Reconfigurable Intelligent Surface Assisted Secure Symbiotic Radio Multicast Communications

In this paper, we propose a reconfigurable intelligent surface (RIS) assisted secure transmission scheme for a symbiotic radio multicast system, where the RIS not only assists the confidential information multicasting from a primary transmitter (PT) to multiple primary users (PUs) to against the information interception by eavesdroppers, but also delivers its own signal to a secondary user (SU) by passive reflections. We formulate a signal-to-noise ratio (SNR) maximization problem for the SU by jointly optimizing the active beamforming at the PT, amplitude reflection coefficients and phase shifts of the RIS. To address the non-convexity of the formulated problem, we propose to decompose the original problem into two sub-problems and solve them independently in an iteratively alternating manner. For the first sub-problem, we adopt the successive convex approximation (SCA) and semidefinite relaxation (SDR) techniques to design the active beamforming by proving the tightness of SDR. For the second sub-problem, the sequential rank-one constraint relaxation (SROCR) technique is adopted to handle the rank-one constraint for reflection coefficients optimization. Numerical results show that compared to the benchmark schemes, the proposed scheme can achieve up to 68.3% performance gain in terms of SNR.