Method and apparatus for generating soft bit values in reduced-state equalizers

A receiver comprising a demodulator configured to:

• update an M-state trellis 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 a sequence of transmitted symbols;
• save 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,
• trace 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;
• a trellis searcher configured to:
• update 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,
• identify the trellis state for the current symbol time n most likely corresponding to a transmitted symbol, and
• use 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;
• a soft value calculator configured to:
• determine 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
• generate soft bit values associated with the detected symbol for symbol time n−D+1 based on the decision-delayed path metrics;
• and
• a buffer configured to store the survivor metrics for the M trellis states at each symbol time over a span of D symbols.

One or more non-transitory computer-readable media containing executable instructions that, when executed, cause a demodulator to:

• update an M-state trellis managed by a 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 a sequence of transmitted symbols;
• save 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;
• trace 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;
• update 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;
• identify the trellis state for the current symbol time n most likely corresponding to a transmitted symbol;
• use 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;
• determine 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
• generate soft bit values associated with the detected symbol for symbol time n−D+1 as a function of the decision-delayed path metrics.