Broadcasting a Common Message with Variable-Length Stop-Feedback Codes

TL;DR

This paper examines how variable-length stop-feedback codes impact the maximum coding rate in a two-user broadcast channel with a common message, revealing that the benefits seen in point-to-point channels do not always apply.

Contribution

The paper demonstrates that, unlike in point-to-point channels, variable-length stop-feedback codes may not eliminate dispersion in broadcast channels with a common message.

Findings

Variable-length stop-feedback codes can have positive dispersion in broadcast channels.

The speed-up in convergence to capacity observed in point-to-point channels may not occur in broadcast channels.

Certain scenarios show no zero dispersion benefit for broadcast channels with common messages.

Abstract

We investigate the maximum coding rate achievable over a two-user broadcast channel for the scenario where a common message is transmitted using variable-length stop-feedback codes. Specifically, upon decoding the common message, each decoder sends a stop signal to the encoder, which transmits continuously until it receives both stop signals. For the point-to-point case, Polyanskiy, Poor, and Verd\'u (2011) recently demonstrated that variable-length coding combined with stop feedback significantly increases the speed at which the maximum coding rate converges to capacity. This speed-up manifests itself in the absence of a square-root penalty in the asymptotic expansion of the maximum coding rate for large blocklengths, a result a.k.a. zero dispersion. In this paper, we show that this speed-up does not necessarily occur for the broadcast channel with common message. Specifically, there exist scenarios for which variable-length stop-feedback codes yield a positive dispersion.