Algorithm for continuous decoding of turbo codes - Electronics Letters

Introduction: Optimum symbol decision algorithms for digital transmission must base their decisions on the maximum a posteriori (MAP) probability. They have been known since the early seventies [l], although they are much less popular than the Viterbi algorithm and almost never applied in practical systems. Only re cently interest in these algorithms has been revived, in connection with the problem of decoding concatenated coding schemes, also known as ‘turbo codes’. The decoding algorithm for ‘turbo codes’ uses an iterative procedure whose heart is an algorithm which computes the sequence of the a posteriori probability (APP) distributions of the information symbols. There have been several attempts to achieve, or at least to approach, this goal. The algorithm originally proposed [2] is based on a modification of the soft-output Viterbi algorithm (SOVA), whereas other approaches consisted of revisiting the original symbol MAP decoding algorithm and simplifying it to a form suitable for implementation [3, 41. These algorithms require the transformation of the parallel concatenated convolutional coding scheme (PCCC) into an equivalent block code. This in turn requires trellis termination of all constituent encoders (thus reducing the code rate) and block synchronisation, and leads to a nonuniform reliability of the decoded bits. In this Letter we propose two new sliding-window versions of the MAP algorithm (the optimum one and its suboptimum version) which allow a continuous decoding of the PCCC, avoiding the problem of trellis termination and leading to a more flexible system solution.