0. Introduction

How to smoothly evolve the world's largest GSM network (more precisely, GSM + GPRS network) into a 3G technology system is very important. This article discusses the strategy of GSM + GPRS evolution to 3G WCDMA and gives its own thinking and experience.

The development of 2G to 3G is not only the business promotion, but also the inevitable development of technology. Of course, there is a complicated process in the middle. First of all, the 2G system needs data communication services. From a single circuit switching, GSM only supports the call service to 2.5G. That is, GPRS and GSM + GPRS are divided into circuit domain (CS) and packet switched domain (PS) in the core network, so that the formed GSM + GPRS mobile communication system can not only support voice communication, but also support low-speed data Communication and short messages.

The broadband CDMA based on GSM technology, that is, WCDMA technology is very different from 2G mobile communication technology in structure, but is similar to the structure of 2.5G. In addition to structural problems, the technology used by 3G is CDMA, This is also different from the FDD TDMA standard adopted by 2G.

TDMA time division multiple access is one of the basic multiple access technologies in communication. It is mostly used in 2G (GSM) mobile communication systems. The basic concept of time division multiple access is to divide time into periodic frames, and then divide each frame. Signals are sent to the base station in several time slots, and transmission and signal reception are performed under the conditions of timing and synchronization.

CDMA code division multiple access, it is a brand-new and mature wireless communication technology developed on the branch of digital technology-spread spectrum communication technology. The principle of CDMA technology is based on spread spectrum technology, which has a certain signal bandwidth to be transmitted The information data is modulated with a high-speed pseudorandom code with a bandwidth much larger than the signal bandwidth, so that the original data signal bandwidth is expanded, and then modulated by the carrier wave and sent out. The receiving end uses the same pseudorandom code, which is the same as the received bandwidth The signal is correlated, the broadband signal is replaced with the narrowband signal of the original information data for correlation processing, and the broadband signal is replaced with the narrowband signal of the original information data to despread to achieve information communication.

GSM + GPRS and 3G are basically the same in system architecture, and are composed of user terminals, wireless network side and core network. The evolution to 3G is mainly reflected in the two parts of the wireless network side and the core network.

1. Evolution of the wireless network

The wireless network side of GSM + GPRS is particularly different from the wireless network side of WCDMA. To complete the evolution from GSM + GPRS to the 3G wireless network side, it is necessary to abandon the original system and rebuild the 3G wireless network system.

The wireless access network part of GSM + GPRS is composed of the base station BTS and the base station controller BSC. The BSC is connected to the core network MSC / VLR through the A interface, and is connected to the SGSN of the core network packet domain through the Gb interface.

The 3G WCDMA wireless network is a network built with a new spectrum efficiency technology in a new frequency band. The basic units RNC and Node B of UTRAN and the core network CN are connected through Iu and Iub interfaces, and both adopt ATM transmission.

They are completely different from the wireless interface and access method of GSM + GPRS, so the wireless access part of WCDMA will be redeployed. The WCDMA wireless network is connected to the user terminal through the air interface. RTT can achieve a high-speed access rate of 144Kbps; at a medium speed, it can achieve an access rate of 384Kbps; indoors, it can achieve an access rate of 2Mbps; Variable bit rate information transmission; one connection can simultaneously support services with different QoS requirements; meet the delay requirements of different services (from real-time voice services to best-effort data services).

Wireless side smooth transition solution:

● The original GSM + GPRS equipment upgrade network solution

For operators with GSM + GPRS networks, there are technical problems in considering solutions for upgrading based on the original GSM + GPRS wireless network. The wireless network controller (ie RNC) upgrade method is adopted as the upgraded networking solution on the wireless side.

The main difficulty in using the above upgrade method is to upgrade the BSC device to a device with RNC function, which is very difficult to implement in technology. First, the wireless network layer protocol of BSC and RNC is very different, and the difference is completely revolutionary; second, the difference between the transmission network layer and the physical layer is also very large, one is based on circuit switching, the other is based on ATM switching; third, The processing of the user plane is completely different. The BSC is basically transparent to the user plane, while the RNC has a very complicated processing of the user plane.

Based on the above analysis, there is no common equipment problem between BSC equipment and RNC equipment. The combination of the two will not have any cost savings, but will increase the complexity of the equipment, so now mainstream manufacturers basically do not consider using this upgrade method .

● Networking solution of superimposing on the original GSM + GPRS equipment

For the evolution of the wireless side network, the currently generally accepted solution is to superimpose the 3G network on the basis of the original GSM + GPRS equipment.

After the superposition method is adopted, there is no direct connection between the GSM + GPRS wireless network and the 3G wireless network. As for the connection with the core network, it is related to the evolution mode adopted by the core network.

It should be noted that although the GSM + GPRS wireless network and the 3G wireless network are not directly connected in terms of networking structure, there is no problem with the shared transmission of the Iub and Abis ports.

2. Core network side evolution solution

2.1 Three solutions for core network side evolution

Unlike the WCDMA system, which cannot evolve smoothly on the wireless side, the core network side can evolve. According to the different evolution methods of the circuit domain and the packet domain on the core network side, three solutions are proposed below.

● Full upgrade of core network

For the existing GSM + GPRS network, some communication equipment manufacturers support the evolution from the GSM + GPRS network to the 3G system, and provide comprehensive solutions. The core network upgrade solutions of these communication equipment manufacturers are based on the original GSM + GPRS core network, through the hardware update and software upgrade to achieve the evolution to WCDMA system.

The GSM + GPRS network has a certain degree of redundancy during construction, and the redundancy plays a certain role in protecting the network, avoiding blocking when the amount of network traffic increases to maintain the stability of the network. This kind of redundancy provides the possibility to upgrade and expand the core network. However, due to the rapid increase in the number of mobile users in recent years, the redundancy of the GSM + GPRS core network is already very small. Forcible upgrade and expansion will inevitably affect the stability of the network; As far as the network side is concerned, even though the upgrade can temporarily meet the needs of the networking, in the future, the network needs to build a 3G core network in the process of evolution to all-IP. From the perspective of network development, the upgrade cannot be an operator Network construction saves costs.

From the perspective of the operator, the stability and reliability of the system and the price-performance ratio of the equipment are the most worthy of concern. The construction of the core network only accounts for about 20% of the entire construction cost of the mobile network. The core network upgrade does not save operators much cost, but it greatly reduces the network performance and lowers the performance-price ratio of the equipment. The system brought by this upgrade is undoubtedly extremely unstable. From a strategic point of view, operators can only win users in fierce competition in the future if they build a high-quality, high-quality network. It is undoubtedly not the first and best solution to upgrade the WCDMA system.

● Overlay and upgrade combination network construction

One way to evolve from the GSM + GPRS network to the WCDMA network is to superimpose and upgrade the combined network construction method. This network construction scheme is mainly a group in which the circuit domain of the original GSM + GPRS network is superimposed and the group domain is upgraded. Network mode.

From a technical point of view, WCDMA network design fully considers the inheritance of the network, and there is no technical barrier to the 3G system superimposed on the core network of GSM + GPRS. The UTRAN of the 3G system can access the core network of the 3G system to meet the needs of the 3G service; the BSS of the GSM + GPRS system can access the existing network or the 3G network at the same time to achieve related business functions.

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July 03, 2020