  In this paper, we consider a multi-hop molecular communication network
consisting of one nanotransmitter, one nanoreceiver, and multiple
nanotransceivers acting as relays. We consider three different relaying schemes
to improve the range of diffusion-based molecular communication. In the first
scheme, different types of messenger molecules are utilized in each hop of the
multi-hop network. In the second and third scheme, we assume that two types of
molecules and one type of molecule are utilized in the network, respectively.
We identify self-interference, backward intersymbol interference
(backward-ISI), and forward-ISI as the performance-limiting effects for the
second and third relaying schemes. Furthermore, we consider two relaying modes
analogous to those used in wireless communication systems, namely full-duplex
and half-duplex relaying. We propose the adaptation of the decision threshold
as an effective mechanism to mitigate self-interference and backward-ISI at the
relay for full-duplex and half-duplex transmission. We derive closed-form
expressions for the expected end-to-end error probability of the network for
the three considered relaying schemes. Furthermore, we derive closed-form
expressions for the optimal number of molecules released by the nanotransmitter
and the optimal detection threshold of the nanoreceiver for minimization of the
expected error probability of each hop.
