# Capacity Results for the K-User Broadcast Channel with Two Nested Multicast Messages

## TL;DR

This paper characterizes the capacity region for the K-user discrete memoryless broadcast channel with two nested multicast messages, introducing new coding strategies like rate-splitting and providing a unified framework that extends previous results.

## Contribution

It establishes the capacity region for several classes of DM broadcast channels with nested messages, using superposition coding and rate-splitting, and unifies previous results under a comprehensive framework.

## Key findings

- Capacity regions are derived for multiple classes of DM BCs.
- Superposition coding achieves capacity in certain classes.
- Rate-splitting enhances achievable rate regions and proves capacity in broader cases.

## Abstract

The K-user discrete memoryless (DM) broadcast channel (BC) with two nested multicast messages is studied in which one common message is to be multicast to all receivers and the second private message to a subset of receivers. The receivers that must decode both messages are referred to as private receivers and the others that must decode only the common message as common receivers. For two nested multicast messages, we establish the capacity region for several classes of DM BCs characterized by the respective associated sets of pair-wise relationships between and among the common and private receivers, each described by the well-known more capable or less noisy conditions. For three classes of DM BCs, the capacity region is simply achieved by superposition coding and the proofs of the converses rely on a recently found information inequality. The achievable rate region is then enhanced through the addition of a splitting of the private message into as many parts as there are common receivers and indirect decoding. A closed-form two-dimensional polyhedral description is obtained for it for a given coding distribution. Through a converse result that relies on the well-known Csiszar sum lemma and the information inequality, a specialization of this region that involves splitting the private message into just two sub-messages is proved to be the capacity region for several classes of DM BCs, beyond those for which superposition coding alone is capacity optimal, thereby underscoring the benefit of rate-splitting. All previously known capacity results for DM BCs with two nested multicast messages for the two and three-receiver DM BCs as well as DM BCs with one private or one common receiver are included in the general framework presented in this work.