Flexible Backhaul Design and Degrees of Freedom for Linear Interference Networks

TL;DR

This paper investigates maximizing degrees of freedom in cellular downlink networks with backhaul constraints, proposing an optimal asymmetric message assignment and zero-forcing beamforming, with extensions to more complex network models.

Contribution

It introduces the optimal asymmetric message assignment strategy and coding scheme for linear interference networks under backhaul constraints, extending to more general network topologies.

Findings

Asymptotic per user DoF is (4B-1)/(4B) for backhaul load B.

Optimal message assignment is asymmetric and locally constrained.

Zero-forcing beamforming suffices for optimal coding scheme.

Abstract

The considered problem is that of maximizing the degrees of freedom (DoF) in cellular downlink, under a backhaul load constraint that limits the number of messages that can be delivered from a centralized controller to the base station transmitters. A linear interference channel model is considered, where each transmitter is connected to the receiver having the same index as well as one succeeding receiver. The backhaul load is defined as the sum of all the messages available at all the transmitters normalized by the number of users. When the backhaul load is constrained to an integer level B, the asymptotic per user DoF is shown to equal (4B-1)/(4B), and it is shown that the optimal assignment of messages to transmitters is asymmetric and satisfies a local cooperation constraint and that the optimal coding scheme relies only on zero-forcing transmit beamforming. Finally, an extension of the presented coding scheme is shown to apply for more general locally connected and two-dimensional networks.