Secrecy Energy Efficiency Maximization for RIS-Assisted UAV Communications via Joint 3D Trajectory and Passive Beamforming Design

Abstract

Unmanned aerial vehicle (UAV) communications show great potential in establishing strong line-of-sight (LoS) links for ground users, which also leads to more susceptible terrestrial eavesdropping. Integrating a reconfigurable intelligent surface (RIS) into UAV communications offers a promising approach for secure transmission. This paper considers a secure uplink system assisted by RIS and UAV, in which a rotary-wing UAV acts as an aerial receiver to collect the information from two ground users under the threat of a terrestrial eavesdropper. A probabilistic LoS (PLoS) channel model is employed to characterize the ground-to-air (G2A) links. We formulate the design objective as maximizing the secrecy energy efficiency (SEE) through the joint optimization of the three-dimensional UAV trajectory, RIS passive beamforming, and user scheduling. The corresponding problem is highly coupled and nonconvex. To effectively solve it, we develop an iterative framework and adopt the method of block coordinate descent (BCD) to decompose the original problem into several tractable subproblems, which are handled via semi-definite relaxation (SDR), Dinkelbach’s algorithm, and successive convex approximation (SCA). Numerical results demonstrate that the proposed design achieves significant SEE gains over benchmark schemes. Index Terms-Unmanned aerial vehicle (UAV), reconfigurable intelligent surface (RIS), secrecy energy efficiency (SEE), probabilistic LoS channel, 3D trajectory design.