The Design And Implementation Of PN Code Iterative Acquisition Method In DS-UWB Systems

Direct Sequence-Ultra WideBand (DS-UWB) systems use low-duty-cycle pulse trains having very short impulse duration to transmit signals. In the PN code acquisition stage, both the time acquisition and PN code phase acquisition must be achieved within the allowable time limits. The traditional acquisition schemes can not realize the requirement of rapid acquisition.To deal with this problem, the iterative message passing algorithms (iMPA) applied for PN code acquisition in DS-UWB systems is researched in this paper, which can satisfy the desire of rapid acquisition and its FPGA implementation is also given.Firstly, the basic principles of DS systems and UWB systems are introduced in this paper, and the mathematic model and the working principle of DS-UWB systems are also researched in this paper, which lay the foundation for the following research.Secondly, the iMPA based on factor graphs are introduced in this paper. And the rapid acquisition scheme based on iMPA for m-sequences is particularly reasearched. To further improve the acquisition performance of iMPA, the iMPA based on redundant factor graphs and the iMPA based on hidden variable factor graphs are given in this paper from the aspect of considering the configuration of the factor graphs, and the simulations of their acquisition performaence are also given.The simulation results show that the iMPA based on hidden variable factor graphs can advance acquisition probability and quicken the speed of astringency; the acquisition time of the iMPA based on hidden variable factor graphs is half of the acquisition time of the iMPA based on factor graphs whitout hidden variable.Thirdly, to deal with the problem of the influence of timing error on the iMPA a two-stage iterative acquisition (TS-IA) scheme for PN code in DS-UWB systems is shown in this paper:First of all, the periodic pulse trains for timing acquisition are joined into the transmitted DS-UWB signal. And in the first stage of the synchronization, the timing information is acquired by correlating the received signal with a replica of the periodic pulse trains, after that the problem of timing error is solved when timing acquisition is finished, and in the second stage the rapid PN code acquisition can be carried out using the iMPA. The performance of this two-stage iterative PN code acquisition scheme is analysed and simulated in terms of both the acquisition probability and the achievable mean acquisition time in this paper. The simulation results verify that the proposed scheme can realize the rapid PN code acquisition in DS-UWB systems.At last, the FPGA implementation of TS-IA algorithm is given in the paper. The TS-IA algorithm is divided by its function into timing acquisition module, iMPA module and correlation-decision module. Detailed designs and implementations of each module are carried out in Altera's QuartusⅡsoftware. Testings of each module are also carried out in the ModelSim software. Finally, all these modules are joined together from the bottom to the top to finish the design and implementation of proposed PN code acquisition algorithm in Altera's QuartusⅡsoftware.