Multi-rate detection for the IS-95A CDMA forward traffic channels using the 13 kbps speech coder

The IS-95A direct sequence code division multiple access (DS-CDMA) system is a US digital cellular standard. In the forward traffic channels (from the base station to the mobiles), the speech encoder rate can be reduced to four different sub-rates, depending on the speech activity in the transmitter. At the receiver, the decoder does not know which one of the four speech encoder sub-rates the transmitter used. This paper focuses on the IS-95A system that uses the 13 kbps 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 as is done by conventional decoders. This can reduce the overall decoder complexity by about a factor of two to fourteen, depending on the speech encoder sub-rate used. It can also reduce the power consumption in the decoder.