Subtraction in a viterbi decoder

Abstract From the Canadian patent

A Viterbi decoder for decoding a convolutional code. For each possible state, an accumulated error AE is maintained at 66. As each codeword Rx-GP is received, the errors between it and the code groups of all the transitions are determined at 65. For each possible new state logic 68 determines the errors , of, the two transitions leading from old states to that new state adds them to , the, accumulated errors of those two old states and determines the smaller of the , two sums. Path logic 67 records the corresponding transition updating a record of , the, path leading to the new state. Tracing back along a path a predetermined and sufficiently large number of transitions the input bit or …