Diffusive Molecular Communications With Reactive Molecules: Channel Modeling and Signal Design

Abstract

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 intersymbol 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. The performance of this genie-aided ML detector yields an upper bound on the performance of any practical detector. 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 reactiondiffusion 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. Simulation results obtained using this CR computation algorithm confirm the effectiveness of the proposed modulation and detection schemes. Moreover, these results show that MC systems with reactive signaling have superior performance relative to those with nonreactive signaling due to the reduction of ISI enabled by the chemical reactions.

Equilibrium Signaling: Molecular Communication Robust to Geometry Uncertainties

“…A key feature of the RDME model is that it provides information about the statistical dependence in the receiver observations over time, which is not the case for the model in [23]. We also note that spatial homogeneity for the diffusion process is assumed in [23] and in the vast majority of other work on molecular communications. In the case of mass-action kinetics and first-order reactions, the probability per unit time that a molecule of S l in voxel i reacts at time t is given by a l i M l i (t) with rate constants a l i .…”

“…One key difference is in the choice of the sampling time: in CSK, the sampling time is typically optimized in order to maximize the average number of molecules to arrive in the receiver; while in equilibrium signaling, the sampling time is chosen such that the system approximately reaches equilibrium. A key requirement for CSK is therefore that information carrying molecules are able to leave the fluid medium; either by being absorbed by the transmitter as in the vast majority of CSK schemes [2]- [12], [14], reacting with substrates in the channel as in [23], [24], or diffusing far away when no boundary is present as in [13]. Moreover, CSK also requires the full solution of the Fokker-Planck equation governing molecular motion [25], which is computationally expensive in complex channels, such as in Fig.…”

“…Equilibrium signaling, as developed in this paper, also bears superficial similarities to MoSK [18] and schemes known as reactive signaling [23], where two species of molecules are employed. In binary MoSK schemes, a different molecule is used to transmit each symbol.…”

“…We remark that an alternative stochastic model has recently been studied in the context of reactive signaling which aims to reduce interference by utilizing the reaction of two molecules [23]. A key feature of the RDME model is that it provides information about the statistical dependence in the receiver observations over time, which is not the case for the model in [23]. We also note that spatial homogeneity for the diffusion process is assumed in [23] and in the vast majority of other work on molecular communications.…”

Characterizing the Probability of Collision Between Information Particles in Molecular Communications

“…However, not enough work exists in the literature that studies the compositing processes of collision between the information molecules and the possible escape through the absorbing boundary [12]. In [13], the authors consider a problem of reactive interaction between the molecules based on bulk concentration of molecules. In the presented work, we consider the interaction between a pair of molecules when a competing process of escape through the absorbing receiver is also present in the system.…”