The evolution of IS-136: voice service

The so-called 136+ features represent the first step in the evolution of the IS-136 standard towards an IMT-2000 system. In this article, we present the voice service of 136+. The main requirements for this service consist of improved quality for speech and non-speech signals, and for background noise and tandem conditions. Performance and implementation considerations lead to the selection of 8-PSK for the 136+ modulation. Listening tests reveal that the US1 vocoder, at 12.2 kb/s, satisfies the 136+ objectives. Furthermore, simulated performance in dispersive environments suggests that an equalizer may not be required. The use of a standard vocoder greatly simplifies the development of 136+, and the proposed scheme is expected to provide a cost-efficient service