Cramér-Rao Bound Optimization for STAR-RIS Enabled ISAC System

We consider a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) enabled integrated sensing and communication (ISAC) system. The whole space is divided into sensing and communication subspaces by the STAR-RIS. The Cramér-Rao bound (CRB) for the estimation of the direction of arrival (DoA) of the radar echo is derived. By simultaneously optimizing the transmit beamforming at the base station and the reflection and transmission vectors of the STAR-RIS, a CRB minimization problem is formulated while the minimal signal-to-interference-plus-noise ratio (SINR) at the communication user equipment is used as a constraint. In this context, the original problem is decomposed using an alternating optimization (AO) algorithm, and two subproblems are alternately optimized using the semi-definite relaxation (SDR) and successive convex approximation (SCA) techniques until the final solution converges. Finally, simulation results confirm the effectiveness of the proposed method.