HRIS-Assisted Integrated Sensing and Communication: CraméR-Rao Bound Optimization

Hybrid reconfigurable intelligent surface (HRIS) as one of the key technologies has the potential to improve the sensing and communication performance in the upcoming sixth-generation systems by creating virtual links among the entities. This paper proposes a HRIS-assisted integrated sensing and communication (ISAC) system to simultaneously perform the sensing and communication by co-designing transmit beamforming at base station and the reflection coefficients at the HRIS. Specifically, we derive the Fisher information matrix on the estimations of transmission delay and angle of departure, and derive the corresponding closedform expression of the Cramér-Rao bound (CRB) to describe the sensing performance. Then, a CRB minimization problem is formulated by taking into account the communication requirements and power constraints, which is non-convex and generally difficult to solve. To address this problem, we propose an algorithm that combines the Schur complement technique with sequential parametric convex approximation to approximate the original problem into a convex version. Finally, numerical results indicate that the proposed method effectively enhances the performance of the ISAC system by appropriately increasing the number of active reflecting elements. It also demonstrates that our proposed HRIS outperforms the conventional passive RIS and active RIS for the ISAC system under limited power budget.