Multi-UAV Assisted Communication Networks: Towards Max-Min Secrecy Rate

This paper investigates a multi-unmanned aerial vehicle (UAV) assisted network, where multiple UAVs send confidential information to multiple authorized access users (AUs), in the existence of an unauthorized access users. In order to improve the AUs' experience quality, an optimization problem is formulated to maximize the minimum secrecy rate of AUs by jointly optimizing user scheduling, UAVs' trajectories and transmit power allocation, subject to on board energy budget and curvature radius constraints. Since the problem is a mixed integer non-convex optimization problem, an efficient algorithm is proposed based on the block coordinate descent (BCD) and successive convex approximation (SCA) theories, which achieves a good suboptimal solution. Simulation results show that the proposed algorithm converges within several iterations, and significant system performance gain is achieved compared to other benchmarks. Moreover, the larger the curvature radius is, the smoother the UAVs' trajectories are. Although it also yields the lower the secrecy rate, introducing curvature radius into trajectory design is an effective approach to make the optimized UAVs' trajectories closer to practical flight. Besides, the higher the propulsion energy stored at UAVs is, the higher the achieved max-min secrecy rate is. Additionally, the max-min secrecy rate is improved by allowing concurrent transmissions of multiple UAVs.