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Introduction; Technologies for 3G evolution; HSPA; LTE and SAE; Performance and concluding remarks
From the first experiments with radio communication by Guglielmo Marconi in the 1890s, the road to truly mobile radio communication has been quite long. To understand the complex 3G mobile-communication systems of today, it is also important to understand where they came from and how cellular systems have evolved from an expensive technology for a few selected individuals to today's global mobile-communication systems used by almost half of the world's population. Developing mobile technologies has also changed, from being a national or regional concern, to becoming a very complex task undertaken by global standards-developing organizations such as the Third Generation Partnership Project (3GPP) and involving thousands of people.
1.1 History and background of 3G
The cellular technologies specified by 3GPP are the most widely deployed in the world, with more than 2.6 billion users in 2008. The latest step being studied and developed in 3GPP is an evolution of 3G into an evolved radio access referred to as the Long-Term Evolution (LTE) and an evolved packet access core network in the System Architecture Evolution (SAE). By 2009-2010, LTE and SAE are expected to be first deployed.
Looking back to when it all it started, it begun several decades ago with early deployments of analog cellular services.
1.1.1 Before 3G
The US Federal Communications Commission (FCC) approved the first commercial car-borne telephony service in 1946, operated by AT&T. In 1947 AT&T also introduced the cellular concept of reusing radio frequencies, which became fundamental to all subsequent mobile-communication systems. Commercial mobile telephony continued to be car-borne for many years because of bulky and power-hungry equipment. In spite of the limitations of the service, there were systems deployed in many countries during the 1950s and 1960s, but the users counted only in thousands at the most.
These first steps on the road of mobile communication were taken by the monopoly telephone administrations and wire-line operators. The big uptake of subscribers and usage came when mobile communication became an international concern and the industry was invited into the process. The first international mobile communication system was the analog NMT system (Nordic Mobile Telephony) which was introduced in the Nordic countries in 1981, at the same time as analog AMPS (Advanced Mobile Phone Service) was introduced in North America. Other analog cellular technologies deployed worldwide were TACS and J-TACS. They all had in common that equipment was still bulky, mainly car-borne, and voice quality was often inconsistent, with 'cross-talk' between users being a common problem.
With an international system such as NMT came the concept of 'roaming,' giving a service also for users traveling outside the area of their 'home' operator. This also gave a larger market for the mobile phones, attracting more companies into the mobile communication business.
The analog cellular systems supported 'plain old telephony services,' that is voice with some related supplementary services. With the advent of digital communication during the 1980s, the opportunity to develop a second generation of mobile-communication standards and systems, based on digital technology, surfaced. With digital technology came an opportunity to increase the capacity of the systems, to give a more consistent quality of the service, and to develop much more attractive truly mobile devices.
In Europe, the telecommunication administrations in CEPT initiated the GSM project to develop a pan-European mobile-telephony system. The GSM activities were in 1989 continued within the newly formed European Telecommunication Standards Institute (ETSI). After evaluations of TDMA, CDMA, and FDMA-based proposals in the mid-1980s, the final GSM standard was built on TDMA. Development of a digital cellular standard was simultaneously done by TIA in the USA resulting in the TDMA-based IS-54 standard, later simply referred to as US-TDMA. A somewhat later development of a CDMA standard called IS95 was completed by TIA in 1993. In Japan, a second-generation TDMA standard was also developed, usually referred to as PDC.
All these standards were 'narrowband' in the sense that they targeted 'low-bandwidth' services such as voice. With the second-generation digital mobile communications came also the opportunity to provide data services over the mobile-communication networks. The primary data services introduced in 2G were text messaging (SMS) and circuit-switched data services enabling e-mail and other data applications. The peak data rates in 2G were initially 9.6 kbps. Higher data rates were introduced later in evolved 2G systems by assigning multiple time slots to a user and by modified coding schemes.
Packet data over cellular systems became a reality during the second half of the 1990s, with General Packet Radio Services (GPRS) introduced in GSM and packet data also added to other cellular technologies such as the Japanese PDC standard. These technologies are often referred to as 2.5G. The success of the wireless data service iMode in Japan gave a very clear indication of the potential for applications over packet data in mobile systems, in spite of the fairly low data rates supported at the time.
With the advent of 3G and the higher-bandwidth radio interface of UTRA (Universal Terrestrial Radio Access) came possibilities for a range of new services that were only hinted at with 2G and 2.5G. The 3G radio access development is today handled in 3GPP. However, the initial steps for 3G were taken in the early 1990s, long before 3GPP was formed.
