Solutions to Smooth Evolution from GSM to WCDMA

1 Driving Forces of Evolution from 2G to 3G

1.1 Ever-Increasing Subscribers

With the rapid increase in the number of mobile subscribers, radio frequency resources and network capacities have been utilized to their maximum limits in some developed countries and regions. 3G networks employ the CDMA technology with much higher frequency efficiency than that of the 2G/2.5G technologies to solve the contradiction between the subscriber increase and limited radio frequency resources. From this point of view, 2G radio technologies will inevitably be replaced by 3G technologies.

1.2 Development of Mobile Services

The development of information technologies and subscriber’s diversified and personalized demands require the mobile communication system to provide richer services such as multimedia and high-speed data services. But the 2G network mainly provides voice and bearer services with a low transmission rate and limited QoS, which cannot meet the requirements of multimedia, e-business and mobile IP services. 3G technology can bring up to 2 Mb/s access rate in a low speed or motionless environment and 384 kb/s in a high speed environment, so 3G can provide various and personalized services and evolve in the directions of multimedia, personalization, intelligent services and packetization.

1.3 Market Competition Among Operators

Due to the decrease of the voice service profit margin, diversified and competitive services have to be developed in order to increase ARPU of carriers. In 2G systems, services are standardized, so it is very difficult to deploy new services offered by the third party and meet the diversified and personalized demands. In a 3G system, a service can be regarded as and realized by the combination of different service features that are reflected by different service capabilities of different bearer networks, which exactly represents the variety and flexibility of 3G-service creation. The carriers can only survive and thrive by fully utilizing the 3G platform to implement differentiated competition.

2 Solutions to Smooth Evolution from GSM to WCDMA

The development of mobile services is a step-by-step procedure. Service development decides the procedure of network construction. The construction of 3G networks is a gradual process. 2G networks will keep playing an important role for some time. So how to efficiently utilize 2G equipment during the construction of 3G networks should be taken into full consideration. Below is an analysis of solutions to the evolutions on the radio side and core network side.

2.1 Solutions to Radio Side Smooth Transition

(1)Upgrading Existing GSM Equipment

Figure 1 illustrates the network architecture with upgraded BSC. It shows that there are some technical problems.

It is technically difficult to upgrade a base station  controller(BSC) to  equipment   with   the function of a radio network controller (RNC). Firstly, the protocol of the radio network layer of RNC differs greatly from that of BSC; secondly, there is a big difference in the transmission network layer and physical layer: for BSC they are based on circuit switch (CS) and for RNC on ATM switch; thirdly, the processing is totally different at user planes, which is transparent for BSC.

Based on the above analysis, the integration of BSC and RNC will not save cost but will need complex equipment. So the mainstream vendor will not employ this evolution solution.

(2)Overlapping Existing GSM Equipment

For the network evolution at the radio side, the solution of overlapping 2G equipment, as shown in Figure 2, is widely accepted.

With the overlapping mode, there is no direct connection between 2G and 3G radio networks. The connection with the core network (CN) depends on situations. An upgraded CN connects RNC in the way shown by the gray line in Figure 2, and an overlapping CN connects RNC in the way shown by the red line.

One thing that should be mentioned here is that although there is no direct connection between 2G and 3G radio networks, Iub and Abis interfaces can be shared for transmission. ZTE’s 3G radio network equipment is capable of providing such a function.

2.2 Solutions to CN Side Smooth Transition

Three solution schemes are designed for the transition of CS domain and packet switch (PS) domain at the CN side.

(1) Overall Upgrade of Existing CN

For the transition of GSM/GPRS to 3G, ZTE provides overall solutions. They are based on the existing GSM/GPRS core networks and through hardware renovation and software upgrade to realize the evolution towards WCDMA.

A certain capacity redundancy is necessary for the GSM/GPRS network, because redundancy can protect the network, avoid traffic block and maintain network stability. This redundancy makes it possible to upgrade and expand the capacity of CN. But due to the rapid subscriber increase, the redundancy margin of 2G networks was greatly decreased. Therefore the upgrade and capacity expansion may harm the stability of the network. Moreover, at the network side, even the current upgrade can only temporarily meet the requirements. New 3G CNs still need to be constructed during the transition process towards all-IP networks. Therefore, from the development point of view, the CN upgrade is not cost-effective.

From the operators’ point of view, they are concerned with the system stability and the cost/performance ratio. The cost of CN construction is only about 20% of the whole investment in the mobile network, so the CN upgrade cannot save enough cost, and it will adversely affect the network performance and decrease the cost/performance ratio. There is no doubt that the upgraded system is an unstable one. From the strategic point of view, it is necessary for an operator to construct a network with high QoS and a high starting point to win more subscribers in future competition. Therefore, the upgrade of current CN networks is obviously not the priority and best solution for the evolution to WCDMA, since it takes risks.

(2)Combination of Upgrade and Overlap

As shown in Figure 3, the combination of upgrade and overlap, one of the solutions to the evolution from GSM/GPRS to WCDMA, is to overlap in the CS domain and upgrade in the PS domain.

