Iterative UAV Trajectory Optimization for Physical Layer Secure Mobile Relaying

With the rapid development and growth of un-manned arial vehicle (UAV) technology and industry, there is increasing research interests in UAV communications. In this work, the mobility and deployment flexibility of UAV is used to provide assistance to terrestrial point-to-point communications. In particular, design of the UAV-enabled mobile relaying system is investigated from the physical layer security perspective to achieve higher overall secrecy capacity. A dynamic channel model that changes with the trajectory is established based on the UAV relay's mobility. By optimizing the UAV's moving trajectory, secrecy capacity of the relay-assisted wireless communication system is maximized. The problem formulated is shown to be non-convex, which is difficult to solve in general. To make the problem tractable, we decompose the original problem and optimize the trajectory to approximate the optimal flight path by optimizing the increments at each trajectory iteration. Simulation results show that the method of finding the optimal trajectory based on the displacement updating iteration is effective and fast converging. When the number of iterations is sufficiently large, the trajectory of the UAV converges, and the optimized trajectory significantly improves the system's secrecy capacity performance. Based on the optimized trajectory, it is investigated through numerical results how the overall secrecy capacity is impacted by the total flight time and the maximum instantaneous UAV speed. It is revealed that higher flight speed is desirable in finding a secure UAV trajectory, and there exists a minimum total flight time to achieve the best average secrecy rate performance.