Patent ID: 8300739

Claim:
A method of generating soft bit values for a sequence of transmitted symbols using a demodulator, the method comprising: updating an M-state trellis managed by the demodulator responsive to a transition from symbol time n−1 to symbol time n, where M is a function of the number of bits per symbol in the sequence of transmitted symbols; saving survivor metrics associated with the M states of the trellis each symbol time so that the demodulator can calculate soft bit values with regard to transitions from symbol time n+D−1 to symbol time n+D, the survivor metrics indicating the probability that each respective state represents the transmitted symbol associated with symbol time n+D−1; tracing back through the trellis to calculate soft bit values for a symbol detected at symbol time n−D based on survivor metrics saved for the M states at symbol time n−D; updating a symbol history for each state in the trellis based on path metrics calculated for the M trellis states at a current symbol time n and G trellis states at a previous symbol time n−1, where G<M; identifying the trellis state for the current symbol time n most likely corresponding to a transmitted symbol; using the symbol history of the identified trellis state for tracing back to the trellis state at symbol time n−D+1 which corresponds to a detected symbol for symbol time n−D+1; determining decision-delayed path metrics for the M trellis states at symbol time n−D and the detected symbol for symbol time n−D+1 based on survivor metrics saved for the M trellis states at symbol time n−D; and generating soft bit values associated with the detected symbol for symbol time n−D+1 as a function of the decision-delayed path metrics.