Degrees of Freedom of Holographic MIMO Channels

We consider spatially-constrained apertures of rectangular symmetry and aim to retrieve the limit to the average number of channel spatial degrees of freedom (DoF), obtained elsewhere through different analyses and tools. Unlike prior works, we use a novel Fourier plane-wave series expansion of the channel, 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 propagation.