmittedtransmission power is less than \(-7\) dBm\(-7\) dBm, the BER of the NBFSK is superior than that of the CBFSK scheme, while the performance of the NBFSK is gradually behind the CBPSK scheme when the transmittedtransmission power is larger than \(-15\) dBm. Based on thhe above observations, we obtain that weather conditions can cause effects on FSO links under the small transmittedtransmission power scenario. Moreover, the BER of the hybrid direct link by the phase modulation scheme is better than that by the frequency modulation. This is because the phase modulation has significant advantages in the environment, where signal suffers from serious attenuations and distortions. From Figure 2, it can be seen that conditions such as clear air, haze, and light fog have a significant impact on the FSO communication links only, while light rain condition has an impact on both FSO and RF communication links. Assuming that the hybrid system is in weather conditions such as atmosphere, haze, and light fog, when the transmission power is low, the BER performance of the hybrid direct links with phase modulation scheme is better than that of frequency modulation. This is because the signal is severely attenuated and distorted during transmission, and phase modulation has a significant advantage in this case. ; Moreover, when the transmittedtransmission power is large, the BER of the hybrid direct link by the coherent modulation scheme is better than that by the non-coherent \begin{table}
\begin{tabular}{c c c c} \hline \hline
**FSO Subsystem** & \multicolumn{3}{c}{**RF Subsystem**} \\ \hline
**Parameters** & **Value** & **Parameters** & **Value** \\ \hline Wavelength, \(\lambda_{x,y}^{FSO}\) & \(1550\) nm & Wavelength of \(60\) GHz RF, \(\lambda_{x,y}^{RF}\) & \(5\) mm \\ \hline Divergence angle, \(\theta_{x,y}\) & \(1\) mrad & Nakagami fading parameter, \(m_{x,y}\) & \(2\) \\ \hline Receiver aperture diameter, \(D_{x,y}\) & \(10\) cm or \(20\) cm & Transmit antenna gain, \(G_{x,y}^{t}\) & \(44\) dBi \\ \hline Responsivity, \(R_{x,y}^{FSO}\) & \(0.5\) A/W & Receive antenna gain, \(G_{x,y}^{t}\) & \(44\) dBi \\ \hline Noise Variance, \(\left(\sigma_{x,y}^{FSO}\right)^{2}\) & \(10^{-14}\) A\({}^{2}\)/Hz & Noise Variance, \(\left(\sigma_{x,y}^{RF}\right)^{2}\) & \(-85\) dBm \\ \hline Transmission distance, \(L_{x,y}\) & \(1\) km & Oxygen attenuation, \(\omega_{x,y}^{oxg}\) & \(15.1\) dB/km \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The UAV-assisted hybrid FSO/RF system parameters [30; 33; 34; 35]. modulation. This is because coherent modulation technology can predict the carrier phase at receivers, which can further improve the system BER when the signal waveform is better. For light rain atmospheric environments, the BER with the phase modulation scheme is better than that of the frequency modulation, and the hybrid direct link with the CBPSK modulation scheme is the best under any weather conditions. Figure 3 reveals the relationship between the BER and the transmittedtransmission power by the CBPSK modulation scheme under different conditions, such as RF fading parameters \(m_{x,y}m\), receiver aperture diameters \(D_{x,y}D\), and weather conditions. Comparing Figure 3a,b, it can be seen that the larger the fading parameter \(m_{x,y}m\), the lower the BER.