Optimal Use of Current and Outdated Channel State Information - Degrees of Freedom of the MISO BC with Mixed CSIT

TL;DR

This paper enhances the understanding of the degrees of freedom in MISO broadcast channels with mixed CSIT by introducing new private messages and establishing a tight outer bound, improving previous results especially in delayed CSIT scenarios.

Contribution

It introduces an improved scheme with additional private messages and derives a tight DoF outer bound, optimizing the use of mixed CSIT without assuming statistically identical fading.

Findings

Achieves higher DoF with the new scheme.

Provides a tight outer bound for DoF.

Removes the need for statistically identical fading assumptions.

Abstract

We consider a multiple-input-single-output (MISO) broadcast channel with mixed channel state information at the transmitter (CSIT) that consists of imperfect current CSIT and perfect outdated CSIT. Recent work by Kobayashi et al. presented a scheme which exploits both imperfect current CSIT and perfect outdated CSIT and achieves higher degrees of freedom (DoF) than possible with only imperfect current CSIT or only outdated CSIT individually. In this work, we further improve the achievable DoF in this setting by incorporating additional private messages, and provide a tight information theoretic DoF outer bound, thereby identifying the DoF optimal use of mixed CSIT. The new result is stronger even in the original setting of only delayed CSIT, because it allows us to remove the restricting assumption of statistically equivalent fading for all users.