8

Summary and Future-Work

8.1 Conclusion

I n this thesis, an overview of macro-scale molecular communication was undertaken. Macro-scale molecular communication is described as an alternative branch of communication engineering, where the applications and the challenges greatly differ from its micro-scale counterpart. Molecular communication at the macro-scale can be used for infrastructure monitoring or communication in underwater or underground environments where EM waves would prove inefficient due to environmental energy absorption.

In Chapter 2 a comprehensive review on the topic of molecular communication is done. The review includes the concept of molecular communications. A detailed description of the differences between macro and micro-scale communication is given. The information theory of the communication is discussed and a brief section on information security is given. A detailed analysis on different types of propagation used in modelling molecular communication is shown for two types of propagation elements: diffusion and advection-diffusion. Moreover, a review on different types of modulation methods proposed and simulated for molecular communications are compared with additional discussion on the ISI, error correction and the receivers. Receiver for molecular communications, both experimental and theoretical are reviewed along with the experimental setups and proposed applications throughout the literature. The review on the field concludes with the simulation platforms and the standardisation efforts made for molecular communications

Throughout Chapter 2 it was shown that majority of the literature focuses on the theoretical aspect of molecular communication with micro-scale being the considered range. Therefore, two major gaps are identified in the literature: theoretical and experimental studies of molecular communication at the macro-scale. Subsequent chapters focused on different aspects of macro-scale molecular communication with experimental studies conducted to validate the mathematical models developed in the thesis.

In Chapter 3 a detailed description of the experimental setup is given that is used in the experiments conducted in Chapters 5, 6 and 7. To generate chemical pulses based on the digital information and transmit them into the transmission medium, an odour generator (OG) was utilised and and evaporation chamber (EC) was used to transmit the signal chemicals into the odour generator. The properties of the chemicals used in the thesis are given which are: acetone, methyl alcohol, cyclopentane and n-hexane. To detect the transmitted chemicals, a membrane inlet mass spectrometer (MIMS) with a quadrupole mass analyser (QMA) was used. One of the defining features of the MIMS is its use of a membrane to separate chemicals at the inlet which is discussed in this chapter. The chapter concludes with detailed information regarding the operational use and the theoretical aspects of the detector.

In Chapter 4 the mathematical model that is used throughout the thesis is described. The model is based on the advection-diffusion equation (ADE). The developed model is used in describing and understanding the macro-molecular propagation and the particles interaction with the membrane. The model is developed for two environments: Open boundary (Cartesian coordinates) and cylindrical boundary (cylindrical coordinates). In open boundary a decay ( ${\lambda }_D$ ) is implemented to simulate the effect of decay and in closed boundary, method of mirror images is used to simulate the boundary in a pipe. Due to the effect of the boundary, the diffusion is calculated based on Taylor-Aris dispersion which the derivation can be seen in Appendix B. Based on the equation, a simulation model was developed and given in detail for both aforementioned environments.

In Chapter 5, the parameter analysis of molecular communication in macro-scale is experimented and studied. The parameters in question are: environmental noise, signal flow, carrier flow, bit duration, open-distance transmission and closed-distance transmission. These aforementioned particles were analysed based on their signal amplitude, signal energy, signal-to-noise and signal shape. The noise present in the communication is analysed and modelled as additive white Gaussian noise (AWGN) present at the detector. The mathematical model derived in Chapter 4 is used to describe and predict the behaviour of the parameters. It was shown that the model accurately predicts the parameters behaviour and its signal shapes.

In Chapter 6, modulation methods on macro-scale molecular communications is studied. $M_C$ -ary modulation of levels 2, 4, 8 and symbol periods of 30s, 60s, 90s were investigated. An experimental transmission of a chemical message ( “Call me Ishmael” ) is conducted. To better understand the effect of leftover chemicals on the background noise (Mo-ISI) transmissions were investigated. All the aforementioned experiments were additionally analysed by the theoretical model developed in Chapter 4. Based on the empirical evidence gathered a model was developed to predict the residual background signal after a transmission is terminated and additional calculations were done on the optimal symbol period of a chemical pulse. Experimental investigations were carried out on the SER performance of the communication and the channel behaviour is defined as a Binary Asymmetric Channel (BAC). Based on the channel definition and the model, theoretical calculations were initiated for channel capacity and SER performance for the following parameters: symbol duration, coefficient of diffusion, transmission distance and advective flow. The chapter concludes with a theoretical study on the bit distribution on the four aforementioned parameters.

