This paper addresses secure transmission in a high-occlusion urban environment, where an intelligent reflecting surface (IRS)-assisted unmanned aerial vehicle (UAV) communication system serves a legitimate user while countering an eavesdropper. The UAV signal is reflected to the base station through the IRS. We study the secure energy efficiency optimization problem. The tightly coupled optimization variables make the problem difficult to solve directly. Therefore, we decompose the original problem into three sub-problems. For the UAV active beamforming design, the closed-form solution can be obtained directly. For the IRS phase shift optimization, we propose an optimization algorithm based on Riemannian manifolds to obtain the optimal solution. Due to the non-convex fractional UAV trajectory optimization, it can be solved by successive convex approximation (SCA) and the Dinkelbach algorithm. Different comparison schemes are designed to evaluate the effectiveness of the proposed algorithm. The simulation results show that the proposed algorithm has improved advantages compared with other schemes.
Lv et al. (Mon,) studied this question.