On the relationship of information theory and electromagnetism

Limitations imposed by the laws of electromagnetism on achievable MIMO channel capacity are studied in its general form. The approach is a two-fold one. First, the channel correlation argument is used 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 the Nyquist sampling theorem in the spatial domain, we show that the laws of electromagnetism limit the minimum antenna spacing to half a wavelength, λ/2, (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 λ/2. 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.