## 4 Results and Discussion
Figure 2: Influence of different modulation schemes and turbulence intensity on ABER performance of direct link. (**a**) The FSO-only direct link. (**b**) The hybrid FSO/RF direct link. Considering the system conditions of CBPSK modulation, RF fading parameter \(m^{RF}=2\), and pointing error parameter \(\rho=6\), Figure 4 describes the relationship between the ABER and the SNR of the multi-hop parallel hybrid FSO/RF system under different relay-assisted structres and turbulence intensity. It can be seen from Figure 4 that under the same relay-assisted structure, the ABER of the multi-hop parallel hybrid system will increase with the increase of turbulence intensity. Under the same turbulence intensity, when the total number of paths \(N\) is constant, the ABER of the multi-hop parallel hybrid system will increase with the increase of the number of hops \(M\), and when the number of hops \(M\) is constant, the ABER of the multi-hop parallel hybrid system decreases with the increase of the total number of paths \(N\). This is because it is assumed that the FSO and RF link channel environments of each hop are the same, and thus the ABER performance of each hop hybrid link is also the same. With the increase of hops, the transmission distance and range can be increased but the cumulative ABER of the system will also increase. Since the signal with the largest SNR is selected for output at the receiver, increasing the number of transmission paths means that the probability of the signal with large SNR received at the receiver is greater; therefore, the end-to-end BER of the system will be reduced. When the RF link fading parameter \(m^{RF}=2\), Figure 5 shows the relationship between the ABER and the SNR of the multi-hop parallel hybrid FSO/RF system under different turbulence intensity, different relay-assisted structures, and different pointing errors. It is obvious from Figure 5 that for any turbulence intensity and pointing errors, the ABER performance of the multi-hop parallel hybrid FSO/RF system is better than that of the hybrid FSO/RF direct link, and increasing the total number of transmission paths \(N\) can significantly improve the ABER performance of the multi-hop parallel hybrid system. In strong turbulence, the ABER of the multi-hop parallel hybrid FSO/RF system will increase rapidly with the increase of the hop number \(M\). For example, when the output SNR of the receiver is 10 dB, for \(\rho=1\) and \(\rho=6\), the ABER of the multi-hop parallel hybrid systems \((N=2,M=5)\) are \(2.312\times 10^{-4}\) and \(1.798\times 10^{-4}\), respectively, while the ABER of the multi-hop parallel hybrid systems \((N=2,M=3)\) are \(1.113\times 10^{-4}\) and \(8.588\times 10^{-5}\), respectively. Figure 3: The ABER of the hybrid FSO/RF direct link under different turbulence intensity and RF fading parameters. In weak turbulence, the multi-hop parallel hybrid FSO/RF system is affected by the pointing errors and hop number \(M\). For example, when the output SNR of the receiver is 10 dB, for \((N=2,M=5)\) and \((N=2,M=3)\), the ABER of the multi-hop parallel hybrid systems with \(\rho=1\) are \(1.13\times 10^{-4}\) and \(8.287\times 10^{-5}\), respectively, while the ABER of the multi-hop parallel hybrid systems with \(\rho=6\) are \(1.204\times 10^{-4}\) and \(5.705\times 10^{-5}\). When the turbulence is moderate and the pointing error parameter is large, the influence of hop number \(M\) on the multi-hop parallel hybrid system is clear. For example, when the output SNR of the receiver is 10 dB, for \((N=2,M=5)\) and \((N=2,M=3)\), the ABER of the multi-hop parallel hybrid systems with \(\rho=6\) are \(1.517\times 10^{-4}\) and \(7.221\times 10^{-5}\), respectively. However, when the turbulence is moderate and the pointing error parameter is small, the influence of hop number \(M\) on the multi-hop parallel hybrid system is not significant.