  In diffusion-based molecular communication (DMC), one important functionality
of a transmitter nano-machine is signal modulation. In particular, the
transmitter has to be able to control the release of signaling molecules for
modulation of the information bits. An important class of control mechanisms in
natural cells for releasing molecules is based on ion channels which are
pore-forming proteins across the cell membrane whose opening and closing may be
controlled by a gating parameter. In this paper, a modulator for DMC based on
ion channels is proposed which controls the rate at which molecules are
released from the transmitter by modulating a gating parameter signal.
Exploiting the capabilities of the proposed modulator, an on-off keying
modulation scheme is introduced and the corresponding average modulated signal,
i.e., the average release rate of the molecules from the transmitter, is
derived in the Laplace domain. By making a simplifying assumption, a
closed-form expression for the average modulated signal in the time domain is
obtained which constitutes an upper bound on the total number of released
molecules regardless of this assumption. The derived average modulated signal
is compared to results obtained with a particle based simulator. The numerical
results show that the derived upper bound is tight if the number of ion
channels distributed across the transmitter (cell) membrane is small.
