Joint Trajectory and Power Design for UAV-Enabled Secure Communications With No-Fly Zone Constraints

Unmanned aerial vehicles (UAVs) have attracted growing interest in wireless communications due to their several superiorities, such as highly controllable mobility, favorable communication links with the ground, on-demand deployment, and low cost. This paper investigates a UAV-ground communication system, where a UAV is dispatched to send classified information to a legitimate user in the presence of an eavesdropper on the ground while simultaneously avoiding flying over the no-fly zones (NFZs). We aim to maximize the average secrecy rate of the system by jointly optimizing the UAV's trajectory and transmit power over a given flight period under the practical constraints on the UAV's maximum speed, the initial and final locations, avoidance of NFZs, as well as the transmit power. Although the existing works have studied a similar secrecy rate maximization problem, they all rely on general-purpose solvers, which leads to considerably high computational complexity. To address this issue, we propose an efficient algorithm by applying the techniques of alternating optimization (AO) and successive convex approximation (SCA) to obtain a suboptimal solution and utilizing the alternating directional method of multipliers (ADMM) under the SCA framework to realize low-complex implementation. The simulation results demonstrate the superior computational efficiency of our proposed algorithm and show the impact of NFZs on the system.