Cramér-Rao Bound Optimization for Near-Field Wideband ISAC: Delay-Phase Precoding for Beam Squint Mitigation

A near-field wideband integrated sensing and communication (ISAC) system is investigated where a base station (BS) simultaneously performs downlink communication and target sensing. To mitigate the beam squint effect induced by wideband transmission, a true-time-delay (TTD)-based beamforming architecture is developed. The close-form Cramér-Rao bound (CRB) for angle and distance estimation in the wideband scenario is derived, which is minimized subject to the user communication requirement. To address the non-convex optimization problem, a penalty-based method is proposed, where the block coordinate descent (BCD) algorithm is employed to decouple the variables for iterative optimization. Numerical results demonstrated that the proposed approach achieves a favorable trade-off between communication and sensing and exhibits improved robustness against bandwidth variations.