  A diffusion molecular channel is a channel with memory, as molecules released
into the medium hit the receptors after a random delay. Coding over the
diffusion channel is performed by choosing the type, intensity, or the released
time of molecules diffused in the environment over time. To avoid intersymbol
interference (ISI), molecules of the same type should be released at time
instances that are sufficiently far apart. This ensures that molecules of a
previous transmission are faded in the environment, before molecules of the
same type are reused for signaling. In this paper, we consider ISI-free
time-slotted modulation schemes. The maximum reliable transmission rate for
these modulations is given by the constrained coding capacity of the graph that
represents the permissible transmission sequences. However, achieving the
constrained coding capacity requires long blocklengths and delays at the
decoder, making it impractical for simple nanomachines. The main contribution
of this paper is to consider modulations with small delay (short blocklength)
and show that they get very close to constrained coding capacity.
