List Viterbi Algorithms for Continuous Transmission of Concatenated Cyclic Redundancy Check and Convolutional Codes

Convolutional codes with Viterbi decoding have been proposed for so-called hybrid In Band on Channel (Hybrid IBOC) systems for digital audio broadcasting compatible with the FM band. For high quality audio signals it is beneficial to use error concealment/error mitigation techniques to avoid the worst type of channel errors. This requires a reliable error flag mechanism (error detection feature) in the channel decoder. A cyclic redundancy check (CRC) on a block of audio information bits provides this mechanism. In this paper, we study the parameter tradeoffs (block length, CRC code rate, flag rate, etc.) for such schemes for the Gaussian channel. We also describe informal listening tests. We present the preferred length of the outer CRC code for 96-kb/sec audio coding. The list Viterbi algorithm (LVA) produces the list of the L best output sequences, according to some path metric, over a certain block length. We show that the List Viterbi Algorithm with a sufficiently long list is the optimum maximum likelihood decoder for the concatenated pair of a convolutional code and a CRC. The CRC is used to select the output. We demonstrate how the LVA can significantly reduce the flag rate for the same transmission parameters. At the expense of complexity. a receiver optional LVA can reduce the flag rate by more than an order of magnitude. The difference in audio quality is dramatic. New LVAs for continuous transmission are proposed, where no termination bits are required for the convolutional code for every CRC block. Finally, we also present optimum and suboptimum list Viterbi algorithms for tailbiting convolutional codes.