PN Code Fast Acquistion Methods Based On Factor Graph Message Passing Algorithms

The principle of iterative PN code acquisition algorithms is using iterative message passing algorithms(MPAs) based on factor graphs(FGs) to acquire PN codes. The process is first building FG models according to the constraint relationship of PN codes, secondly performing MPA on FGs using soft-channel-messages. After iterative processing, the Maximum a Posteriori(MAP) is got, from which local codes are chosen, thereby the PN code has been synchronized. Because of its short acquisition time and low complexity, this method has got vast attention. Researchs of this method are newly started. On the basis of previous work, further researchs has been made on the algorithms by theoretical analysis and simulation in the thesis, and some valuable improvements has been got.For getting MPAs of arbitrary FGs, on the basis of nondirectional graphs and its MPAs, using brink functions and expression trees as analyzing tools, computing rule of sum-product algorithms on FGs was exported. Then two applying models of iterative PN code acquisition algorithms were expressed by uniform signal model.For the purpose of using MPAs to acquire m-sequense, a building method of FGs was showed. Then the PN code acquisition computing steps with different MPAs and different FG structure when single station exists was analized, and some simulation results were got. Finally, the condision of multi-path existing was discussed. Two improved methods were proposed, one is multi-path union initial message evolution method, the other is multi-iterative method, and a multi-path iterative acquisition scheme was showed.On the basis of m-sequence iterative acquisition, the method using Gold codes was discussed. First a method of calculating equivalent sparse checking relationship was proposed, characteristics of the checking relationship were analyzed, and a route to acquire optimal checking relationship is found. Then contraposing redundancy checking acquisition, improved computing steps called ordinal checking method was proposed. Contraposing hiberachy checking acquisition, an optimizing method of hiberachical FGs was given.After simulation analysis of each iterative acquisition algorithms, the performance of the algorithms was researched in detail theoretically. First two kinds of convergence analytical methods of iterative acquisition algorithms were showed, one is density evolution method, the other is EXIT graph method. Secondly, acquisition time and complexity for different vector reckoning methods were disscussed. Finally carrier synchronization and sample rate in the process of acquiring initial soft-channel- messages were discussed.Since iterative methods use soft-channel-messages to initialize, symbol timing is the premise condition of acquisition methods. In the thesis, the influence on iterative acquisition made by symbol phase error was analyzed, and then two useful symbol phase synchronization methods were showed after presenting the searching method, one is re-sampling method, the other is three-order correlation method. Simultaneously, applying scale for each method was showed.At the end of the thesis, two instances of iterative acquisition algorithm were showed. In wireless navigation instance, an improved method was proposed in which wave synchronization and symbol timing can work together to reduce complexity, and 3-user iterative acquisition scheme was also presented. In UWB instance, the best Gold codes which suit in iterative acquisition was got by computing, and then an interative acquisition scheme for UWB systems was presented.