Research On Intersymbol Interference Mitigation In Molecular Communication

With the rapid development of nanotechnology,nanomachines have been applied in many fields such as biological detection,intelligent drug delivery and environmental monitoring.However,the constraint of the ratio of antenna size to electromagnetic signal wavelength makes the application of electromagnetic communication in nanomachines infeasible.Inspired by the communication in nature,a feasible way to solve this problem is molecular communication(MC),in which chemical signals are employed as the carrier of information during the information transmission.In MC,the released molecules diffuse from the transmitter to the receiver by Brownian motion,result in the molecules transmitted at the current bit interval cannot all arrive at the receiver within a given bit inreval.Residual molecules remain in the channel will cause severe intersymbol interference(ISI),which affects the accurate judgment of subsequent time slot information and greatly reduces the communication rate and communication reliability.The existing ISI mitigation methods based on statistical information such as the number of molecules at the receiver under constant transmit power do not fully consider the statistical information such as the number of residual molecules in the channel or the change of channel parameters caused by the change of transceiver distance,which reduces the accuracy of detection decisions;in addition,under the imperfect transmitter,due to the presence of interfering molecules in the emitted molecules,the existing detection and decision methods based on the molecular threshold at the receiver cannot fully consider the influence of interfering molecules at the transmitter,which reduces the performance of ISI mitigation.Therefore,this thesis conducts research on the above problems,and the main contributions obtained are summarized as follows:In the MC system with the fixed transceiver position,the ISI mitigation method based on chemical reaction reduces the number of interfering molecules in the channel by transmitting specially selected molecules to chemically react with the remaining interfering molecules in the channel,to improve the impact of ISI on the detection performance.However,at a fixed transmit power,the number of residual interference molecules in the channel is not fully considered,resulting in a large fluctuation in the number of received molecules,which reduces the performance of ISI mitigation.To solve this problem,we make full use of the statistical information of residual interfering molecules in the channel,and propose two power control schemes based on chemical reaction,and design the transmit power of each time slot to mitigate ISI.In the first scheme,only when the transmitted bit information jumps,the transmitter controls the number of transmitted molecules based on the number of residual molecules in the channel and the expected number of molecules at the receiver.Thus,the transmitted molecules eliminate the residual interfering molecules by the chemical reaction in the channel,and the expected number of molecules at the receiver is satisfied.On the basis of the first scheme,in the second scheme,in order to make full use of the residual molecules in the channel,the power control is performed no matter the continuous same bit information or the bit information is hopping.The simulation results show that the two methods proposed in this chapter can effectively mitigate ISI.In particular,when the residual molecules in the channel completely participate in the chemical reaction,these two methods achieve the same bit error rate(BER)performance with the Mo SK without ISI.Compared with the first scheme,the second scheme increases molecular utilization,however higher computational complexity is required.In the mobile MC,the position of the nanomachine moves under the Brownian motion and flow,making the transceiver position is time-varying.In the existing detection method under fixed transmit power,the distance information obtained by distance estimation is outdated,and when the distance changes greatly,the number of detected molecules fluctuates widely,reducing the performance of the detection.Therefore,we firstly models the movement of the transceiver nanomachines,and according to the established nonlinear motion model,an Extended Kalman Filter(EKF)is employed to predict the distance between the transceivers in the next time slot.In order to hold the number of received molecules at a stable level,we control the transmit power according to the predicted distance obtained by the extended Kalman filter,the number of molecules remaining in the channel,and the expected number of molecules at the receiver.This scheme makes full use of the residual molecules in the channel,avoids the large fluctuation of the received molecules under the fixed transmission power,and therefore ISI is mitigated effectively.Moreover,the power control scheme based on EKF maintains a detection threshold.Meanwhile,the bit error rate of the power control method based on EKF is much better than the fixed transmission power but the receiver adopts the dynamic optimal detection threshold.In the above MC scenarios,the molecules released by the transmitter are all of the desired molecular types.However,in a more realistic imperfect transmitter model,the molecules in the reservoirs are a mixture of the two types of molecules,resulting in the presence of interfering molecules in the emitted molecules.In this case,the released interfering molecules and the remaining interfering molecules in the channel result in more serious ISI.To mitigate ISI under the imperfect transmitter,first,we start from the perspective of thermodynamics,moving one type of molecule from one reservoir to another,by consuming energy.Making the difference in the concentration ratio of different types of molecules in the two reservoirs,then,an imperfect transmitter is created.Then,the performance of the created imperfect transmitter in relation to the energy consumption and the physical parameters of the transmitter(total number of molecules in the transmitter reservoirs,the number of molecules released per bit,etc.)is analyzed.On this basis,we analyze the ISI under the effects of imperfect transmitter and residual molecules in the channel.At the receiver,a detection method is proposed according to the ratio of different types of molecules,reducing the influence of interfering molecules on the detection,and meanwhile mitigating the ISI.Theoretical and simulation results show that with the increase of energy consumption,the concentration difference of the two molecule types increases,and the transmitter is closer to the perfect transmitter.And the greater the concentration difference between the two molecules type,the better performance of the detection scheme achieved which is proposed in this chapter.