10. WCDMA Network Solution 10.1 Overview of WCDMA Evolution

10.1.1  Overview of Standard Evolution

The WCDMA technology has gone through R99/R4/R5/R6 stages since it comes into being. The R99 protocol was functionally frozen in March 2000 (December 1999 in 3GPP official document) and almost became mature after two years of improvement. The R4 protocol was functionally frozen in March 2001. The R5 protocol was functionally frozen in March 2002 (with some functions frozen in June), and the R6 protocol is estimated to finalize its functions in December 2004.

Compared with GSM and GPRS networks, the most significant change of the WCDMA system is the change of the radio network. In the WCDMA network, the Radio Access Network (RAN) is used to replace the Base Station Subsystem (BSS).
The WCDMA core network in the R99 version can be regarded as a combination of GSM and GPRS core networks in terms of networking, namely, the R99 core network is classified into the Circuit Switched (CS) domain and the Packet Switched (PS) domain. The architectures of the CS domain and the GSM core network are basically the same, so are the architectures of the PS domain and the GPRS core network.
In contrast to the R99 version, the biggest change of the R4 core network is that the network element function of MSC in the CS domain of the R99 core network is fulfilled by the MSC Server and MGW in the R4 version, where the MSC Server processes the signaling while the MGW processes the voice. There is no change in PS domain. For details, refer to the architecture description in Chapter 3.
The core network using the R4 protocol has two networking modes: TDM and IP. When the TDM mode is adopted, the R4 network planning and construction are identical with that of R99 to a large degree. For example, in constructing tandem and signaling networks, many considerations are just the same. When the IP mode is adopted, the R4 network planning and construction are largely different from that of R99.
In contrast to the R4 version, an IMS (IP Multimedia Subsystem) domain is added in the R5 core network, together with the corresponding equipment and interfaces, but there is nearly no change in the network structure of CS and PS domains. Meanwhile, some equipment functions are upgraded due to the enhancement of the network functions.

The IMS domain is overlaid with the original PS/CS domain. It is used to control the subscribers’ services. Subscribers can use all kinds of access techniques of the PS/CS domain to access the IMS domain. In the future, new services based on the 3GPP R5 IMS domain will have nothing to do with the access techniques that subscribers adopt. At that time, no matter what technique is adopted, the service needs to be developed only once by the service developers.
The IMS domain adopts SIP as the basic session protocol. As the SIP is used to unify the session model of voice/data services, the IMS domain provides more flexible and simple support to multimedia services.
It also provides abundant service development interfaces. Operators can even provide open session messages to the trusted service providers. Therefore, the services will become even more open and flexible.

10.1.2  Network Construction Solutions

It becomes a persisting issue for operators and equipment providers to construct a WCDMA network with powerful functions and stable performance and also to consider the factors ranging from engineering technical difficulties, capital investment and the network compatibility to evolution.

1. WCDMA network construction

For operators, there are two solutions to building a WCSDMA network: Upgrading the old one and establishing a new one.
As to new mobile operators, they will usually choose to establish a new WCDMA network. For GSM and GPRS operators, they need to consider issues such as the network compatibility and roaming. However, gradually they will also choose to establish a new one. The following is a comparison between these two solutions.
Solution 1: Upgrading from GSM to the WCDMA R99 network:
This solution will turn the 2G GSM network into a network with 2G/3G coexistence by upgrading MSC, HLR and GMSC of the GSM network into 3G MSC, 3G HLR, 3G GMSC.
l   The upgraded MSC should support access of WCDMA and GSM radio network equipment at the same time.
l   The MSC should support the lu interface based on ATM and the A interface based on TDM.
l   Support AMR voice, H.324M multimedia calls and TC equipment upgrading.
l   Support double authentication via GSM/WCDMA, 3G MAP, internal handover and compatibility.
l   Support CAMEL3 VAS and the open capability of MSC OSA.
l   Support the integration of 2G/3G charging systems.
Advantages of the upgrade solution:
l   Minimize inter-MSC handover at the initial stage.
Disadvantages of the upgrade solution:
l   Have a big impact on the stability of the existing GSM network.
l   Require a heavy workload and increase difficulties in the compatibility test.
l   Cause the decrease of the existing network capacity, since 3G services are even more complex and demand better processing capability.
l   Affect the service provisioning and cause difficulties in smooth evolution, especially the provisioning of NGN technology, IN service and the third-party applications.
l   Require high costs of upgrading and large investment. The 3G MSC adopts the highly-advanced technical platform with large volume and high integration. Its construction cost is lower than that of the 2G MSC; therefore, it would be not worthwhile to upgrade the small-capacity 2G MSC.
l   The upgrading of the original charging system will have a big impact on the network operation. The interworking and compatibility of multiple manufacturers and nationwide roaming need to be verified.
This solution will give a full-scale evaluation to the problems such as the impact on the stability of the existing network, the continuity of service capabilities, service processing capacity, capacity, level of integration and whether the seamless transition of GSM-R99-R4-R5-R6 is available after the upgrading. It is hard to know whether the traditional architecture of 2G switches meets the above requirements, therefore, it is also impossible to know the investment utilization rate of the original GSM equipment.
Solution 2: Establishing a new WCDMA network
This solution is to establish a new 3G MSC and a new 3G RAN based on the original GSM network. The GSM and WCDMA networks will coexist in a certain period of time with service interoperability and the WCDMA network will gradually replace the GSM network.
Requirements for the core network equipment:
l   Establish a new WCDMA RAN and a new MSC to support CAMEL2, 3 IN service and the access of 2G/3G networks simultaneously.
l   The existing SGSN and GGSN have relatively small traffic and can be upgrade into 3G equipment. They should be compatible with GPRS access and applications and support CAMEL3 IN services.
l   For the WCDMA HLR, both solutions (upgrading the old one and building a new one) are available. As there are few HLRs, the upgrading is quite easy.
l   The backbone and core networks of TMSC and GMSC should be shared. GMSC upgrading can be disregarded at the initial stage but later they may support 3G MAP and CAP protocols and the trigger of 3G IN services.
l   The signaling network equipment LSTP/HSTP of 2G SS7 should be shared.
l   The 2G MSC should stop capacity expansion and BSS should be connected to the new 3G MSC.
Advantages of the solution for building a new WCDMA network:
l   As a new network, it is easier for the entire network to plan the resources and configurations uniformly in order to have a clear network architecture.
l   With large capacity and less offices, it can simplify the network architecture to facilitate centralized maintenance and management.
l   The test becomes more convenient and comprehensive.
l   Have a small impact on the existing network during the implementation.
Disadvantages of the solution for building a new WCDMA network:
l   There may be coordination problems with the handover between 2G and 3G systems. If the manufacturers of the existing network do not set obstacles deliberately, it is easy to solve this problem.

