  To support the extremely high spectral efficiency and energy efficiency
requirements, and emerging applications of future wireless communications,
holographic multiple-input multiple-output (H-MIMO) technology is envisioned as
one of the most promising enablers. It can potentially bring extra
degrees-of-freedom for communications and signal processing, including spatial
multiplexing in line-of-sight (LoS) channels and electromagnetic (EM) field
processing performed using specialized devices, to attain the fundamental
limits of wireless communications. In this context, EM-domain channel modeling
is critical to harvest the benefits offered by H-MIMO. Existing EM-domain
channel models are built based on the tensor Green function, which require
prior knowledge of the global position and/or the relative distances and
directions of the transmit/receive antenna elements. Such knowledge may be
difficult to acquire in real-world applications due to extensive measurements
needed for obtaining this data. To overcome this limitation, we propose a
transmit-receive parameter separable channel model methodology in which the
EM-domain (or holographic) channel can be simply acquired from the
distance/direction measured between the center-points between the transmit and
receive surfaces, and the local positions between the transmit and receive
elements, thus avoiding extensive global parameter measurements. Analysis and
numerical results showcase the effectiveness of the proposed channel modeling
approach in approximating the H-MIMO channel, and achieving the theoretical
channel capacity.
