Channel Coding Scheme Of Turbo Code In Td-scdma System Application And Implementation Of

Noises and disturbance are involved in digital communication system inevitably, so error control coding must be included in the system. Shannon published his paper"A mathematical theory of communication"in 1948, which has been arresting zealous researches from academia of coding and information area.Turbo code was first discovered in 1993. Because of its near Shannon limit performance, turbo code has been the focus of channel coding area quickly. Along with the development of fundamental theory, turbo code is coming into application area. Because of high complexity of algorithm, long time delay and large store space about the turbo decoder, it's one of important researches about turbo code to design simple and effective decoding algorithm to get excellent performance decoder.In this paper, according to the basic structure of Turbo encoder and thought of iterative decoder, some factors that can affect the performance of Turbo-code can be got from lots of computer simulation results. Besides, two kinds of decoding algorithm are compared. One is the method of maximum a posteriori algorithm(MAP) and the other is the Soft-Output Viterbi algorithm. We mainly focus on the derivation of the MAP algorithm and from which we can get some reduction algorithm, such as Log-MAP, MAX-Log-MAP etc. Some decoding algorithm of Turbo-code in common use are compared in their capability and complexity.In order to implement the turbo decoder on DSP, the iterative decoding algorithm and some technical problems in decoder inplement are thoroughly researched. The following parts are included in this paper:Firstly, the structures of encoder and decoder are introduced and the iterative process of decoder as well. The interleaver is analyzed, especially the interleaver algorithm of 3GPP.Secondly, the principle of MAP algorithm and MAX-Log-MAP algorithm, the deducing and the step for calculation of algorithm are discussed. By programming emulation, this paper analyzes the parameters influence based on Matlab simulation, including the iterative decoding times, length of interleaver, weight code etc. Then, according to the previous researchs, we can get some valuable conclusion.Finally, some important problems are considered in order to implement the decoder with MSC8144 fix-point digital signal processor(DSP), such as fix-point quantification, over-flow of calculation. A way to avoid over-flow of calculation is presented, and it can avoid over-flow of calculation effectively. With the way to deal with some important problems mentioned above, Turbo decoder is implemented with fix-point digital signal processor(DSP). The test result shows the performance of the decoder is close to that of float-point one.