  As the backbone of the fifth-generation (5G) cellular network, massive
multiple-input multiple-output (MIMO) encounters a significant challenge in
practical applications: how to deploy a large number of antenna elements within
limited spaces. Recently, holographic communication has emerged as a potential
solution to this issue. It employs dense antenna arrays and provides a
tractable model. Nevertheless, some challenges must be addressed to actualize
this innovative concept. One is the mutual coupling among antenna elements
within an array. When the element spacing is small, near-field coupling becomes
the dominant factor that strongly restricts the array performance. Another is
the polarization of electromagnetic waves. As an intrinsic property, it was not
fully considered in the previous channel modeling of holographic communication.
The third is the lack of real-world experiments to show the potential and
possible defects of a holographic communication system. In this paper, we
propose an electromagnetic channel model based on the characteristics of
electromagnetic waves. This model encompasses the impact of mutual coupling in
the transceiver sides and the depolarization in the propagation environment.
Furthermore, by approximating an infinite array, the performance restrictions
of large-scale dense antenna arrays are also studied theoretically to exploit
the potential of the proposed channel. In addition, numerical simulations and a
channel measurement experiment are conducted. The findings reveal that within
limited spaces, the coupling effect, particularly for element spacing smaller
than half of the wavelength, is the primary factor leading to the inflection
point for the performance of holographic communications.
