Space-Time Interference Alignment and Degrees of Freedom Regions for the MISO Broadcast Channel with Periodic CSI Feedback

TL;DR

This paper introduces space-time interference alignment for MISO broadcast channels with periodic CSI feedback, improving degrees of freedom by leveraging both current and outdated CSI, and characterizing feedback delay effects.

Contribution

It proposes a novel space-time interference alignment method and provides a comprehensive analysis of DoF regions considering periodic CSI feedback and delays.

Findings

Significant sum-DoF gain when using both current and outdated CSI.

Inner bound of sum-DoF region as a function of feedback frequency.

Optimal DoF gain achievable with feedback delay less than a fraction of coherence time.

Abstract

This paper characterizes the degrees of freedom (DoF) regions for the multi-user vector broadcast channel with periodic channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-time interference alignment is proposed, which exploits both the current and past CSI jointly. Using the proposed alignment technique, an inner bound of the sum-DoF region is characterized as a function of a normalized CSI feedback frequency, which measures CSI feedback speed compared to the speed of user's channel variations. One consequence of the result is that the achievable sum-DoF gain is improved significantly when a user sends back both current and outdated CSI compared to the case where the user sends back current CSI only. Then, a trade-off between CSI feedback delay and the sum-DoF gain is characterized for the multi-user vector broadcast channel in terms of a normalized CSI feedback delay that measures CSI obsoleteness compared to channel coherence time. A crucial insight is that it is possible to achieve the optimal DoF gain if the feedback delay is less than a derived fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligible.