Wireless transmission technology in IMT2000
On the basis of comparing the characteristics of various RTTs and combining with the needs of China's mobile communications development, it puts forward the views on the development of China's third-generation mobile communications based on the interests of the country, operators, national communications manufacturing industry and the majority of users: first develop And use the CDMA TDD technology and products formed based on the TD-SCDMA RTT proposal proposed by China to provide third-generation mobile communication services based on the GSM core network in large and medium-sized cities. Then, consider the issues of building an all-IP core network and using FDD technology and systems.
Abstract: On the basis of a comparison of the characterisTIcs of different RTTs and taking into account the needs of the development of China's mobile communicaTIons and the interests of the country, its operators and its users, the author expresses the following view concerning the development of 3G in the country: First to develop and use CDMA TDD technologies and products developed on the basis of China's proposal on TD-SCDMA RTT and provide 3G services based on the GSM core network in large- and medium-sized ciTIes, and then to consider building a full IP core network and using FDD technology and systems.
Keywords: wireless transmission technology (RTT); third-generation mobile communications; core network
Key words: radio transmission technology (RTT); 3G; core network
1 Introduction
At the ITU-R plenary meeting in May 2000, the standard for wireless transmission technology (RTT) of the third generation mobile communication was adopted [1] -ITU recommendation ITU-R M.1457 This standard is only the first preliminary version. In the future, it will be improved and updated every year under the organization of the ITU-R WP8F Working Group. For the mainstream technology CDMA, actual work will be carried out in two international standardization organizations: 3GPP completes the standards of CDMA-DS (WCDMA) and CDMA TDD; In 3GPP, the main content of work in 2000 is to complete the core network standard of all IP and complete the integration of CDMA TDD standard (IntegraTIon). For the above two contents, in the first half of 2001, 3GPP will adopt a new standard version: R'00. The ITU-R WP8F will complete the new version of ITU-R M.1457, the third-generation mobile communication standard officially used.
China now has the world's largest GSM mobile communication network, and it is still increasing at a rate of more than 2 million users per month. Next year, it will become the country with the largest number of mobile communication users in the world. In major cities, the spectrum resources of the GSM system are about to dry up, mobile data services are exponentially increasing, wireless Internet access has risen, and operators are hoping to obtain third-generation mobile communications licenses and provide third-generation as early as possible Mobile communication business, in order to meet market demand while occupying a favorable position in market competition.
China's mobile communications manufacturing industry also regards the third-generation mobile communications as a rare opportunity in 10 years, hoping to completely change the disadvantageous situation of foreign companies monopolizing the Chinese market in the third-generation mobile communications industry. Each company started with three CDMA technologies for third-generation mobile communications, and made technical preparations and product developments. From 2001, the manufacturing industry will cooperate with the requirements of operators to carry out field trials of third-generation mobile communications in China. In this article, we will give a brief introduction to the situation of the third-generation mobile communication standard, explain the characteristics of the TDD mode and the TD-SCDMA standard and technology. view.
2 Several issues of the third generation mobile communication standard
2.1 Core network problems
Among the standards of the third-generation mobile communication, the core network standard is a relatively slow part. The core network standards (see Figure 1) established in 3's R'99 since the standards began in the mid-1990s are all based on circuit-switched networks. In packet switching, they are based on ATM technology. This was the technological development at that time. Determined by history. As shown in Figure 1, the wireless access network accesses circuit-switched services through Iuc and packet-switched services through Iup, and the core network itself is an expansion and upgrade based on the GSM MAP core network (with GPRS function). The requirements for the third-generation mobile communications are obviously far-reaching, and are also a transitional standard version. I am afraid that this kind of core network is only used by Japan in order to rush to open the WCDMA system.
But at the same time, due to the progress of IP technology, many companies have strongly proposed the concept of an all-IP core network since 1999, which has been recognized by the International Organization for Standardization and will be standardized in R'00.
2.2 Technology after the third generation
At the first meeting of ITU WP8F (March 2000), it took into account the development of new technologies and raised the issue of wireless transmission technology after the third generation. Currently, two main technologies are considered: Smart / adaptive Antenna and Software Defined Radio. Countries are required to make suggestions on the application of these two technologies in IMT-2000.
In the IMT-2000 RTT, only the TD-SCDMA standard recommended by China clearly proposes the use of smart antennas, and the physical layer technology is designed with smart antennas. ITU and many representatives hope to use this technology in FDD systems to comprehensively improve the performance and capacity of the third generation mobile communication system. The software radio is a device, mainly the implementation of user terminal equipment. Don't develop a dedicated chip for each mode, but based on a common hardware platform, implemented in software. In the future, multimode terminals must be implemented using software radio technology.
