  A distribution matcher (DM) encodes a binary input data sequence into a
sequence of symbols (codeword) with desired target probability distribution.
The set of the output codewords constitutes a codebook (or code) of a DM.
Constant-composition DM (CCDM) uses arithmetic coding to efficiently encode
data into codewords from a constant-composition (CC) codebook. The CC
constraint limits the size of the codebook, and hence the coding rate of the
CCDM. The performance of CCDM degrades with decreasing output length. To
improve the performance for short transmission blocks we present a class of
multi-composition (MC) codes and an efficient arithmetic coding scheme for
encoding and decoding. The resulting multi-composition DM (MCDM) is able to
encode more data into distribution matched codewords than the CCDM and achieves
lower KL divergence, especially for short block messages.
