Secure Transmission for STAR-RIS Aided NOMA Against Internal Eavesdropping

Abstract

Simultaneous transmitting and reflecting reconfig- urable intelligent surface (STAR-RIS) has attracted extensive attentions due to its prominent advantages to future wireless networks. This correspondence considers a downlink multi-input single-output non-orthogonal multiple access system, in which the STAR-RIS is utilized to improve the secrecy performance. To restrain the internal eavesdropping, we formulate a secrecy rate maximization problem and propose a secure transmission design via the joint active and passive beamforming optimization with optimum power allocation. To decouple optimization variables, the original non-convex problem is divided into two subproblems, which are transformed into convex forms via successive convex approximation and solved by alternating optimization. Simula- tion results demonstrate the superiority of the proposed scheme in the security enhancement.