Study On The Near/Far Interference Mitigation For Bei Dou Pseudolite Signals

BeiDou pseudolites are ground-based BeiDou signal transmitters which have been proposed as a solution to the problem of navigation in scenarios where the reception of BeiDou satellite signals is problematic.By transmitting high power signals,pseudolites have the capability to augment the BeiDou constellation,providing better geometry for greater positioning accuracy,reliability,availability,continuity,and integrity monitoring.However,the near/far problem occurs when using signals from the BeiDou pseudolites.Due to the widely-varying srtength,the pseuolite signals may jam the relatively weak satellite signals,leading to signal acquisition failure and total denial of the navigation service.Therefore,it has an important theoretical guiding significance and a practical application value to study the interference mitigation to the BeiDou pseudolite near/far problem.According to the problems and the status quo of the near/far interference mitigation methods,this dissertation concretely studies design of a pseudo-random pulsing scheme,implementation of adaptive pulse blanking and optimization of wavelet packet based interference mitgation method,to meet the demand of different receiver’s working modes.The main research outcomes are as following:(1)A comprehensive analysis of the cause and the influence of the near/far problem is conducted based on the BeiDou pseudolite signal characteristics.According to the BeiDou signal specificaion,a special design of pseudolite signal structure is proposed,including the navigation message format and the pseudo-random(PRN)code structure.To meet the signal requirements of the navigation service,eight optimal PRN code with 3-tapping phase generator are assigned to the BeiDou pseudolites.Signal characteristics,especially the cross-correltion interference is the major contributing factor to the near/far problem,where the near/far range ratio of the BeiDou pseudolite signal is determined by the cross-correlation margin.A non-coherent integration acquisition model considering the near/far interference is established to analyze the impact on the acqusition threshold.Simulations are carried out to testify the performance loss caused by the change of the acqusition threshold.(2)A signal pulsing scheme dedicated to the BeiDou pseudolite is designed for the stand-alone pseudolite navigation mode.According to the pulsing scheme criteria,there are two critical parameters to define a successful pulsing schem,one is the pulse duty cycle which determines the duration of the pulse,and the other is the transmission order of the pulse sequence(Pulse Pattern).Althugh low duty cycle is preferred,an acquisition model taking into account of the BeiDou PRN code’s partial-correlation propertis is built to test the lower bound of the duty cycle.An optiaml duty cycle is choosen based on the model.In order to provie better multi-access performance,a pseudo-random pulse pattern based upon the theory of linear congruence is proposed.The spectral and temporal characteristics of the proposed pulse pattern are analyzed and compared with the random pulse pattern.From the simulation analysis,it emerges that the proposed scheme features better signal characteristics to facilitate multiple psearudolite transmission.Field tests using the BeiDou pseudolite demonstration system are also conducted to test the usability and positioning accuracy of the BeiDou pseudolite pulsed signals.Test results show that the acqusition performance and multi-access perfoemance are comparable to the BeiDou satellite signal.The pulsing scheme is qualilfied for a stand-alone pseudolite system.(3)An adaptive pulse blanking threshold algrithm is propsed to lower the quantization SNR(Signal to Noise Ratio)loss introduce by the pulse blanking.An overview of the signal and front-end models of the BeDou receiver is presented,in which the automatic gain control settings and the quantization schems are two main parameters that need careful configurations to minimise the SNR loss.An analytical model of the SNR loss accounts for the quantization scheme and the AGC(Automatic Gain Control)scaling is developed,to quantify the optimal AGC gain with regard to the number of bits used for the quantization and the corresponding AGC recovery time.The SNR loss model is then extended to obtain the optimal pulse blanking threhold.An adaptive quantization algorithm baesd on the hierarchical clustering algorithm is propsed to adaptively decrease the quantization interval to further reduce the SNR loss.Simulation results prove the effectiveness and performance improvement in SNR loss,signal acqusition and system robustness in signal variation.(4)The pulsed interference mitigation method based on the wavelet packet analysis is studied and optimized.The theoretical fundamentals of the wavelet transformation are presented,introducing the continuous and the discrete wavelet transformations,and its application to the mitigation of pulsed interferences is demonstrated.In order to provide more precise transformation of high frequency component of the pulsed signal,the wavelet packet transformation is employed.The fundamental steps and critcal parameters including the decoposition level,wavelet family selection and filter bandwith are discussed,optimazition to the BeiDou pulsed signal is implemented by means of parameter setting comparsion.Simulation results show that the optimized settings have more capacity to split the useful signal from the interfering component,leading to better singal acqusition performance.