Research And Application Of PN Code Rapid Acquisition Based On Iterative Detection Algorithm On Factor Graph

Exact synchronization of PN code is the precondition of the excellences of spread spectrum communicating system, among them how to rapidly and effectively make the PN code to exact synchronization becomes the key of the system. The traditional PN code acquisition methods, which include serial capture, capture and mixed parallel capture, etc, are mostly built on the basis of a large number of correlations. Although they have their own advantages, the major computational complexity has not been changed. To resolve conflicts between implementation complexity and rapid acquisition with lower hardware complexity, some pundits began to consider the pseudo-random sequence as a special way of channel coding, using SISO (single input single output) iterative decoding method to achieve rapid acquisition with low-complexity. Chugg and Zhu showed m sequences in factor graph, and applied message passing algorithm (MPA) to the m sequence acquisition, and proposed iterative Message Passing Algorithm (iMPA). Compared to the traditional methods, the PN code acquisition based on iMPA achieved good performance in terms of speed and complexity, giving the field of acquisition of pseudo-random sequence a new research direction. The studies are based on the ideas of Iterative Detection, factory graph and as iMPA algorithm the center, conduct in-depth study, improve and expand iMPA's applications. And the main thesis is as follows:First, through the iterative detection theory, factor graph theory and through proving that m sequence is a special kind of linear block codes, we explained the principles of that the iMPA used for pseudo-code acquisition in details; Through the establishment of behavior model and probability models of m sequence, the general representation of factor graph of m sequence is given; then we show that the comparative analysis of complexity between iMPA algorithm and the traditional algorithms; And then we proposes an estimation vector decision choice algorithm based on bit-by-bit comparison, which increasing the detection performance importantly and reducing the computational complexity. Reference to the performance analysis of traditional PN code acquisition methods, the acquisition process of iMPA algorithm is analyzed using Markov chain, and the analytical expression capture about average acquisition time is given. At the same time, the introduction of density evolution, we analysis the convergence process of the iMPA; and through analysis of probability density function of soft message in iterative acquisition process, we calculate the analytical expression of acquisition probability.Second, for the synchronization problems of PN code rapid acquisition based on iMPA,we analysis the initial channel information and detection performance under the given timing error and phase error. At the same time, using open-loop maximum likelihood estimator with low complexity to eliminate timing errors; and because of the mean of output soft information of iterative PN code acquisition is increasing with the reducing of error, the information symmetrized about zero-error, the symbol timing error estimation methods based on "early - late" threshold algorithm is proposed.Third, for the PN code sequence acquisition based on iterative detection under fading channel, we proposed the joint channel estimation and PN code acquisition using iterative detection. Firstly, we analysis Rician and Rayleigh fading channel model and modify the initial channel soft information under Rayleigh channel. And then we used AR random process modeling the Raleigh fading channel; By studying the factor graph theory, the factorization of optimal signal receive model is obtained, and the joint factor graph model included channel estimation and iterative PN code acquisition is proposed; and using the classical Kalman smoothing algorithm to calculate the iterative channel estimation; the message passing rules of the joint factor graph is given.Forth, aiming at the low acquisition probability of PN code acquisition using iMPA, redundancy iMPA algorithm based on increase the equivalence constraints is proposed, namely R-iMPA. Gave the factor graph model and the message passing rules of R-iMPA algorithm, Meanwhile, according to the idea of redundant constraints, the hierarchical factor graph structure of Gold codes is proposed and the message passing rules is given; And for the "non-sparse" factory graph with multi-tap which will seriously affect the performance of rapid acquisition based on iMPA, the paper proposes a new structure which use the equivalent high-order "sparse" polynomial to generating redundancy factor graph with the original generating polynomial, and improved the acquisition probability. We also gave the general Zech algorithm for searching the equivalent "sparse" higher-order polynomials.Finally, the iMPA rapid acquisition algorithm is applied to the marine radio-navigation system for sky-wave interference cancellation. Sky-wave signal is a typical time-varying signal with highly amplitude and rapidly changing. Traditional PN code acquisition is based on sliding correlation method are some limitations; In this paper, a multiple PN code signal acquisition algorithm based on auxiliary iMPA, under rapid PN code acquisition using iMPA contain high acquisition speed has been proposed, this method can effectively solve the rapid parallel spread-spectrum signal acquisition under different phases of the same spread spectrum.