3G-Universal Mobile Telecommunication Systems (UMTS), was conceived by the European Telecommunication Standards Institute (ETSI) as one of the 3rd generation standards. Initially, 2G supported a maximum data rate of 384 Kbps (commercially). To increase the system capacity so that more and more number of users can be served with better data rates, UMTS came into existence. UMTS supports up to 2 Mbps data rates for users from the outset.
UMTS integrates packet and circuit data transmission, thus providing virtual connectivity to the network at all times. UMTS is backwards compatible with GSM/GPRS CS. New services like IMS is introduced. UMTS networks provide QoS to satisfy customer’s application requirements.
UMTS uses CDMA for its air interface. It operates at 3.8 Mcps in a FDD mode with a 5 MHz channel that includes guard bands. UMTS uses spread spectrum technology to transmit data over the same wideband channel. Each user’s data is spread across the entire bandwidth using a unique digital code that identifies the user. At receiver, the received data is demodulated and de-spread using the same code used for spreading and the signal from other users is rejected as noise.
UMTS uses channelization codes and scrambling codes. Orthogonal Variable Spreading Factor (OVSF) code is used to provide separation of channels in both uplink and downlink directions. Scrambling codes are used to identify the transmit source.
UMTS also uses power control which controls the power transmitted to and from the UE. UMTS uses both hard and soft handover to switch to a better cell for efficient provision of services. Soft handovers are unique to the CDMA-based system, but there are pros/cons in its operation too. In 3G, UTRAN has three layers of channel functionality- 1) Logical Channels, 2) Transport Channels, 3) Physical Channels.
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