The ability to set the initial states to zero in the Viterbi Decoder block is not available in Communications Blockset.
To work around this issue, set the 'Operation mode' field in the Viterbi Decoder block dialog to 'Terminated'. In this mode, the traceback will start from the all-zeros state.
The key to get this to work is to ensure that the encoder also ends at the zero state for each packet of data encoded. This is achieved by setting the 'Operation mode' field in the Convolutional Encoder block dialog to "Terminate trellis by appending bits".
