Sequence detection

A path metric unit for calculating path metrics, associated with respective states of an n-state trellis, by accumulating branch metrics in a sequence detector, each path metric being represented by N bits plus a wrap-around bit for indicating wrap-around of the N-bit value of that path metric, the path metric unit comprising:

• a set of pipeline stages for receiving said branch metrics and adapted to select, for each state of the trellis, a pair of candidate path metrics from a set of partial path metrics, produced using said branch metrics, for that state;
• and
• an output pipeline stage connected to receive said pair of candidate path metrics for each state and adapted to produce for that state a select signal, for selecting an optimum one of the pair as the path metric for that state, by comparing each of the N-bit values and the wrap-around bits of the pair;
• wherein said candidate path metrics and select signal for each state are fed back from the output pipeline stage to said set of pipeline stages which is further adapted to produce pairs of speculative partial path metrics for each state by adding corresponding branch metrics to the pairs of candidate path metrics, and to select a said partial path metric from each pair of speculative partial path metrics in dependence on the select signal for the candidate path metrics from which that pair was produced.