Multiple-antenna technologies are advancing toward the development of extremely large aperture arrays and the utilization of extremely high frequencies, driving the progress of next-generation multiple access (NGMA). This evolution is accompanied by the emergence of near-field communications (NFC), characterized by spherical-wave propagation, which introduces additional range dimensions to the channel and enhances system throughput. In this context, a tutorial-based primer on NFC is presented, emphasizing its applications in multiuser communications and multiple access (MA). The following areas are investigated: \romannumeral1) the commonly used near-field channel models are reviewed along with their simplifications under various near-field conditions. \romannumeral2) Building upon these models, the information-theoretic capacity limits of NFC-MA are analyzed, including the derivation of sum-rate capacity and capacity region, and their upper limits for both downlink and uplink scenarios. \romannumeral3) A detailed investigation of near-field multiuser beamforming design is presented, offering low-complexity and effective NFC-MA design methodologies in both the spatial and wavenumber (angular) domains. Throughout these investigations, near-field MA is compared with its far-field counterpart to highlight its superiority and flexibility in terms of interference management, thereby laying the groundwork for achieving NGMA.

翻译：多天线技术正朝着极大孔径阵列的构建与极高频谱的利用方向发展，驱动着下一代多址接入（NGMA）的演进。这一发展伴随着近场通信（NFC）的出现，其以球面波传播为特征，为信道引入了额外的距离维度并提升了系统吞吐量。在此背景下，本文提出一篇基于教程的近场通信导论，重点阐述其在多用户通信与多址接入（MA）中的应用。研究涵盖以下方面：\romannumeral1) 回顾常用近场信道模型及其在不同近场条件下的简化形式。\romannumeral2) 基于这些模型，分析近场通信多址接入的信息论容量极限，包括推导下行链路与上行链路场景的和速率容量、容量区域及其上界。\romannumeral3) 详细探究近场多用户波束赋形设计，提出在空间域与波数（角度）域中实现低复杂度且高效的近场通信多址接入设计方法。在整个研究过程中，通过将近场多址接入与其远场对应方案进行比较，突显其在干扰管理方面的优越性与灵活性，从而为实现下一代多址接入奠定基础。