Channel coding techniques for adaptive multi rate speech transmission

A variable channel coding scheme for adaptive multi rate (AMR) speech transmission over mobile radio channels is proposed. Although it was developed for the GSM (Global System for Mobile Communications), the basic concept of variable channel coding can be adopted to other digital radio systems. The new AMR concept allows almost wire-line speech quality even for poor channel conditions by dynamically splitting the gross bit rate between source (speech) and channel coding according to the channel quality. In this study we show some new aspects relating to AMR and show the advantage of recursive systematic convolutional (RSC) codes for mobile speech transmission. With some modification they improve not only the bit error rate but also the frame erasure rate. It is derived that RSC codes can be decoded with a “standard” non-systematic Viterbi decoder and a simple transformation. A new powerful approach is employed for the detection of the currently used mode (in-band signaled information) where the mode bits are integrated in a long block and codes gain with their length