Multiple-antenna capacity in correlated Rayleigh fading with channel covariance information

For a block fading multiple input multiple output (MIMO) wireless link comprising M transmit and N receive antennas operating in a spatially correlated Rayleigh flat fading environment with coherence time T and only knowledge of channel correlations at the transmitter and receiver, we show that the channel capacity is independent of the smallest M-T eigenvalues of the transmit fade covariance matrix. We characterize the structure of the input signal that achieves capacity. In contrast to the previously reported results for the spatially white fading model where adding more transmit antennas beyond the coherence interval length (M>T) does not increase capacity, we find that additional transmit antennas always increase capacity as long as their channel fading coefficients are spatially correlated, making the case for smaller spacing between transmit antennas. For fast fading channels the capacity gained from multiple transmit antennas is bounded by the array gain 10 log M.