Error-trapping Viterbi decoding for type-I hybrid-ARQ protocols

The partial path metrics generated during Viterbi decoding provide side information that can be used in the development of a type-I hybrid-ARQ error control system. Each path through the Viterbi decoding trellis is equivalent to a path through a weighted directed graph. Error patterns in decoded information blocks thus correspond to cycles in the graph. A sliding window is used to trap these cycles. The change in partial path metric across the window determines the reliability of the maximum likelihood path. If the maximum likelihood path is deemed unreliable, a retransmission request is generated. Implementation of this error-trapping ARQ scheme requires a minor modification of the Viterbi decoder. It is shown that the reliability of the data can be greatly improved while incurring only a small reduction in the throughput.