  This article conceives a unified representation for near-field and far-field
holographic multiple-input multiple-output (HMIMO) channels, addressing a
practical design dilemma: "Why does the angular-domain representation no longer
function effectively?" To answer this question, we pivot from the angular
domain to the wavenumber domain and present a succinct overview of its
underlying philosophy. In re-examining the Fourier plane-wave series expansion
that recasts spherical propagation waves into a series of plane waves
represented by Fourier harmonics, we characterize the HMIMO channel employing
these Fourier harmonics having different wavenumbers. This approach, referred
to as the wavenumebr-domain representation, facilitates a unified view across
the far-field and the near-field. Furthermore, the limitations of the DFT basis
are demonstrated when identifying the sparsity inherent to the HMIMO channel,
motivating the development of a wavenumber-domain basis as an alternative. We
then present some preliminary applications of the proposed wavenumber-domain
basis in signal processing across both the far-field and near-field, along with
several prospects for future HMIMO system designs based on the wavenumber
domain.
