Patent ID: 6597743
Filing Date: 2003-07-22
Classification: H03M

Abstract:
A method for decoding a received sequence to obtain an estimate of a transmitted sequence comprising:forming a trellis comprising a plurality of nodes and a plurality of branches, wherein each node represents a particular state at one instant of time and wherein each branch represents a transition between states at different time instants and corresponds to a possible transmitted symbol associated with said transition; assigning branch metrics to said branches of said trellis; performing a forward recursion through said trellis starting from a beginning state and, at each stage in said forward recursion: forming a set of candidate states, wherein said candidate states are those states having entry branches leading from a survivor state in the previous recursion; computing the forward path metric for each candidate state based on said branch metrics; forming a set Q of forward survivor states selected from said set of candidate states based on the forward path metrics for said candidate states, wherein the number of members in Q is less than the number of candidate states; storing the forward path metric for each member state in Q; performing a backward recursion through said trellis starting from an ending state and, at each stage in said backward recursion; forming a set of candidate states, wherein said candidate states are those states having entry branches leading from a survivor state in the previous recursion; computing the backward path metric for each candidate state based on said branch metrics; forming a set R of backward survivor states selected from said set of candidate states based on the backward path metrics for said candidate states, wherein the number of members in R is less than the number of candidate states; storing the backward path metric for each member state in Q; for each stage k of said trellis, computing a soft value based on said accumulated forward path metrics for the survivor states in the set Qkâˆ’1, the accumulated backward path metric for the survivor states in the set Rk, and the branch metrics for the branches in the set B of branches connecting survivors states in Qkâˆ’1 and Rk.