Covert RIS-Based Symbiotic Radio in Artificial Noise-Aided Secure Communications Systems

This work is focused on safeguarding the security of a reconfigurable intelligent surface (RIS) based symbiotic radio (SR) system. In the considered system, a primary link coexists with an RIS-based backscatter link. For the primary link, physical layer security is used to ensure the confidentiality of the transmitted data, for which a null-space artificial noise scheme is adopted. For the RIS-based backscatter link, we design the reflection coefficients of the RIS so that it becomes covert to a third-party. Under the condition that the backscatter link is covert and its signal-to-noise ratio is above a pre-given threshold, the secrecy rate of the primary link is maximized by jointly optimizing the beamforming vector of the primary transmitter and the reflection coefficients of the RIS. The considered optimization problem is non-convex, and we present a penalty-based iterative method to handle with it, along with an alternating-optimization based initialization method. Numerical results reveal that even though the backscatter link interferes with the primary link, the secrecy rate of the primary link could still be increased as compared with the case without an RIS, which demonstrates the superiority of the RIS-SR technology.