Optimal prefix codes for pairs of geometrically-distributed random variables

TL;DR

This paper investigates optimal prefix codes for pairs of geometrically-distributed variables, revealing their singular nature and providing specific codes for certain parameter values to reduce redundancy.

Contribution

It introduces the concept of singular optimal codes for two-dimensional geometric distributions and characterizes these codes for specific parameter sequences.

Findings

Optimal codes are singular and depend on the parameter q.

Codes for q=2^{-1/k} and q=2^{-k} cover the entire parameter range.

Reduces redundancy compared to one-dimensional codes while maintaining low complexity.

Abstract

Optimal prefix codes are studied for pairs of independent, integer-valued symbols emitted by a source with a geometric probability distribution of parameter $q$, $0{<}q{<}1$. By encoding pairs of symbols, it is possible to reduce the redundancy penalty of symbol-by-symbol encoding, while preserving the simplicity of the encoding and decoding procedures typical of Golomb codes and their variants. It is shown that optimal codes for these so-called two-dimensional geometric distributions are \emph{singular}, in the sense that a prefix code that is optimal for one value of the parameter $q$ cannot be optimal for any other value of $q$. This is in sharp contrast to the one-dimensional case, where codes are optimal for positive-length intervals of the parameter $q$. Thus, in the two-dimensional case, it is infeasible to give a compact characterization of optimal codes for all values of the parameter $q$, as was done in the one-dimensional case. Instead, optimal codes are characterized for a discrete sequence of values of $q$ that provide good coverage of the unit interval. Specifically, optimal prefix codes are described for $q=2^{-1/k}$ ($k\ge 1$), covering the range $q\ge 1/2$, and $q=2^{-k}$ ($k>1$), covering the range $q<1/2$. The described codes produce the expected reduction in redundancy with respect to the one-dimensional case, while maintaining low complexity coding operations.