Secure wireless communications for STAR-RIS-assisted millimetre-wave NOMA uplink networks

This paper explores the potential application of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) in improving the physical layer security (PLS) of millimetre wave (mmWave) non-orthogonal multiple access (NOMA) uplink communications. In particular, the legitimate users on both sides of STAR-RIS send confidential information simultaneously to the base station (BS) while keeping it secret from the eavesdroppers near the BS by exploiting the STAR-RIS to proactively regulate the electromagnetic propagation environment. By jointly designing the transmit power, the active beamforming at the BS, and the reflecting/transmitting coefficients at the STAR-RIS, the authors' goal is to maximize the minimum secrecy capacity subject to the successive interference cancellation decoding order constraints. Due to the non-convexity of the formulated problem, an efficient algorithm is proposed by capitalizing on alternating optimization, a penalty-based approach, successive convex approximation, and semi-definite relaxation. Firstly, a two-layer iterative algorithm based on the penalty for the joint beamforming optimization sub-problem is proposed. Then, the non-convex problem is transformed into a convex positive semi-definite programming problem for the transmit power sub-problem. Numerical simulation results reveal that the STAR-RIS-aided system has more tremendous advantages and effectiveness in improving the PLS of mmWave NOMA uplink communication compared with the benchmark schemes.