Precoder partitioning in closed-loop MIMO systems

Authors:
Helka-Liina Määttänen;Karol Schober;Olav Tirkkonen;Risto Wichman
Affiliations:
Department of Communication and Networking, Helsinki University of Technology, Finland;Department of Signal Processing and Acoustics, Helsinki University of Technology, Finland;Department of Communication and Networking, Helsinki University of Technology, Finland and Nokia Research Center;Department of Signal Processing and Acoustics, Helsinki University of Technology, Finland
Venue:
IEEE Transactions on Wireless Communications
Year:
2009

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Abstract

We study unitary precoding for multistream MIMO systems with partial channel state information at the transmitter. We introduce a quantization scheme in which the full space of non-equivalent precoding matrices is partitioned into Grassmannian and orthogonalization parts. The Grassmannian part is used for maximizing the power after precoding and the orthogonalization part is used for removing cross talk between the data streams. We show that orthogonalization improves the attainable capacity when the receiver is linear. We give a parametrization for the non-equivalent orhogonalization matrices and a metric which measures the orthogonality of the transmission. Optimal orthogonalization codebooks for two-stream transmission are presented. When feedback is limited, the optimal partitioning of feedback bits between Grassmannian and orthogonalization parts becomes an issue. In correlated scenarios, the number of feedback bits may be significantly reduced by investing bits into the orthogonalization part.