Hybrid RIS-Assisted MIMO Integrated Sensing and Communication System.

The reconfigurable intelligent surface (RIS), with the ability to program the radio environment, is considered as an essential component of the integrated sensing and communication (ISAC) system. In this paper, we propose a hybrid RIS (HRIS)-assisted multiple-input multiple-output (MIMO) ISAC system, where the HRIS is externally controllable and serves the dual function of reflecting the transmit signal to communication users and receiving radar echoes. Using this new RIS architecture, we characterize the tradeoff between communication and radar sensing performance by jointly designing the base station (BS)'s beamforming vectors and the HRIS's parameters. Our objective is to maximize the signal-to-interference-and-noise ratio (SINR) of radar while ensuring the communication SINR requirement. To solve the corresponding non-convex joint beamforming design problem, we propose an alternating optimization approach, where the auto gradient descent (AGD) is applied to effectively find the best HRIS configuration, and the semidefinite relaxation (SDR) is used to obtain the closed-form BS beamforming solution. Our numerical results are provided to evaluate the quality of radar and communication, as well as the effectiveness of the provided algorithm,