Coarse Network Coding: A Simple Relay Strategy to Resolve Interference

TL;DR

This paper introduces a simple relay coding strategy that uses a binning scheme to improve the sum rate in interference channels, achieving near-optimal performance and practical gains with LDPC codes.

Contribution

It proposes a novel scalar quantization-binning relay strategy that asymptotically reaches the cut-set bound in interference channels with limited relay rate.

Findings

Achieves the cut-set bound asymptotically at high SNR.

Provides significant SNR gains in matched scenarios.

Offers rate improvements beyond power increases in mismatched scenarios.

Abstract

Reminiscent of the parity function in network coding for the butterfly network, it is shown that forwarding an even/odd indicator bit for a scalar quantization of a relay observation recovers 1 bit of information at the two destinations in a noiseless interference channel where interference is treated as noise. Based on this observation, a coding strategy is proposed to improve the rate of both users at the same time using a relay node in an interference channel. In this strategy, the relay observes a linear combination of the two source signals, and broadcasts a common message to the two destinations over a shared out-of-band link of rate R0 bits per channel use. The relay message consists of the bin index of a structured binning scheme obtained from a 2^R0-way partition of the squared lattice in the complex plane. We show that such scalar quantization-binning relay strategy asymptotically achieves the cut-set bound in an interference channel with a common out-of-band relay link of limited rate, improving the sum rate by two bits for every bit relayed, asymptotically at high signal to noise ratios (SNR) and when interference is treated as noise. We then use low-density parity-check (LDPC) codes along with bit-interleaved coded-modulation (BICM) as a practical coding scheme for the proposed strategy. We consider matched and mismatched scenarios, depending on whether the input alphabet of the interference signal is known or unknown to the decoder, respectively. For the matched scenario, we show the proposed strategy results in significant gains in SNR. For the mismatched scenario, we show that the proposed strategy results in rate improvements that, without the relay, cannot be achieved by merely increasing transmit powers. Finally, we use generalized mutual information analysis to characterize the theoretical performance of the mismatched scenario and validate our simulation results.