Two-step soft output viterbi algorithm decoder using modified trace back

A two-step soft output Viterbi algorithm decoder, comprising:

• a branch metric unit for calculating branch metrics for received data from the degree of similarity between data symbols, which appear to be received if a transmission channel has no error, and the actually received data symbols;
• a state metric memory for storing state metrics for all states of the received data symbols;
• an add-compare-select unit for adding the branch metrics of the branches for each state of the current time unit and the state metrics for each state of the previous time unit stored in the state metric memory, storing the results of the additions, which satisfies a predetermined condition, as the state metrics of the current time unit in the state metric memory, and outputting a difference between the results of the additions as a delta value, and as many selector bits as the number of bits of the received data symbols so as to select a path;
• a survivor state output unit for outputting a survivor state satisfying a predetermined condition among the current state metrics for all states output from the add-compare-select unit;
• and
• a soft output Viterbi algorithm-state metric unit (SOVA-SMU) for back tracing a first survivor path for a first time unit from the survivor state output from the survivor state output unit, using the delta values for all states and the selector bits, to obtain the survivor state at the end of the first survivor path and a soft decision value of the first survivor path, for performing a double trace back for a second time unit from the survivor state at the end of the first survivor path to find a second survivor path and a concurrent path, for updating a soft decision value of the second survivor path by the soft decision value of the first survivor path if the hard decision values of the second survivor path and the concurrent path for the second time unit are different and if a soft decision value of the second survivor path is larger than the soft decision value of the first survivor path, and for outputting the hard and soft decision values of the second survivor path at the end of the second time unit.