  Molecular communication (MC) has emerged as a promising paradigm employing
molecules to transfer information at the nano-scale. In a recent paper, prefix
source coding was introduced into the field, through an MC-adapted version of
the Huffman coding. We first demonstrate that, although MC-adapted Huffman
coding improves symbol error rates compared to Huffman coding, it does not
always produce an optimal MC-adapted prefix codebook in terms of expected
length and power. Accordingly, we utilise a straightforward brute-force
algorithm to find an optimal MC-adapted prefix (MoPC$^{*}$) codebook for any
given alphabet.
  In the context of MC source coding, the major finding of this paper is the
Molecular Arithmetic Coding (MoAC) whose algorithmic implementation and
code-structure is non-arbitrarily different than that of the widely-known
classical arithmetic coding (AC). MoAC is designed to mitigate inter-symbol
interference (ISI) for alphabets with known symbol probabilities through, in a
highly efficient way, avoiding consecutive 1-bits. However, due to bit
precision limitations, unique-decodability of MoAC is not guaranteed.
Accordingly, a uniquely-decodable new coding scheme named Molecular Arithmetic
with Prefix Coding (MoAPC) is introduced.
  On two exemplary nucleotide alphabets, we show that MoAPC has a better
compression performance than MoPC$^{*}$. Simulation results demonstrate the
highly superior word error rate (WER) and symbol error rate (SER) values of
MoAPC compared to its arithmetic contenders: AC, and the Substitution
Arithmetic Coding (SAC), which is introduced for the first time in this paper,
and is a simple and trivial adaption of AC to molecular communication.
Simulation results show that on the first alphabet, MoAPC surpasses all given
methods in WER and asymptotically in SER; while on the second alphabet,
MoPC$^{*}$ surpasses all in SER and WER.
