Channel Characterisation for Molecular Communication Systems with Practical Transmitters and Receivers

Abstract

Molecular communication (MC) has emerged as a promising paradigm to facilitate microscale or nanoscale communications. In MC, information is encoded into small particles that are released by a transmitter (TX) into a fluid medium and propagate until they arrive at a receiver (RX). In particular, MC possesses important characteristics, such as low energy consumption and potential for biocompatibility, which makes it suitable for many in vivo applications, e.g., health monitoring and targeted drug delivery. This thesis focuses on designing practical transmission/reception mechanisms and characterising the channel between the TX and RX from the communication and signal processing perspectives. Specifically, this thesis fills the current research gaps by i) designing practical TXs, ii) designing practical RXs, iii) investigating the co-existence of multiple non-transparent RXs, iv) proposing novel parameter estimation methods, and v) designing strategies for enhancing the energy efficiency. First, we propose a novel imperfect TX model, namely the membrane fusion (MF)-based TX, that adopts MF between a vesicle and the TX membrane to release molecules encapsulated within the vesicle in Chapter 3. Incorporating a fully absorbing RX, the channel impulse response (CIR) is derived for two scenarios: 1) Both TX and RX are static, and 2) both TX and RX are diffusion based mobile. Moreover, simulation results show that a low MF probability or low vesicle mobility slows the release of molecules and reduces the molecule hitting probability at the RX. Second, we propose an absorbing RX covered by multiple non-overlapping heterogeneous receptors that may have different sizes and arbitrary locations in Chapter 4. We consider two types of TX, which are a point TX and an MF-based TX. For each type of TX, we analyse the expected molecule hitting rate at the RX as a function of the sizes and locations of the receptors. Exploiting our numerical results, we show that the expected number of absorbed molecules at the RX increases with the number of receptors, when the total area on the RX surface covered by receptors is fixed. Third, we develop a one-dimensional (1D) diffusion-based MC system to analyse CIR between a single TX and two fully absorbing RXs in Chapter 5. In particular, we derive rigorous analytical expressions for i) the fraction of molecules absorbed, ii) he corresponding hitting rate, and iii) the asymptotic fraction of absorbed molecules as time approaches infinity at each RX. Moreover, we consider constant flow and noisy molecules in this environment, and investigate the estimation of different parameters, e.g., propagation distance and flow velocity. A novel estimation method, namely difference estimation (DE), is proposed to eliminate the effect of noise by using the difference between the received signals at two RXs. For DE, the Cramer Rao lower bound (CRLB) on the variance of estimation is derived. Furthermore, numerical results show that DE attains the CRLB and is less sensitive to the change of noise than independent estimation at each RX. Finally, in Chapter 6, we design a molecule harvesting TX model, where the surface of a spherical TX is covered by heterogeneous receptors with different sizes and arbitrary locations. If molecules hit any receptor, they are absorbed by the TX immediately. Within the TX, molecules are stored in vesicles that are continuously generated and released by the TX via the MF process. Considering a transparent RX and molecular degradation during the propagation from the TX to the RX, we derive the molecule release rate and the fraction of molecules absorbed by the TX as well as the received signal at the RX. Numerical results show that different vesicle generation rates result in the same number of molecules absorbed by the TX, but different peak received signals at the RX.