Fundamental Limits of Communications in Interference Networks-Part II: Information Flow in Degraded Networks

TL;DR

This paper characterizes the sum-rate capacity of degraded interference networks, demonstrating the optimality of successive decoding and message subset transmission, and extends these results to derive tighter outer bounds for general networks.

Contribution

It provides a full characterization of sum-rate capacity for degraded networks, introduces algorithms for message subset selection, and derives tighter outer bounds for arbitrary interference networks.

Findings

Successive decoding is sum-rate optimal in degraded networks.

Transmitting only a subset of messages suffices to achieve sum-rate capacity.

Outer bounds are tighter than existing cut-set bounds for many network scenarios.

Abstract

In this second part of our multi-part papers, the information flow in degraded interference networks is studied. A full characterization of the sum-rate capacity for the degraded networks with any possible configuration is established. It is shown that a successive decoding scheme is sum-rate optimal for these networks. Also, it is proved that the transmission of only a certain subset of messages is sufficient to achieve the sum-rate capacity in such networks. Algorithms are presented to determine this subset of messages explicitly. According to these algorithms, the optimal strategy to achieve the sum-rate capacity in degraded networks is that the transmitters try to send information for the stronger receivers and, if possible, avoid sending the messages with respect to the weaker receivers. The algorithms are easily understood using our graphical illustrations for the achievability schemes based on directed graphs. The sum-rate expression for the degraded networks is then used to derive a unified outer bound on the sum-rate capacity of arbitrary non-degraded networks. Several variations of the degraded networks are identified for which the derived outer bound is sum-rate optimal. Specifically, noisy interference regimes are derived for certain classes of multi-user/multi-message interference networks. Also, for the first time, network scenarios are identified where the incorporation of both successive decoding and treating interference as noise achieves their sum-rate capacity. Finally, by taking insight from our results for degraded networks, we establish a unified outer bound on the entire capacity region of the general interference networks. These outer bounds for a broad range of network scenarios are tighter than the existing cut-set bound.