Multi-User Diversity vs. Accurate Channel State Information in MIMO Downlink Channels

TL;DR

This paper investigates the trade-off between multi-user diversity and channel feedback quality in MIMO downlink channels, concluding that high-quality feedback from fewer users yields better sum rates than low-rate feedback from many users.

Contribution

It demonstrates that, under feedback constraints, prioritizing high-quality channel information from fewer users outperforms collecting coarse feedback from many in terms of sum rate.

Findings

High-quality feedback from fewer users improves sum rate.

Multi-user diversity is less effective than accurate channel information.

Optimal feedback strategy favors fewer, high-quality feedback links.

Abstract

In a multiple transmit antenna, single antenna per receiver downlink channel with limited channel state feedback, we consider the following question: given a constraint on the total system-wide feedback load, is it preferable to get low-rate/coarse channel feedback from a large number of receivers or high-rate/high-quality feedback from a smaller number of receivers? Acquiring feedback from many receivers allows multi-user diversity to be exploited, while high-rate feedback allows for very precise selection of beamforming directions. We show that there is a strong preference for obtaining high-quality feedback, and that obtaining near-perfect channel information from as many receivers as possible provides a significantly larger sum rate than collecting a few feedback bits from a large number of users.