Multi-rate Detection for the Is-95 Cdma Forward Traffic Channels

The IS-95 Direct Sequence Code Division Multiple Access (DS-CDMA) system has become a U.S. digital cellular standard. In the forward traffic channels (from the base station to the mobiles), the speech encoder rate can vary, depending on the speech activity in the transmitter. At the receiver, the decoder does not know which one of the speech encoder rate the transmitter used. This paper focuses on the IS-95 system that uses the 8kbps speech encoder and describes an algorithm to determine the encoder rate used for the transmitted sequence, with high confidence, so that the appropriate Viterbi decoder can be applied only once and not four times. This can reduce the overall decoder complexity and increase the throughput by a factor of two. It can also reduce the power consumption in the decoder.