The Evolution of Third-generation Cellular Standards
Phillip Ames, Wireless Communications and Computing Group, Intel Corporation
John Gabor, Wireless Communications and Computing Group, Intel Corporation
developer.intel.com
THE MIGRATION TO 3G
The genesis of today's wireless technology began in the early 1980's with the introduction of the first mobile cellular handsets. These systems utilized analog interface technology and supported voice-only capabilities. This technology is still used in many parts of the world; however, it is limited in bandwidth and is low in quality. With the high demand for cell phones and the increased need for enhanced quality and more features, the Second Generation was introduced. 2G is primarily voice only, but does provide higher bandwidth, better voice quality, and limited data services that use packet data technology. 2G systems are currently in wide deployment with enhanced 2G systems currently available on the market. The 3G standardization process is coming to closure, with the recent completion of the IMT-2000 radio interface recommendations.
First-Generation Mobile Standards
The first generation of cellular wireless communications was based on analog technology and progressively became available to the consumer during the late 1970's and early 1980's. The most successful analog systems are based on the following standards, all of which are still in demand today:
Nordic Mobile Telephone (NMT) was the first commercially available analog system, introduced in Sweden and Norway in 1979.
Advanced Mobile Phone Service (AMPS) was launched in 1982. This has proven to be the most successful analog standard of all. AMPS networks are widely deployed and can be found on all continents.
Total Access Communications System (TACS) was originally specified for the United Kingdom and is based on AMPS. The original TACS specification was extended and is known as ETACS. ETACS is primarily deployed in Asia Pacific regions.
Second-Generation Mobile Standards
The second-generation (also known as 2G) introduced digital wireless standards that concentrated on improving voice quality, coverage, and capacity. The 2G standards were defined and designed to support voice and low-rate data-only Internet browsing was in its infancy during the definition stage. The world's four primary mobile digital wireless standards currently deployed around the world are GSM, TDMA (IS-136), CDMA (IS-95-B), and PDC, all supporting data rates up to 9.6kbps.
Global System for Mobile phone communications (GSM) was the first commercially available digital standard, introduced in 1992. GSM relies on circuit-switched data. The basic development of supporting data at low bit-rates ( < 9.6 kbps ) was introduced at the beginning of commercial services and has been predominantly used for e-mailing from laptop computers. [2]
Time Division Multiple Access, originally IS-54 and now IS-136 (TDMA IS-136), is sometimes referred to as the "North American" digital standard; however, it is also deployed in Latin America, Asia Pacific, and Eastern Europe.
Personal Digital Communications (PDC) is the primary digital standard in Japan.
IS-95 is based on "narrowband" (referred to as narrowband because of the limited amount of information that can flow through these networks) Code Division Multiple Access (CDMA) technology. It has become popular in South Korea and North America.
Enhanced Second-Generation Mobile Standards
Enhanced second-generation (sometimes referred to as 2.5G or 2+G) builds upon the second-generation standards by providing increased bit-rates and bringing limited data capability. Data rates range from 57.6kbps to 171.2kbps.
High-speed Circuit-switched Data (HSCSD) provides access to four channels simultaneously, theoretically providing four times the bandwidth (57.6) of a standard circuit-switched data transmission of 14.4kbps.
D-AMPS IS-136B Time Division Multiple Access (TDMA) is the intermediate step to Universal Wireless Communication (UWC-136), a third-generation standard. The first phase of D-AMPS will provide up to 64kbps. The second phase will provide up to 115kbps in a mobile environment.
General Packet Radio System (GPRS) is an evolutionary path for GSM and IS-136 TDMA to UWC-136. It is a standard from the European Telecommunications Standards Institute (ETSI) on packet data in GSM systems. The Telecommunications Industry Association (TIA), as the packet-data SDO for TDMA-136 systems, has also accepted GPRS. GPRS supports theoretical data rates up to 171.2kbps by utilizing all eight channels simultaneously. This data rate is roughly three times faster than today's fixed telecommunication networks and about ten times as fast as current circuit-switched data services on GSM networks. GPRS is a universal packet-switched data service in GSM. It involves overlaying a packet-based air interface on the existing circuit-switched GSM network. Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Using GPRS, the information is split into separate but related packets before being transmitted and subsequently reassembled at the receiving end. GPRS is a non-voice-added service that allows information to be sent and received across multiple mobile telephone networks. It supplements today's circuit-switched data and short messaging service. GPRS uses packet data technology, a fundamental change from circuit-switched technology, to transfer information. It also facilitates instant connection capability, sometimes referred to as "always connected." Immediacy is one of the key advantages of GPRS. Immediacy enables time-critical application services [5][6].
Third-Generation Mobile Standards
Third-generation systems will provide wide-area coverage at 384kbps and local area coverage up to 2Mbps. The primary motivation for the development of third-generation wireless communications is the ability to supplement standardized 2G and 2G+ services with wideband services. Essentially, this offers voice plus data capability.