2.3 ITU's strategy for improving and improving standards
As a generation of mobile communication standards, I hope that this standard can be used until 2020 or even 2025, and will inevitably encounter a contradiction between the relative stability of the standard and the continuous emergence of new technologies. In order to solve this contradiction, the standards of the third generation mobile communication have a longer life. Within the ITU and the International Organization for Standardization, they are handled in the following ways:
· Continuous research on new technologies, leaving enough time and space for discussion and research on various new technologies and possible development directions;
· The revision and update of the standard version is basically once a year to ensure the relative stability of the standard;
· Minor modifications can be made every year and approved by ITU-R WP8F. This modification must be forward compatible to ensure the interests of users;
· Major revisions are proposed by standardization organizations, discussed in ITU-R WP8F, and adopted by ITU-R conference;
· Major revisions, that is, completely new technologies, must be approved by the ITU-R Conference, proposed by various standardization organizations and ITU-R WP8F, and then approved by the ITU-R Conference.
Obviously, with the progress of technology in the future, the third generation mobile communication standard will be constantly updated and improved.
3 TDD and TD-SCDMA
In the process of formulating the third-generation mobile communication standard, the TDD duplex mode in wireless communication was given high priority internationally for the first time, and the TDD standard was formulated in both CDMA and TDMA systems. Operators in many countries have expressed their desire to choose the TDD system first. The main reason is that under the premise of also meeting the requirements of IMT2000, the TDD system has the following characteristics:
· For the third-generation mobile communications, countries need about 400MHz of spectrum resources. Below 3GHz, frequency resources are very difficult. TDD can use various frequency resources without the need for paired frequencies;
· Among the third-generation mobile communications services, data services will dominate; and among data services, asymmetric IP-based data services are the most important. The TDD mode is particularly suitable for services with asymmetric uplink and downlink and different data transmission rates;
· TDD uplink and downlink work at the same frequency, symmetrical radio wave propagation characteristics make it easy to use new technologies such as smart antennas, to achieve the purpose of improving performance and reducing costs
· The cost of TDD system equipment is lower, which may be 20% ~ 50% lower than FDD system.
Traditionally, the communication industry generally believes that the main problems of the TDD system are in the terminal's moving speed and coverage distance. Because the TDD system uses a multi-slot discontinuous transmission method, the ability to resist fast fading and the Doppler effect is better than continuous transmission. The FDD mode is poor; on the other hand, the ratio of average power and peak power in the TDD system increases with the increase of the number of time slots. Considering power consumption and cost factors, the transmission power of the user terminal cannot be very large, so the communication distance ( Cell radius) is relatively small. For example, the cell radius of the FDD system may reach tens of kilometers and the TDD system generally does not exceed ten kilometers. In this article, we will explain that the above two problems have been basically solved in the TD-SCDMA system.
3.1 The problem of moving speed
The main limitation of the moving speed is the frequency shift and fast fading generated by the Doppler effect, and they all need to be overcome by baseband digital signal processing technology. In the TD-SCDMA system, the baseband digital signal processing technology is based on smart antennas and joint detection, which is limited to the contradiction between the device's baseband digital signal processing capability and algorithm complexity. Our recent achievements are: under the mobile speed of 250km / h and UMTS (3GPP) mobile environment, when all code channels of the TD-SCDMA system are used, the link simulation results show that the system can be supported. This result is comparable to the WCDMA system, proving that TD-SCDMA can also work in high-speed mobile conditions.
3.2 The problem of communication distance
The communication distance is explained by link estimation. Under the same transmission power, the same fading environment and the same receiving sensitivity, different CDMA systems should have substantially the same communication distance. In this regard, there is no significant difference between TD-SCDMA and WCDMA.
Another special requirement of the TDD system is that there must be a guard time slot between upstream and downstream, which is reserved for long-distance user terminals to achieve upstream synchronization. At present, the protection time slot of the TD-SCDMA system can provide a communication distance of 12 km. However, when a base station with a communication radius of more than 10 km is required, the network management system settings can be improved. If one less uplink slot is used, the cell radius will be reduced. Only depends on the transmission power, and the same as FDD system. Although one less uplink is used, the system capacity is reduced by 14%, but it is still more than 50% higher than other systems.
The above shows that the traditional argument that FDD is the mainstream in mobile communications has been challenged, and the position of the TDD system in the third generation of mobile communications has been unshakable. The prospect of the third generation mobile communication network is a common network (global IP network). The satellite mobile communication system is used to complete global seamless coverage. FDD or TDD systems can be used to build national and international mobile communication networks. TDD systems It is used to provide high-density and high-capacity voice, data, and multimedia services in areas with concentrated urban population. It has obvious advantages. Dual-mode or even multi-mode user terminals are used to achieve global roaming.
As you know, in CDMA TDD mode, there are two options for different chip rates: UTRA TDD and TD-SCDMA. These two standards are the same in high-level signaling and some physical layer technologies. However, the design starting points of these two technologies are different, and the technologies used are also different, resulting in different main characteristics and uses. By comparison, TD-SCDMA is more prominent in the aforementioned advantages of TDD, and has a much higher spectrum utilization rate than other systems. More importantly: UTRA TDD is a supplement to the WCDMA (FDD) system, is a system used in the indoor environment to provide data and multimedia; and TD-SCDMA is designed based on all ITU requirements for IMT-2000 It solves the problems of TDD such as moving speed and cell radius. It can form a complete cellular network itself.
