Research On Blind Despreading Method Of Direct Sequence Spread Spectrum Signal

In non-cooperative communication scenarios,the study of blind despreading methods for direct sequence spread spectrum signals is helpful to ensure communication security and improve the capability of military confrontation.The research content of blind despreading of Direct Sequence-Code Division Multiple Access(DS-CDMA)signals mainly involves the detection of direct spread signals,estimation of characteristic parameters,estimation of user spreading code sequences,and despreading and despreading.The estimation of the spreading code sequence is the most difficult and core part of the whole process.Thesis mainly studies the burst direct spread signal,and has carried out the following aspects:1.The blind estimation method of multi-user spreading code sequence based on Fast-Independent Component Analysis(Fast-ICA)is studied,and the estimation method of synchronous DS-CDMA signal code sequence when the out-of-synchronization time is known is studied.The improved method improves the bit error rate of code sequence estimation and reduces the complexity of signal segmentation.Since the code sequence of blind estimation contains errors,Thesis also studies the methods of sequence type identification,generator polynomial estimation,initial state estimation and sequence reconstruction for Linear Feedback Shift Register(LFSR)sequences.The sequence can be effectively identified and reconstructed when the bit error rate is lower than 20%,and it has good performance.The improvement of demodulation performance by code sequence error correction is verified by simulation experiments.2.A fast code sequence detection method based on matching search is proposed.Simulation experiments show that the method has good performance and can detect user spreading code sequences in real time when the polynomial is known.3.For the Orthogonal Variable Spreading Factor(OVSF)sequence used in Wideband Code Division Multiple Access(WCDMA),Thesis uses the pre-despreading symbol average power and near-zero rate As the decision basis,blind estimation of the OVSF sequence is realized.Through simulation experiments,the effectiveness of the algorithm for OVSF sequence estimation in multi-user scenarios is verified.4.A method for estimating time-frequency difference parameters of DS-CDMA signal based on separation and reconstruction is proposed,which effectively avoids the problem of multi-peak ambiguity.Simulation experiments show that the time-frequency difference parameters obtained by the method proposed in thesis have high accuracy.5.Based on the theoretical research of each module,a set of blind despreading system for direct-spreading signals suitable for both single-user and multi-user is designed.The offline detection mode or online detection mode can be selected.Finally,the performance of the whole system is tested with a certain satellite signal actually collected.