  We study non-coherent detection schemes for molecular communication (MC)
systems that do not require knowledge of the channel state information (CSI).
In particular, we first derive the optimal maximum likelihood (ML)
multiple-symbol (MS) detector for MC systems. As a special case of the optimal
MS detector, we show that the optimal ML symbol-by-symbol (SS) detector can be
equivalently written in the form of a threshold-based detector, where the
optimal decision threshold is constant and depends only on the statistics of
the MC channel. The main challenge of the MS detector is the complexity
associated with the calculation of the optimal detection metric. To overcome
this issue, we propose an approximate MS detection metric which can be
expressed in closed form. To reduce complexity even further, we develop a
non-coherent decision-feedback (DF) detector and a suboptimal blind detector.
Finally, we derive analytical expressions for the bit error rate (BER) of the
optimal SS detector, as well as upper and lower bounds for the BER of the
optimal MS detector. Simulation results confirm the analysis and reveal the
effectiveness of the proposed optimal and suboptimal detection schemes compared
to a benchmark scheme that assumes perfect CSI knowledge, particularly when the
number of observations used for detection is sufficiently large.
