Spatial multiplexing over correlated MIMO channels with a closed-form precoder

This paper addresses the problem of multiple-input-multiple-output (MIMO) spatial multiplexing (SM) systems in the presence of antenna fading correlation. Existing SM schemes (e.g., V-BLAST) 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 nonzero capacity that is present even in highly correlated spatial channels. The authors make the simple but key point that only 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-quadratic amplitude modulation (4-QAM) signals form a 16-QAM). The concept builds upon constellation multiplexing (CM) (D. Gesbert and J. Akhtar, Smart Antennas in Europe-State-of-the-Art, 2005) 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. The authors 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.