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.

翻译：本文研究一种混合可重构智能表面辅助的集成感知与通信系统，其中每个RIS单元可在有源与无源模式间灵活切换。在满足每个通信用户的信干噪比约束、以及基站与有源RIS单元的发射功率约束条件下，以最大化多个目标间的最小波束方向图增益为目标，我们联合优化了用于ISAC的基站发射波束成形、每个RIS反射单元的模式选择以及RIS反射系数矩阵。该联合混合RIS辅助的ISAC设计问题是一个混合整数非线性规划问题，可分解为两个低维子问题并通过交替方式求解。具体而言，通过采用半定松弛技术结合秩一波束成形构造过程，我们高效地获得了基站处最优的ISAC发射波束成形设计。借助半定松弛与逐次凸逼近技术，我们联合确定了每个RIS单元的有源/无源模式选择及其反射系数。数值结果表明，所提出的设计方案显著优于现有的基线方案。