Security Enhancement for STARS with an Untrusted User

Abstract

A secure transmission framework is proposed for simultaneously transmitting and reflecting surface (STARS) networks in presence of an untrusted user. The active and passive secure beamforming optimization problem is addressed. A double-loop alternating optimization (DLAO) algorithm is proposed for maximizing the achievable sum secure capacity (SC). More particularly, the inner loop is for beamforming optimization by utilizing successive convex approximation approaches, while the outer loop is for the rank-one constraint recovery by utilizing penalty-based optimization approaches. Numerical results demonstrate that: 1) the proposed DLAO algorithm converges within a few iterations; 2) the proposed framework is capable of achieving enhanced SC compared to conventional reflecting/transmitting-only reconfigurable intelligent surfaces.