What also set the stage for 3G was the internationalization of cellular standardization. GSM was a pan-European project, but quickly attracted worldwide interest when the GSM standard was deployed in a number of countries outside Europe. There are today only three countries worldwide where GSM is not deployed. A global standard gains in economy of scale, since the market for products becomes larger. This has driven a much tighter international cooperation around 3G cellular technologies than for the earlier generations.
1.1.2 Early 3G discussions
Work on a third-generation mobile communication started in ITU (International Telecommunication Union) in the 1980s. The radio communication sector ITU-R issued a first recommendation defining Future Public Land Mobile Telecommunications Systems (FPLMTS) in 1990, later revised in 1997 . The name for 3G within ITU had by then changed from FPLMTS to IMT-2000. The World Administrative Radio Congress WARC-92 identified 230 MHz of spectrum for IMT-2000 on a worldwide basis. Of these 230 MHz, 2 x 60 MHz were identified as paired spectrum for FDD (Frequency Division Duplex) and 35 MHz as unpaired spectrum for TDD (Time Division Duplex), both for terrestrial use. Some spectrum was also set aside for satellite services. With that, the stage was set to specify IMT-2000.
Task Group 8/1 within ITU-R developed a range of recommendations for IMT-2000, defining a framework for services, network architectures, radio interface requirements, spectrum considerations, and evaluation methodology. Both a terrestrial and a satellite component were defined.
Task Group 8/1 defined the process for evaluating IMT-2000 technologies in ITU-R recommendation M.1225 . The evaluation criteria set the target data rates for the 3G circuit-switched and packet-switched data services:
Up to 2 Mbps in an indoor environment.
Up to 144 kbps in a pedestrian environment.
Up to 64 kbps in a vehicular environment.
These numbers became the benchmark that all 3G technologies were compared with. However, already today, data rates well beyond 2 Mbps can be seen in deployed 3G systems.
1.1.3 Research on 3G
In parallel with the widespread deployment and evolution of 2G mobile-communication systems during the 1990s, substantial efforts were put into 3G research activities. In Europe the partially EU-funded project Research into Advanced Communications in Europe (RACE) carried out initial 3G research in its first phase. 3G in Europe was named Universal Mobile Telecommunications Services (UMTS). In the second phase of RACE, the CODIT project (Code Division Test bed) and the ATDMA project (Advanced TDMA Mobile Access) further developed 3G concepts based on Wideband CDMA (WCDMA) and Wideband TDMA technologies. The next phase of related European research was Advanced Communication Technologies and Services (ACTS), which included the UMTS-related project Future Radio Wideband Multiple Access System (FRAMES). The FRAMES project resulted in a multiple access concept that included both Wideband CDMA and Wideband TDMA components.
At the same time parallel 3G activities were going on in other parts of the world. In Japan, the Association of Radio Industries and Businesses (ARIB) was in the process of defining a 3G wireless communication technology based on Wideband CDMA. Also in the US, a Wideband CDMA concept called WIMS was developed within the T1.P1 committee. Also Korea started work on Wideband CDMA at this time.
The FRAMES concept was submitted to the standardization activities for 3G in ETSI, where other multiple access proposals were also introduced by the industry, including the Wideband CDMA concept from the ARIB standardization in Japan. The ETSI proposals were merged into five concept groups, which also meant that the Wideband CDMA proposals from Europe and Japan were merged.
1.1.4 3G standardization starts
The outcome of the ETSI process in early 1998 was the selection of Wideband CDMA (WCDMA) as the technology for UMTS in the paired spectrum (FDD) and TD-CDMA (Time Division CDMA) for the unpaired spectrum (TDD). There was also a decision to harmonize the parameters between the FDD and the TDD components.
The standardization of WCDMA went on in parallel in ETSI and ARIB until the end of 1998 when the Third Generation Partnership Project (3GPP) was formed by standards-developing organizations from all regions of the world. This solved the problem of trying to maintain parallel development of aligned specifications in multiple regions. The present organizational partners of 3GPP are ARIB (Japan), CCSA (China), ETSI (Europe), ATIS (USA), TTA (Korea) and TTC (Japan).
1.2.1 The standardization process
Setting a standard for mobile communication is not a one-time job, it is an ongoing process. The standardization forums are constantly evolving their standards trying to meet new demands for services and features. The standardization process is different in the different forums, but typically includes the four phases illustrated in Figure 1.1 :
1. Requirements, where it is decided what is to be achieved by the standard.
2. Architecture, where the main building blocks and interfaces are decided.
3. Detailed specifications, where every interface is specified in detail.
4. Testing and verification, where the interface specifications are proven to work with real-life equipment.
These phases are overlapping and iterative. As an example, requirements can be added, changed, or dropped during the later phases if the technical solutions call for it. Likewise, the technical solution in the detailed specifications can change due to problems found in the testing and verification phase.
