## 5 Numerical Results
Assuming that the BER of the system is 20 dB and strong turbulence, the BER of the system is \(4.187~{}\times~{}10^{-5}\) and \(1.073~{}\times~{}10^{-5}\) when \(\xi\) is 1 and 2, respectively. Figure 3: Relationship between BER and SNR of the hybrid system under different turbulence intensity and pointing errors. Figure 2: The relationship between BER and SNR of the hybrid system and single FSO system under different modulation mo des. When the RF fading parameters are (\(\alpha_{m}=2\), \(\mu_{m}=1\)) and the decision threshold is \(\gamma_{th}\) = 0 dB, Figure 4 describes the relationship between the OP and the average SNR of the hybrid FSO/RF system and the only FSO communication system under different turbulence intensity, different pointing errors, and different detection techniques. Comparing Figure 4,b, it is obviously found that the OP performance of the system with HD technique is better than that of DD technique whether it is an only FSO system or a hybrid system. As can be seen from Figure 4, compared with the only FSO system, the OP of the hybrid FSO/RF system is smaller, and the OP of the two systems decreases as the SNR increases. Assuming that the SNR is 20 dB, HD technique, \(\xi=2\), and the atmospheric turbulence intensity changes from weak to strong, the OP of the only FSO system are \(2.66~{}\times~{}10^{-3}\) and \(2.173~{}\times~{}10^{-2}\), respectively, while the OP of hybrid system are \(3.307~{}\times~{}10^{-5}\) and \(3.066~{}\times~{}10^{-4}\), respectively. From Figure 4, we can also find that the pointing errors have an impact on the OP performance of both the hybrid system and the single FSO system. Assuming that the SNR is 20 dB, HD technique, strong turbulence, and \(\xi\) changes from 1 to 2, the OP of the only FSO system are \(6.418~{}\times~{}10^{-2}\) and \(2.173~{}\times~{}10^{-2}\), respectively, while the OP of hybrid system are \(1.065~{}\times~{}10^{-3}\) and \(3.066~{}\times~{}10^{-4}\), respectively. Atmospheric turbulence, pointing errors, and detection techniques have a significant impact on the outage performance of both the only FSO system and the hybrid system, and the outage performance of the hybrid system is significantly better than that of the only FSO system. When the FSO link environment is moderate turbulence, HD technique, RF fading parameters (\(\alpha_{m}=1\), \(\mu_{m}=1\)), and different pointing errors, Figure 5 shows the relationship between the ECC and the average SNR of the hybrid FSO/RF system with \(\overline{\gamma}^{RF}\)=10 dB, the only FSO system, and the only RF system. Obviously, the atmospheric turbulence environment and pointing errors have no impact on the communication performance of the RF link, while the ECC of the hybrid FSO/RF system and only FSO system is affected by pointing errors. It can be seen from Figure 5 that the increase of \(\xi\) value means that the probability of beam alignment increases, resulting in the increase of channel capacity of the only FSO system and the hybrid FSO/RF system. It can be seen that when the FSO link is in a turbulent environment, the capacity of the hybrid FSO/RF system with MRC combination is much higher than that of the only FSO system and the only RF system. Figure 4: Relationship between OP and SNR of the hybrid system and only FSO system under different turbulence intensity, different pointing errors, and different detection techniques. (**a**) HD. (**b**) DD. Without losing generality, considering that the alignment of the FSO eavesdropping link is worse than that of the FSO communication link, assuming that the average SNR received by the eavesdropper is \(-5\) dB and the target rate is \(0.1\) bit/s/Hz, (\(\alpha_{m}=2\), \(\mu_{m}=1\)), \(\overline{\gamma}^{RF}=10\) dB, \(\xi=2\) and \(\xi^{E}=1\), Figure 5 shows that the relationship between the SOP of the hybrid system with FSO and RF link eavesdropping and the SNR of the FSO channel under different turbulence conditions.