## 2 System and Channel Model
### One-Hop Hybrid FSO/RF Link Based on Selective Combination Scheme
Comparing Figure 7a,b, it can be seen that the outage probability of the relay-aided hybrid system and the hybrid direct link with heterodyne detection is better than that with direct detection. Figure 5: Relationship between the ABER and the SNR of the relay-aided hybrid FSO/RF cooperation system under different pointing errors and relay-aided structures. Figure 6: Relationship between the ABER and the SNR of the relay-aided hybrid FSO/RF system under different turbulence intensity and relay-aided structures. When the pointing errors parameter \(\xi=1\), decision threshold \(\gamma_{\text{th}}=10\) dB, and with strong turbulence, Figure 8 shows the relationship between the outage probability and the SNR of the relay-aided hybrid system with different structures and the hybrid direct link under different RF fading parameters. It can be seen from Figure 8 that the relay-aided hybrid system can effectively improve the outage performance compared with the hybrid direct link with different RF fading parameters \(m\). The outage probability of the relay-aided hybrid FSO/RF system and the hybrid direct link will decrease with the increase in \(m\). When the pointing errors parameter \(\zeta=1\), RF fading parameter \(m=2\), and with strong turbulence, Figure 9 shows the relationship between the outage probability and the SNR of the relay-aided hybrid system with different structures and the hybrid direct link at different decision thresholds. It can be seen from Figure 9 that the outage probability of the relay-aided hybrid system and the hybrid direct link is very sensitive to the decision  
Figure 8: Relationship between the outage probability and the SNR of the relay-aided hybrid FSO/RF system with different structures and the hybrid direct link under different RF fading parameters. Figure 7: Relationship between the outage probability and the SNR of the relay-aided hybrid FSO/RF system and hybrid direct link under different turbulence intensity and relay-aided structures. **(a)** Heterodyne detection (HD). **(b)** Direct detection (DD). threshold. Compared with the hybrid direct link, the relay-aided hybrid system with different decision thresholds can effectively improve the outage performance. The outage probability of the relay-aided hybrid system and the hybrid direct link will decrease as the decision threshold decreases. Assume that the average SNR of each bit in the FSO and RF sub-links is equal, that is, \(\overline{\gamma}^{\text{FSO}}=\overline{\gamma}^{\text{RF}}\), the distance of each hop is 1 km and the direct link is 2 or 3 km. With weak turbulence, CBPSK modulation, heterodyne detection, the RF fading parameter \(m=2\), the pointing errors parameter \(\xi=1\), and the decision threshold \(\gamma_{\text{th}}=10\) dB, Figure 10 shows the relationship between the performance metrics and the SNR of the relay-aided hybrid PSO/RF system under different direct distances and relay-aided structures. Similar to the previous conclusion, it can be seen from Figure 10 that as the direct distance increases, the performance of the hybrid PSO/RF system will slightly decrease, that is, the transmission distance has little impact on the performance of hybrid direct links and relay-aided hybrid systems. The hybrid direct communication system can effectively improve the reliability of the communication system. Furthermore, we can see that the relay-aided hybrid system can significantly improve the system performance, and as the number of paths increases, the system's performance can be significantly improved, while an increase in hops slightly reduces the system's performance. Assume that the average SNR of each bit in the FSO and RF sub-links is unequal, that is, \(\overline{\gamma}^{\text{FSO}}\
eq\overline{\gamma}^{\text{RF}}\), the distance of each hop is 1 km and the direct link is 2 or 3 km.