Impact of Coupling on Multiple-Antenna Capacity in Correlated Fast-Fading Environments

Recent work has demonstrated that for a multiple-antenna system operating in a rapidly fading environment where the transmitter and receiver know the channel covariance information, the capacity increases as the transmit signal spatial correlation increases. This finding means that the performance improves as the transmit antenna element spacing decreases, an implication that is physically nonintuitive. This correspondence applies electromagnetic considerations to show that this gain stems from increased radiated power when the array spacing is reduced. It further shows that when the radiated power is explicitly limited and impractical supergain solutions are properly constrained, a more physically intuitive result emerges.