Performance Analysis for Near-Field ISAC: A Holographic MIMO Design

A near-field holographic multiple-input multiple-output (MIMO) based integrated sensing and communications (ISAC) framework is proposed for both downlink and uplink scenarios, where spherical wave-based model is considered to capture the characteristics of the near field. The coupling effect introduced by the densely spaced antennas of the holographic MIMO are characterized by spatially correlated Rayleigh fading. Based on the proposed framework, by considering both instantaneous channel state information (CSI) and statistical CSI, closed-form expressions are derived for sensing rates (SRs), communication rates (CRs), and outage probabilities under different ISAC designs. Further insights are gained by examining high signal-to-noise ratio slopes and diversity orders. Specifically, 1) for the downlink case, a sensing-centric (S-C) design and a communications-centric (C-C) design are investigated based on different beamforming strategies, and a Pareto optimal design is proposed to characterize the attainable SR-CR region; and 2) for the uplink case, the S-C design and the C-C design are distinguished by the interference cancellation order of the communication signal and the sensing signal, and the rate region is obtained through a time-sharing strategy. Numerical results reveal that the proposed ISAC system achieves more extensive rate regions than the conventional frequency-division sensing and communications system, highlighting its superior performance.