Near-field Integrated Sensing and Communication: Beamforming Design and Subarray Selection

In this paper, we investigate a near-field integrated sensing and communication (ISAC) system, where the base station is equipped with a uniform linear array composed of several widely-spaced subarrays. Due to the near-field target sensing, we consider extended targets (ETs) instead of single-point targets. In particular, we model the ET by its center point and contour points, and then derive the expression of the Cramér-Rao bound (CRB) on the ET parameter estimation. Aiming to maximize the CRB, we propose a dual-loop triple alternating optimization (DTAO) scheme based on penalty dual decomposition to jointly optimize subarray selection, digital beamformer and analog beamformer, subject to the constraints of communication-to-interference ratio, total transmit power and binary constraints of subarray selection and constant modulus of analog beamformer. In the inner loop, the digital beamformer, analog beamformer and subarray selection vector are alternately optimized, while in the outer loop, the dual variables and penalty coefficients are iteratively updated until the convergence condition is met. Numerical results show the validity of the near-field CRB on the ET parameter estimation, and reveal the proposed DTAO scheme can enhance the sensing performance of the ISAC system.