Secure Downlink Transmission with NOMA-Based Mixed FSO/RF Communications in Space–Air–Ground Integrated Networks

Security is paramount in space–air–ground integrated networks (SAGINs) due to their inherent openness and the broadcast characteristics of wireless transmission. In this paper, we propose a secure downlink transmission scheme with NOMA-based mixed FSO/RF communications for SAGINs. Specifically, the satellite communicates with two ground users through an unmanned aerial vehicle (UAV) relay, where FSO and RF transmissions are adopted for the satellite–relay and relay–user links, respectively. Furthermore, the NOMA technique is integrated to further enhance secrecy performance. Subsequently, exact closed-form expressions for the secrecy outage probability of the downlink transmission link in SAGINs are derived. Finally, Monte Carlo simulations are performed to validate the effectiveness of the proposed secure downlink transmission scheme and the accuracy of the analytical expressions.