The existing array of incompatible second-generation technologies, together with the restricted amount of information that can be transferred over these narrowband systems, prompted the ITU to work towards defining a new global standard for the next-generation broadband mobile telecommunication systems. Known as IMT-2000 (International Mobile Telecommunications-2000), the project was started to attain authorship of a set of globally harmonized standards for broadband mobile communications. The first set of IMT-2000 recommendations was recently approved by the ITU.
IMT-2000 is the term used by the International Telecommunications Union for this set of globally harmonized standards. The initiative was to define the goal of accessing the global telecommunication infrastructure through both satellite and terrestrial mobile systems. IMT-2000 has reflected the explosion of mobile usage and the need for future high-speed data communications, with wideband mobile submissions. IMT-2000 is a flexible standard that allows operators around the world the freedom of radio access methods and of core networks so that they can openly implement and evolve their systems. How they do it depends on regulations and market requirements.
The recent IMT-2000 recommendation highlights five distinct mobile/terrestrial radio interface standards:
IMT-MC: CDMA Multi-carrier (known as cdma2000 or IS-2000).
IMT-DS: CDMA Direct Spread (known as Wideband CMDA or WCDMA-FDD).
This standard is intended for applications in public macro-cell and micro-cell environments. The Frequency Division Duplex (FDD) mode is used for symmetrical applications, i.e., those requiring the same amount of radio resources in the uplink as in the downlink. This standard is well supported by Japan's ARIB and GSM network operators and vendors.
IMT-TC: CDMA TDD (WCDMA-TDD).
Time Division Duplex (TDD) targets public micro-cell and pico-cell environments, and, due to severe interference-related considerations, is intended primarily for indoor use. This standard is optimized for symmetrical and asymmetrical applications with high data rates.
IMT-SC: TDMA Single Carrier (known as UWC-136 and EDGE).
UWC-136 (Universal Wireless Communications) and EDGE (Enhanced Data Rates for GSM Evolution) will provide extended data services, with no changes to channel structure, frequency, or bandwidth. IMT-SC is the evolutionary path for GSM and TDMA-136, achieved by building upon enhanced versions of GSM and TDMA-136 technology. EDGE is a radio-based high-speed mobile data standard with aggregate transmission speeds of up to 384kbps when all eight timeslots are used.
IMT-FT: TDMA Multi-carrier (well known as DECT, Digital Enhanced Cordless Telecommunication).
The IMT-2000 recommendations encompass three CDMA and two TDMA radio air interface standards.
WIDEBAND CODE DIVISION MULTIPLE ACCESS (WCDMA) SHOULD NOT BE CONFUSED WITH NARROWBAND CDMA; THEY ARE COMPLETELY DIFFERENT PROTOCOLS. WCDMA is a younger technology, defined specifically to deliver high-speed data services and Internet-based packet-data at 3G data rates. WCDMA supports both packet and circuit-switched communications, such as Internet access and landline telephone services; however, WCDMA was defined with no requirements on second-generation backward compatibility.
WCDMA makes very efficient use of the available radio spectrum. No frequency planning is needed, since one-cell re-use is applied. Using techniques such as adaptive antenna arrays, hierarchical cell structures, and coherent demodulation, network capacity can be increased. In addition, circuit and packet-switched services can be combined on the same channel, allowing true multimedia services with multiple packet or circuit connections on a single terminal. WCDMA capacity is approximately double that of narrowband CDMA. The wider bandwidth and the use of both coherent demodulation and fast power control in the uplinks and the downlinks allow a lower receiver threshold. WCDMA uses a network protocol structure (signaling) similar to that of GSM; therefore, it will be able to use the existing GSM network as the core network infrastructure. [4]
In CDMA2000, a range of RF channel bandwidths are supported: 1.25, 3.75, 7.5, 11.25, and 15MHz. This range allows for support of a range of data rates as well as a high number of users.
In order to support higher bandwidth channels, CDMA2000 has defined two configuration options: Direct Spread (DS) and Multi-carrier (MC). The DS option is similar to IS-95B and uses the entire bandwidth to spread the data for radio transmissions. In the MC option, user data is encoded as a single stream and de-multiplexed into multiple streams. Each stream carries part of the user data using a different carrier frequency signal, hence the name Multi-carrier. The receiver will multiplex the received signals together before demodulation is carried out. Both the DS and MC options are available in the forward link only. The reverse link supports only the DS option. [3]
Time Division Multiple Access
One approach to reducing the number of confusing options to the end user and to improve the overall functionality of time-division cellular technology is to combine TDMA and CDMA radio air interface technology into one system. This combined approach, referred to as TD-CDMA, would retain some of the fundamental GSM-TDMA design parameters, such as frame and time-slot structure, which are key factors for interoperability and evolution. At the same time, the CDMA technology would add better interference averaging and frequency diversity. The combined approach would also merge the excellent spectral efficiency of CDMA, while retaining the robustness, planning principles, and well understood characteristics of TDMA-based GSM. [1]
In addition to the improvements of data throughput and interworking, 3G will provide an additional spectrum for the operators. The increase in 3G spectrum efficiency will also provide the operator with more throughput over limited resources. The transition from the existing 2G networks to 3G capabilities will evolve over time. Dual-mode terminals will attempt to provide seamless hand-over and roaming capabilities. |
