Communicating on MIMO Channels with Covariance Information: Antenna Correlation and Coupling

A recent study on correlated block-fading MIMO channels with transmit covariance information indicates guaranteed capacity growth with additional transmit elements, in contrast to previous results for uncorrelated channels. Also, for very rapidly fading channels, the results indicate the optimality of placing antenna elements as close together as possible. In this work, application of radiated power considerations indicates that transmit beamforming only increases capacity when this gain results from correlation inherent in the channel, not correlation that is simply offset by increased antenna coupling. The new analysis reveals that when multipath is directionally biased, antenna spacings of 0.3 to 0.6 wavelengths are optimal, and that when no such bias is present, antennas should be placed as far apart as possible. As expected, the electromagnetic analysis shows that at zero element separation, the array yields no capacity gain over a single antenna.