Joint Mode Selection and Beamforming Designs for Hybrid-RIS Assisted ISAC Systems

TL;DR

This paper proposes a joint optimization framework for hybrid-RIS assisted ISAC systems, enabling flexible mode switching of RIS elements to enhance sensing and communication performance under power and SINR constraints.

Contribution

It introduces a novel joint design method for mode selection and beamforming in hybrid-RIS ISAC systems, formulated as a mixed-integer nonlinear program and solved efficiently.

Findings

Proposed method outperforms baseline solutions in simulations.

Efficiently optimizes beamforming and RIS mode selection.

Achieves significant improvements in beampattern gain.

Abstract

This paper considers a hybrid reconfigurable intelligent surface (RIS) assisted integrated sensing and communication (ISAC) system, where each RIS element can flexibly switch between the active and passive modes. Subject to the signal-to-interference-plus-noise ratio (SINR) constraint for each communication user (CU) and the transmit power constraints for both the base station (BS) and the active RIS elements, with the objective of maximizing the minimum beampattern gain among multiple targets, we jointly optimize the BS transmit beamforming for ISAC and the mode selection of each RIS reflecting element, as well as the RIS reflection coefficient matrix. Such formulated joint hybrid-RIS assisted ISAC design problem is a mixed-integer nonlinear program, which is decomposed into two low-dimensional subproblems being solved in an alternating manner. Specifically, by using the semidefinite relaxation (SDR) technique along with the rank-one beamforming construction process, we efficiently obtain the optimal ISAC transmit beamforming design at the BS. Via the SDR and successive convex approximation (SCA) techniques, we jointly determine the active/passive mode selection and reflection coefficient for each RIS element. Numerical results demonstrate that the proposed design solution is significantly superior to the existing baseline solutions.