QPSK Based Spread Spectrum Communication Technology And Implementation In FPGA

At present,spread spectrum communication is an important research direction in the field of modern communication technology,and it is also a widely used communication approach,which plays an important role in the satellite communication,military communication and mobile communication.The spread spectrum communication has excellent anti-interference ability and anti eavesdropping ability,which can meet the requirements of modern communication for confidentiality and anti-interference.Digital modulation and demodulation technology is one of the core technologies in modern communication with high data throughput.QPSK is widely used because of its band utilization and excellent antiinterference ability.With the rapid development of software radio technology,the use of programmable FPGA to replace the traditional hardware equipment greatly reduces the cost and improves the universality of the system.Therefore,it is of great significance to study the design method of direct sequence spread spectrum communication system and Its FPGA implementation.This paper first studies the basic theory of spread spectrum despreading and DQPSK modulation and demodulation,including the principle of acquisition and tracking in PN code synchronization,and the principle of QPSK modulation and demodulation.The transmitter of DSSS system designed in this paper consists of code conversion module,differential coding module,shaping filter and carrier modulation;the receiver mainly includes digital down conversion,carrier synchronization,pseudo-random code synchronization and bit timing synchronization.On this basis,this paper introduces the design principle of each module and FPGA implementation scheme in detail,and focuses on the design scheme of PN code synchronization and demodulation algorithm in DSSS system.In this paper,the sequence phase search method and delay locked loop are used to form the PN code synchronization,and the coherent demodulation based on Costas loop is used in the demodulation part.Based on the principle simulation and verification of each module in the decision of modulation and demodulation of DSSS signal,the function design and simulation verification of each module are carried out in vivado.Finally,the proposed algorithm is verified in a Xilinx Zynq-7000 FPGA demo board.The original data is generated and modulated,then the demodulated data after the system transmission is analyzed.The final test results show that the system can reliably and effectively complete the task of data transmission,and greatly improve the anti-interference ability of the system.