3. WCDMA System Structure 3.1 Overview

The UMTS (Universal Mobile Telecommunications System) is the third generation mobile telecommunication system by using the WCDMA air interface technology, usually called the WCDMA telecommunication system. It adopts a structure similar to the second generation mobile telecommunication system, including the RAN (Radio Access Network) and the CN (Core Network). The RAN is used to process all the radio-related functions, while the CN is used to process all voice calls and data connections within the UMTS system, and implements the function of external network switching and routing. Logically, the CN is divided into the CS (Circuit Switched) Domain and the PS (Packet Switched) Domain. UTRAN, CN and UE (User Equipment) together constitute the whole UMTS system.

From the point of view of the 3GPP R99 standard, the UE and the UTRAN (UMTS Terrestrial Radio Access Network) are composed of new protocols, and the design is based on WCDMA radio technologies. However, the CN adopts the definition of GSM/GPRS, so it not only can implement smooth transition of the network, but also can implement global roaming at the initial phase of 3G network construction.

3.1.1  Composition of the UMTS Network System

The UMTS network system includes the following parts:

1. UE (User Equipment)

As the user terminal equipment, the UE exchanges data with network equipment through the Uu interface, and provides such kinds of services within CS and PS domains as common voice, data communication, mobile multi-media and Internet application (For example, E-mail, WWW browse and FTP).
UE includes the two parts below:
l   ME (Mobile Equipment): Providing application and services.
l   USIM (UMTS Subscriber Module): Providing subscriber identification.

2. UTRAN (UMTS Terrestrial Radio Access Network)

UTRAN is divided into Node B and RNC (Radio Network Controller).
l   Node B
Node B is the base station of the WCDMA system (i.e. radio transceiver), and it interconnects with RNC via the standard Iub interface and processes the physical layer protocols of the Uu interface. Its main functions include spreading/de-spreading, modulation/demodulation, channel coding/decoding, and conversion between baseband signals and RF signals.
l   RNC (Radio Network Controller)
RNC (Radio Network Controller) implements such functions as connection establishment and release, handover, macro diversity and the management and control of radio resources. The details are given as follows:
1)      Provides the system information broadcast and system access control functions
2)      Provides such mobility management functions as handover and RNC transition
3)      Provides radio resource management and control functions such as macro diversity combination, power control and radio bearer allocation

3. CN (Core Network)

CN (Core Network) is responsible for connecting other networks as well as communicating and managing UEs. The core network equipment of different protocol versions in the WCDMA system differ. Generally, the R99 core network is divided into the CS domain and the PS domain. The R4 core network is the same as the R99 core network, but in the R4 core network, the MSC function of R99 CS is implemented by the two separate entities: MSC Server and MGW. The R5 core network is the same as the R4 core network except that R5 has been added with an IP multi-media domain.
The R99 core network has the following function entities:
1)      MSC/VLR
MSC/VLR is a functional node of the CS domain in the WCDMA core network. It connects with UTRAN via the Iu-CS interface, with external networks (such as PSTN and ISDN) via the PSTN/ISDN interface, with HLR/AUC via the C/D interface, with MSC/VLR, GMSC or SMC via the E interface, with SCP via the CAP interface, and with SGSN via the Gs interface. Its main functions are call control, mobility management, authentication and ciphering of the CS domain.
2)      GMSC
GMSC is the gateway node between the CS domain of the WCDMA mobile network and external networks, and it is an optional functional node. It connects with external networks (PSTN, ISDN and other PLMN) through the PSTN/ISDN interface, connects with HLR through the C interface and connects with SCP through the CAP interface. It implements the routing function of incoming calls in the VMSC function and inter-network settlement function of such external networks as fixed networks.
3)      SGSN
SGSN (Serving GPRS Support Node) is a functional node of the PS domain in the WCDMA core network. It connects with UTRAN through the Iu-PS interface, with GGSN through the Gn/Gp interface, with HLR/AUC through the Gr interface, with MSC/VLR through the Gs interface, with SCP through the CAP interface, with SMC through the Gd interface, with CG through the Ga interface and with SGSN interface through the Gn/Gp interface. And its main functions are route forwarding, mobility management, authentication and ciphering of the PS domain.
4)      GGSN
GGSN (Gateway GPRS Supporting Node) is a functional node of the PS domain in the WCDMA core network. It connects with SGSN through the Gn/Gp interface and with external data networks (Internet/Intranet) through the Gi interface. It provides the routing and encapsulation of data packets between the WCDMA mobile network and the external data networks. Its major functions are to provide interfaces to external IP packet networks. It needs to provide the gateway function for UE to access external packet networks. From the point of view of external networks, GGSN looks as if it were a router of all user IP networks in the addressable WCDMA mobile network, and it needs to exchange routing information with external networks.
5)      HLR
HLR (Home Location Register) is a functional node shared by the CS and PS domains in the WCDMA core network. It connects with MSC/VLR or GMSC through the C interface, with SGSN through the Gr interface, and with GGSN through the Gc interface. And its main functions are to store subscription information for subscribers, support new services and provide the enhanced authentication function. 

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