  We study diversity in one-shot communication over molecular timing channels.
We consider a channel model where the transmitter simultaneously releases a
large number of information particles, while the information is encoded in the
time of release. The receiver decodes the information based on the random time
of arrival of the information particles. The random propagation is
characterized by the general class of right-sided unimodal densities. We
characterize the asymptotic exponential decrease rate of the probability of
error as a function of the number of released particles, and denote this
quantity as the system diversity gain. Four types of detectors are considered:
the maximum-likelihood (ML) detector, a linear detector, a detector that is
based on the first arrival (FA) among all the transmitted particles, and a
detector based on the last arrival (LA). When the density characterizing the
random propagation is supported over a large interval, we show that the simple
FA detector achieves a diversity gain very close to that of the ML detector. On
the other hand, when the density characterizing the random propagation is
supported over a small interval, we show that the simple LA detector achieves a
diversity gain very close to that of the ML detector.
