Spatial multiplexing over correlated MIMO channels with a closed form precoder

This 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 that is present even 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 (e.g. two 4-QAM signals form a 16-QAM)! The concept builds upon constellation multiplexing (CM) [1] whereby distinct QAM streams are superposed to form a higher-order constellation with rate equivalent to the sum of rates of all original streams. In contrast to SM transmission, the substreams in CM schemes are differentiated through power scaling rather than through spatial signatures. 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. At the extreme correlation case (identical channels), the scheme behaves equivalent to sending a single higher-order modulation whose independent components are mapped to the different antennas.