Keyholes, correlations, and capacities of multielement transmit and receive antennas

Multielement system capacities are usually thought of as limited only by correlations between elements. It is shown here that degenerate channel phenomena called keyholes may arise under realistic assumptions which have zero correlation between the entries of the channel matrix H and yet only a single degree of freedom. Canonical physical examples of keyholes are presented. For outdoor environments, it is shown that roof edge diffraction is perceived as a keyhole by a vertical base array that may be avoided by employing instead a horizontal base array.