Using Direct Analog Feedback for Multiuser MIMO Broadcast Channel

We consider the bit error rate (BER) performance of a multiuser MIMO system with minimum mean-square error (MMSE) precoding. In contrast to most studies on this topic which fading is assumed static and the feedback channel state information (CSI) is assumed ideal and/or digital, we consider time-selective ("fast") fading with imperfect analog (or unquantized) CSI feedback. To attain multi-user diversity in this operating environment, we propose two user selection strategies: snap-shot (SS) and frame-based (FB) selection. Based on the BER performance obtained through simulation, we found that the proposed SS-MMSE and FB-MMSE precoders with analog feedback can effectively mitigate the effect of time-selective fading. In contrast, the unitary precoders (inherently digital feedback) proposed in the literature have disappointing performance in a "fast" fading environment. The conclusion is reached that analog CSI feedback, in conjunction with prediction-based MMSE precoding, should be given serious consideration for future generation MIMO broadcast systems.