Secure Transmission in Active RIS-Assisted Cell-Free MU-MISO Networks

To combat the cascade channel fading effect in passive reconfigurable intelligent surface (RIS)-assisted systems, active RIS has gained growing attention and research focus. In this paper, we investigate the secure transmission in active RIS-assisted cell-free multi-user multiple input single output (MU-MISO) networks based on physical layer security. To maximize the weighted sum of secrecy rate (WSSR) of legitimate users, we jointly optimize the beamforming (BF) at the base stations (BSs) and the reflection matrix of the RIS. To handle the non-convex problem, we propose an alternating optimization to decouple the original problem into two sub-problems. The first sub-problem is to optimize the BF at the BSs by using the semidefinite relaxation and successive convex approximation. The second sub-problem is to optimize the reflection matrix of the RIS by using the semidefinite relaxation and the Gaussian randomization method. Simulation results verify that the proposed scheme shows a better performance in improving the system security than the passive RIS scheme under the conditions of power restrictions. Compared with the passive RIS scheme, the proposed scheme increases the WSSR up to 17% and effectively enhances the security of the cell-free networks.