## 5 Numerical Results  
In this section, numerical results will be provided to analyze the communication performance and security performance of the hybrid RF/FSO system. The main simulation parameters of the hybrid FSO/RF with eavesdropper are shown in Table 3[39; 51]. Numerical results are validated by Monte Carlo simulations performed using the software package Matlab 2019(b). Without loss of generality, it is assumed that the average SNR of FSO and RF link output is equal from Figures 2-4, that is, \(\overline{\gamma}^{FSO}=\overline{\gamma}^{RF}\). Figure 2 shows the relationship between the average BER and the average SNR when the hybrid FSO/RF parallel transmission system and the only FSO system adopt three sub-carrier modulation modes (CBPSK, QPSK, 4 QAM) and the HD technique, respectively. The simulation conditions in Figure 2 are that the FSO link is in strong turbulence, fading parameters are \(\alpha_{m}=1\), \(\mu_{m}=3\), and the pointing error parameter is \(\xi=1\). As can be seen from Figure 2, compared with the only FSO link, the hybrid system can significantly improve BER performance. From Figure 2, whether in the only FSO system or the hybrid FSO/RF system, it is evident that the system using the CBPSK modulation has the best BER performance, QPSK is the second, and 4 QAM is the worst. This is because the higher the modulation order, the greater the error probability of decoding, so the BER of a high-order modulation system is higher than that of a low-order modulation system. When the hybrid system adopts the CBPSK modulation and the fading parameters are \(\alpha_{m}=2\), \(\mu_{m}=1\), Figure 3 shows the relationship between the average BER and average SNR of the hybrid system under different pointing error parameters \(\xi\) and different atmospheric turbulence intensity. Figure 3 shows that when the FSO link is in weak turbulence, the greater the pointing error parameter \(\xi\) value, the more obvious the impact on the BER performance, and vice versa. Assuming that the average SNR of the system is 20 dB and the FSO link is in weak turbulence, the system BER is \(2.043~{}\times~{}10^{-5}\) and \(1.12~{}\times~{}10^{-6}\) when the pointing error parameter \(\xi\) is 1 and 2, respectively. When the FSO link environment is moderate turbulence and strong turbulence, the change of pointing error parameter \(\xi\) has an obvious effect on the BER of the hybrid system. For example, assuming that the BER of the system is 20 dB and moderate turbulence, the system BER is \(2.578~{}\times~{}10^{-5}\) and \begin{table}
\begin{tabular}{c c} \hline \hline
**Information Transmission Link** & **Channel Model and Parameters** \\ \hline FSO communication link & Malaga turbulence, (\(\alpha\), \(\beta\)) \\ RF communication link & \(\alpha-\mu\) fading, (\(\alpha_{m}\), \(\mu_{m}\)) \\ FSO eavesdropping link & Malaga turbulence, (\(\xi^{E}\), \(\xi^{E}\)) \\ RF eavesdropping link & \(\alpha-\mu\) fading, (\(\underline{\mu}^{E}_{m},\underline{\mu}^{E}_{m}\)) \\ \hline \hline
**FSO Link** & **Parameter Value** \\ \hline Wavelength & 785 nm \\ Link range & 1 km \\ Laser type & Fabry–Perot laser diodes \\ Beam type & Gaussian beam \\ Detector type & Avalanched photodiode \\ Weak turbulence & (\(\alpha\), \(\beta\)) = (\(\alpha^{E}\), \(\beta^{E}\)) = (8, 4) \\ Moderate turbulence & (\(\alpha\), \(\beta\)) = (\(\alpha^{E}\), \(\beta^{E}\)) = (4.2, 3) \\ Strong turbulence & (\(\alpha\), \(\beta\)) = (\(\alpha^{E}\), \(\beta^{E}\)) = (2.296, 2) \\ \hline
**RF Link** & **Parameter Value** \\ \hline Carrier frequency & 60 GHz \\ \(\alpha-\mu\) fading & (\(\alpha_{m}\), \(\mu_{m}\)) = (\(\alpha_{m}^{E}\), \(\mu_{m}^{E}\)) = (3,1) or (2,1) or (1,1) \\ \hline
**Parameters of Pointing Errors** & **Symbol** \\ \hline FSO communication link & \(\xi=1\) or 2 or 4 \\ FSO eavesdropping link & \(\xi^{E}=1\) or 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation parameters of the hybrid FSO/RF with eavesdropper. \(42.88~{}\times~{}10^{-6}\) when the pointing error parameter \(\xi\) is 1 and 2, respectively.