Revisiting Multi-User Downlink in IEEE 802.11ax: A Designers Guide to MU-MIMO

TL;DR

This paper analyzes the conditions under which downlink MU-MIMO in IEEE 802.11ax improves network efficiency, revealing that factors like CSI overhead and spatial correlation often hinder its benefits, and provides guidelines for optimal usage.

Contribution

It offers a comprehensive understanding of the factors affecting DL MU-MIMO performance and proposes practical guidelines for when to enable or disable it in various scenarios.

Findings

CSI overhead and spatial correlation can negate MU-MIMO benefits

Turning on MU-MIMO does not always increase efficiency

Guidelines for optimal MU-MIMO activation based on scenario

Abstract

Downlink (DL) Multi-User (MU) Multiple Input Multiple Output (MU-MIMO) is a key technology that allows multiple concurrent data transmissions from an Access Point (AP) to a selected sub-set of clients for higher network efficiency in IEEE 802.11ax. However, DL MU-MIMO feature is typically turned off as the default setting in AP vendors' products, that is, turning on the DL MU-MIMO may not help increase the network efficiency, which is counter-intuitive. In this article, we provide a sufficiently deep understanding of the interplay between the various underlying factors, i.e., CSI overhead and spatial correlation, which result in negative results when turning on the DL MU-MIMO. Furthermore, we provide a fundamental guideline as a function of operational scenarios to address the fundamental question "when the DL MU-MIMO should be turned on/off".