Limited Feedback-based Block Diagonalization for the MIMO Broadcast Channel

TL;DR

This paper analyzes the impact of limited feedback on block diagonalization in MIMO broadcast channels, demonstrating that linear scaling of feedback bits with SNR maintains near-capacity performance and showing quantized feedback's advantages over analog feedback.

Contribution

It quantifies throughput loss due to limited feedback and establishes that linear feedback bit scaling with SNR preserves performance, comparing quantized and analog feedback schemes.

Findings

Feedback bits must scale linearly with SNR to prevent rate loss

Quantized feedback outperforms analog feedback in MIMO broadcast channels

Limited feedback can achieve near-capacity performance with proper scaling

Abstract

Block diagonalization is a linear precoding technique for the multiple antenna broadcast (downlink) channel that involves transmission of multiple data streams to each receiver such that no multi-user interference is experienced at any of the receivers. This low-complexity scheme operates only a few dB away from capacity but requires very accurate channel knowledge at the transmitter. We consider a limited feedback system where each receiver knows its channel perfectly, but the transmitter is only provided with a finite number of channel feedback bits from each receiver. Using a random quantization argument, we quantify the throughput loss due to imperfect channel knowledge as a function of the feedback level. The quality of channel knowledge must improve proportional to the SNR in order to prevent interference-limitations, and we show that scaling the number of feedback bits linearly with the system SNR is sufficient to maintain a bounded rate loss. Finally, we compare our quantization strategy to an analog feedback scheme and show the superiority of quantized feedback.