Secure NOMA Based RIS-UAV Networks: Passive Beamforming and Location Optimization

Radio signals are electromagnetic waves that are propagated in freespace. This nature makes it vulnerable to be attacked from eavesdroppers. Fortunately, with the aid of the reconfigurable intelligent surface (RIS), which passively reflects the incident signal, the spatial distribution of the signal strength can be customized to benefit legitimate users. In this work, we propose a RIS aided non-orthogonal multiple access (NOMA) transmission scheme to provide secure links for two users, where the unmanned aerial vehicle (UAV) equipped with RIS serves as a relay to change radio coverage flexibly. In the proposed scheme, the NOMA transmit power of base-station (BS), the UAV's location, and the RIS phase shift are jointly optimized to maximize the secure transmission rate, which is a nonconvex optimization problem. For this non-convex problem, we first decompose it into three subproblems. Then an efficient iterative algorithm is proposed, where the transmit power and UAV's location are optimized through the successive convex approximation (SEA) method, and the phase shift is optimized through the semi-definite relaxation (SDR) strategy. Numerical results verify the secrecy superiority of the proposed scheme compared with the current schemes.