Multiuser MIMO Achievable Rates with Downlink Training and Channel State Feedback

TL;DR

This paper analyzes achievable rates in multiuser MIMO broadcast channels considering downlink training and various feedback schemes, demonstrating that well-designed digital feedback can significantly enhance throughput even with practical constraints.

Contribution

It compares unquantized and quantized feedback schemes, showing digital feedback's advantages and how exploiting MIMO-MAC uplink channels improves feedback efficiency.

Findings

Digital feedback can outperform analog when feedback per channel state exceeds 1.

Properly designed digital feedback minimizes errors even with simple modulation.

Exploiting MIMO-MAC uplink channels improves feedback scaling with antennas.

Abstract

We consider a MIMO fading broadcast channel and compute achievable ergodic rates when channel state information is acquired at the receivers via downlink training and it is provided to the transmitter by channel state feedback. Unquantized (analog) and quantized (digital) channel state feedback schemes are analyzed and compared under various assumptions. Digital feedback is shown to be potentially superior when the feedback channel uses per channel state coefficient is larger than 1. Also, we show that by proper design of the digital feedback link, errors in the feedback have a minor effect even if simple uncoded modulation is used on the feedback channel. We discuss first the case of an unfaded AWGN feedback channel with orthogonal access and then the case of fading MIMO multi-access (MIMO-MAC). We show that by exploiting the MIMO-MAC nature of the uplink channel, a much better scaling of the feedback channel resource with the number of base station antennas can be achieved. Finally, for the case of delayed feedback, we show that in the realistic case where the fading process has (normalized) maximum Doppler frequency shift 0 < F < 1/2, a fraction 1 - 2F of the optimal multiplexing gain is achievable. The general conclusion of this work is that very significant downlink throughput is achievable with simple and efficient channel state feedback, provided that the feedback link is properly designed.