4 TD-SCDMA is the technology of choice for the evolution from the second generation to the third generation
According to most predictions at home and abroad, in the first decade of the 21st century or so, the market development of the third generation mobile communication may be divided into at least the following three stages:
· Early period, 2001 ~ 2005. Its characteristics are: the second generation mobile communication network (mainly GSM and IS-95CDMA) continues to develop and expand, and on the basis of the second generation mobile communication network, the third generation mobile is provided in local areas (city and other user concentrated areas) For communication services, the data service rate is limited to 384 kbit / s and below, and regional or international roaming depends on the second-generation mobile communication system;
· Mid-term, 2004 ~ 2010. It is the high-speed growth period of the third-generation mobile communication system. Its characteristics are: the development of the second-generation mobile communication network and system has ceased, the construction of the third-generation mobile communication network with successful national or global coverage has been achieved, and it fully meets the requirements of IMT2000.
· Later, after 2010, more than 25% of the world's population used third-generation mobile communication systems, and fourth-generation mobile communication devices began to enter the market to provide higher-rate multimedia services.
This market development forecast is also the basis for formulating the evolution strategy of mobile communications from the second generation to the third generation. The so-called evolution is to consider the above initial situation and formulate countermeasures.
The evolution process proposed in this article can be divided into the following two stages:
The first stage is to provide third-generation mobile communication services in the second-generation network. For example, for the expansion of the existing GSM network, the BSS (Base Station Subsystem) of TD-SCDMA is used. At the same time, in the area where users are concentrated, TD-SCDMA base stations are added to the site of the existing GSM base station. Using TD-SCDMA / GSM dual-band dual-mode user terminals, these initial 3G users can enjoy 3G services within the coverage area of ​​TD-SCDMA base stations. Outside the coverage area, GSM works. Obviously, when users of this initial system enjoy 3G services, they can only implement handover between TD-SCDMA base stations of the same BSS, and the functions of the GSM network will not be affected.
In this way, you can use a lower investment than simply expanding the network capacity of the GSM system, which not only expands the capacity, but also provides users with mobile communication services in areas where users are dense, and solves the capacity problem caused by insufficient spectrum resources. Moreover, it also provides third-generation mobile communication services for the most urgently needed areas. At the same time, it also laid the foundation for the future transition to the third generation.
At this step, the coverage of third-generation mobile communications may gradually reach the city level. There will be dozens to hundreds of third-generation BTSs in large cities, and the system will support automatic handoff between different BSSs. Of course, national and global coverage has not yet been achieved, and automatic roaming still depends on the GSM system.
The second stage is the transition to the third generation mobile communication network. By 2004 ~ 2005, the third-generation mobile communication will enter a high-speed growth period, and all countries and operators will begin to build a complete third-generation mobile communication network. I think this network will be based on all IP. At this time, the all-IP core network may be entered by the RNC through the Iu interface, or it may be directly entered by the base station (Node B) through the Iub interface. For the equipment used in the first stage, the only part that needs to be updated is the BSS software upgrade to meet the requirements of the all-IP core network. The largest part of the investment in the network: BTS has been built in the first phase, only the interface software upgrade, without increasing the hardware investment. At this time, there will be not only TDD base stations but also FDD base stations in the network. This 3G network will be a complete network with national coverage and international roaming. The process of evolution or transition from the second generation to the third generation will be completed in the above process without substantially increasing new investment.
5 Summary
Based on the above analysis, it is not difficult to get the following conclusions:
(1) The third generation mobile communication standard (IMT_RSPC) is the result of more than ten years of cooperation and hard work in various countries around the world, and is a reflection of the development of modern wireless communication technology. Moreover, this standard is also in the process of being gradually improved and updated.
(2) In R'00 completed by 3GPP by mid-2001, the most important thing is to complete the standard integration of the all-IP core network and CDMA TDD mode. In this way, when the construction of the third-generation mobile communication network begins in 2002-2003, the all-IP network can be reached in one step; there is only one TDD standard among the mainstream CDMA standards.
(3) In the third-generation mobile communication technology, the advantages of the TDD mode wireless transmission technology in terms of spectrum flexibility, providing asymmetric services and low prices have been recognized internationally. The advantages in providing mobile data services, especially IP-based services, make the TDD mode system highly competitive in cities and suburbs.
(4) The TD-SCDMA technology proposed by China was originally designed as a complete system, and advanced technologies such as smart antennas were adopted, which is recognized as having obvious advantages in technology. Moreover, it can be used in a high-speed mobile environment and may increase the cell radius by increasing the transmission power, which basically overcomes the shortcomings of the TDD mode. The equipment developed according to this standard can achieve the goal of providing high spectrum utilization, flexibility and low cost, and will have strong competitiveness in the market.
(5) In the early part of this century (2001 ~ 2004), in the second-generation mobile communication network where users are dense or areas requiring data and multimedia services (mainly large and medium-sized cities), the second-generation mobile communication network opened The third-generation mobile communication service is mainly based on the TD-SCDMA system. The use of dual-band dual-mode terminals will receive huge economic benefits and is a better way to smoothly transition to the third-generation mobile communication.
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