Joint COST 273/284 Workshop on Antennas and Related System Aspects in Wireless Communications, Chalmers

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 three-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 planewave 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 . The number of antennas and, consequently, the MIMO capacity is limited for a given aperture size. This is a scenario-independent limit. Finally, we study the MIMO capacity of waveguide and cavity channels. The rationale for this is three-fold: (i) waveguide / cavity models can be used to model corridors, tunnels and other confined space channels, (ii) this is a canonical problem; its analysis allows to develop appropriate techniques, which can be further used for more complex problems, (iii) it allows to shed light on the relation between information theory and electromagnetism and, in particular, to establish the limits imposed by the laws of electromagnetism on achievable channel capacity.