Information tone handling in the half-rate GSM voice activity detector

A problem has been identified with the handling of information tones by the full-rate GSM voice activity detector (VAD). The paper describes the modifications made in the half-rate GSM VAD which significantly improve the transmission of information tones, thus enabling the implementation of discontinuous transmission (DTX) in the GSM down-link. The modification consists of the addition of a tone detection algorithm to the basic full-rate GSM VAD architecture. The underlying assumptions behind the tone detector are discussed and the potential performance limitations identified. The half-rate GSM VAD successfully detects almost all tones that are likely to be encountered in networks across the world.