Extremely large-scale multiple-input-multiple-output (XL-MIMO) is a promising technology to empower the next-generation communications. However, XL-MIMO, which is still in its early stage of research, has been designed with a variety of hardware and performance analysis schemes. To illustrate the differences and similarities among these schemes, we comprehensively review existing XL-MIMO hardware designs and characteristics in this article. Then, we thoroughly discuss the research status of XL-MIMO from "channel modeling", "performance analysis", and "signal processing". Several existing challenges are introduced and respective solutions are provided. We then propose two case studies for the hybrid propagation channel modeling and the effective degrees of freedom (EDoF) computations for practical scenarios. Using our proposed solutions, we perform numerical results to investigate the EDoF performance for the scenarios with unparallel XL-MIMO surfaces and multiple user equipment, respectively. Finally, we discuss several future research directions.

翻译：超大规模多输入多输出（XL-MIMO）是一项有望赋能下一代通信的 promising 技术。然而，XL-MIMO目前仍处于研究初期，其硬件设计与性能分析方案呈现多样化。为阐明这些方案间的异同，本文全面综述了现有XL-MIMO硬件设计与特征。随后，我们从“信道建模”、“性能分析”及“信号处理”三个维度深入探讨了XL-MIMO的研究现状，介绍了当前面临的若干挑战并提供了相应的解决方案。进一步，针对混合传播信道建模及实际场景下的有效自由度（EDoF）计算，我们提出了两项案例研究。基于所提解决方案，我们分别针对非平行XL-MIMO表面及多用户设备场景进行了EDoF性能的数值分析。最后，本文讨论了若干未来研究方向。