Physical Layer Security of HAPS-Based Space–Air–Ground-Integrated Network With Hybrid FSO/RF Communication

In this article, we study the physical layer security performance of a downlink space–air–ground-integrated network (SAGIN), where a satellite communicates with the ground user/destination via a high-altitude-platform-station-based relay in the presence of an eavesdropper. In particular, we propose to use hybrid free-space optical (FSO)/radio frequency (RF) transmission to enhance the security of the considered system. Herein, the FSO and RF links are assumed to experience Gamma–Gamma and Shadowed-Rician distributions, respectively. Specifically, we derive exact and asymptotic expressions of secrecy outage probability for the considered SAGIN by employing intensity modulation/direct detection. We provide the numerical and simulation results to validate our analysis.