Imperfect Delayed CSIT can be as Useful as Perfect Delayed CSIT: DoF Analysis and Constructions for the BC

TL;DR

This paper demonstrates that even imperfect delayed channel state information at the transmitter can achieve similar degrees-of-freedom gains as perfect delayed CSIT in a two-user broadcast channel, through novel multi-phase schemes.

Contribution

It introduces new multi-phase broadcasting schemes that utilize imperfect delayed and current CSIT to match the optimal DoF region of perfect delayed CSIT.

Findings

Imperfect delayed CSIT can achieve DoF gains similar to perfect delayed CSIT.

Proposed schemes effectively utilize imperfect CSIT to approach optimal DoF.

Insights into when improving CSIT quality is practically beneficial.

Abstract

In the setting of the two-user broadcast channel, where a two-antenna transmitter communicates information to two single-antenna receivers, recent work by Maddah-Ali and Tse has shown that perfect knowledge of delayed channel state information at the transmitter (perfect delayed CSIT) can be useful, even in the absence of any knowledge of current CSIT. Similar benefits of perfect delayed CSIT were revealed in recent work by Kobayashi et al., Yang et al., and Gou and Jafar, which extended the above to the case of perfect delayed CSIT and imperfect current CSIT. The work here considers the general problem of communicating, over the aforementioned broadcast channel, with imperfect delayed and imperfect current CSIT, and reveals that even substantially degraded and imperfect delayed-CSIT is in fact sufficient to achieve the aforementioned gains previously associated to perfect delayed CSIT. The work proposes novel multi-phase broadcasting schemes that properly utilize knowledge of imperfect delayed and imperfect current CSIT, to match in many cases the optimal degrees-of-freedom (DoF) region achieved with perfect delayed CSIT. In addition to the theoretical limits and explicitly constructed precoders, the work applies towards gaining practical insight as to when it is worth improving CSIT quality.