The invention provides an iterative
massage passing (IMP) PN code capturing method based on a sum-product
algorithm on the condition of a low
signal to
noise ratio. According to the method, an m sequence is modelized into a special linear
block code; a sum-product decoding
algorithm is executed on a
Tanner graph to obtain maximum posteriori
estimation of the m sequence; and an initial state is selected by introducing a posteriori log-likelihood ratio to generate a local m sequence; and at last, serial correlation
verification is carried out with reception data and capturing is completed. The greatest
advantage of the method provided in the invention is as follows: maximum posteriori
estimation of an m sequence is directly decoded by utilization of a sum-product decoding
algorithm and a posteriori log-likelihood ratio is introduced to select an initial state. Besides, the method has advantages of rapid capturing speed,
low complexity and excellent performance under a low
signal to
noise ratio. According to the invention, a fundamental principle on IMP PN code capturing is mainly introduced and then an IMP PN code capturing method based on a sum-product algorithm is provided; and an analysis on a selection process of an initial state of an m sequence is emphasized and a detailed
flow chart is provided; at last, a capturing performance and a complexity of the method are analyzed though examples.