Limited Feedback Strategy For MU-MIMO Systems Under Spatially Correlated Channels

This paper studies limited feedback transmission strategy for downlink multiuser multiple-input multiple-output (MU-MIMO) systems under spatially correlated channels. To exploit the channel statistics for improving the performance of precoding at the base station (BS) with quantized channel direction information (CDI), we propose a two-level weighted DFT codebook (WDFT). It is composed of a long term DFT codebook representing the average CDIs and a short term codebook serving as an instantaneous weighting vector on them. The proposed codebook can adapt to various correlated channels without the need of channel correlation matrix at the BS. To mitigate the sum rate ceiling in high signal-to-noise ratio region caused by the quantized CDI with fixed size codebooks in less correlated channels, we propose a channel quality information (CQI) feedback strategy where each user helps the BS schedule users. By comparing its own CDI quantization error with a pre-determined threshold, each user feeds back one bit to indicate its preference of the transmission mode together with a corresponding CQI. With these information the BS first selects users separately from two user sets and finally selects users for downlink transmission. Simulation results show that the proposed limited feedback strategy including the WDFT codebook and the CQI feedback strategy outperforms existing schemes in various correlated channels.