System for decoding digital data using a variable decision depth

Abstract

In a digital data decoder such as a Viterbi algorithm decoder, digital data transmitted over a communications channel are estimated by a receiver using a variable decision depth technique to thereby improve bit error rate (BER) and channel estimation and to reduce decision delay. In the context of conventional trellis structures used for sequence estimation, the decoder assigns group flags to each state for each time interval being considered and stores those group flags along with corresponding accumulated path metrics associated with the states. More specifically, from a just decoded state, flags from different groups, e.g. each group representing a color, number, etc., are assigned to successor states connected by a survivor path to the just decoded state. Each offspring state of a parent state is assigned the parent's group flag. The decoder detects where in the trellis all of the survivor states have the same group flag. At this point where all the flags agree, the decoder back traces through the trellis to the parent state having that same group flag. That parent state is then decoded as the estimate of the symbol(s). Thereafter, group flags are reassigned and the procedure repeated using this newly decoded state as the new just decoded state.