Research On Key Technologies For Asynchronous Molecular Communication System

Molecular communications are the biological-inspired communication paradigms which utilize chemical signals as information carriers in nano-networks,bringing about potential applications in plenty of research areas.One of the most promising paradigms is diffusion-based molecular communications(MC),in which information molecules are released at the transmitter,propagated in a fluid media and captured by the receiver,and the receiver recovers information based on the received molecules.In diffusion-based molecular communications,time synchronization is a major reason for the increase of system structure complexity.Improving the transmission reliability of asynchronous molecular communication systems has significant research meanings.The main research work in this thesis includes three sections as follows:(1)Modulation and Demodulation Scheme for Asynchronous Molecular Communication Systems with No Clock at ReceiversIn this study,we consider an asynchronous receiver design for molecular communications with information symbols conveyed in the time of released molecules.The main contribution of section is that we develop a detector called clock-free asynchronous receiver design(CFARD),in which the receiver recovers the information symbols without measuring the arrival time of molecules.The theoretical analysis indicates that compared with the synchronous receiver designs,the proposed scheme considerably lowers the structure complexity for information demodulation,which is of great significance to the feasibility of nano-scale molecular communications systems with the limitation of energy and size.The numerical results show that in the comparison of bit error ratio(BER)performance,the proposed asynchronous receiver design outperforms the synchronous linear average filter(LAF)detector and approaches to the synchronous maximum likelihood(ML)detector and first arrival(FA)detector.(2)Modulation and Demodulation Scheme for Asynchronous Molecular Communication Systems with Clocks at ReceiversIn this study,a novel time-based modulation scheme is proposed in the time-asynchronous channel with clocks.Based on this modulation scheme,we demonstrate that the sample variance of information molecules' arrival time approximately follows a non-central chi-squared distribution.According to its conditional probability density function(PDF),the asynchronous receiver designs are deduced based on the maximum likelihood(ML)detection,with or without background noise in the channel environment.Since the proposed schemes can be applied to the case of transmitting multiple information molecules,simulation results reveal that the BER performance improves with the increase of the number of released information molecules per bit.Furthermore,when the background noise is not negligible,our proposed asynchronous scheme outperforms the asynchronous modulation techniques based on encoding information on the time between two consecutive release of information molecules.(3)Decoding Algorithm in Crossover Channel for Asynchronous Molecular Communication SystemsThe inter-symbol interference(ISI)is a main reason for molecular communication system performance degradation,which is caused by the random movement and out-of-order arrival of molecules.In this study,a new metric called Crossover Distance is introduced to measure the distance between the input and output sequences in difusion-based molecular communication channels.We use Crossover Distance for the decoding of convolutional codes with ISI errors,which can enhance the communication reliability significantly.The simulation results show that compared with uncoded systems and some existing channel codes,the proposed convolutional codes delivery better performance under same throughput.