  In this paper, a novel time-based modulation scheme is proposed in the
time-asynchronous channel for diffusion-based molecular communication systems
with drift. Based on this modulation scheme, we demonstrate that the sample
variance of information molecules' arrival time approximately follows a
noncentral chi-squared distribution. According to its conditional probability
density function (PDF), the asynchronous receiver designs are deduced based on
the maximum likelihood (ML) detection, with or without background noise in the
channel environment. Since the proposed schemes can be applied to the case of
transmitting multiple information molecules, simulation results reveal that the
bit error ratio (BER) performance improves with the increase of the number of
released information molecules. Furthermore, when the background noise is not
negligible, our proposed asynchronous scheme outperforms the asynchronous
modulation techniques based on encoding information on the time between two
consecutive release of information molecules.
