Secure Transmission for IRS-on-UAV-Assisted Wireless Networks

Combined with the unmanned aerial vehicle (UAV), intelligent reflecting surface (IRS) can leverage the adjustable mobility and flexible deployment, thus enhancing the quality of wireless transmission via line-of-sight (LoS) links. However, IRS-assisted UAV network may also encounter security issues in the physical layer. To realize the secure transmission and improve transmission performance, we investigate an IRS-assisted UAV network against an eavesdropper in this paper, where the IRS is mounted on the UAV to leverage its mobility. Our goal is to maximize the secrecy rate while improving overall transmission performance by jointly optimizing the transmit/jamming beamforming vectors, phase shifting matrix, and the hovering location of IRS. Owing to the non-convexity of the summarized optimization problem, we decompose the optimization problem into three subproblems and solve them alternately to derive the optimal beamforming vectors, UAV location and phase shifting matrix. First, transmit/jamming beamforming vectors are derived with given IRS location and phase shifting matrix. Then, we adopt the first-order Taylor expansion to change the non-convex location optimization subproblems into a mathematical solvable convex version, and then solve it through successive convex approximation (SCA) with given phase shifting matrix and beamforming vetors. Next, with given UAV hovering location and and beamforming vetors, the phase shifting matrix is optimized via semidefinite relaxation (SDR) and SCA. Simulation results are provided to evaluate the effectiveness of our proposed secure IRS-on-UAV transmission scheme.