SOURCE-CONTROLLED CHANNEL DECODING

Source and channel coding have been treated separately in most cases. It can be observed that most source coding algorithms for voice, audio and images still have correlation in certain bits, Transmisson errors in these bits usually account for the significant errors in the reconstructed source signal. This paper proposes a modification of the Viterbi decoding algorithm (VA) for binary trellises which uses a priori or a posteriori information about the source bit probability for better decoding in addition to soft inputs and channel state information. Analytical upper bounds for the BER of convolutional codes for this modified VA (APRI-VA) are given, The algorithm is combined with the Soft Output Viterbi algorithm (SOVA) and an estimator for the residual correlation of the source bits to achieve source-controlled channel decoding for framed source bits. The description is simplified by an algebra for the log-likelihood ratio L(u)= log(P(u = +1)/P(u = -1)) which allows a clear definition of the ''soft'' values of source-, channel-, and decoded bits as web as a simplified description of the traceback version of the SOVA. Applications are given for PCM transmission and the full rate GSM speech codec. For an PCM coded oversampled bandlimited Gaussian source transmitted over Gaussian and Rayleigh channels with convolutional codes the decoding errors are reduced by a factor of 4 to 5 when the APRI-SOVA is used instead of the VA, This results in much less signal distortion. The GSM decoder is modified for those few significant bits only which still have correlation between consecutive 20 ms speech frames, A simple dynamic Markov correlation estimator is used, With these receiver-only modifications the channel SNR in a bad mobile environment can be lowered by 2 to 3 dB resulting in the same voice quality, Further applications are briefly discussed.