Aligned Interference Neutralization and the Degrees of Freedom of the 2 User Interference Channel with Instantaneous Relay

TL;DR

This paper demonstrates that a 2-user interference channel with an instantaneous relay can achieve 3/2 degrees of freedom, surpassing the classical limit, using a novel interference neutralization scheme without relay memory.

Contribution

It introduces a new scheme showing that instantaneous relays increase the degrees of freedom in the 2-user interference channel, achieving 3/2 DoF with memoryless relays.

Findings

Achieves 3/2 DoF with an instantaneous relay.

Uses aligned interference neutralization scheme.

Relays with no memory suffice for DoF gain.

Abstract

It is well known that the classical 2 user Gaussian interference channel has only 1 degree of freedom (DoF), which can be achieved by orthogonal time division among the 2 users. It is also known that the use of conventional relays, which introduce a processing delay of at least one symbol duration relative to the direct paths between sources and destinations, does not increase the DoF of the 2 user interference channel. The use of instantaneous relays (relays-without-delay) has been explored for the single user point-to-point setting and it is known that such a relay, even with memoryless forwarding at the relay, can achieve a higher capacity than conventional relays. In this work, we show that the 2 user interference channel with an instantaneous relay, achieves 3/2 DoF. Thus, an instantaneous relay increases not only the capacity but also the DoF of the 2 user interference channel. The achievable scheme is inspired by the aligned interference neutralization scheme recently proposed for the 2X2X2 interference channel. Remarkably the DoF gain is achieved with memoryless relays, i.e., with relays that have no memory of past received symbols.