Joint Trajectory and RIS-NOMA Optimization for Multi-User UAV Secure Communications

Unmanned aerial vehicles (UAVs) have become key components of sixth-generation (6G) wireless networks and have significantly promoted the rapid development of the lowa-ltitude economy (LAE). However, UAV communications are inherently vulnerable to eavesdropping attacks owing to the open and broadcast nature of wireless channels, which makes transmission secrecy a critical challenge. This paper investigates an air-to-ground secure transmission system in which a UAV-mounted reconfigurable intelligent surface (RIS) assists a base station (Alice) while simultaneously serving multiple ground users (Bobs) under the surveillance of an eavesdropper (Eve). We mount the RIS on the UAV to jointly exploit UAV mobility and RIS reconfigurability for physical-layer security (PLS). First, we consider a single-user case in which we formulate a secrecy throughput maximization problem by jointly optimizing the UAV trajectory and RIS phase shifts. The framework is then extended to a multi-user non-orthogonal multiple access (NOMA) system, where the additional transmit power allocation between users is optimized. To address the resulting nonconvex problems, we develop an efficient alternating optimization (AO) framework by leveraging successive convex approximation (SCA) and semidefinite relaxation (SDR). The numerical results show that the proposed method achieves rapid convergence and delivers significant improvements in secrecy performance over benchmark schemes with random RIS phase shifts and without UAV trajectory planning. Our results highlight the importance of UAV–RIS cooperation in achieving secure and scalable multi-user communications for future wireless networks.