A Spread Spectrum Transmitting System Based On DQPSK Modulation

Spread spectrum technology has the advantages of strong anti-interference ability,good confidentiality,high spectrum utilization.So it is widely used in different communications fields and is a very promising technology.Software defined radio is used widely in wireless communication in recent years.Its central idea is to realize different functions through programmable software on an open hardware platform.The design is flexible and convenient,and it can greatly reduce requirements for analog hardware.Field Programmable Gate Arrays(FPGA)is a very fast growing technology that can perform a variety of complex digital signal processing functions on an FPGA chip and have great advantages in designing and developing large-scale systems.This paper comprehensively utilizes the above-mentioned technology,has designed a kind of spread spectrum transmission system,this system not only can modulate baseband signal with low sampling rate to the intermediate frequency signal that meet the requirements of high sampling rate ADC,but also can broaden the spectrum of the signal,making it more reliable and more efficient in the transmission process.This paper first introduces the basic theory and key technologies of software radio and spread spectrum technology,and describes in detail the signal sampling technology,decimation and interpolation,modulation method,digital filter,direct digital synthesizer and pseudo random code.Secondly,using the Simulink simulation platform in MATLAB to model the entire spread spectrum transmitter,the feasibility of this scheme is discussed at the system level.In this platform,the design of the spread spectrum module,differential encoding module,modulation module,filter module,and up-converter module was completed in this paper.At the same time,each module is analyzed and simulated in detail.The implementation of this part provides a good foundation for the next FPGA implementation.In addition,in order to guarantee the system to realize better,the modulation mode adopts DQPSK modulation,this kind of modulation can effectively avoid the problem of carrier phase ambiguity in QPSK,the utilization rate of the frequency band is higher,very suitable for applying in the high-speed digital communication system.Finally,the clock distribution among the signals in the system working process is analyzed in detail.At the same time,the change rate of the sampling rate is studied.The design of the entire spread spectrum transmitter is implemented in the Quartus software,and simulations are performed using Modelsim and MATLAB.In this design,the sampling rate of the signal is changed 64 times.The 10 k Hz low sampling rate input signal can be converted to a high sampling rate output signal of 81.6 MHz.At the same time,the bandwidth of the signal is expanded 255 times,which not only satisfies the requirement of AD high sampling.but also make the signal transmission more reliable.It has certain reference value and significance in the actual digital communication system.