  Two new asynchronous modulation techniques for molecular timing (MT) channels
are proposed. One based on modulating information on the time between two
consecutive releases of indistinguishable information particles, and one based
on using distinguishable particles. For comparison, we consider the
synchronized modulation scheme where information is encoded in the time of
release and decoded from the time of arrival of particles. We show that all
three modulation techniques result in a system that can be modeled as an
additive noise channel, and we derive the expression for the probability
density function of the noise. Next, we focus on binary communication and
derive the associated optimal detection rules for each modulation. Since the
noise associated with these modulations has an infinite variance, geometric
power is used as a measure for the noise power, and we derive an expression for
the geometric SNR (G-SNR) for each modulation scheme. Numerical evaluations
indicate that for these systems the bit error rate (BER) is constant at a given
G-SNR, similar to the relation between BER and SNR in additive Gaussian noise
channels. We also demonstrate that the asynchronous modulation based on two
distinguishable particles can achieve a BER performance close to the
synchronized modulation scheme.
