LAUSR

—A secure transmission framework is proposed for simultaneously transmitting and reflecting surface (STARS) net- works 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 par- ticularly, 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.