Channel Modeling and PLS Performance Analysis of RIS-Assisted Multi-User Hybrid FSO/RF Systems

In this letter, we investigate the physical layer security (PLS) of hybrid free-space optical (FSO)/radio frequency (RF) systems in space-air-ground integrated network (SAGIN). Considering the impact of imperfect channel state information (CSI), we propose a novel unmanned aerial vehicle (UAV)-mounted reconfigurable intelligent surface (RIS)-assisted multi-user channel modeling method. To enhance system PLS performance, an opportunistic user scheduling (OUS) scheme is adopted at the high-altitude platform (HAP) relay node. We derive closed-form expressions for secrecy outage probability (SOP) and secrecy throughput (ST), along with an asymptotic SOP analysis in the high signal-to-noise ratio (SNR) regime. Numerical results reveal that imperfect CSI induces an irreducible SOP floor, while the proposed OUS scheme significantly enhances security performance compared with existing benchmark schemes.