On the Degrees of Freedom of the MISO Interference Broadcast Channel with Delayed CSIT

TL;DR

This paper investigates the degrees of freedom in a multi-cell MISO interference broadcast channel with delayed CSIT, revealing interference coupling issues and proposing an uncoupled MAT scheme to improve DoF performance.

Contribution

It introduces the uMAT scheme that uncouples interference across multiple cells, achieving improved DoF bounds in multi-cell MISO IBCs with delayed CSIT.

Findings

MAT scheme fails due to interference coupling in IBCs

uMAT scheme uncouples interference, improving DoF bounds

Achieves the best known DoF inner bound for most settings

Abstract

The Maddah-Ali and Tse (MAT) scheme is a linear precoding strategy that exploits Interference Alignment and perfect, but delayed, channel state information at the transmitters (delayed CSIT), improving the degrees of freedom (DoF) that can be achieved for the broadcast channel (BC). Since its appearance, many works have extended the concept of Retrospective Interference Alignment (RIA) to other multi-user channel configurations. However, little is known about the broadcast channel with multiple cells, i.e. the interference broadcast channel (IBC). In this work, the DoF are studied for the $K$-user $C$-cell multiple-input single-output (MISO) IBC with delayed CSIT (with $K/C$ users per cell). We show that the straightforward application of the MAT scheme over the IBC fails because it requires all interference to be received from the same source. Hence, in this case not all the interference can be cancelled, thus blocking the decoding of the received messages. We call this phenomenon as \textit{interference coupling}, forcing to use the MAT scheme by serving just one cell at a time. In this work, we propose an extension, namely the uncoupled MAT scheme (uMAT), exploiting multiple cells, uncoupling the interference, and achieving the best known DoF inner bound for almost all settings.