RIS-Assisted Integrated Sensing and Symbiotic Radio Systems

In this letter, we propose a reconfigurable intelligent surface assisted integrated sensing and symbiotic radio (ISASR) system. Our goal is to establish symbiotic radio (SR) communications constituted by simultaneous transmissions of primary and secondary signals, while ensuring target sensing performance. To address the absence of direct links for SR communications, we consider two RIS modulation schemes, namely, the RIS partitioning (RP) scheme that divides two sub-surfaces for assisting primary transmission and enabling secondary transmission separately, and the hybrid phase-shifting (HPS) scheme that achieves the above functionalities through coupled phase shift design. For each scheme, we formulate an optimization problem to maximize the beampattern gain at the sensing target through joint optimization of transmit beamforming and RIS phase shifts. Although the problem is non-convex and challenging to solve, we propose an efficient alternating optimization algorithm based on semidefinite relaxation and successive convex approximation techniques to find its sub-optimal solution. Simulation results show that the HPS scheme can achieve superior performance over the RP scheme due to its generalized form.