Joint Port Selection and Cramér-Rao Bound Optimization for ISAC in Fluid Antenna System

Integrated sensing and communication (ISAC) has been envisioned as a key enabler for next-generation wireless systems. Meanwhile, fluid antenna system (FAS) has emerged as a promising flexible antenna technology, offering enhanced multiple access capabilities and significant spatial diversity gains. This paper studies the problem of joint port selection and Cramér-Rao Bound (CRB) optimization in an ISAC-enabled FAS. Specifically, the objective is to minimize the CRB while ensuring compliance with communication quality of service (QoS) requirements, transmit power constraints, and port selection limitations. To address this problem, we propose an iterative optimization algorithm leveraging the majorization-minimization (MM) and block coordinate descent (BCD) methods. Numerical results validate the effectiveness of the proposed algorithm, demonstrating its ability to achieve near-optimal solutions with efficient search.