Research And FPGA Implementation Of On-board BTM Signal Demodulation Based On OQPSK Technology

Due to the rapid development of computer technology,the development of communication has pushed the whole human society into a new information age.For the increasingly mature digital communication technology and the rapid development of radio theory,the digital wireless communication system is becoming more and more common.In railway communication,Frequency Shift Keying and Phase Shift Keying signals are commonly used in railway point positioning system signal modulation.How to quickly realize the demodulation of FSK and PSK signals is an important problem in railway communication.The demodulation of FSK and PSK signals focuses on two indicators.One is the accuracy of demodulation.Due to the influence of amplitude,period and burr of actual working signals,the accuracy of demodulation will be reduced,thus affecting the availability of equipment.The second is the speed of demodulation.Point equipment is used for real-time train positioning,so it is necessary to quickly demodulation data.At present,demodulation is usually adopted in the initial filtering and amplification,the use of intermediate frequency amplifier chip to extract and amplify the original signal,and then demodulation of the obtained analog signal.The main advantages of the modulation technique of Offset Quadrature Phase Shift Keying are that it can achieve constant envelope characteristics,good spectral characteristics and high spectrum utilization and power utilization.Thus in the practical application environment of data transmission system provides a more reliable solution.The application of OQPSK modulation and demodulation technology in wireless data transmission system has important theoretical significance and practical value,especially in the ground mobile communication and satellite communication system,it is more widely used and has important research value.This paper choose alternative OQPSK FSK scheme,compared with FSK has high spectrum efficiency,and the advantages of nonlinear resistance is strong,can meet the demand of the practical application environment of data transmission.The task of this paper is to study the OQPSK modulation and demodulation technology,according to the requirements of the task,design and implement the OQPSK signal receiver system,and has been well realized in the project.This paper first introduces the application background and research status of the subject,and then introduces OQPSK on the basis of the principle of FSK modulation and demodulation technology,and then makes a brief analysis of the high-speed train transponder system,focusing on the basic composition and working principle of the transponder system,including transponder transmission module and antenna,ground transponder,trackside electronic unit,Then the principle of OQPSK modulation and demodulation is analyzed,and its key technologies are studied in depth,including digital down conversion,molding filter,carrier synchronization,symbol synchronization,etc.In order to make the train-ground communication in the high speed data transmission can more quickly and accurately,therefore widely used in engineering is adopted in phase of orthogonal ring(Costas)and the interpolation algorithm to the correction of the error and carrier synchronization take Costas loop can restrain carrier signal tracking of low SNR,guarantees the carrier of the input and output of the carrier to keep the small frequency offset and differ,The interpolation algorithm can obtain the sampling value signal of the optimal interpolation time through interpolation,that is,the synchronization of the local clock frequency and the data sampling rate is ensured,the symbol synchronization is realized,and the signal output is guaranteed to be stable.Second,the train-ground communication transceiver FPGA design simulation and testing system.In the process of FPGA design,the first is the overall scheme of OQPSK demodulation system is described,then the system development environment to carry on the detailed introduction,adopted streamlined design from part to the overall design of FPGA.FPGA chip is selected because FPGA chip has advantages such as high-speed processing capability,reconfigurable performance,low development cost and short development cycle,making it an ideal choice for OQPSK modulation and demodulation hardware chip.Then,the module function is co-simulated by Modelsim software,and finally the hardware platform of communication receiver is designed.It mainly introduces the circuit part of the hardware core,including the coupling amplifier circuit for amplifying the circuit,and A/D analog-to-digital sampling circuit,D/A analog-to-digital conversion circuit,and the power supply circuit for supplying power to the circuit,then also introduces the transceiver FPGA hardware equipment and configuration circuit,and finally build a platform to verify the correctness of its function.The results show that the demodulation of OQPSK signal can be realized by FPGA chip,which proves the feasibility of vehicle BTM demodulation of OQPSK signal based on FPGA.The scheme meets the requirements of communication work.