Abstract
Motivated by potential applications to mobile radio, we studied variable-bit-rate speech communication through Gaussian-noise and Rayleigh-fading channels. For convenience we used a constant signaling rate of 32 kb/s and adjusted the source-coding and channel-coding rates in response to changing transmission quality. When the channel quality was good enough, we used all 32 kb/s for speech transmission. When the channel quality was lower, we reduced the source rate to 24 or 16 kb/s and introduced channel coding to control distortion due to transmission errors. We concentrated on specific source and channel codes that could be implemented with hardware of modest complexity. The source code was embedded differential pulse code modulation, which is amenable to variable-bit-rate operation and economical to implement. For error control we introduced punctured convolutional codes and a Viterbi decoder with only 16 states. Although the source/channel codec was simple, it offered good performance. Speech quality was at the level of normal telephony when the channel was good; the error-correcting codes extended by up to 13 dB the range of channel signal-to-noise ratios that support adequate quality. Our performance estimates were based on a new analysis of transmission errors in embedded differential pulse code modulation and on computer simulations of speech transmission through fixed and fading channels.
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