The basic requirements of 3G systems are that they should be able to support a variety of application data rates and operation environments. This means that there must be support for quality of service and operation from mega-cells to pico-cells. The forward channels are referred to as transport channels in the UTRA W-CDMA standard proposed by 3GPP. The forward channel modifications are as follows.
IN W-CDMA, the BSs can operate in an asynchronous fashion that obviates the need of GPS availability to synchronize base stations, W-CDMA employs what is known as the orthogonal variable spreading factor (OVSF) Codes. OVSF codes allow a variable spreading factor technique that maintains orthogonality between spreading codes of different lengths. The logical channels are called transport channels in W-CDMA.
Cdma 2000 employs multiple carriers to provide a higher data rate compared with W-CDMA. It employs N carriers (N=1, 3, 6, 9) for an overall chip rate on N X 1.2288 which is 3.6864 Mcps for N=3. Alternatively, a single carrier can be employed to chip at the larger chipping rate. The Former mode of operation is suitable for overlaying cdma 2000 over existing IS-95 systems. Walsh codes from 128 chips to 4 chips are employed to provide spreading and processing gains. All N carriers use the same single code for scrambling. The BSs still need to be synchronized and used for fast acquisition and handoff as before. QPSK modulation is employed before spreading with the walsh codes to increase the number of usable walsh codes. In addition to the pilot, synch and paging channels, auxiliary pilot channels can be used to supply beam-forming information if smart antennas are employed. A dedicated MAC channel (DMCH) may be available for sending MAC layer message to specific mobiles.