Optimal Reconfigurable Intelligent Surface Deployment for Secure Communication in Cell-Free Massive Multiple-Input Multiple-Output Systems with Coverage Area

• Published in: Electronics (Basel) 2025-01, Vol.14 (2), p.241
• Main Authors: Zhao, Jie, Zhang, Qi, Ai, Tianyu, Wei, Xianhu, Peng, Fengqiang
• Format: Article
• Language: English
• ISSN: 2079-9292; 2079-9292
• DOI: 10.3390/electronics14020241

This paper investigates the secure communication in the reconfigurable intelligent surface (RIS)-aided cell-free massive multiple-input multiple-output (CF-mMIMO) system in the presence of an eavesdropper (Eve). Since the RIS can only reflect the incident signal from its front, we define the RIS coverage and non-coverage area based on whether the incident signals can be reflected. The RIS coverage area is affected by the deployment position and rotation angle, and thus, we take both of these two factors into account and a closed-form approximation for the ergodic secrecy rate of the legitimate user is derived. Based on it, the optimal RIS deployment position and phase shift are obtained through an alternating iteration method, and the optimal RIS angle is achieved through an exhaustive enumeration of angles with a certain interval. Simulations confirm that our optimal RIS deployment can achieve a superior secrecy rate. We find that to guarantee the best secrecy rate, the RIS should be placed near the target user, and its rotation angle should be adjusted to make as many access points (APs) as possible within the RIS coverage area.