Improving Dual-UAV Aided Ground-UAV Bi-Directional Communication Security: Joint UAV Trajectory and Transmit Power Optimization

This paper investigates a dual-unmanned aerial vehicle (UAV) aided communication system to improve the security of the communication between ground devices and UAVs. Different from the existing works which ignored ground devices mobility and just considered one-way communication security between ground devices and UAVs, we allow the devices to be mobile and consider bi-directional ground-UAV communication security. Specifically, one UAV server communicates with mobile ground devices, and the other UAV jammer is invoked to confuse eavesdroppers. Our objective is to maximize the worst-case average secrecy rate by the joint optimization of UAV trajectory and sender transmit power. To achieve it, we first formulate the worst-case average secrecy rate maximization problem as a constrained Markov decision process (CMDP) under the constraints of UAV flight space, flight speed, energy capacity, anti-collision, and peak transmit power. Then, we design a Deep Deterministic Policy Gradient (DDPG) based algorithm to solve the CMDP. Experiment results demonstrate that our joint optimization scheme can enhance the communication security in terms of the secrecy rate in both UAV-to-ground (U2G) case and ground-to-UAV (G2U) case. Besides, it is observed that UAV trajectory and sender transmit power have different impacts on the communication security in U2G case and G2U case.