On Mimo Channel Capacity, Spatial Sampling and the Laws of Electromagnetism

In this paper, we study the limitations imposed by the laws of electromagnetism on achievable MIMO channel capacity in its general form. Our approach is a two-fold one. First, we use the channel correlation argument to demonstrate that the minimum antenna spacing under any scattering conditions is at least half a wavelength. Secondly, using a plane-wave spectrum expansion of a generic electromagnetic wave combined with Nyquist sampling theorem in the spatial domain, we show that the laws of electromagnetism limit the minimum antenna spacing to half a wavelength, / 2 λ , (in the case of 1-D antenna apertures) only asymptotically, when the number of antennas n → ∞ . For a finite number of antennas, this limit is slightly less than / 2 λ . In any case, the number of antennas and, consequently, the MIMO capacity is limited for a given aperture size. This is a scenario-independent limit.