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 examine exact and approximate near-field channel models for 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 motivate and discuss low-complexity signal processing schemes 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）因能提供巨大的空间自由度，已成为第六代（6G）无线移动网络潜在的关键使能技术。随着其重要性日益凸显，机遇与挑战并存。本文对XL-MIMO无线系统的研究进行了全面综述。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、采用贴片天线或点天线的基于均匀平面阵列（UPA）的XL-MIMO，以及基于连续孔径（CAP）的XL-MIMO。我们全面分析并探讨了它们的特性及相互关联性。随后，我们研究了XL-MIMO的精确与近似近场信道模型。鉴于近场通信独特的电磁特性，我们呈现了一系列信道模型以展示XL-MIMO的优势。我们进一步探讨并论证了低复杂度信号处理方案，以促进XL-MIMO的实际部署。此外，我们探索了XL-MIMO与其他新兴6G技术之间的相互作用。最后，我们概述了未来XL-MIMO无线通信系统若干引人注目的研究方向。