The technical trends for the next-generation wireless network significantly extend the near-field region, necessitating a reevaluation for the performance of integrated sensing and communications (ISAC) to account for the effects introduced by the near field. In this paper, a near-field ISAC framework is proposed with a more accurate channel model than the three conventional models (TCMs): uniform plane wave, uniform spherical wave, and non-uniform spherical wave. Based on the proposed model, sensing and communication (S\&C) performance in both downlink and uplink scenarios are analyzed. For the downlink case, three different designs are investigated: the sensing-centric (S-C) design, the communications-centric (C-C) design, and the Pareto optimal design. For the uplink case, the S-C design, the C-C design and the time-sharing strategy are considered. Within each design, sensing rates (SRs) and communication rates (CRs) are derived. To gain further insights, high signal-to-noise ratio slopes and rate scaling laws concerning the number of antennas are also examined. Finally, the attainable SR-CR regions of the near-field ISAC are characterized. Numerical results reveal that 1) as the number of antennas grows, the SRs and CRs of the proposed model converges to constants, while those of the TCMs increase unboundedly; 2) ISAC achieves a more extensive rate region than the conventional frequency-division S\&C in both downlink and uplink cases.

翻译：下一代无线网络的技术趋势显著扩展了近场区域，这使得重新评估集成感知与通信（ISAC）性能时需考虑近场引入的影响。本文提出一种近场ISAC框架，其信道模型比三种传统模型（TCMs）——均匀平面波、均匀球面波和非均匀球面波——更为精确。基于所提模型，分析了下行和上行场景中的感知与通信性能。对于下行链路，研究了三种不同设计：感知中心设计、通信中心设计以及帕累托最优设计。对于上行链路，考虑了感知中心设计、通信中心设计以及时分复用策略。针对每种设计，推导了感知速率和通信速率。为深入分析，还研究了高信噪比斜率及速率随天线数量的缩放律。最后，刻画了近场ISAC的可达SR-CR区域。数值结果表明：1）随着天线数量增加，所提模型的SR和CR趋于常数，而传统模型的相应速率无界增长；2）在下行和上行场景中，ISAC相比传统频分感知与通信系统能实现更广阔的速率区域。