Joint Beamforming and Trajectory Design for Aerial Intelligent Reflecting Surface-Aided Secure Transmission

Abstract

This paper studies the secure transmission challenge confronted with future communication systems. In our considered model, the confidential communication between legitimate users is strengthened by an aerial intelligent reflecting surface (AIRS) deployed on aerial platforms such as an unmanned aerial vehicle (UAV). The average secrecy rate for all time slots is first investigated to improve information security during AIRS flights. Then, the transmit beamforming, phase-shifting matrix, and trajectory of AIRS are jointly designed, aiming to maximize the average secrecy rate performance between legitimate users. On account of the non-convexity of the formulated objective function and the coupled three key variables, we resort to an alternative strategy that converts the original objective into three sub-problems and solves them recursively. In particular, the transmit beamforming is designed on the basis of the generalized eigenvalue optimization method, and the closed-form solution is derived. For the AIRS-related optimization, a Minorization-Maximization (MM)-based algorithm and a deep deterministic policy gradient (DDPG)-based method are proposed to derive the solutions of phase shift matrix and trajectory, respectively. Simulation results show that AIRS assistance can obtain nearly twice the secrecy performance of terrestrial intelligent reflecting surface (TIRS) systems.