Secrecy outage analysis for RIS-assisted hybrid FSO-RF systems with NOMA

In this paper, we investigate the physical-layer security performance for a reconfigurable intelligent surface (RIS) assisted free-space optical (FSO) communication-radio frequency (RF) system with non-orthogonal multiple access (NOMA). In particular, the source (S) transmits an optical NOMA signal to the relay (R), which forwards the optical-to-electrical converted signal to both users in the presence of an eavesdropper (E). Furthermore, for the far user, the scenario that there is no direct path from R and it can only receive the signal from R via the RIS is considered. While for the near user, three signal transmission scenarios are taken into account: 1) only the direct path from R exists; 2) only the reflected paths from RIS exist; 3) both the direct and the reflected paths exist and superimpose together. In addition, for the first hop of information transmission, E may or may not locate in the region of laser beam divergences of S. Therefore, both scenarios that the first hop may or may not be overheard by E are taken into consideration. Considering the FSO link is with a Gamma-Gamma distribution and the RF channels experience the Nakagami-m fading, the analytic and asymptotic expressions of secrecy outage probability for both users are presented. Monte-Carlo simulations are also conducted to validate the accuracy of the derived expressions.