Trajectory Optimization for Physical Layer Secure Buffer-Aided UAV Mobile Relaying

In this work, we study the buffer-aided relaying mechanism in a UAV-enabled mobile relaying system assisting the terrestrial communications. Optimal UAV trajectory design against a randomly located eavesdropper is investigated from the physical layer (PHY) security perspective considering the wireless channel dynamics as the UAV relay moves in the air. Specifically, we maximize the sum secrecy rate by optimizing the discrete trajectory anchor points based on the information causality and UAV mobility constraints. To make the non- convex problem tractable, the increments of the trajectory anchor points are optimized instead through an iterative updating procedure, and successive convex approximation technique is applied for progressive optimization. The convergence of the proposed iterative optimization technique is proved by introducing additional rate bound constraints and employing the squeeze principle. Simulation results show that the proposed optimal trajectory finding algorithm is effective and fast converging. Simulation results also reveal that the distribution of the eavesdropper location has a significant impact on the PHY security performance.