Experimental and computational study of molecular communication in turbulent fluid environments

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Abstract

Molecular communication (MC) is a type of communication and networking in the electromagnetic (EM)-denied environments. MC is concerned with information transfer by preserving information in the structure of chemical flow through molecular diffusion, advection or reaction. Hence, the information transmission in MC is closely associated with the physics of fluid dynamics. The mechanism of MC, i.e., using chemical substances for information exchange, is prevalent in nature among organisms at various length scales, from intra-cell signaling and bacterial communication to airborne and waterborne pheromone signals.

At nano-scale the physical conditions are such that the main mechanism of transport is mass diffusion. Therefore fluid turbulence, for which other transport mechanisms are relevant, have hitherto hardly been considered at all in the context of MC. Nevertheless, MC is obviously not restricted to nano-scales, as demonstrated by insect and crustacean pheromone signaling. Here turbulence does become a crucial issue affecting the reliability of the message transfer. The goal of this thesis is to draw on turbulence theory to assess implications of relevance to MC at macro scale.

The results show that in turbulent channels, viscous shear stresses hinder a reliable transfer of the molecular information between the transmitter and the receiver which results in severe inter-symbol-interference (ISI). In order to mitigate the ISI in turbulent channels, vortex ring are proposed as coherent structures representing a means for modulating information symbols onto them. Each vortex ring can propagate approximately 100× the diameter of the transmission nozzle without losing its compact shape. It is shown that by maintaining a coherent signal structure, the signal-to-inference (SIR) ratio is higher over conventional puffs.

Moreover, the results show that the received signals of the same transmitted symbols vary due to the presence of the underlying noise in turbulent channels. To understand the behaviour of the noise in turbulent channels, both of the additive and jitter noises distributions characterised statistically, and a new channel model is proposed. Thereafter, this channel model is used to quantify the mutual information in turbulent channels. Finally, the waterborne chemical plumes are investigated as a paradigm for a means of molecular communication at macro scales. Results from the Richardson’s energy cascade theory are applied and interpreted in the context of MC to characterise an information cascade and the information dissipation rate. The results show that the information dissipation rate decreases with increasing the Reynolds number and distance d from transmitter. This may appear counter intuitive because stronger turbulence levels at higher Reynolds numbers increases energy dissipation rates. However, increased turbulence leads to more efficient scalar mixing and, therewith, the power of the molecular signal quickly reduces to low levels. Accordingly the information dissipation rate necessarily reduces due to the remaining low information content available.

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