Patent ID: 7512868
Filing Date: 2009-03-31
Classification: H03M

Abstract:
1. Signal processing method using a MAP (Maximum A Posteriori) type algorithm to determine a likelihood ratio A k X of a set of states X of a lattice at instant k, each of the said states being associated with at least one intermediate variable, belonging to the group including a so-called “forward” variable and a so-called “backward” variable, propagated by the said MAP algorithm and calculated recursively, in a direct direction by a first Viterbi decoder and in an indirect direction by a second Viterbi decoder respectively at the said instant k with respect to the said lattice, wherein the method comprises the following steps performed by a communication signal receiver: reducing the number of states selected by the said MAP type algorithm so as to calculate the said likelihood ratio; calculating said likelihood ratio for the selected states; assigning at least one determined value to the corresponding said forward variable calculated by the first Viterbi decoder and/or backward variable calculated by the second Viterbi decoder, so as to calculate an approximate likelihood ratio, at least for some non-selected states; and calculating said approximate likelihood ratio at least for some non-selected states; and wherein at a given instant k, with 0≦k≦K, said at least one determined value a(k) assigned to said forward variable is smaller than a minimum value of said forward variable at said instant k and/or said at least one determined value b(k) assigned to said backward variable is smaller than a minimum value of said backward variable at said instant k.