  This article investigates a novel electrophoretic molecular communication
mechanism that utilizes a time-varying electric field, which induces
time-varying molecule velocities and in turn improves communication
performance. For a sinusoidal field, we specify favorable signal parameters
(e.g., phase and frequency) that yield excellent communication link
performance. We also analytically derive an optimized field function by
formulating an appropriate cost function and solving the Euler-Lagrange
equation. In our setup, the field strength is proportional to the molecular
velocity; we verify this assumption by solving the Basset-Boussinesq-Oseen
equation for a given time-varying electric field (forcing function) and
examining its implications for practical physical parameterizations of the
system. Our analysis and Monte-Carlo simulation results demonstrate that the
proposed time-varying approach can significantly increase the number of
information-carrying molecules expected to be observed at the receiver and
reduce the bit-error probability compared to the constant field benchmark.
