Extremely large-scale multiple-input multiple-output (XL-MIMO) is a promising technology for the sixth-generation (6G) mobile communication networks. By significantly boosting the antenna number or size to at least an order of magnitude beyond current massive MIMO systems, XL-MIMO is expected to unprecedentedly enhance the spectral efficiency and spatial resolution for wireless communication. The evolution from massive MIMO to XL-MIMO is not simply an increase in the array size, but faces new design challenges, in terms of near-field channel modelling, performance analysis, channel estimation, and practical implementation. In this article, we give a comprehensive tutorial overview on near-field XL-MIMO communications, aiming to provide useful guidance for tackling the above challenges. First, the basic near-field modelling for XL-MIMO is established, by considering the new characteristics of non-uniform spherical wave (NUSW) and spatial non-stationarity. Next, based on the near-field modelling, the performance analysis of XL-MIMO is presented, including the near-field signal-to-noise ratio (SNR) scaling laws, beam focusing pattern, achievable rate, and degrees-of-freedom (DoF). Furthermore, various XL-MIMO design issues such as near-field beam codebook, beam training, channel estimation, and delay alignment modulation (DAM) transmission are elaborated. Finally, we point out promising directions to inspire future research on near-field XL-MIMO communications.

翻译：超大规模多输入多输出（XL-MIMO）是第六代（6G）移动通信网络中的一项前景技术。通过将天线数量或尺寸显著提升至当前大规模MIMO系统至少一个数量级，XL-MIMO有望前所未有地增强无线通信的频谱效率和空间分辨率。从大规模MIMO到XL-MIMO的演进不仅是阵列尺寸的增加，还面临近场信道建模、性能分析、信道估计及实际实现等方面的全新设计挑战。本文对近场XL-MIMO通信进行了全面的教程式概述，旨在为应对上述挑战提供有益指导。首先，考虑非均匀球面波和空间非平稳性的新特性，建立了XL-MIMO的基本近场模型。其次，基于近场建模，进行了XL-MIMO性能分析，包括近场信噪比缩放规律、波束聚焦模式、可达速率及自由度。此外，详细阐述了各种XL-MIMO设计问题，如近场波束码本、波束训练、信道估计和延迟对齐调制传输。最后，指出了富有前景的研究方向，以激发近场XL-MIMO通信的未来研究。