  Molecular communication is an expanding body of research. Recent advances in
biology have encouraged using genetically engineered bacteria as the main
component in the molecular communication. This has stimulated a new line of
research that attempts to study molecular communication among bacteria from an
information-theoretic point of view. Due to high randomness in the individual
behavior of the bacterium, reliable communication between two bacteria is
almost impossible. Therefore, we recently proposed that a population of
bacteria in a cluster is considered as a node capable of molecular transmission
and reception. This proposition enables us to form a reliable node out of many
unreliable bacteria. The bacteria inside a node sense the environment and
respond accordingly. In this paper, we study the communication between two
nodes, one acting as the transmitter and the other as the receiver. We consider
the case in which the information is encoded in the concentration of molecules
by the transmitter. The molecules produced by the bacteria in the transmitter
node propagate in the environment via the diffusion process. Then, their
concentration sensed by the bacteria in the receiver node would decode the
information. The randomness in the communication is caused by both the error in
the molecular production at the transmitter and the reception of molecules at
the receiver. We study the theoretical limits of the information transfer rate
in such a setup versus the number of bacteria per node. Finally, we consider
M-ary modulation schemes and study the achievable rates and their error
probabilities.
