Rate decision at the IS-95A CDMA handset receiver

In the IS-95A CDMA standard speech is encoded at a variable rate vocoder that generates data at rates of 1.2, 2.4, 4.8 or 9.6 kbit/s depending on speech activity. As the frame duration is fixed at 20 milliseconds, the number of bits per frame varies according to the vocoder rates. At the base station transmitter half-rate convolutional encoding with constraint length of 9 doubles the traffic rate to give rates from 2.4 to 19.2 kbit/s. To ensure that the rate is always 19.2 kbit/s, data repetition is appropriately used at the lower speech rates. Since the transmitted data does not contain any explicit information about the data rate the receiver has to determine the data rate by analysing the received data. This paper describes two algorithms for determining the data rate at the receiver.