Abstract
Integrated sensing and communication (ISAC) has emerged as a critical technique of the sixth generation (6G) wireless communication. Preferable radio environments for ISAC systems can be created by leveraging reconfigurable intelligent surfaces (RISs). However, the sensing performance seriously deteriorates when the line-of-sight (LoS) signals are blocked. The RIS with sensing capabilities, namely hybrid RIS (HRIS), is capable of enhancing the sensing performance by simultaneously reflecting and receiving the sensing signals rather than just reflecting the sensing signals. In this paper, we focus on an ISAC system enabled by an HRIS under the non-line-of-sight (NLoS) condition. The communication rate is maximized by the proposed transmit beamforming and HRIS parameter design methods, which can also facilitate estimating target angles based on the atomic norm minimization. Specifically, the optimal HRIS amplitude factor is obtained by analyzing the Cramér-Rao bound (CRB) expression; the transmit and HRIS beamforming matrices are obtained by the proposed alternative optimization based on semidefinite relaxation (AO-SDR) algorithm. Simulation results demonstrate that the proposed methods can accurately estimate the target angles without deteriorating the communication rate excessively.