Distributed Structure: Joint Expurgation for the Multiple-Access Channel

TL;DR

This paper demonstrates that structured linear and lattice codes can improve error exponents in multiple-access channels, especially for additive and nearly additive cases, surpassing previous bounds.

Contribution

It introduces a joint expurgation approach leveraging code structure to enhance error exponents in MAC channels, including non-additive and Gaussian cases.

Findings

Linear codes achieve better error exponents in MAC channels with additive structure.

Transformation to nearly additive channels can still yield performance gains.

Lattice codes improve error exponents for Gaussian MAC at certain rates.

Abstract

In this work we show how an improved lower bound to the error exponent of the memoryless multiple-access (MAC) channel is attained via the use of linear codes, thus demonstrating that structure can be beneficial even in cases where there is no capacity gain. We show that if the MAC channel is modulo-additive, then any error probability, and hence any error exponent, achievable by a linear code for the corresponding single-user channel, is also achievable for the MAC channel. Specifically, for an alphabet of prime cardinality, where linear codes achieve the best known exponents in the single-user setting and the optimal exponent above the critical rate, this performance carries over to the MAC setting. At least at low rates, where expurgation is needed, our approach strictly improves performance over previous results, where expurgation was used at most for one of the users. Even when the MAC channel is not additive, it may be transformed into such a channel. While the transformation is lossy, we show that the distributed structure gain in some "nearly additive" cases outweighs the loss, and thus the error exponent can improve upon the best known error exponent for these cases as well. Finally we apply a similar approach to the Gaussian MAC channel. We obtain an improvement over the best known achievable exponent, given by Gallager, for certain rate pairs, using lattice codes which satisfy a nesting condition.