Secrecy Rate Optimization for STAR-RIS-Enhanced UAV Communications

In this paper, a novel unmanned aerial vehicle (UAV) secure communication system enhanced by the passive coupled phase-shift simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR- RIS) is investigated. Considering the multiple-user and multiple-eavesdropper systems, we aim to maximize the minimum average secrecy rate by the joint design of the UAV base station's (UAV-BS's) beamforming, the UAV's trajectory, and the STAR-RIS's transmission and reflection (T&R) coefficients. Since the formulated optimization problem is highly non-convex with intricately coupled variables, we decompose it into three subproblems, and resolve them alternately by semi-definite relaxation, successive convex approximation, and penalty-based approach. Simulation results show that: 1) the practical coupled phase-shift model of passive STAR-RIS suffers a security performance degradation compared with the ideal independent phase-shift model, but it still outperforms the conventional RIS; 2) the UAV's trajectory tends to be closer to the STAR-RIS to fully exploit the benefits of STAR-RIS; 3) the amplitudes of T&R coefficients highly depend on the UAV's real-time trajectory.