In Chapter 7, to improve the throughput of the communication, multiple chemicals were employed and experimented. A proof-of-concept transmission of multiple chemical was investigated. The environmental noise characteristics of multiple-chemical transmission was also undertaken and was shown to demonstrate Gaussian behaviour with different mean and variance for each chemical. Three types of modulation methods were then developed. The first one is based on quadrature amplitude modulation (QAM) named molecular-QAM (MQAM). Theoretical analysis were done on the channel capacity and the SER performance with additional analysis on the scattering distributions. The second modulation method is based on the relative time different between distinguishable chemical signals and an experimental transmission was conducted based on this proposed modulation method. A Final modulation method was experimented based on the ratios of the propagating chemicals and was shown to be possible for use in long range molecular communications.

8.2 Future Research Directions

The field of molecular communication in the macro-scale is still in its infancy in regards to its understanding and its exploitation. Based on the research done in this thesis there are a few areas in which the work presented here can be carried out.

8.2.1 Different Receivers

In Chapter 3 the MS with a membrane inlet (MIMS) was used as the receiver. MS are an industry standard in detecting and quantifying chemical species. However, the response rate of the sensor can be better. Therefore, alternative chemical sensor can be employed that can be better choice for fast-transmission molecular communication.

In Chapter 2 alternative sensors for macro-scale molecular communication were discussed. One of these being electronics noses. These machines, intended to detect odours or flavours, can be used as a detector for the communication. However, to decrease the cost of the setup, cheaper alternatives can be sought after, for example Tin-Oxide ( $Sn{O}_2$ ) sensors. The behaviour of these sensors in the macro-scale are yet to be fully understood and further studies can show the optimal cost-effective use of these sensors for different environments.

Additional future-work can be focused on designing a molecular antenna to detect the direction in which the message is coming and if possible, triangulate the source of the transmission.

8.2.2 Turbulent Environments

In Chapter 4 a mathematical description of a molecular communication in macro-scale in isotropic environment was developed. In future, the mathematical model can be defined for describing the environments where turbulence (Re > 4000) plays a prominent role. Turbulence is defined as a fluid motion with chaotic changes in pressure and flow velocity and is encountered in everyday phenomena such as fast flowing rivers, smokes from a chimney and majority of the fluid flows occurring in nature. The complex motion of the fluid with rapid changes in its properties make it a challenge to understand and still is an unsolved problem in physics.

Since the majority of fluid flow in nature happens on Re > 4000, an understanding of this motion would open new possibilities of implementing macro-scale molecular communication to already existing propagation methods.

8.2.3 Complex Geometries

In Chapter 5 the parameters of macro-scale molecular communications were analysed. The closed distance transmission was experimented and mathematically modelled. However, only a straight cylindrical geometry (i.e., pipe) was considered. Geometries that have curvature would increase the complexity of the solution and analytical solutions to these geometries may not exist and only approximations can be made. Different types of methods can be used to achieve this, such as asymptotic analysis or centre manifold theorem. Derivation of these complex geometries may open up macro-molecular communication to new applications such as drug-delivery for in-vivo systems or molecular communication in riverbeds.

8.2.4 Secure Chemical Communications

In Chapter 7 communication throughput was shown to be able to be increased by the use of multiple chemical propagation. The prospect opens up new possibilities for implementing known technologies (i.e., security) to be implemented.

8.2.5 Novel Modulation Methods

In Chapter 7 it was shown that novel modulation methods can be developed that have the potential to increase the communication throughput or its reliability. Future-work on this topic is to analyse other modulation methods that also exploit multiple chemical transmission. One approach is the optimisation of the ratio modulation discussed in Chapter 7. While it was shown that it is possible to send chemicals with their ratios staying relatively stable, the choice of the chemicals that are capable of accomplishing this task needs to be chosen carefully. Understanding the chemicals properties may yield better combinations which may increase the range and the throughput of the modulation scheme.