A closed-form precoder for spatial multiplexing over correlated MIMO channels

The paper addresses the problem of MIMO spatial-multiplexing (SM) systems in the presence of antenna fading correlation. Existing SM (V-BLAST and related) schemes rely on the linear independence of transmit antenna channel responses for stream separation and suffer considerably from high levels of fading correlation. As a result, such algorithms simply fail to extract the non-zero capacity present in highly correlated spatial channels. We make the simple, but key, point that just one transmit antenna is needed to send several independent streams if those streams are appropriately superposed to form a high-order modulation (such as QAM). We build on this idea to present a new transmission scheme based on a precoder adjusting the phase and power of the input constellations in closed-form as a function of the antenna correlation. This yields a rate-preserving MIMO multiplexing scheme that can operate smoothly at any degree of correlation.