Precoding by using a finite-set codebook at transmitters is an effective approach to improve the performance of multiple-input multiple-output (MIMO) systems with reduced feedback overhead. K-Best sphere decoders are popular for high-throughput spatially-multiplexed MIMO signal detection at receivers. In this paper, we propose a precoding-matrix-index (PMI) selection criterion suitable for K-best MIMO detectors by minimizing the trace of the upper triangular matrix after QR decomposition (QRD). To further improve the system performance, fully-sorted QRD, per-layer sorted QRD and partially-sorted QRD are also proposed to be incorporated in the minimum-trace PMI selection criterion. Simulation results based on the MIMO codebooks of 3GPP-LTE/LTE-A systems show the improvements of the proposed scheme compared to the non-precoding MIMO schemes and the precoding schemes using the conventional PMI selection criterions.
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