The Research On Receiver Design For Ambient Backscatter Communications Based On Channel Estimation

5G communication technology has promoted the realization of the goal of Internet of Everything,and also promoted the development of Internet of Things technology.Large-scale connection is an important feature of the Internet of Things system.Millionlevel connections per square kilometer brings three challenges to the Internet of Things system: high energy consumption,high hardware costs,and the shortage of spectrum resources.The ambient backscatter communication technology has attracted extensive attention in academia and industry in recent years,because its passive communication mode meets the development needs of the Internet of Things system.The main information transmission equipment of the ambient backscatter communication system is the passive tag,which communicates with the receiver by reflecting the signals present in the environment.This communication mode does not require active RF signal transmission originals,at the same time,it reduces the hardware cost of the tag,what's more,it does not require additional spectrum,these characteristics make the ambient backscatter communication technology can well meet the three challenges of the Internet of Things system.In an ambient backscatter communication system,the reflective link carries the tag's information,while the strength of the reflective link is much lower than the strength of the direct link,which brings great challenges to the receiver design.In this paper,we study the design method of the receiver of the ambient backscatter communication system for three settings: ambient signal full blind,ambient signal semi-blind and OFDM excitation signal source.The main research contents are as follows:Firstly,for the blindness of ambient signals,we studied the receiver design methods for MPSK and QAM for environmental signal modulation.We studied the phase distribution characteristics of the MPSK signal,designed a power channel phase estimation method,and proposed two channel amplitude estimation methods,and then proposed a direct link average detector based on log-likelihood detection criteria,finally,we demodulated the tag signal.For the QAM modulation of environmental signals,we propose another channel estimation and signal detection method.First,based on the maximum likelihood probability criterion of the received sequence,we derive the relationship between the channel response and the received signal for a given ambient signal sequence.Secondly,we apply a grid search algorithm to demodulate the ambient signal sequence and obtain channel estimates.Finally,we propose a low complexity distance detector to demodulate the tag signal.Through simulation experiments,the effectiveness of our proposed algorithm is verified.The numerical results show that the performance of the receiver is related to the signal-to-noise ratio of the direct link and the difference between the signal-to-noise ratio of the reflected link and the direct link.Secondly,considering the limitation of the ambient signal's completely blind setting on the receiver's performance,we studied the design method of the ambient signal's semiblind receiver.Corresponding to the content of the first part,the research is conducted when the ambient signal is modulated by MPSK and QAM.For the ambient signal MPSK modulation,we simultaneously use the received signal correspond to the period of the ambient pilot signal and the ambient data signal to estimate channel information.This method eliminates the channel estimation phase ambiguity.And then,we apply a distance detector to demodulate the tag signal.In addition,in order to further improve the performance of detecting bit error rate,we propose two feedback distance receivers.For QAM modulation of ambient signals,firstly,with the help of ambient pilot signals,the starting route of grid search is located,and the optimal path found corresponds to the accurate demodulation value of the ambient signal sequence.The estimated channel value is unbiased,and a low complexity distance detector is directly used to demodulate the tag signal.Through simulation experiments,the effectiveness of our proposed algorithm is verified.Finally,we studied receiver design methods for OFDM excitation signal sources.We assume that the subcarriers of the ambient OFDM signal are all modulated by MPSK.First,we studied the generation process of the ambient OFDM signal and the process of loading the tag signal to the ambient OFDM carrier,and converted the received signal in the time domain to the frequency domain for demodulation.In this study,the value of the tag signal is combined into the expression of the cascade channel,so the channel information is used to represent different tag signals.Based on this,we propose two channel parameter estimation methods,and use a low complexity distance detector to demodulate the tag signal.Through simulation experiments,the performance of the two receiver algorithms is compared,and the effectiveness of the algorithm is verified.Numerical results prove that the greater the number of OFDM signal subcarriers,the better the receiver performance.