  This paper focuses on molecular communication (MC) systems using two types of
signaling molecules which may participate in a reversible bimolecular reaction
in the channel. The motivation for studying these MC systems is that they can
realize the concept of constructive and destructive signal superposition, which
leads to favorable properties such as inter-symbol interference (ISI) reduction
and avoiding environmental contamination due to continuous release of signaling
molecules into the channel. This work first presents a general formulation for
binary modulation schemes that employ two types of signaling molecules and
proposes several modulation schemes as special cases. Moreover, two types of
receivers are considered: a receiver that is able to observe both types of
signaling molecules (2TM), and a simpler receiver that can observe only one
type of signaling molecules (1TM). For both of these receivers, the maximum
likelihood (ML) detector for general binary modulation is derived under the
assumption that the detector has perfect knowledge of the ISI-causing sequence.
In addition, two suboptimal detectors of different complexity are proposed,
namely an ML-based detector that employs an estimate of the ISI-causing
sequence and a detector that neglects the effect of ISI. The proposed
detectors, except for the detector that neglects ISI for the 2TM receiver,
require knowledge of the channel response (CR) of the considered MC system.
Moreover, the CR is needed for performance evaluation of all proposed
detectors. However, deriving the CR of MC systems with reactive signaling
molecules is challenging since the underlying partial differential equations
that describe the reaction-diffusion mechanism are coupled and non-linear.
Therefore, we develop an algorithm for efficient computation of the CR and
validate its accuracy via particle-based simulation.