2. Evolution from R99 to R4

Evolving from the R99 network to the R4 network, you need replace the MSC NE of R99 with the MSC Server and MGW of R4 from the perspective of the protocol layer. However, in terms of the specific implementation, the construction solution of the R99 network at the initial stage is closely related with the amount of investment needed for evolution and the difficulty of upgrading.
1)      Solution 1: Upgrading the R99 network to the R4 network
If the MSC NE of the R99 network has already taken into account the idea of processing the signaling and voice separately (that is, the MSC based on the design philosophy of separating bearer from control is adopted to build the R99 network), the evolution will be very easy no matter upgrading from R99 to the R4 of TDM networking, or even to the R4 of IP networking and only the equipment needs to be upgraded.
l   The original R99 should be capable of separating bearer from control to facilitate the transition to the R4 architecture.
l   The upgraded equipment should support all the signaling interfaces of the R4 network.
Following is a comparison between the advantages and disadvantages of this solution:
l   Advantages: It is easy to upgrade the network. You need only upgrade the original equipment instead of purchasing any MSC Server or MGW. It is also easy to upgrade the ATM/IP-based network. The most important thing is that it brings very big flexibility to the construction and planning of the network.
l   Disadvantages: The R99 network at the initial stage should meet higher requirements. The R99 network upgraded directly from the GSM network probably will not meet these requirements. In this regard, you have to establish a new MSC Server and MGW.
2)      Solution 2: Establishing a new R4 network
If the MSC of the R99 network fails to take into account the idea of processing the signaling and voice separately at the later stage (or if this R99 network is upgraded directly from the GSM network), probably the bearer and control modules cannot be well separated. In that case, the evolution from R99 to R4 cannot be completed through upgrading the MSC, or we can say it is not worthwhile. In this regard, we have to purchase additional MSC Server and MGW.
Following is a comparison between the advantages and disadvantages of this solution:
l   Advantages: The new network has powerful functions and is capable of smooth transition to the later all-IP networking as well as R5.
l   Disadvantages: It is required to establish a new network, which makes the cost higher than the upgrade solution.
To summarize, it is a continuous evolution process from GSM to R99 and then from R99 to R4 in terms of the whole version evolution. During the WCDMA network construction at the earlier stage, if the solution to upgrading the GSM network to the R99 network is adopted, you need rebuild the MSC Server and MGW NE in upgrading from R99 to R4. However, if you establish new R99 network with the MSC that is able to separate bearer from control, the cost at the initial stage may be higher than that of the upgrade solution, but it is conducive to the later smooth transition to R4 and R5. Therefore, the comprehensive investment efficiency of the solution for establishing a new R4 network will be much higher.

3. Evolution from R4 to R5

In R5, an IMS domain is added. The logic block diagram of the IMS domain is, where the Go interface is interconnected with the GGSN while the Mb interface is used for the external network to access the IMS domain.

The functional and logic entities of the IMS include BGCF, CSCF, MGCF, IM-MGW, MRFC, MRFP, HSS and SLF. Some logic entities can be evolved from the R4 function entities while others must be added.

Therefore, it is better to establish new equipment of the IMS domain for evolution from R4 to R5 and upgrade the functions of the equipment in the original CS and PS domains. 

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