Joint Trajectory and Beamforming Optimization for UAV-RIS Assisted Secure Communications

This paper investigates a UAV-RIS-assisted secure communication system, where a unmanned aerial vehicle (UAV) equipped with a reconfigurable intelligent surface (RIS) is deployed to assist the secure communication between a ground base station and a ground user in the presence of an eavesdropper. Due to the absence of a line-of-sight (LOS) channel between the base station and user, the UAV dynamically adjusts its flight trajectory and RIS phase shift matrix to achieve a reliable and secure transmission. A secrecy rate maximization problem is formulated by jointly optimizing the UAV trajectory and RIS reflection coefficients. An alternating optimization algorithm based on Dinkelbach's method is developed to tackle the non-convex problem. Numerical results illustrate the effectiveness of the proposed approach, achieving a secrecy capacity of approximately 9 bits/s/Hz at a transmit power of 10 W. The optimized UAV trajectory effectively balances proximity to Bob while maintaining a safe distance from Eve, highlighting the importance of joint trajectory and phase-shift optimization in enhancing physical layer security.