Rate-Splitting and Sum-DoF for the K-User MISO Broadcast Channel with Mixed CSIT and Order-(K-1) Messages

TL;DR

This paper introduces a novel rate-splitting scheme with retrospective interference alignment to characterize the sum-DoF in a K-user MISO broadcast channel with mixed CSIT and order-(K-1) messages, revealing saturation effects for K>2.

Contribution

It provides the first sum-DoF characterization for K-user MISO channels with mixed CSIT and order-(K-1) messages, including new bounds and insights for K>2.

Findings

Sum-DoF is saturated with CSIT quality for K>2.

Proposed rate-splitting scheme achieves the sum-DoF lower bound.

Matching upper bound established via genie signals and extremal inequality.

Abstract

In this paper, we propose a rate-splitting design and characterize the sum-degrees-of-freedom (DoF) for the K-user multiple-input-single-output (MISO) broadcast channel with mixed channel state information at the transmitter (CSIT) and order-(K-1) messages, where mixed CSIT refers to the delayed and imperfect-current CSIT, and order-(K-1) message refers to the message desired by K-1 users simultaneously. In particular, for the sum-DoF lower bound, we propose a rate-splitting scheme embedding with retrospective interference alignment. In addition, we propose a matching sum-DoF upper bound via genie signalings and extremal inequality. Opposed to existing works for K=2, our results show that the sum-DoF is saturated with CSIT quality when CSIT quality thresholds are satisfied for K>2.