Integrated Sensing and Communication Beamforming Design in RIS-Assisted Symbiotic Radio System

This paper aims to facilitate the integration of integrated sensing and communication (ISAC) and symbiotic radio (SR), which studies a reconfigurable intelligent surface (RIS)-assisted ISAC-SR system in single-user and multi-user scenarios. In the ISAC-SR system, a base station (BS) transmits the downlink signal to the user while sensing multiple targets. The RIS reflects the BS signal by adjusting its reflection coefficient and embeds its data for user transmission. We aim to maximize the communication rate of RIS by optimizing the transmit beamformers and RIS phase shift matrix while meeting the minimum quality of service (QoS) requirement for BS data transmission and targets sensing. Due to the non-convexity of the formulated problem, in the single-user case, we develop an alternating optimization (AO) algorithm using a semidefinite relaxation (SDR) and the Dinkelbach method to transform it into a convex problem. In the multi-user case, we leverage SDR and successive convex approximation (SCA) to obtain a suboptimal solution and prove that a rank-one solution is guaranteed. Numerical results validate the effectiveness of our proposed schemes.