Research And Implementation On Key Technology For DSSS System Based On Low Power Wide Area Network

Low Power Wide Area Network is designed to achieve low power consumption,long distance,low rate,large connection of the Internet of things applications.Institute of Electrical and Electronics Engineers has developed IEEE802.15.4k standard for this demand in low-rate wireless personal area network.This standard is suitable for the application of low power critical infrastructure monitoring(LECIM:Low Energy Critical Monitoring)network.This thesis is based on the DSSS LECIM PHY(Direct Sequence Spread Spectrum LECIM Physical Layer)of IEEE 802.15.4k standard.The key technologies of each physical layer transceiver link module are studied deeply,and the whole transceiver link is designed and built.In practical engineering applications.Due to cost constraints,common and cheap crystal is used,whose frequency offset is up to 40PPM or more.The receiving performance deteriorates with crystal's frequency drift,which causes the difficulty of correctly recovering the transmitted data in the low signal noise rate environment,so it is of great academic value and practical application significance to study the synchronization techniques of receiver link in the physical layer.To solve the problem above,firstly,the long pseudo-random code sequence acquisition technology with random frequency offset of the low SNR environment in the baseband receiver is studied.This thesis focuses on the time-frequency domain two-dimensional search algorithm based on FFT operation and acquisition algorithm based on differential processing.The traditional acquisition algorithm cannot meet the LECIM network application requirements,so an M-order auto-correlation pseudo code synchronization algorithm based on differential processing is proposed.First of all,the received baseband intermediate frequency signal and the pseudo code are multiplied to eliminate the pseudo code information,then the M-order differential auto-correlation is used to eliminate the carrier frequency offset and suppress the noise at the same time.Finally,the algorithm proposed in this thesis is compared with traditional differential acquisition algorithms under the same conditions.The experimental results show that the proposed algorithm has better anti-noise performance,whose acquisition performance is improved by 4-5dB.The signal-to-noise ratio is improved to a large extent,which more,the receiver sensitivity of the baseband system is improved.Therefore,the algorithm presented is more suitable for low SNR environment.Secondly,to achieve the high accuracy frequency offset estimation technique in the ultra-low SNR environment,the algorithm based on the FFT operation and the classical Fitz algorithm based on time domain are studied.Based on the classical Fitz algorithm,the strategy of signal preprocessing before the correlation operation is put forward,which can suppress the noise by means of partial self-correlation and iterative method without reducing the frequency offset estimation range.The estimation of the frequency offset can applied in the low SNR environment and improves the estimation accuracy.As the method is stable and reliable,it is very suitable for IEEE802.15.4k standard under the long pseudo-random code sequence,ultra-low signal to noise ratio,long-distance communication,and ultra-high reception sensitivity of the application requirements.Finally,based on the whole baseband system built on the MATLAB platform,the corresponding FPGA(Field Programmable Gate Array)function is implemented for the frequency offset correction module and the channel decoding module.The correctness of the system is verified by the analysis results.