On the Fundamental Feedback-vs-Performance Tradeoff over the MISO-BC with Imperfect and Delayed CSIT

TL;DR

This paper establishes fundamental bounds on the tradeoff between feedback quality and performance in multiuser MISO broadcast channels, providing new theoretical limits and minimum feedback requirements for optimal degrees-of-freedom.

Contribution

It introduces a novel DoF outer bound for K-user MISO BC with partial current CSIT, bridging existing cases and identifying minimal feedback needed for optimal performance.

Findings

Derived a new DoF region outer bound for partial CSIT

Identified minimum CSIT feedback for optimal sum DoF in specific cases

Bridged the gap between no CSIT and full CSIT scenarios

Abstract

This work considers the multiuser multiple-input single-output (MISO) broadcast channel (BC), where a transmitter with M antennas transmits information to K single-antenna users, and where - as expected - the quality and timeliness of channel state information at the transmitter (CSIT) is imperfect. Motivated by the fundamental question of how much feedback is necessary to achieve a certain performance, this work seeks to establish bounds on the tradeoff between degrees-of-freedom (DoF) performance and CSIT feedback quality. Specifically, this work provides a novel DoF region outer bound for the general K-user MISO BC with partial current CSIT, which naturally bridges the gap between the case of having no current CSIT (only delayed CSIT, or no CSIT) and the case with full CSIT. The work then characterizes the minimum CSIT feedback that is necessary for any point of the sum DoF, which is optimal for the case with M >= K, and the case with M=2, K=3.