Research On Acquisition Process Of DSSS Receiver Under High Dynamic And Low SNR Circumstances

In recent years,countries all over the world have shown great interest in near space and the near-space technology has developed rapidly.The direct sequence spread spectrum(DSSS)technology has shown tremendous application prospects in the measurement and control and information transmission system of near-space high dynamic flight carrier due to its good anti-jamming and anti-interception capability.The high dynamic and low signal-to-noise ratio(SNR)environment where the near-space flight carrier works brings new challenges to the design of spread spectrum receiver which is supposed to be efficient and reliable.It is essential for spread spectrum systems to de-spread the received signal correctly which requires the coarse synchronization of the carrier frequency and PN code phases between the incoming signal and local signal.Once the incoming signal has been acquired,a finer synchronization maintains the alignment of the carrier frequency and PN codes which is achieved by using phase-lock technology.In this paper only the PN code acquisition of DSSS signal under the high dynamic and low SNR circumstances will be considered.In this paper the received DSSS signal can be modelled and analyzed by the introduction of Doppler dilation coefficient which connects the carrier Doppler and code Doppler.Based on the form of noncoherent detector,the effects of residual Doppler offset,residual code phase offset and residual Doppler dilation coefficient on the received signals are studied by combining formula derivation and simulation analysis.Then by introducing a high dynamic mathematical model,signal effects of velocity,acceleration and jerk on the detector output are studied.Also,efforts have been taken to build the connection between the Doppler dilation coefficient,Doppler offset and their derivatives.The results show that the effects on signal acquisition under high dynamic circumstances are significant,mainly on the power attenuation,auto-correlation main lobe spread and the peak bias.Doppler dilation coefficient in the signal model indicates that the effects of carrier Doppler and code Doppler are coupled together,and the existing research shows that decoupling analysis can be carried out under certain conditions.Thus,this paper first studies the rapid acquisition algorithms of DSSS signals under large frequency uncertainty and low SNR circumstances,including the classical digital matched filter,the correlation peak noncoherent cumulative acquisition based on the frequency uncertainty division and partial matched filter(PMF)algorithm based on Fast Fourier Transform(FFT).The algorithm structure,output gain and performance parameters of PMF-FFT are analyzed.PN code phase offset accumulated over time by the code Doppler will bring power attenuation on the correlation output which causes acquisition performance degradation.Then this paper studies the estimation and compensation methods of code frequency offset,including a parallel search method based on uncertainty division and a direct estimation method.In addition,a PN code acquisition method is studied which can eliminate the residual code phase offset effects.Last but not least,this paper proposes a parallel acquisition scheme under large frequency uncertainty and low SNR circumstances based on frequency uncertainty domain division and PMF-FFT algorithm and a design of acquisition head sequence is presented.The scheme is verified by MATLAB simulation and the result shows that the scheme works well under the high dynamic and low SNR condition.