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 expression for the outage probability, ergodic channel capacity, and ergodic mutual information are derived. Finally, for each aspect, the 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多天线技术支持第六代（6G）网络的三个明确趋势。为在通信系统设计中恰当考虑这三个趋势所引入的新特性，需采用区别于经典远场平面波模型的近场球面波传播模型。因此，近场通信（NFC）将成为6G网络的关键技术。本教程涵盖NFC的三个核心方面：1）信道建模：首先回顾基于球面波的空间离散（SPD）天线近场信道模型，随后讨论均匀球面波（USW）与非均匀球面波（NUSW）模型，进而介绍针对SPD天线的通用近场信道模型以及基于格林函数的连续孔径（CAP）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，讨论适用于SPD和CAP天线的NFC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NFC的完整性能分析框架。针对近场视距信道，推导接收信噪比与功率缩放定律；针对统计近场多径信道，提出通用分析框架，并基于该框架推导中断概率、遍历信道容量及遍历互信息的解析表达式。针对每个方面，讨论未来研究课题。