  Molecular Communications via Diffusion (MCvD) is a promising paradigm which
enables nano-machines to communicate with each other. However, the reliability
of existing systems degrade rapidly as the distance between the transmitters
and the receivers grows. To solve this issue, relaying schemes must be
implemented in practice. In this paper, we study two relaying schemes: In the
first case, the relay node decodes the incoming signal symbol and forwards it
to the receiver using a different type of molecule. Then, the receiver detects
the information bits by only considering the molecules from the relay node. In
the second case, the receiver considers both the types of molecules sent from
the transmitter and the relay node. For these two scenarios, the optimal
location of the relay node is obtained. We assume Quadruple Concentration Shift
Keying (QCSK) modulation in which the signal is encoded into the four level
concentrations of molecules emitted by the nano-machines. Our simulation
results indicate that adding a relay improves the performance by 10dB and 15dB
in the first and the second schemes, respectively.
