Degrees of Freedom of Holographic MIMO Channels

TL;DR

This paper derives the fundamental limit on the average number of spatial degrees of freedom in holographic MIMO channels using a Fourier plane-wave series expansion, providing a physics-based statistical model.

Contribution

It introduces a novel Fourier-based statistical modeling approach to determine the DoF limit, differing from previous methods and applicable to isotropic scattering environments.

Findings

Derived the limit of spatial degrees of freedom for holographic MIMO channels.

Used a Fourier plane-wave series expansion to model small-scale fading.

Applicable to isotropic scattering environments, with potential extension to non-isotropic cases.

Abstract

We consider spatially-constrained apertures of rectangular symmetry and aim to retrieve the limit to the average number of spatial degrees of freedom (DoF), obtained elsewhere through different analyses and tools. Unlike prior works, we use the Fourier plane-wave series expansion, recently introduced in [1], where a statistical model for the small-scale fading in the far-field is developed on the basis of a continuous-space and physics-based orthonormal expansion over the Cartesian spatial Fourier basis. This expansion yields a set of statistically independent random coefficients whose cardinality directly gives the limit to the average number of DoF. The treatment is limited to an isotropic scattering environment but can be extended to the non-isotropic case through the linear-system theoretic interpretation of plane-wave propagations.