STAR-RIS Enabled Downlink Secure NOMA Network under Imperfect CSI of Eavesdroppers

Reconfigurable intelligence surface (RIS) is regarded as one of the key technologies for the future wireless communications. Recently, simultaneous transmission and reflection (STAR) RIS is proposed to overcome the geographical constraint of the conventional reflection only RIS. In this letter, we model the STAR-RIS enabled downlink secure non-orthogonal multiple access systems under imperfect channel state information (CSI) of eavesdroppers. Specifically, we propose the block coordinate descent based optimization framework to maximize the worst secrecy rate with active beamforming of downlink NOMA at the base station and STAR-RIS coefficients, where the uncertainty of eavesdropper's CSI is transformed by S-Procedure. The successive convex approximation and a penalty-based method are utilized to address the non-convexity of the problem. Finally, simulation results demonstrate that the worst secrecy rate of the proposed algorithm outperforms that of the conventional RIS and the other benchmark schemes.