Clock Synchronization For Molecular Communication

Molecular communication is a short range communication methodology at nanoscale which uses chemical or biological molecules as the information carriers. It is a novel inter-disciplinary communication methodology which combines with biology, computer technology and communication technologies. Molecular communication has emerged as an interesting and important research hot spot in the last few years. It has an extensive research prospect in the applications of biomedical engineering, environmental monitoring, industrial production and military.The entire molecular communication process consists of modulation, emission, transmission, reception and demodulation. In a point-to-point molecular communication system, a transmitter nanomachine senses a trigger signal and modulates the information molecules based on their physical or chemical properties. Those modulated information molecules are released into the information channel. The receiver nanomachine senses and receives the information molecules, and operates for demodulation. Since the molecular communication paradigm is different from the conventional radio communication at the physical layer such as information carriers, signal type, medium condition, propagation distance and signal propagation, the corresponding communication techniques and networking protocols need to be reconsidered and redesigned. This dissertation will combine the communication theory in electronics to study the modulation schemes in diffusion-based molecular communication system. Based on probability theory and mathematical statistics, such as probability density function, maximum likelihood estimation, mean square error and Cramer-Rao lower bound, the clock synchronization technology for molecular communication is studied. The main contributions are as follows.1. Digital baseband modulation schemes for diffusion-based molecular communication are evaluated. These modulation methods, including unipolar, polar, bipolar and Manchester methods, are investigated. Firstly, we describe four digital baseband modulation schemes for molecular communication systems, which modulate digital signals into the concentration of molecules. Then, the influences of the parameters such as symbol rate, distance, diffusion coefficient, are evaluated. The numerical results reveal that Manchester modulation scheme outperforms others in terms of bit error rate.2. Estimator of clock offset between nanomachines is studied. In molecular communication system including two nanomachines with the same clock skew, diffusive random delay is modeled as inverse Gaussian distribution. Based on the statistical delay model of the molecular diffusion, a two-way message exchange model is proposed for the clock synchronization. The maximum-likelihood estimator of the clock offset is derived. The convergence and the bias of the estimator are analyzed. The simulation results show that the proposed estimator is effective for the offset compensation for the clock synchronization.3. Clock synchronization under inverse Gaussian random delay model is discussed. In molecular communication based on fluid medium, an asymmetrical clock synchronization method based on two-way message exchange mechanism is proposed. The clock offset and clock skew are estimated by the maximum likelihood estimation. Simulation results show that the mean square errors of the estimated clock offsets and the estimated clock skews can be reduced and converge with a number of rounds of message exchanges. The comparison of the proposed scheme with a clock synchronization method based on symmetrical propagation delay demonstrates that our proposed scheme can achieve a better performance in terms of accuracy.4. Clock synchronization issue for the molecular communication system with the presence of Gaussian distributed propagation delay is studied. In diffusion-based molecular communication system, a Gaussian distributed propagation delay is proposed and a two-way message exchange mechanism is designed. To eliminate the nuisances, the two random variables are translated into a variable by using arithmetic operation. Then the closed-form expressions of maximum likelihood estimators for the relative clock offset and the relative clock skew are derived. The convergence and the Cramer-Rao lower bound of the estimators are analytically analyzed. Simulation results further demonstrate the effectiveness of the proposed estimators.