Keyholes, Correlations and Capacities of Multi-element Transmit and Receive Antennas

Multi-element system capacities are usually thought of a 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.