Joint 3D Trajectory and Tilt Angle Optimization for UAV-Assisted Secure Communications

To exploit the full spatial degrees of freedom (DoFs) and maximize the average secrecy rate in unmanned aerial vehicle (UAV)-assisted communication systems, this paper proposes a novel physical layer security (PLS) architecture that integrates the UAV’s 3D kinematic trajectory with its tilt angle control. This joint design is particularly effective in the challenging scenario where the UAV, the legitimate receiver, and the eavesdropper are geometrically aligned. Furthermore, we develop an efficient alternating optimization (AO) framework to decouple the highly non-convex secrecy rate maximization problem, in which the tilt angle and the flight trajectory are iteratively optimized via a two-stage coarse-to-fine strategy and the successive convex approximation (SCA) method, respectively. Extensive simulation results demonstrate that by incorporating UAV tilt angle control, the proposed architecture liberates the UAV from taking powerconsuming spatial detours, yielding a substantial 11.7% enhancement in the average secrecy rate compared to conventional trajectory-only baseline schemes.