Aerial IRS-Enabled Secure Mobile Communications: Joint 3-D Trajectory and Beamforming Design

This letter investigates a novel aerial intelligent reflecting surface (IRS)-assisted secure mobile communication system. In particular, the IRS is mounted on a unmanned aerial vehicle (UAV) to help a source transmit its confidential messages to a legitimate mobile user in the presence of a mobile eavesdropper. The aerial IRS can adjust its trajectory and phase-shift to track the moving user and provide safer communication services. Furthermore, due to the mobility of the UAV, user and eavesdropper, the effect of Doppler shifts is also taken into consideration in the channel model. Under such a setup, we formulate an average secrecy rate maximization problem to jointly optimize the 3D trajectory of the UAV and the phase-shift matrix of the aerial IRS. To deal with this non-convex optimization problem, we decompose the original problem into two subproblems and propose an iterative algorithm to determine its suboptimal solution. Numerical results show that the proposed aerial IRS-assisted 3D joint design can significantly improve the secrecy rate compared to the benchmark schemes.