 a reverse dedicated common control channel. The next stage in the evolution of CDMA2000 is 1xEV. As both cdmaOne and CDMA2000 1X were developed with the primary intention of increasing voice capacity they are primarily optimized for circuit switched rather than packet switched data. The next evolution of CDMA2000 is intended to make the system more suitable for high speed data transmission and as a result multiple access packet switching is incorporated. CDMA2000 1xEV-DO uses a hybrid CDMA/TDMA approach to provide high speed data only services. This requires a dedicated 1.25 MHz carrier on the forward link while the reverse link retains many of the features of the cdma2000 format. However, if voice services are to be provided as well an additional CDMA2000 carrier is required. The reason for adopting an evolution in this way is because it was envisaged that 1x EV-DO would be used in conjunction with data only applications with a PC where no voice link would be required. This means that if voice is required then two carriers are needed. One of the main features of the 1xEV system is the inclusion of link adaptation. This means that users with a good signal at the base station will be able to have a higher data rate than a station further away for which the signal strength is much lower. Therefore stations are only able to use a higher data rate in situations where the system can succeed without a high rate of errors. By slowing the data rate down the errors are reduced. In addition to this the form of modulation is also adapted changing between QPSK, 8PSK and 16QAM, etc. to make the most efficient use of the bandwidth available. For the highest data rates 16QAM is used, although for this to operate with a low bit error rate a better signal to noise ratio is required than can be accepted by the other forms of modulation. Accordingly when the link quality falls, the data rate is slowed as other forms of modulation are used. Cellular telecommunications 275 To achieve the level of link adaptation that is required, th