The increasing demand for wireless data transfer has been the driving force behind the widespread adoption of Massive MIMO (multiple-input multiple-output) technology in 5G. The next-generation MIMO technology is now being developed to cater to the new data traffic and performance expectations generated by new user devices and services in the next decade. The evolution towards "ultra-massive MIMO (UM-MIMO)" is not only about adding more antennas but will also uncover new propagation and hardware phenomena that can only be treated by jointly utilizing insights from the communication, electromagnetic (EM), and circuit theory areas. This article offers a comprehensive overview of the key benefits of the UM-MIMO technology and the associated challenges. It explores massive multiplexing facilitated by radiative near-field effects, characterizes the spatial degrees-of-freedom, and practical channel estimation schemes tailored for massive arrays. Moreover, we provide a tutorial on EM theory and circuit theory, and how it is used to obtain physically consistent antenna and channel models. Subsequently, the article describes different ways to implement massive and dense antenna arrays, and how to co-design antennas with signal processing. The main open research challenges are identified at the end.

翻译：无线数据传输需求的持续增长，推动了大规模多输入多输出（MIMO）技术在5G中的广泛部署。为应对未来十年新型用户设备与服务带来的数据流量与性能预期，下一代MIMO技术正在研发中。向"超大规模MIMO（UM-MIMO）"的演进不仅涉及天线数量的增加，还将揭示仅通过联合运用通信、电磁（EM）及电路理论领域的见解才能处理的新型传播与硬件现象。本文全面概述了UM-MIMO技术的关键优势及相应挑战，探讨了由辐射近场效应促成的大规模复用、表征空间自由度的方法，以及针对大规模阵列优化的实际信道估计方案。此外，我们提供了电磁理论与电路理论的教程，阐述如何利用它们获得物理一致的天线与信道模型。随后，本文描述了大规模密集天线阵列的不同实现方式，并讨论了天线与信号处理的协同设计方法。最后指出了主要开放研究挑战。