Improving Physical Layer Security with RIS-Assisted Symbiotic Radio

Reconfigurable intelligent surface (RIS) has been widely exploited for secure communications in physical layer security (PLS) by destructing the eavesdropper's channel via reflect beamforming. In this paper, we investigate RIS-aided secure communications with a novel RIS design scheme. The proposed design leverages RIS to increase the achievable secrecy rate via transmitting the artificial noise (AN) instead. To do so, RIS modulates its information over the incident signal and reflects it to the legitimate receiver and eavesdropper. The RIS modulation scheme is a prior knowledge available at the legitimate receiver, but not available at the eavesdropper. Thus, the reflected signal through the RIS is naturally an additional multi-path component for the legitimate user but a type of AN for the eavesdropper, yielding a mutualistic symbiosis between the RIS and the legitimate user but a parasitic symbiosis between the RIS and the eavesdropper as demonstrated in symbiotic radio (SR). From this SR perspective, we consider an achievable secrecy rate maximization problem by optimizing the RIS reflect beamforming. To this end, we use the path-following algorithm to solve the problem iteratively. Further-more, the comparison of the conventional destruct-channel (DC)- RIS design and the proposed AN - RIS design is conducted. Finally, simulation results show that the proposed AN - RIS design outperforms the DC- RIS design in general cases where the reflecting link is weaker than the direct link of eavesdropper.