Secure Communications Employing Intelligent Reflecting Surfaces

This paper focuses on secure transmission with multiple intelligent reflecting surfaces (IRS) for millimeter-wave (mmWave) and terahertz (THz) systems. Specifically, it explores the scenario where a base station (BS) and a user (D) utilize multi-IRS for transmission while having active eavesdropping from an eavesdropper (E). There is no direct path between the BS and the user, nor between the eavesdropper and BS. Considering hardware implementation, the conditions for discrete phase shifts of IRS reflections are designed. Under these conditions, we maximize the system’s secrecy rate of the mmWave/THz systems. We solve the non-convex problem by decomposing it into two sub-problems: optimizing beamforming and then optimizing the reflection phase shift matrix. Then, these sub-problems are jointly optimized. For the beamforming optimization problem, we propose a closed-form optimal solution. For the optimization problem of the reflection phase shift matrix, we propose a method based on semidefinite relaxation programming (SDR) and element-wise block coordinate descent (E-BCD) to optimize the matrix. Simulation results show the effectiveness of the proposed methods.