## 6 Simulation Results and Analysis
Figure 7 presents the correlation between \(\lambda_{SR}\) and the RIS-aided mixed RF/FSO system SOP under the influence of different fading severity parameter values. Figure 7 shows that the system SOP decreases continuously as \(\lambda_{SR}\) increases. The number of reflecting elements N is fixed, and the mixed system SOP is significantly decreased by increasing \(m\) from 2 to 6. When \(\lambda_{SR}=35\) dB and N = 2, the system SOP is about 9.85 \(\times\) 10\({}^{-7}\) and 1.05 \(\times\) 10\({}^{-7}\) for \(m\) equal to 2 and 6, respectively. It is shown that increasing the fading severity parameter value improves the quality of the interfering link fading, providing better secrecy performance for the mixed system. Similarly, for \(\lambda_{SR}=35\) dB and N = 4, the values of SOP corresponding to \(m\) equal to 2 and 6 are about 2.82 \(\times\) 10\({}^{-7}\) and 6.47 \(\times\) 10\({}^{-9}\), respectively; they differ by two orders of magnitude. As \(m\) varies, the mixed system SOP with N = 4 is better than that of the mixed system with N = 2 in terms of security performance, indicating that the increase of \(m\) improves the system security performance more significantly with a higher N compared to the system with a lower N.  
Figure 6: Simulation diagram of the SOP with different RIS reflection before and after link distances \(d_{IR^{e}},d_{RE}\) in the RIS-aided mixed RF/FSO system. Figure 7: Simulation diagram of SOP under different fading severity parameters \(m\) of RIS in the RIS-aided mixed RF/FSO system. Figure 8 shows the correlation between \(\lambda_{SR}\) and the RIS-aided mixed RF/FSO system ASC with the effect of a varying number of RIS-reflecting elements. As the simulation diagram shows, as \(\lambda_{SR}\) increases, there is a corresponding increase in ASC. When \(\lambda_{SR}=35\) dB, the ASC values are about 3.26, 3.44, and 3.75 for 4, 6, and 8 reflecting elements, respectively. The results show that the ASC is maximum at N = 8. This is because the RIS can reflect more interference signals with the increase of N that can greatly limit the receiver side rate of the eavesdropper, consequently increasing the ASC. Therefore, in the RIS-aided mixed RF/FSO system, an appropriate increase in N can effectively increase the ASC of the entire communication system. Figure 9 investigates the correlation between \(\lambda_{SR}\) and the RIS-aided mixed RF/FSO system ASC for different link distances before and after the RIS reflection. As the instantaneous SNR \(\lambda_{SR}\) increases, the system ASC increases as \(d_{I{R^{\prime}}},d_{{R^{\prime}}E}\) decreases. When \(\lambda_{SR}=35\) dB, the ASC values of the system are about 2.86 and 3.26 for \(d_{I{R^{\prime}}}=10\) m, \(d_{{R^{\prime}}E}=10\) m and \(d_{I{R^{\prime}}}=5\) m, \(d_{{R^{\prime}}E}=5\) m, respectively. When \(d_{I{R^{\prime}}}\) and \(d_{{R^{\prime}}E}\) increase simultaneously, the interference signal loss in the link becomes higher, the effect of interference becomes worse, and the ASC decreases. On the other hand, when both distances are relatively small, the interference signal loss is low, the interference signal is received with high quality by the RIS, the interference effect is enhanced, and thus, the ASC increases. Therefore, a proper reduction of the link distances before and after reflection in the RIS-aided mixed RF/FSO system can effectively improve the physical layer security performance. Figure 10 shows the correlation between \(\lambda_{SR}\) and the RIS-aided mixed RF/FSO system ASC for different fading severity parameters of the RIS. It can be concluded based on the figure that the system ASC gradually increases as \(\lambda_{SR}\) increases. Considering that \(N\) is fixed, the mixed system ASC shows a prominent increase when m increases. When \(\lambda_{SR}=35\) dB, \(N=2\), the ASC values are about 2.72, and 3.55 for \(m\) equal to 2 and 6, respectively. Similarly, when \(\lambda_{SR}=35\) dB, \(N=4\), the ASC values are about 3.00, and 3.77 corresponding to \(m\) equal to 2 and 6, respectively.