Log domain implementation of the dual-APP algorithm

For high rate codes, an a posteriori probability (APP) algorithm working on the dual code is preferable. The challenge involved in log domain implementation of this algorithm due to the presence of negative valued metrics is highlighted. To represent these metrics, a new representation system is proposed in place of the existing sign-magnitude representation. A generalised model of a trellis based implementation is presented, over which the log domain implementation of two approaches, one based on likelihood ratios (LRs) and the other based on reflection coefficients (RCs) are compared in complexity. It is found that the approach based on LRs is less complex