Secure Transmission in STAR-RIS-Assisted Cell-Free Networks: Maximizing Weighted Sum Secrecy Rate

This letter focuses on the secure transmission problem in simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted cell-free networks. Previous research has primarily focused on optimizing the transmission and reflection coefficients of STAR-RIS independently. However, we take into account the practical coupled phase shift characteristics of STAR-RIS. We design a secure beamforming scheme by combining the distributed base station of the cell-free network, aiming to maximize the weighted sum secrecy rate (WSSR). To solve the highly non-convex problem, we employ the Augmented Lagrangian (AL) method by linearizing the original problem, enabling us to obtain a closed-form solution for the coupled phase shift of STAR-RIS in each iteration. The simulation results show that compared with the traditional RIS-assisted cell-free system, the proposed scheme has significantly improved security performance.