Empowered by the latest progress on innovative metamaterials/metasurfaces and advanced antenna technologies, holographic multiple-input multiple-output (H-MIMO) emerges as a promising technology to fulfill the extreme goals of the sixth-generation (6G) wireless networks. The antenna arrays utilized in H-MIMO comprise massive (possibly to extreme extent) numbers of antenna elements, densely spaced less than half-a-wavelength and integrated into a compact space, realizing an almost continuous aperture. Thanks to the expected low cost, size, weight, and power consumption, such apertures are expected to be largely fabricated for near-field communications. In addition, the physical features of H-MIMO enable manipulations directly on the electromagnetic (EM) wave domain and spatial multiplexing. To fully leverage this potential, near-field H-MIMO channel modeling, especially from the EM perspective, is of paramount significance. In this article, we overview near-field H-MIMO channel models elaborating on the various modeling categories and respective features, as well as their challenges and evaluation criteria. We also present EM-domain channel models that address the inherit computational and measurement complexities. Finally, the article is concluded with a set of future research directions on the topic.

翻译：依托新型超材料/超表面与先进天线技术的最新进展，全息多输入多输出（H-MIMO）作为实现第六代（6G）无线网络极端目标的一项重要技术应运而生。H-MIMO采用的天线阵列包含大量（可能达到极端规模）天线单元，这些单元以小于半波长的间距密集排列并集成于紧凑空间内，实现了近乎连续的孔径。得益于其预期的低成本、小尺寸、轻重量和低功耗特性，此类孔径有望大规模应用于近场通信。此外，H-MIMO的物理特性使其能够直接在电磁（EM）波域和空分复用中进行操控。为充分发挥这一潜力，特别是从电磁学角度出发的近场H-MIMO信道建模具有至关重要的意义。本文综述了近场H-MIMO信道模型，详细阐述了各类建模方法及其特征、面临的挑战与评估准则。同时，我们提出了能够解决内在计算与测量复杂性的电磁域信道模型。最后，本文总结了该领域未来的一系列研究方向。