  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.
