1. A method of performing Viterbi decoding function comprising the steps of: calculating candidate path metrics for states at time T n based on previously calculated path metrics for states at time T n−1 and branch metrics associated with transitions between said states at time T n−1 and states at time T n according to a first trellis; selecting path metrics for states at time T n from said candidate path metrics; calculating candidate path metrics for states at T n&plus;1 based on said selected path metrics for states at T n according to a second trellis, different from said first trellis.