Secure Rate Splitting in STAR-RIS Assisted Downlink MISO Systems

This paper explores a secure downlink multiple-input single-output (MISO) system using simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided rate splitting multiple access (RSMA), where a base station transmits confidential signals to users, combating eavesdroppers in both transmission and reflection spaces of the RIS. Assuming known channel state information (CSI) at the base station, the RSMA strategy involves splitting user messages into common and private parts, aiming to maximize the sum secrecy rate under power and beamforming constraints. We address the non-convex optimization challenge through alternating optimization and the sequential convex approximation (SCA) method, focusing on both active and passive beamforming. Our simulations highlight notable enhancements in secrecy rate and spectral efficiency via STAR-RIS, underscoring the effectiveness of our algorithms. Furthermore, our approach outperforms the non-orthogonal multiple access (NOMA) technique.