Variable-rate sub-band speech coding and matched channel coding for mobile radio channels

The effect of digital transmission errors on a family of variable-rate embedded subband speech coders has been analyzed. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most sensitive and the least sensitive bits of the speech coder. As a result, a family of rate-compatible punctured convolutional codes with flexible unequal error protection capabilities has been matched to the speech coder. On a Rayleigh fading channel with 4-DPSK modulation, as much as 5-dB gain in channel signal-to-noise ratio can be obtained by using four levels of error protection compared to only two levels in more conventional designs. The gain is achieved at no extra bandwidth requirement and at a negligible complexity increase. Among the results, analysis and informal listening tests show that with a four-level unequal error protection scheme, transmission of 12-kb/s speech is possible with little degradation in quality over speech is possible with little degradation in quality over a 16-kb/s channel with an average bit error rate of 2×10^-2 at a vehicle speed of 60 mph