STAR-RIS Assisted Secure MIMO Communication Networks: Transmit Power Minimization for Perfect and Imperfect CSI

In this paper, we investigate the secure transmission design for simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted multiple-input multiple-output (MIMO) systems. By considering both perfect and imperfect channel state information (CSI) scenarios, we jointly optimize the covariance matrix of the transmitter and the transmitting and reflecting coefficients of the STAR-RIS and formulate two transmit power minimization problems. For the optimization problem in the perfect CSI scenario, we develop a penalty-based alternating optimization (AO) algorithm to handle it. For the optimization problem in the imperfect CSI scenario, this paper is the first work to study the robust beamforming design for STAR-RIS-assisted secure MIMO systems. To address this challenging problem, we first use the inequalities of the determinant to transform it into an equivalent form. Then, we use the generalized S-procedure to handle the worst-case constraints. Finally, we develop a penalty-based AO algorithm. Performance evaluation results show that the two proposed optimization algorithms significantly reduce the transmit power compared to other baseline schemes.