Standardization starts with the requirements phase, where the standards body decides what should be achieved with the standard. This phase is usually relatively short.
In the architecture phase, the standards body decides about the architecture, i.e. the principles of how to meet the requirements. The architecture phase includes decisions about reference points and interfaces to be standardized. This phase is usually quite long and may change the requirements.
After the architecture phase, the detailed specification phase starts. It is in this phase the details for each of the identified interfaces are specified. During the detailed specification of the interfaces, the standards body may find that it has to change decisions done either in the architecture or even in the requirements phases.
Finally, the testing and verification phase starts. It is usually not a part of the actual standardization in the standards bodies, but takes place in parallel through testing by vendors and interoperability testing between vendors. This phase is the final proof of the standard. During the testing and verification phase, errors in the standard may still be found and those errors may change decisions in the detailed standard. Albeit not common, changes may need to be done also to the architecture or the requirements. To verify the standard, products are needed. Hence, the implementation of the products starts after (or during) the detailed specification phase. The testing and verification phase ends when there are stable test specifications that can be used to verify that the equipment is fulfilling the standard.
Normally, it takes one to two years from the time when the standard is completed until commercial products are out on the market. However, if the standard is built from scratch, it may take longer time since there are no stable components to build from.
The Third-Generation Partnership Project (3GPP) is the standards-developing body that specifies the 3G UTRA and GSM systems. 3GPP is a partnership project formed by the standards bodies ETSI, ARIB, TTC, TTA, CCSA and ATIS. 3GPP consists of several Technical Specifications Groups (TSGs), (see Figure 1.2).
A parallel partnership project called 3GPP2 was formed in 1999. It also develops 3G specifications, but for cdma2000, which is the 3G technology developed from the 2G CDMA-based standard IS-95. It is also a global project, and the organizational partners are ARIB, CCSA, TIA, TTA and TTC.
3GPP TSG RAN is the technical specification group that has developed WCDMA, its evolution HSPA, as well as LTE, and is in the forefront of the technology. TSG RAN consists of five working groups (WGs):
1. RAN WG1 dealing with the physical layer specifications.
2. RAN WG2 dealing with the layer 2 and layer 3 radio interface specifications.
3. RAN WG3 dealing with the fixed RAN interfaces, for example interfaces between nodes in the RAN, but also the interface between the RAN and the core network.
4. RAN WG4 dealing with the radio frequency (RF) and radio resource management (RRM) performance requirements.
5. RAN WG 5 dealing with the terminal conformance testing.
The scope of 3GPP when it was formed in 1998 was to produce global specifications for a 3G mobile system based on an evolved GSM core network, including the WCDMA-based radio access of the UTRA FDD and the TD-CDMA-based radio access of the UTRA TDD mode. The task to maintain and develop the GSM/EDGE specifications was added to 3GPP at a later stage. The UTRA (and GSM/EDGE) specifications are developed, maintained and approved in 3GPP. After approval, the organizational partners transpose them into appropriate deliverables as standards in each region.
In parallel with the initial 3GPP work, a 3G system based on TD-SCDMA was developed in China. TD-SCDMA was eventually merged into Release 4 of the 3GPP specifications as an additional TDD mode.
The work in 3GPP is carried out with relevant ITU recommendations in mind and the result of the work is also submitted to ITU. The organizational partners are obliged to identify regional requirements that may lead to options in the standard. Examples are regional frequency bands and special protection requirements local to a region. The specifications are developed with global roaming and circulation of terminals in mind. This implies that many regional requirements in essence will be global requirements for all terminals, since a roaming terminal has to meet the strictest of all regional requirements. Regional options in the specifications are thus more common for base stations than for terminals.
The specifications of all releases can be updated after each set of TSG meetings, which occur 4 times a year. The 3GPP documents are divided into releases, where each release has a set of features added compared to the previous release. The features are defined in Work Items agreed and undertaken by the TSGs. The releases up to Release 8 and some main features of those are shown in Figure 1.3 . The date shown for each release is the day the content of the release was frozen. For historical reasons, the first release is numbered by the year it was frozen (1999), while the following releases are numbered 4, 5, etc.
Excerpted from 3G Evolution by Erik Dahlman Stefan Parkvall Johan Sköld Per Beming Copyright © 2008 by Erik Dahlman, Stefan Parkvall, Johan Sköld and Per Beming. Excerpted by permission.
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