Energy-Efficient Secure Communication in Coupled Phase-Shift STAR-RIS Aided MU-MISO Systems

This paper explores the energy-efficient secure communication design enabled by simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS). In particular, we consider a coupled phase-shift STAR-RIS aided downlink network within a multi-user multiple-input single-output (MU-MISO) system, where eavesdroppers are present. To optimize the system’s secrecy energy efficiency (SEE), we jointly design the secure beamforming and STAR-RIS coefficients. An alternating optimization (AO) framework is proposed to tackle the non-convexity of the problem. Specifically, the beamforming is configured via the successive convex approximation (SCA) algorithm, while closed-form solutions for the coupled STAR-RIS coefficients are derived using the penalty dual decomposition (PDD) framework. Numerical results validate the algorithm’s effectiveness in enhancing both energy efficiency and security, and reveal the trade-off between hardware feasibility and performance.