Research On Multiple-Access Interference Effects And Mitigation Techniques In Satellite Navigation Systems

With the development of Global Navigation Satellite System(GNSS),it is predicted that over 120 navigation satellites will be applied in future.As a result,the Multiple Access Interference(MAI)caused by shared spectrum between signals has become a main restriction for system performance of GNSS,especially when the satellite signal power enhanced or there are pseudolite signals.The MAI also seriously affects the BeiDou inter-station system and Radio Determined Satellite System(RDSS),both systems need to perform high precision ranging and highly reliable communication.However,the matched spectrum interference caused by MAI will reduce not only system capacity,but also ranging accuracy.In order to address this issue,methods of evaluating navigation receiver performance and MAI suppression techniques for pseudo-code ranging signal are studied in this work.The dissertation also proposes solutions to improve the spectrum efficiency of ranging and communication integrated satellite system when a certain ranging accuracy is guaranteed.The main innovations and achievements in this dissertation are listed as following:1.Traditional MAI evaluation method simplifies the calculation of Spectral Separation Coefficient(SSC)and Code Tracking Spectral Sensitivity Coefficient(CT-SSC)by using continuous power spectrum.However,the method is not suitable for periodic code spread spectrum signal with discrete power spectrum.Therefore,the formulas of SSC and CT-SSC for periodic code spread spectrum signal are deduced,which reveals that the pseudo-code tracking error and its standard deviation vary as a function of difference of Doppler shift,carrier phase,signal power and pseudo-code phase between interfering and expected signal changes,respectively.Formulas also show that the zero-value of pseudo-code tracking caused by MAI is determined by the behavior of sidelobe of neighboring chip in cross-correlation function.Moreover,SSC and CT-SSC of period code spread spectrum signal can be calculated by continuous spectrum power signal approximately if the data transmission rate is lower than pseudo-code chip rate,and the offset error is about 0.2 dB.Finally,the above findings are verified by software receiver simulations.2.The formula of pseudo-code tracking error for a bandwidth-limited spread spectrum system is deduced as a function of channel bandwidth,pseudo-code rate and MAI.To minimize pseudo-code tracking error,the optimal pseudo-code chip rate to channel bandwidth ratio is obtained.It shows that for an ideal low-pass-filtered channel with its bandwidth-limited,the optimal pseudo-code chip rate to channel bandwidth ratio is about 1.Moreover,for non-ideal channel with group delay fluctuation,the optimal ratio is between 0.5 and 1,and the more intense the group delay fluctuates,the smaller optimal pseudo-code chip rate is required.For such a bandwidth-limited channel,the performance of pseudo-code tracking MAI suppression,which is based on Parallel Interference Cancellation(PIC),degrades.To solve this problem,an optimized channel-matching filtering method for the local reconstructed MAI signal is proposed.The method enhances the equivalent Carrier to Noise Ratio(CNR)by 2 dB,and reduces pseudo-code tracking standard deviation by 1 dB.This provides theoretical basis and practical implementation for a better design of interference cancellation technique.3.Although the PIC can reduce Bit Error Rate(BER)and ranging error,it requires very large hardware resources.In order to improve receiver ranging accuracy under the restriction of hardware resources,an interference suppression technique based on multi-correlator for pseudo-code ranging signal is proposed.Compared with the traditional method,Early Late Slope(ELS)and Double Difference Correlator(DDC)reduce pseudo-code tracking error and its standard deviation by 14 times.It also reveals that if the power difference between MAI and the expected signal is smaller than 10 d B,DDC has the same performance with the single-stage PIC,while its implementation complexity is only 2/K times of that of PIC(K is signal number).This greatly simplifies implementation complexity of the receiver MAI suppression technology.4.The above methods of evaluating MAI are applied to signal design of Beidou ranging and communication integrated system.To enhance the inbound capacity while satisfying inbound signal ranging accuracy and BER requirements,spread spectrum partial separation method for bandwidth limited channel is proposed.To be more specific,in this method,the original single spread spectrum carrier is separated into several partially overlapped subcarriers,each with a smaller bandwidth.In the cases of two and three subcarriers,their optimal pseudo-code chip rates are analyzed respectively,and the corresponding system inbound capacity increases by 29% and 37%.In order to improve the whole network capacity while satisfying the ranging accuracy and BER of BeiDou inter-station time synchronization and data transmission system,it proposes to use spread spectrum partial separation first,then employ PIC for intra-carrier MAI.Compared with the strategy of applying PIC directly for the whole spectrum,the implementation complexity of the new method is halved at the cost of less than 1 dB decline in the equivalent CNR after interference cancellation.Achievements in this dissertation have been employed to the compatibility evaluation and high-precision navigation receiver design for civil signal of global navigation satellite system,and signal design of high capacity RDSS system and interstation system for BeiDou global system.