Extremely large-scale antenna arrays, tremendously high frequencies, and new types of antennas are three clear trends in multi-antenna technology for supporting the sixth-generation (6G) networks. To properly account for the new characteristics introduced by these three trends in communication system design, the near-field spherical-wave propagation model needs to be used, which differs from the classical far-field planar-wave one. As such, near-field communication (NFC) will become essential in 6G networks. In this tutorial, we cover three key aspects of NFC. 1) Channel Modelling: We commence by reviewing near-field spherical-wave-based channel models for spatially-discrete (SPD) antennas. Then, uniform spherical wave (USW) and non-uniform spherical wave (NUSW) models are discussed. Subsequently, we introduce a general near-field channel model for SPD antennas and a Green's function-based channel model for continuous-aperture (CAP) antennas. 2) Beamfocusing and Antenna Architectures: We highlight the properties of near-field beamfocusing and discuss NFC antenna architectures for both SPD and CAP antennas. Moreover, the basic principles of near-field beam training are introduced. 3) Performance Analysis: Finally, we provide a comprehensive performance analysis framework for NFC. For near-field line-of-sight channels, the received signal-to-noise ratio and power-scaling law are derived. For statistical near-field multipath channels, a general analytical framework is proposed, based on which analytical expressions for the outage probability, ergodic channel capacity, and ergodic mutual information are obtained. Finally, for each aspect, topics for future research are discussed.

翻译：超大规模天线阵列、极高频率以及新型天线是支撑第六代（6G）网络的多天线技术的三大明确趋势。为在通信系统设计中恰当考虑这三大趋势引入的新特性，必须采用近场球面波传播模型——该模型与经典的远场平面波传播模型截然不同。因此，近场通信将成为6G网络的关键技术。本教程涵盖近场通信的三大核心方面：1）信道建模：首先回顾基于球面波的空间离散天线近场信道模型，继而讨论均匀球面波模型与非均匀球面波模型，随后介绍面向空间离散天线的一般性近场信道模型以及基于格林函数的连续孔径天线信道模型；2）波束聚焦与天线架构：重点阐述近场波束聚焦特性，讨论面向空间离散天线与连续孔径天线的近场通信天线架构，并介绍近场波束训练的基本原理；3）性能分析：最后提供近场通信的全面性能分析框架。针对近场视距信道，推导接收信噪比与功率缩放律；针对统计近场多径信道，提出一般性分析框架，并基于该框架获得中断概率、遍历信道容量与遍历互信息的解析表达式。最后，针对每个方面探讨未来研究方向。