Active Hybrid RIS Enabled Integrated Sensing and Communications Systems: Design and Optimization

In this paper, we propose an active hybrid reconfigurable intelligent surface (AHRIS) structure, which can reflect and receive the incident signals while amplifies them to mitigate multiplicative attenuation. Based on the properties of AHRIS, we consider an AHRIS-assisted integrated sensing and communication (ISAC) system to evaluate the performance enhancement of AHRIS. In the downlink (DL), the ISAC base station (BS) performs radar sensing and wireless communication with assistance of AHRIS, which first amplifies the composite signals sent by the ISAC BS and then reflects them to the sensing targets (STs) and communication users (CUs). During the uplink (UL), the radar echoes from the detecting zone are received by the AHRIS, then the received signals are amplified and aggregated at the AHRIS, and finally forwarded to the ISAC BS. Under the considered ISAC system, we aim to maximize the minimum sensing signal-to-interference-plus-noise ratio (SINR) among all STs by jointly optimizing sensing beamforming matrix, communication beamforming matrix, reflected matrix and received vector of AHRIS, and radar receive beamforming matrix, subject to the quality of service (QoS) constraints for CUs. To tackle the non-convex optimization problem, we first translate the objective function into a more tractable form. Then, an alternate optimization (AO) algorithm framework is utilized to optimize all the parameters based on the transformed problem. Numerical results verify the validity of the proposed AHRIS structure and optimization solution in enhancing the minimum sensing SINR while guaranteeing the communication constraints.