Secrecy Rate Analysis for RIS-Aided Multi-User MISO System over Rician Fading Channel

The reconfigurable intelligent surface (RIS), which is composed of multiple passive reflective components, is now considered as an effective mean to improve security performance in wireless communications, as it can enhance the signal of legitimate users and suppress the power leakage at eavesdroppers by adjusting signal phases. In this paper, we maximize the downlink ergodic secrecy sum rate of a RIS-aided multi-user system over Rician fading channels, where we assume that only imperfect channel state information (CSI) is available at the base station (BS). Firstly, we obtain the deterministic approximate expression for the ergodic secrecy sum rate by resorting to the large-system approximation theory. Then the problem is formulated to maximize the downlink ergodic secrecy sum rate by optimizing the regularization coefficient of regularized zero-forcing (RZF) precoding and the phase-shifting matrix of the RIS. By using the particle swarm optimization (PSO) method, we propose an alternate optimization (AO) algorithm to solve this non-convex problem. Finally, the numerical simulations illustrate the accuracy of our large-system approximate expression as well as the effectiveness of the proposed algorithm.