Extremely large-scale multiple-input-multiple-output (XL-MIMO), which offers vast spatial degrees of freedom, has emerged as a potentially pivotal enabling technology for the sixth generation (6G) of wireless mobile networks. With its growing significance, both opportunities and challenges are concurrently manifesting. This paper presents a comprehensive survey of research on XL-MIMO wireless systems. In particular, we introduce four XL-MIMO hardware architectures: uniform linear array (ULA)-based XL-MIMO, uniform planar array (UPA)-based XL-MIMO utilizing either patch antennas or point antennas, and continuous aperture (CAP)-based XL-MIMO. We comprehensively analyze and discuss their characteristics and interrelationships. Following this, we introduce several electromagnetic characteristics and general distance boundaries in XL-MIMO. Given the distinct electromagnetic properties of near-field communications, we present a range of channel models to demonstrate the benefits of XL-MIMO. We further discuss and summarize signal processing schemes for XL-MIMO. It is worth noting that the low-complexity signal processing schemes and deep learning empowered signal processing schemes are reviewed and highlighted to promote the practical implementation of XL-MIMO. Furthermore, we explore the interplay between XL-MIMO and other emergent 6G technologies. Finally, we outline several compelling research directions for future XL-MIMO wireless communication systems.

翻译：超大规模多输入多输出（XL-MIMO）通过提供巨大空间自由度，已成为第六代（6G）无线移动网络中具有潜在关键支撑作用的技术。随着其重要性日益凸显，机遇与挑战并存。本文全面综述了XL-MIMO无线系统的研究成果。具体而言，我们介绍了四种XL-MIMO硬件架构：基于均匀线性阵列（ULA）的XL-MIMO、基于均匀平面阵列（UPA）的XL-MIMO（采用贴片天线或点天线），以及基于连续孔径（CAP）的XL-MIMO。我们深入分析并讨论了这些架构的特性及其相互关系。随后，我们介绍了XL-MIMO中的若干电磁特性与通用距离边界。鉴于近场通信独特的电磁特性，我们提出了一系列信道模型以展示XL-MIMO的优势。此外，我们进一步探讨并总结了XL-MIMO的信号处理方案。值得注意的是，本文重点评述了低复杂度信号处理方案与深度学习赋能的信号处理方案，以推动XL-MIMO的实际部署。我们还探讨了XL-MIMO与其他新兴6G技术之间的协同关系。最后，我们指出了未来XL-MIMO无线通信系统中若干值得关注的研究方向。