  In this paper, we consider bidirectional relaying between two diffusion-based
molecular transceivers (bio-nodes). As opposed to existing literature, we
incorporate the effect of direct diffusion links between the nodes and leverage
it to improve performance. Assuming network coding type operation at the relay,
we devise a detection strategy, based on the maximum-likelihood principle, that
combines the signal received from the relay and that received from the direct
link. At the same time, since a diffusion-based molecular communication channel
is characterized by high inter-symbol interference (ISI), we utilize a decision
feedback mechanism to mitigate its effect. Simulation results indicate that the
proposed setup incorporating the direct link can achieve notable improvement in
error performance over conventional detection schemes that do not exploit the
direct link and/or do not attempt to mitigate the effect of ISI.
