Double-RIS-Aided Secure and Covert Communications for Symbiotic Radio Systems

Abstract

In this paper, we propose a double-cooperative reconfigurable intelligent surfaces (RIS)-aided secure and covert framework in symbiotic radio (SR) systems in the presence of an eavesdropper and a warden. In particular, one RIS is deployed to establish the primary communication link, while another backscatter device (BD)-based RIS modulates a signal onto the primary link to convey covert symbols via the backscatter link. We formulate an average bit error rate (BER) minimization problem via optimizing the transmit beamforming and RISs passive beamforming, while satisfying BD link covertness, security requirement of primary link and power budget constraints. We develop a penalty-based block coordinate descent (BCD) algorithm, in which the system configurations are efficiently solved by combining lagrange duality, successive convex approximation (SCA), and majorization-minimization (MM) method. Simulation results are provided to verify the effectiveness of our proposed algorithm by comparing it to benchmark strategy.