  Holographic MIMO is a spatially-constrained MIMO system with a massive number
of antennas, possibly thought of, in its ultimate form, as a
spatially-continuous electromagnetic aperture. Accurate and tractable channel
modeling is critical to understanding the full potential of this technology.
This paper considers arbitrary spatially-stationary scattering and provides a
4D plane-wave representation in Cartesian coordinates, which captures the
essence of electromagnetic propagation and allows to evaluate the capacity of
Holographic MIMO systems with rectangular volumetric arrays. The developed
framework generalizes the virtual channel representation, which was originally
developed for uniform linear arrays.
