Secrecy Rate Optimization for Secure communication in IRS-Aided UAV Systems

Intelligent reflecting surface (IRS), composed of a large number of hardware-efficient passive elements, is deemed as a potential technique for future wireless communications. Meanwhile, Unmanned Aerial Vehicles (UAVs) have recently been found applications in areas besides military missions because of their mobility, maneuverability, flexibility, ease of deployment, and low cost. In this paper, we study secure communications in an IRS-aided UAV system, which consists of a UAV, a passive eavesdropper, an IRS, and multiple ground users. In this system, we formulate a secrecy rate maximization problem taking into account the power control, phase of the IRS, UAV trajectory, user scheduling, respectively. In previous research, multiple users scenarios are not taken into account in secure communication in IRS-aided UAV systems. Since this problem is non-convex, we decompose this problem into four sub-problems. Each subproblem optimizes power control, phase, UAV trajectory, and user scheduling respectively. This problem is subsequently solved via the Successive Convex Approximation (SCA) method, and an approximation solution is then obtained. Simulation shows that the system has a significant gain in average secrecy rate compared to the case without IRS. After optimizing user scheduling, the system performs better than no scheduling in terms of secrecy performance. Meanwhile, the trajectory of the UAV is different from the single-user case because the UAV has different optimal locations to maximize the average secrecy rate for each user.