On Medium Chemical Reaction in Diffusion-Based Molecular Communication: A Two-Way Relaying Example

Abstract

Chemical reactions are a prominent feature of molecular communication (MC) systems, with no direct parallels in wireless communications. While chemical reactions may be used inside the transmitter nodes, receiver nodes or the communication medium, we focus on its utility in the medium in this paper.Such chemical reactions can be used to perform computation over the medium as molecules diffuse and react with each other (physical-layer computation). We propose the use of chemical reactions for the following purposes: (i) to reduce signal-dependent observation noise of receivers by reducing the signal density, (ii) to realize molecular physical-layer network coding (molecular PNC) by performing the natural XOR operation inside the medium, and (iii) to reduce the inter-symbol interference (ISI) of other transmitters by canceling out the remaining molecules from previous transmissions. To make the ideas formal, we consider an explicit two-way relaying example with a transparent receiver (which has a signal-dependent noise). The proposed ideas are used to define a modulation scheme (which we call the PNC scheme). We compare the PNC with a previously proposed scheme for this problem where the XOR operation is performed at the relay node (using a molecular logic gate). We call the latter, the straightforward network coding (SNC). It is observed that in addition to the simplicity of the proposed PNC scheme, it outperforms the SNC scheme especially when we consider ISI. I. INTRODUCTIONWhile traditional wireless communication systems employ energy carriers (such as electromagnetic or acoustic waves) for communication, Molecular Communication (MC) utilizes physical molecules as This work was in part presented in the 2016 Iran Workshop on Communication and Information Theory (IWCIT) [1].DRAFT 2 its carriers of information. In diffusion-based MC system, the transmitter and the receiver are biological/engineered cells or electronic systems that release or receive molecules, while the channel is assumed to be a fluid medium in which molecules diffuse. Electromagnetic waves and molecular diffusion share similarities and differences. Both the electromagnetic wave equation and the Fick's second law of macroscopic diffusion are second-order linear partial differential equations. As a result, both lead to linear system models that satisfy the superposition property. However, there are also differences between electromagnetic waves and molecular diffusion. Notably, the degradation and attenuation of transmitted signals are more pronounced in molecular diffusion-based channels and seriously limit the transmission distance between the transmitter and the receiver [2]. Relaying is a solution for increasing the range of communication and has been utilized by nature in intracellular communication [3, Chapter 15]. In addition, while the measurement noise of a wireless receiver may be modeled by an additive Gaussian noise (the AWGN channel), some of the most promising molecular receptors, such as the ligand receiver and the transparent recei...

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