Outage-Constrained Secrecy Rate Maximization for STAR-RIS With Energy-Harvesting Eavesdroppers and Imperfect CSI

Abstract

This paper proposes a novel approach to enhancing secure wireless communication using a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) in a multiple-input single-output system. Unlike conventional RIS, which is limited to half-space coverage, STAR-RIS enables full 360-degree coverage, making it a promising technology for next-generation secure wireless communication. In the presence of energy-harvesting eavesdroppers, this study aims to maximize the sum secrecy rate while ensuring strict energy harvesting constraints, an area that has not been widely explored, particularly under imperfect channel state information (CSI) conditions. To tackle this challenge, we formulate a complex non-convex optimization problem, which is efficiently solved using a penalty concave-convex procedure combined with an alternating optimization algorithm. This optimization framework jointly optimizes beamforming and STAR-RIS transmission and reflection coefficients, striking a balance between secure communication and energy harvesting requirements. Numerical simulations show that the proposed method outperforms conventional RIS-based approaches, demonstrating its effectiveness even in the presence of CSI uncertainty. Furthermore, the results confirm that optimizing STAR-RIS under imperfect CSI leads to superior security and energy efficiency compared to assuming perfect CSI, highlighting the practical significance of our approach in real-world wireless networks.