Variable Rate Speech Coding for Cellular Networks

Abstract

A central objective in the design of a cellular network for mobile or personal communication is to maximize capacity while maintaining an acceptable level of voice quality under varying traffic and channel conditions. Conventional FDMA and TDMA techniques, dedicate a channel or time slot to one unidirectional speech signal regardless of the fact that a speaker is silent roughly 65% of the time in a two-way conversation. Furthermore, when speech is present, the short-term rate- distortion trade-off varies quite widely with the changing phonetic character. Thus, the number of bits needed to code a speech frame for a given perceived quality varies widely with time. The speech quality of coders operating at a fixed bit rate is largely determined by the worst-case speech segments, i.e., those that are the most difficult to code at that rate. Variable rate coding can achieve a given level of quality at an average bit-rate R a that is substantially less than the bit rate R f that would be required by an equivalent quality fixed rate coder. Efficient multiple-access systems, such as CDMA, directly translate this rate reduction into a corresponding increase in network capacity.