The problem of decoding received, encoded data sequences can be formulated and solved in a number of ways. In this chapter we provide a general framework for the decoding problem. The basic concepts of measures and metrics are introduced. It is then shown how metrics can be used in different ways to realize different decoding algorithms, with an emphasis on the BCJR and Viterbi algorithms. These two algorithms are unified within a general framework that we call the “Generalized Viterbi Algorithm”. This is followed by a detailed description of turbo decoding: a low-complexity, suboptimal means for decoding serial and parallel concatenated codes. The chapter concludes with a discussion of mismatched decoding: a problem that arises when the channel statistics are not known or have been inaccurately specified. It is shown that turbo decoders are remarkably robust under such conditions, and can be made more so by including channel estimation within the decoding process.
KeywordsConvolutional Code Viterbi Algorithm Iterative Decode Turbo Decode Soft Decision
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