  Instantaneous or statistical channel state information (CSI) is needed for
most detection schemes developed in the molecular communication (MC)
literature. Since the MC channel changes, e.g., due to variations in the
velocity of flow, the temperature, or the distance between transmitter and
receiver, CSI acquisition has to be conducted repeatedly to keep track of CSI
variations. Frequent CSI acquisition may entail a large overhead whereas
infrequent CSI acquisition may result in a low CSI estimation quality. To cope
with these issues, we design codes which facilitate maximum likelihood sequence
detection at the receiver without instantaneous or statistical CSI. In
particular, assuming concentration shift keying modulation, we show that a
class of codes, referred to as strongly constant-weight (SCW) codes, enables
optimal CSI-free sequence detection at the cost of decreasing the data rate.
For the proposed SCW codes, we analyze the code rate and the error rate.
Simulation results verify our analytical derivations and reveal that the
proposed CSI-free detector for SCW codes outperforms the baseline coherent and
non-coherent detectors for uncoded transmission.
