Blind Estimation Approaches For Low Probability Of Intercept DS/SS Signal

Direct sequence spread spectrum (DS/SS) signals have been widely used in secure communication and mobile communication known as Code Division Multiple Access (CDMA) system for the reason is that DS/SS signals have many advantages such as anti-jamming capability, low probability of interception and multiple access capability. The interception and radio monitoring for spread spectrum communication have been of great research interests. The problem of estimating DS/SS & DS/CDMA signals direct from the received signals has been of great research interest with the development of the field of wide-band weak signal processing and the field of communication antagonism. The dissertation focus on detecting and estimating the DS/SS and direct sequence code division multiple access (DS/CDMA) signals hidden in the noise.For the last several decades, many methods for the estimation the DS/SS & DS/CDMA signal estimation methods has been developed. Most of these methods are based on the theory of conventional signal detection and estimation. The DS/SS &DS/CDMA signals are wide-band weak signals, they are not in conformity with the theory of conventional signal processing, when the signal to noise ratios of the received signals becomes lower, the performance of the methods take a turn for the worse. Though DS/SS &DS/CDMA signals are widely used in the field of military and civil, recent researches on the DS/SS &DS/CDMA signal estimation appeared in the literature. The development of DS/SS &DS/CDMA signal estimation is very slow, it is almost in study of conception and the research on concrete method is far from comprehensive and mature. Due to the challenge of DS/SS &DS/CDMA signal estimation without any prior knowledge of the pseudo-noise (PN) sequence, the study of DS/SS &DS/CDMA signal estimation is necessary.The main contributions of this dissertation include two aspects. One is parameter estimation of the DS/SS &DS/CDMA signal, include period estimation and chip interval estimation of the PN sequence etc., the other one is PN sequence estimation of the DS&DS/CDMA signals.Several valuable and important results which bring forth new ideas are achieved and listed as follows:A union approach based on the various spectrum estimation algorithms has been presented. Through the new approach, we can estimate some characteristic parameter, including carrier frequency, code length and chip rate of the DS and DS/CDMA signal. Besides, these methods can make up for their defects each other.In order to solve the estimation problem of PN sequence itself, a matrix decomposition approach to estimate the PN sequence itself has been presented. Through decomposition of the correlation matrix of DS&DS/CDMA signal, we can estimate the PN sequence from the largest and the second largest left singular vector. The final correlation matrix R, used in Eigen value decomposition (EVD) or singular value decomposition (SVD), can be denoted as the received vector multiplies itself transposition. A new method of constructing the correlation matrix is proposed in this dissertation. The proposed algorithm can enhance the signal Eigen values' resolving power and resolve the problem that traditional subspace methods can not be applied in low SNR. Then the proposed new method is applied to the direct sequence spread spectrum (DS/SS) signal's signature sequence estimation. Its performance is analyzed, and some illustrative simulations are presented.Having not the apriority knowledge about the DS/SS signal in the non-cooperation condition, we apply self-organizing feature map (SOFM) neural network theory to detect and identify the signal parameter and PN sequence. The computer simulation and experiment test have demonstrated that the algorithm is effective. Comparing the traditional slip-correlation method, the BER of SOFM algorithm and implementation complexity is lower.An approach for solve the estimation of users number has been presented. This algorithm is based on the delay-multiply. According to the different phase and amplify of each mobile phone and the different transmission condition, each user can be separated in the time field.A blind separation algorithm for instantaneous mixed DS/CDMA signal with unknown spreading codes is discussed in this dissertation. The reappearance of mixing users' phase information is utilized to separate the users' sequence by the simply plus and subtract operation in low SNR. The proposed method is different for ICA that not only saved the users' amplitude information but also calculation complication is independent of chip length L. The implement condition of the algorithm is analyzed and the performance of parameter estimation is measured by computer simulation.An improved and more complex BSS algorithm for separating linear convolved mixtures of no stationary signals in CDMA system is presented. This algorithm relies on the no-stationary nature of the sources to achieve separation. Most of which assume statistically stationary sources as well as instantaneous mixtures of signals. In practicality, the CDMA sources received are no-stationary and linear convolute mixing. A more complex BSS algorithm is required to achieve better source separation. The algorithm is based on minimizing the "average squared cross output channel correlation". The mixture coefficients are totally unknown, while some knowledge about temporal model exists. The simulation results show the effectiveness of the method in the blind detection of DS/CDMA signals.This methods above mentioned are suited for blind condition that is all the parameter not been known. Through the computer simulation, all of these algorithms can work in lower SNR.