  In this paper, we perform receiver design for a diffusive molecular
communication environment. Our model includes flow in any direction, sources of
information molecules in addition to the transmitter, and enzymes in the
propagation environment to mitigate intersymbol interference. We characterize
the mutual information between receiver observations to show how often
independent observations can be made. We derive the maximum likelihood sequence
detector to provide a lower bound on the bit error probability. We propose the
family of weighted sum detectors for more practical implementation and derive
their expected bit error probability. Under certain conditions, the performance
of the optimal weighted sum detector is shown to be equivalent to a matched
filter. Receiver simulation results show the tradeoff in detector complexity
versus achievable bit error probability, and that a slow flow in any direction
can improve the performance of a weighted sum detector.
