Relay-Assisted Interference Channel: Degrees of Freedom

TL;DR

This paper analyzes the degrees of freedom in a K-user interference channel with a MIMO relay, showing that a simple two-hop strategy achieves K/2 degrees of freedom, and exploring conditions for full degrees of freedom with high relay power.

Contribution

It establishes the degrees of freedom for the interference channel with a MIMO relay and explores encoding strategies to exploit direct links and high relay power.

Findings

K/2 degrees of freedom with basic strategies

Full degrees of freedom K achievable with high relay power

Hybrid encoding strategies improve sum-rate performance

Abstract

This paper investigates the degrees of freedom of the interference channel in the presence of a dedicated MIMO relay. The relay is used to manage the interference at the receivers. It is assumed that all nodes including the relay have channel state information only for their own links and that the relay has M (greater than or equal to K) antennas in a K-user network. We pose the question: What is the benefit of exploiting the direct links from the source to destinations compared to a simpler two-hop strategy. To answer this question, we first establish the degrees of freedom of the interference channel with a MIMO relay, showing that a K-pair network with a MIMO relay has K/2 degrees of freedom. Thus, appropriate signaling in a two-hop scenario captures the degrees of freedom without the need for the direct links. We then consider more sophisticated encoding strategies in search of other ways to exploit the direct links. Using a number of hybrid encoding strategies, we obtain non-asymptotic achievable sum-rates. We investigate the case where the relay (unlike other nodes) has access to abundant power, showing that when sources have power P and the relay is allowed power proportional to O(P^2), the full degrees of freedom K are available to the network.