  Distributed Arithmetic Coding (DAC) is an effective implementation of
Slepian-Wolf coding, especially for short data blocks. To research its
properties, the concept of DAC codeword distribution along proper and wrong
decoding paths has been introduced. For DAC codeword distribution of
equiprobable binary sources along proper decoding paths, the problem was
formatted as solving a system of functional equations. However, up to now, only
one closed form was obtained at rate 0.5, while in general cases, to find the
closed form of DAC codeword distribution still remains a very difficult task.
This paper proposes three kinds of approximation methods for DAC codeword
distribution of equiprobable binary sources along proper decoding paths:
numeric approximation, polynomial approximation, and Gaussian approximation.
Firstly, as a general approach, a numeric method is iterated to find the
approximation to DAC codeword distribution. Secondly, at rates lower than 0.5,
DAC codeword distribution can be well approximated by a polynomial. Thirdly, at
very low rates, a Gaussian function centered at 0.5 is proved to be a good and
simple approximation to DAC codeword distribution. A simple way to estimate the
variance of Gaussian function is also proposed. Plenty of simulation results
are given to verify theoretical analyses.
