Performance of analysis-by-synthesis low-bit rate speech coders in mobile radio channel

Low bit rate coding of voice is critical for accommodating more users on channels that have inherent limitations of bandwidth or power-like cellular radio or satellite links. Analysis-by-Synthesis (AbS) coders are the candidate coders for producing good quality speech in the range from 4.8 to 16 kb/s. The multi-pulse excited linear predictive coder (MPE-LPC), and the regular-pulse excited linear predictive coder (RPE-LPC) are among very large number of (AbS) speech coders. The MPE and RPE can produce high quality speech at bit rate lower than 9.6 kb/s with a fairly moderate complexity. In this paper we introduce a novel approach for achieving variable-bit rates below 9.6 kb/s by using switched quantization. The issues of transmitting a 9.6 kb/s MPE arid RPE analysis-by-synthesis predictive coded speech over a mobile radio-fading channel are discussed. The coder robustness and the sensitivity of speech-carrying bits to channel errors are also presented. Further, the performance of a π/4 differential quadrature phase shift keying (π/4 DQPSK) method over Rayleigh fading mobile radio channel is presented by the coder output. Objective and subjective tests are used to show the effect of the Rayleigh fading mobile radio channel on the performance of the coder, and also determine the amount of the minimum value of channel signal to noise power ratio (SNR dB) required to obtain a good speech quality.