  Holographic Multiple-Input and Multiple-Output (MIMO) is envisioned as a
promising technology to realize unprecedented spectral efficiency by
integrating a large number of antennas into a compact space. Most research on
holographic MIMO is based on isotropic scattering environments, and the antenna
gain is assumed to be unlimited by deployment space. However, the channel might
not satisfy isotropic scattering because of generalized angle distributions,
and the antenna gain is limited by the array aperture in reality. In this
letter, we aim to analyze the holographic MIMO channel capacity under practical
angle distribution and array aperture constraints. First, we calculate the
spectral density for generalized angle distributions by introducing a
wavenumber domain-based method. And then, the capacity under generalized angle
distributions is analyzed and two different aperture schemes are considered.
Finally, numerical results show that the capacity is obviously affected by
angle distribution at high signal-to-noise ratio (SNR) but hardly affected at
low SNR, and the capacity will not increase infinitely with antenna density due
to the array aperture constraint.
