  Molecular communication between biological entities is a new paradigm in
communications. Recently, we studied molecular communication between two nodes
formed from synthetic bacteria. Due to high randomness in behavior of bacteria,
we used a population of them in each node. The reliability of such
communication systems depends on both the maximum concentration of molecules
that a transmitter node is able to produce at the receiver node as well as the
number of bacteria in each nodes. This maximum concentration of molecules falls
with distance which makes the communication to the far nodes nearly impossible.
In order to alleviate this problem, in this paper, we propose to use a
molecular relaying node. The relay node can resend the message either by the
different or the same type of molecules as the original signal from the
transmitter. We study two scenarios of relaying. In the first scenario, the
relay node simply senses the received concentration and forwards it to the
receiver. We show that this sense and forward scenario, depending on the type
of molecules used for relaying, results in either increasing the range of
concentration of molecules at the receiver or increasing the effective number
of bacteria in the receiver node. For both cases of sense and forward relaying,
we obtain the resulting improvement in channel capacity. We conclude that
multi-type molecular relaying outperforms the single-type relaying. In the
second scenario, we study the decode and forward relaying for the M-ary
signaling scheme. We show that this relaying strategy increases the reliability
of M-ary communication significantly.
