On the Degrees-of-Freedom of the K-user Distributed Broadcast Channel

TL;DR

This paper investigates the Degrees-of-Freedom in a multi-transmitter wireless system with distributed, imperfect channel information, proposing a new scheme that achieves optimal DoF under certain CSI quality conditions.

Contribution

It introduces a novel robust transmission scheme for distributed CSI scenarios and establishes a regime where the upper bound on DoF is achieved.

Findings

The genie-aided upper bound for DoF is computed.

The proposed scheme achieves the upper bound in the Weak-CSIT regime.

Optimal DoF depends only on the best-informed transmitter's CSI quality.

Abstract

We study the Degrees-of-Freedom (DoF) in a wireless setting in which K Transmitters (TXs) aim at jointly serving K users. The performance is studied when the TXs are faced with a distributed Channel State Information (CSI) configuration in which each TX has access to its own multi-user imperfect channel estimate based on which it designs its transmit coefficients. The channel estimates are not only imperfectly acquired but they are also imperfectly shared between the TXs. Our first contribution consists of computing a genie-aided upper bound for the DoF of that setting. Our main contribution is then to develop a new robust transmission scheme that leverages the different qualities of CSI available at the TXs to improve the achieved DoF. We show the surprising result that there is a CSI regime, coined the Weak-CSIT regime, in which the genie-aided upper bound is achieved by the proposed transmission scheme. Interestingly, the optimal DoF in the Weak-CSIT regime only depends on the CSI quality at the best informed TX and not on the CSI quality at all other TXs.