  Significant inter-symbol interference (ISI) challenges the achievement of
reliable, high data-rate molecular communication via diffusion. In this paper,
a hybrid modulation based on pulse position and concentration is proposed to
mitigate ISI. By exploiting the time dimension, molecular concentration and
position modulation (MCPM) increases the achievable data rate over conventional
concentration and position-based modulations. In addition, unlike
multi-molecule schemes, this hybrid scheme employs a single-molecule type and
so simplifies transceiver implementations. In the paper, the optimal sequence
detector of the proposed modulation is provided as well as a reduced complexity
detector (two-stage, position-concentration detector, TPCD). A tractable cost
function based on the TPCD detector is proposed and employed to optimize the
design of the hybrid modulation scheme. In addition, the approximate
probability of error for the MCPM-TPCD system is derived and is shown to be
tight with respect to simulated performance. Numerically, MCPM shows improved
performance over standard concentration and pulse position-based schemes in the
low transmission power and high bit-rate regime. Furthermore, MCPM offers
increased robustness against synchronization errors.
