Comparing the Bit-MAP and Block-MAP Decoding Thresholds of Reed-Muller Codes on BMS Channels

TL;DR

This paper investigates the decoding thresholds of Reed-Muller codes on BMS channels, providing a new approach to relate bit-MAP and block-MAP error probabilities, and extends capacity-achieving results beyond erasure channels.

Contribution

It introduces an alternative method based on weight distribution analysis to connect bit-MAP and block-MAP thresholds for Reed-Muller codes on BMS channels.

Findings

Block-MAP error probability converges to 0 if bit-MAP error decays polynomially.

The approach applies to any binary memoryless symmetric channel.

Extends capacity-achieving proofs of Reed-Muller codes beyond erasure channels.

Abstract

The question whether RM codes are capacity-achieving is a long-standing open problem in coding theory that was recently answered in the affirmative for transmission over erasure channels [1], [2]. Remarkably, the proof does not rely on specific properties of RM codes, apart from their symmetry. Indeed, the main technical result consists in showing that any sequence of linear codes, with doubly-transitive permutation groups, achieves capacity on the memoryless erasure channel under bit-MAP decoding. Thus, a natural question is what happens under block-MAP decoding. In [1], [2], by exploiting further symmetries of the code, the bit-MAP threshold was shown to be sharp enough so that the block erasure probability also converges to 0. However, this technique relies heavily on the fact that the transmission is over an erasure channel. We present an alternative approach to strengthen results regarding the bit-MAP threshold to block-MAP thresholds. This approach is based on a careful analysis of the weight distribution of RM codes. In particular, the flavor of the main result is the following: assume that the bit-MAP error probability decays as $N^{-\delta}$, for some $\delta>0$. Then, the block-MAP error probability also converges to 0. This technique applies to transmission over any binary memoryless symmetric channel. Thus, it can be thought of as a first step in extending the proof that RM codes are capacity-achieving to the general case.