MIMO capacity and antenna array design

It is generally accepted that correlation reduces the capacity of the MIMO radio channel. Recently, however, it has been pointed out that several sources of correlation exist. Whereas most often in communications, correlation is interpreted in terms of spatial correlations due to the propagation environment, also the coupling between transmit and receive antennas has impact on the capacity of a given communication channel. Against intuition, it has been claimed that in certain cases antenna coupling has a beneficial effect. In this paper, we derive a general expression for the correlation matrix of the MIMO channel in dependence on both antenna coupling and spatial correlation. We show how the capacity of the MIMO channel depends on the eigenvalues of the antenna coupling matrices and study the interplay between spatial correlation and antenna coupling in the high SNR regime.