The inheritance of the 2G/2.5G network has been taken into full consideration during the design of the WCDMA network, so there are no technical difficulties in the overlap of the 3G and 2G networks. The UMTS terrestrial radio access network (UTRAN) may access 3G CN to meet the need of 3G services, while the 2G base station system (BSS) may access both the 2G or 3G networks to realize related service functions.

For the CS domain in 3G, the bearer layer is ATM, which makes it impossible to upgrade from 2G to 3G. In hot areas, the traffic has already reached 800 Erl/km2 and is expected to reach 2 500-3 000 Erl/km2 in three years with the introduction of data services, but the initial design capacity for the GSM network is up to only 1 000 Erl/km2. So the capacity in the hot areas needs to be expanded. Due to completely different bearers, the CS domain cannot realize the transition from 2G to 3G. The overlap of CS domains is the right way to do it.

As far as the investment of operators is concerned, the PS domain upgrade is an available solution. Taking ZTE’s products for example, the same hardware platform is used for PS domains of both GPRS and 3G, and the network can be evolved just through the addition of some new interface hardware and the software upgrade. In this way, operators’ investment is to the utmost extent protected, and the stability of the 3G network is maintained, which is convenient for the network to transit to R4/R5.

From investors’ point of view, ZTE’s solution, which is based on the transition of the GPRS network, can ensure good performance of the network while saving investment cost in the signal network, transport network, intelligent network (IN) and network administration, power system, and HLR.

In this solution, the stability of current GPRS networks will be affected, and more investment will be needed later as the number of subscriber increases. So far, much serving GPRS support node (SGSN) equipment still employs the narrowband transmission system, which will become a bottleneck for the high-speed data interchange in the WCDMA network and the development of new services.

(3)Overlapping Existing CN

As shown in Figure 4, another way of evolution from 2G to 3G is the total overlap of the networks.

For the local CS domain network, an entire overlap is adopted. Since the long-distance call network of the WCDMA network usually functions for call transfer just like the GSM network, it is suggested that the two networks share the long-distance call network. This will have only a minor impact on the existing networks. Besides, WCDMA R99 and GSM can also share the No.7 signal network.

For the PS domain, WCDMA PS and GPRS share the same backbone network. It is required to construct new provincial SGSN and GGSN. With similar working process and protocols, WCDMA PS can also share the charging gateway (CG), domain name system (DNS) and routers of the existing GPRS.

The advantages of CN overlap solution include avoidance of the transition influence on current 2G services, ease of networking, guarantee of the stability of the existing network and freedom from the capacity limit of the 2G network.

By overlapping the CNs of 2G and 3G, this solution introduces the broadband access and new resources of radio frequencies and CN resource to separate voice and data services and promote the mobile business.

It is notable that overlapped GSM/GPRS and 3G networks keep a cooperative and complementary relationship but not a competitive relationship. In order to make full use of the 3G radio frequencies, the voice service can use the 3G network with priority when the network is idle. The overlap solution not only facilitates the step-by-step construction of the 3G network, but also clears holdback in the network evolution to the all-IP network based on R4/R5.

2.3 Proposals on Evolution Strategies

In general, operators who have 3G licenses have three choices for the construction of 3G etworks: constructing new networks, and upgrading or overlapping the existing networks. The fact is that they usually follow a combination of two or three choices.

Those who do not have 2G systems can use the latest technology to build a brand-new 3G network and add new CN and RNS network elements to provide 3G services. For these operators, ZTE proposes to directly build new CN and UTRAN of the 3G system. ZTE can provide total end-to-end WCDMA solutions and all system equipment, such as Node B, RNC, MSC/VLR, SGSN, GGSN, HLR, CG, OMC, SC, MMSC and the integrated service management platform.

What concerns operators with existing GSM networks is how to smoothly and seamlessly evolve their GSM/GPRS networks to 3G. They pay special attention to how to decrease the impact on the operation of GSM/GPRS networks caused by the introduction of 3G networks, make full use of current network resources, protect the new investment in the network evolution, ensure smooth transition to the all-IP network and keep the GSM services available on WCDMA networks.

For operators who have considered the compatibility of their GPRS networks with 3G PS and prepared their networks well for the transition to 3G via software upgrade, ZTE proposes the evolution solution of overlap in CS domain and upgrade in PS domain. However, the fact is that most existing GPRS networks lack capabilities of transiting the existing network to 3G PS only by software upgrade. ZTE, therefore, proposes the solution of total overlap. The solution is to overlap a 3G core network on the existing 2G local network and make 3G UTRAN and 2G BSS simultaneously accessible. With this solution, operators can easily evolve their networks to all-IP networks. The total overlap solution has the minimum impact on current network operation, and at the same time, it can enhance the quality of network operation and services and widen the development area for operators.

3 Summary

Whatever solutions are adopted for the evolution from GSM to WCDMA, how to improve and utilize the existing 2G equipment, such as the long distance backbone network, signal network, IN network, GW, HLR/AUC, SC and so on, has to be taken into full consideration. This will not only avoid the waste of operators’ investment, and also make it easier for operators to develop 3G services, optimize their network resources, simplify network structure and enhance network operation and management. This fully reflects the succession and protection of the WCDMA system to the GSM/GPRS network.