In order to improve the confidentiality and performance of the mixed Radio Frequency (RF)/Free Space Optical (FSO) communication systems, it is necessary to study the security performance of the Reconfigurable Intelligent Surface (RIS)-assisted mixed RF/FSO system to obtain the relationship between system security performance and parameters such as RIS deployment location and the degree of correlation between different antennas branches.

In the article, a single eavesdropper is considered to eavesdrop against RF links and FSO links respectively, with RF links obeying an arbitrarily correlated Nakagami-m distribution and FSO links obeying a Gamma-Gamma distribution. Closed expressions for the Security Outage Probability (SOP) and Strictly Positive Security Capacity (SPSC) probability of the communication system for the two eavesdropping scenarios are derived using the decode-and-forward protocol, and the SOP is analyzed asymptotically. Simulation experiments are also conducted using the Matlab software to verify the accuracy of the derived formulas through Monte Carlo simulation.

Simulation results show that deploying the RIS close to the receiver reduces the security outage probability of the system. It is also shown that an increase in the degree of antenna correlation increases the SOP of the system when the channel quality of the RF main channel is better compared to the eavesdropping channel.

The RIS-assisted FSO link has better security compared to the RF link. Although the multi-antenna diversity technique can effectively improve the security performance of the system, antenna correlation, similar to the fading caused by pointing error and atmospheric turbulence, can also damage the security performance of the system. It is also shown that the security of the system can be effectively improved by reducing the degree of antenna correlation.