Add compare select circuit and method implementing a viterbi algorithm

A method of performing an Add-Compare-Select (ACS) function to provide a current state metric value of a current state from a set of previous states, each having a respective previous state metric value, the method comprising the steps of:

• a) combining each respective previous state metric value of the set of previous states with a respective branch metric value defined for a transition between the respective one of the set of previous states and the current state, thereby to provide a respective updated state metric value, wherein each branch metric value being equivalent to one another;
• b) comparing each previous state metric value of the set of previous states to determine one previous state metric value being a minimum value;
• c) providing a select signal corresponding to the one previous state metric value being a minimum value;
• and
• d) selecting, responsive to the select signal, as the current state metric value the respective updated state metric value provided from the combination of the one minimum previous state metric value with the respective branch metric value,
• wherein the current and previous states are states of a portion of a trellis structure, a sequence of states of the trellis structure corresponding to sequence of received symbols representing data and a number of possible states for each transition determined by the number of possible values for the symbol, and
• wherein the ACS function implements a double state trellis structure derived from augmentation of the number of possible states of the trellis structure.

A method of performing an Add-Compare-Select (ACS) finction to provide a current state metric value of a current state k, k an integer, from a set of previous states, each of the set of previous states having a respective previous state metric value (SM.sub.t), t an integer, the method comprising the steps of

• a) calculating a respective branch metric value (BM.sub.k) for each transition from each of the set of previous states to the current state k based on a received channel output sample of a channel output sample sequence;
• b) implementing a double state trellis structure for the set of previous states, the double state trellis structure defined such that for a set of M previous states, M an integer, of the set of previous states which transition to the current state k, each of the respective transitions to the current state k having a respective branch metric value BM.sub.k being equivalent to one another;
• and
• c) calculating the current state metric value (SM.sub.k) as:
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