Construction and Analysis of Posterior Matching in Arbitrary Dimensions via Optimal Transport

TL;DR

This paper extends the posterior matching scheme to higher dimensions using optimal transport, establishing conditions for reliable message recovery without shared randomness, with applications in human-computer interfaces and multi-antenna systems.

Contribution

It generalizes posterior matching to arbitrary dimensions without shared randomness using optimal transport and establishes ergodicity as the key condition for achieving rates below capacity.

Findings

Ergodicity of a generated process is necessary and sufficient for message recovery.

The scheme achieves any rate below capacity under the ergodicity condition.

Applications demonstrated in human-computer interfaces and multi-antenna communications.

Abstract

The posterior matching scheme, for feedback encoding of a message point lying on the unit interval over memoryless channels, maximizes mutual information for an arbitrary number of channel uses. However, it in general does not always achieve any positive rate; so far, elaborate analyses have been required to show that it achieves any positive rate below capacity. More recent efforts have introduced a random "dither" shared by the encoder and decoder to the problem formulation, to simplify analyses and guarantee that the randomized scheme achieves any rate below capacity. Motivated by applications (e.g. human-computer interfaces) where (a) common randomness shared by the encoder and decoder may not be feasible and (b) the message point lies in a higher dimensional space, we focus here on the original formulation without common randomness, and use optimal transport theory to generalize the scheme for a message point in a higher dimensional space. By defining a stricter, almost sure, notion of message decoding, we use classical probabilistic techniques (e.g. change of measure and martingale convergence) to establish succinct necessary and sufficient conditions on when the message point can be recovered from infinite observations: Birkhoff ergodicity of a random process sequentially generated by the encoder. We also show a surprising "all or nothing" result: the same ergodicity condition is necessary and sufficient to achieve any rate below capacity. We provide applications of this message point framework in human-computer interfaces and multi-antenna communications.