## 2 System and Channel Model
### One-Hop Hybrid FSO/RF Link Based on Selective Combination Scheme
Figure 2: Relationship between ABER and SNR of hybrid FSO/RF direct link under different modulation modes and detection schemes. Assuming that the average SNR of each bit in the FSO and RF sub-links is equal, that is, \(\overline{\gamma}^{\text{FSO}}=\overline{\gamma}^{\text{RF}}\), and the distance of each hop and direct link is 1 km for Figures 5-9, under CBPSK modulation with heterodyne detection, with the RF fading parameter \(m=2\) and weak turbulence conditions, Figure 5 shows the relationship between the ABER and the SNR of the relay-aided hybrid FSO/RF system with different pointing errors and relay-aided structures. Figure 5 shows that the pointing errors have no impact on the ABER performance of the relay-aided hybrid system, that is, the relay-aided hybrid system can compensate for the adverse impact of pointing errors on the system. According to Figure 5, it can also be seen that different structures of the relay-aided hybrid system have different abilities to improve the ABER performance, that is, the ABER performance will be improved with the increase in the path, and will be deteriorated with the increase in the number of hops. This is because when the transmission path increases, the probability of receiving a large SNR signal at the receiving end of the system increases, resulting in the improvement of the ABER performance. When the number of hops increases, the cumulative ABER of the signal received by the receiving end of the system will increase, so the overall ABER of the system will decline. For example, when the SNR is 20 dB, the ABERs of the relay-aided hybrid systems with (\(N=2\), \(M=3\)), (\(N=2\), \(M=5\)), and (\(N=4\), \(M=3\)) structures are \(7.98\times 10^{-12}\), \(2.206\times 10^{-11}\), and \(6.925\times 10^{-16}\), respectively. Comparing Figures 4 and 5, it can be further found that the relay-aided hybrid system can significantly improve the ABER performance of the system compared with the hybrid direct link. With CBPSK modulation, heterodyne detection, the RF fading parameter \(m=2\), and the pointing errors parameter \(\xi=1\), Figure 6 depicts the relationship between the ABER and the SNR of the relay-aided hybrid FSO/RF system under different turbulence intensity and relay-aided structures. Figure 6 shows that the relay-aided hybrid FSO/RF system with a suitable structure can better compensate for the negative impact of turbulence and improve the ABER performance. For example, when the SNR is 20 dB, the ABERs of the relay-aided hybrid system with (\(N=2\), \(M=3\)), (\(N=2\), \(M=5\)), and (\(N=4\), \(M=3\)) structures under strong turbulence are \(4.088\times 10^{-11}\), \(1.129\times 10^{-10}\), and \(6.197\times 10^{-15}\), respectively, and the ABERs under moderate turbulence are \(8.742\times 10^{-12}\), \(2.417\times 10^{-11}\), and \(7.723\times 10^{-16}\), respectively. Figure 4: Relationship between the ABER and the SNR of the hybrid FSO/RF direct link under different turbulence intensity, detection schemes, and RF fading parameters. When the pointing errors parameter \(\xi=3\), the RF fading parameter \(m=2\), and the decision threshold \(\gamma_{\text{th}}=10\) dB, Figure 7 shows the relationship between the outage probability and the SNR of the relay-aided hybrid FSO/RF system and hybrid direct link with heterodyne detection and direct detection, respectively. According to Figure 7, it is easy to see that compared with the hybrid direct link, the relay-aided hybrid system can significantly improve the outage performance. In Figure 7, we can also see that a different turbulence intensity still has a certain impact on the relay-aided hybrid FSO/RF system. Different structures of the relay-aided hybrid system have different effects on the outage performance of the system, that is, the outage probability of the system will increase with the increase in hops and decrease with the increase in paths.