A Research On Technology Of Channel Code In Dual-Mode Transponder

Because of the unideal transmitting characteristic of satellite channel and a large amount of noises and interfere, the digital signal will be distorted when the transponder communicates with ground stations or user satelliates. In order to resume original messages correctly, a concatenated code, composed of RS code and convolutional code, is introduced to control errors in the received signal.Firstly, this paper analyzes the key technology which is adopted in RS encoding and decoding. And according to the characteristic of Galois Field and the platform where RS encoding is carried out, multiplicative operation in GF is realized by two-table method. With two-table method and symmetry, an encoder realized in time field has been designed. RS decoding is achieved in frequent field. Calculating the syndrome and solving wrong values is simplified by using iterative algorithm. For determining the wrong position, this paper induces a kind of simple algorithm from the classical BM algorithm. And iterative times is reduced greatly by methods of modified BM algorithm, which leads to reduce operational quantity and shorten the iterative period.Convolutional encoding and decoding is accomplished by methods of fragmenting, realized in DSP. Seeing that DSP has the characteristic that it is convenient to operate based on words, this paper designs a parallel shift xor algorithm ' to realize convolutional encoding, which has improved the encoding efficiency greatly. In the receiver, soft judgement is adopted in decoding. On the basis of analyzing the butterfly structure of trellis, complex branch metric calculation is simplified. In ACSU and SMUS, this paper realizes add-compare-select and register-exchange by pointer, which reduces the required time and saves the storage greatly. Because adopting fragment decoding, the route metric is limited which make the metric overflow unconsidered .After digital signal passing the satellite channel, decoder can't synchronize with transmitted message. So synchronizing code groups are inserted in transmitted code groups. Through searching the synchronizing code group in the receiver, the starting moment is determined. For raising searching speed and reduce synchronizing setting time, this paper proposes a method that synchronizing code groups are searched in two ways in the same time and the relativity between synchronizing code group and received code sequence is calculated by parallel xor algorithm.