Not Too Delayed CSIT Achieves the Optimal Degrees of Freedom

TL;DR

This paper characterizes how limited feedback delay affects the optimal degrees of freedom in multi-user broadcast channels and introduces a new space-time interference alignment method to exploit current and past CSI.

Contribution

It provides a precise trade-off analysis for CSIT delay and proposes a novel transmission scheme leveraging both current and past CSI.

Findings

Optimal DoF is achievable if delay is below a certain threshold.

Introduces space-time interference alignment for improved interference management.

Characterizes the delay-DoF gain trade-off in multi-user channels.

Abstract

Channel state information at the transmitter (CSIT) aids interference management in many communication systems. Due to channel state information (CSI) feedback delay and time-variation in the wireless channel, perfect CSIT is not realistic. In this paper, the CSI feedback delay-DoF gain trade-off is characterized for the multi-user vector broadcast channel. A major insight is that it is possible to achieve the optimal degrees of freedom (DoF) gain if the delay is less than a certain fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligeable. To show this, a new transmission method called space-time interference alignment is proposed, which actively exploits both the current and past CSI.