Secrecy Communication for Simultaneous Transmission and Reflection (STAR) RIS Aided Systems With Multiple Eavesdroppers

In this paper, we investigate the physical layer security for simultaneous transmission and reflection (STAR) reconfigurable intelligent surface (RIS) aided systems. We consider a more practical case where multiple eavesdroppers can wiretap the transmitted signals from the access point (AP) to the legitimate users located in the transmission and reflection regions, respectively. An artificial noise (AN) aided secrecy transmission scheme is then proposed, aiming to improve the secrecy performance. The transmit beamforming and AN vectors of AP as well as the transmission and reflection coefficients are jointly optimized to maximize the sum secrecy rate. To effectively solve the formulated problem, it is firstly divided into two subproblems, and then the methods of semi-definite relaxation and Sion's minimax theorem are employed to deal with the subproblems. By alternatively solving the sub-problems, the optimal solutions are obtained. Simulation results demonstrate the effectiveness of the proposed scheme compared to the conventional scheme without AN and STAR-RIS, and also bring useful insights into the design of secrecy communications in STAR-RIS aided systems with multiple eavesdroppers.