Multi-User MIMO with outdated CSI: Training, Feedback and Scheduling

TL;DR

This paper explores how multi-user MIMO systems can still achieve spectral efficiency gains even with outdated or imperfect channel information, by leveraging recent theoretical insights and proposing practical scheduling strategies.

Contribution

It demonstrates that the Maddah-Ali and Tse scheme enables effective MU-MIMO operation with outdated CSIT, and proposes scheduling methods to improve real-world performance.

Findings

Outdated CSIT can still be used for efficient MU-MIMO with proper scheduling.

The MAT scheme outperforms conventional MU-MIMO under feedback delay and channel dynamics.

Scheduling strategies enhance MU-MIMO performance with imperfect CSI.

Abstract

Conventional MU-MIMO techniques, e.g. Linear Zero-Forced Beamforming (LZFB), require sufficiently accurate channel state information at the transmitter (CSIT) in order to realize spectral efficient transmission (degree of freedom gains). In practical settings, however, CSIT accuracy can be limited by a number of issues including CSI estimation, CSI feedback delay between user terminals to base stations, and the time/frequency coherence of the channel. The latter aspects of CSIT-feedback delay and channel-dynamics can lead to significant challenges in the deployment of efficient MU-MIMO systems. Recently it has been shown by Maddah-Ali and Tse (MAT) that degree of freedom gains can be realized by MU-MIMO even when the knowledge of CSIT is completely outdated. Specifically, outdated CSIT, albeit perfect CSIT, is known for transmissions only after they have taken place. This aspect of insensitivity to CSIT-feedback delay is of particular interest since it allows one to reconsider MU-MIMO design in dynamic channel conditions. Indeed, as we show, with appropriate scheduling, and even in the context of CSI estimation and feedback errors, the proposed MAT scheme can have performance advantages over conventional MU-MIMO in such scenarios.