Safeguarding MIMO Communications with Reconfigurable Metasurfaces and Artificial Noise
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Wireless communications empowered by Reconfigurable Intelligent (meta)Surfaces (RISs) are recently gaining remarkable research attention due to the increased system design flexibility offered by RISs for diverse functionalities. In this paper, we consider a Multiple Input Multiple Output (MIMO) physical layer security system including one legitimate passive and one eavesdropping passive RIS with the former being transparent to the eavesdropper and the latter's presence being unknown at the legitimate link. We first focus on the eavesdropping subsystem and present a joint design of the eavesdropper's combining vector and the reflection coefficients of the eavesdropping RIS. Then, focusing on secrecy rate maximization, we propose a transmission scheme that jointly designs the legitimate precoding vector and Artificial Noise (AN) covariance matrix, as well as the reflection coefficients of the legitimate RIS. Our simulation results reveal that, in the absence of a legitimate RIS, AN and precoding are incapable of offering nonzero secrecy rates even for eavesdropping RISs with small numbers of unit elements. However, when a L-element legitimate RIS is deployed, confidential communication can be safeguarded against cases with even more than a 3L-element eavesdropping RIS.
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