Abstract: The reconfigurable intelligent surface (RIS) is an effective technique that can improve wireless signal propagation environment and enhance communication security by adjusting the reflected signal through its controlled reflecting elements. The RIS-assisted secure communication in a non-orthogonal multiple access system is considered, where multiple users simultaneously send confidential information to a single-antenna base station in a non-orthogonal multiple access way in the presence of a single-antenna eavesdropper. Our objective is to maximize the total security rate of the system by optimizing the transmit power of the users and the phase shifts of the reflecting elements of the RIS while satisfying the constraints of transmit power and the quality-of-service for each user, as well as the constant modulus constraint of each reflecting element of the RIS. Although the considered optimization problem is non-convex and difficult to solve, an efficient algorithm based on the alternate optimization and semidefinite relaxation techniques is proposed to obtain a near-optimal solution. Simulation results validate that as compared with two benchmark schemes, the proposed algorithm significantly improves the total security rate of the system. It provides a reference for the secure communication research of reconfigurable intelligent surface assisted non orthogonal multiple access system.

Key words: reconfigurable intelligent surface, physical-layer security, secrecy rate