ITU IMT-2000

Recently, the International Telecommunication Union (ITU) has advanced the concept of Mobile Telecommunication in the year 2000 (IMT-2000) as its generalized view of future wireless/nomadic networks. To support high-speed data rates, and to support multimedia services, the ITU — Radio Communication Sector (ITU-R) undertook the task of defining a set of recommendations for IMT-2000, which is outlined in Figure 9-6.

Figure 9-6: IMT-2000: ITU’s concept for future networks

IMT-2000 is “the ITU globally coordinated definition of 3G covering key issues such as frequency spectrum use and technical standards.” Some of the basic desiderata for IMT-2000 are as follows:

• Speeds of 2 Mbps (indoors) and 144 Kbps (outdoors) or better

• Circuit- and packet-switched (IP) services

• Good voice quality comparable with wire-line quality (with Mean Opinion Score [MOS] of 4.0 or thereabouts)

• Increased capacity and improved spectrum efficiency

• Global roaming between different operational environments

1G mobile telecommunication systems that were introduced in the 1980s were analog. These systems, which are still in use, do not intrinsically have data transport capabilities. To provide data services in these analog systems, a capability such as Cellular Digital Packet Data (CDPD) has to be added to the analog. Naturally, this arrangement supports only slow-speed data. 2G systems (IS-136, cdmaOne, and Global System for Mobile Communications [GSM]) are digital and have intrinsic data transport capabilities. However, the data support is still limited: GSM supports short messaging services and data at rates only up to 9.6 Kbps, and IS-95B provides data rates in the neighborhood of 64 to 115 Kbps in increments of 8 Kbps over a 1.25 MHz channel. 2.5G General Packet Radio Service (GPRS) was developed in the late 1990s as a further enhancement: in GPRS, each slot can handle up to 20 Kbps and since each user may be allocated up to 8 slots, data rates up to about 160 Kbps per user are therefore achievable. Yet none of these break the 256 Kbps barrier. The goal of 3G is to do just that.

IMT-2000 includes a variety standards, the most significant being: (1) IMT-DS (Direct Spread) (Wideband Code Division Multiple Access [W-CMDA]), (2) IMT-MC (Multicarrier) (CDMA2000), (3) IMT-TC (Time Code) (Time Division Synchronous Code Division Multiple Access [TD-SCDMA]), (4) IMT-SC (Single Carrier): UWC-136/Enhanced Data GSM Environment (EDGE), and (5) IMT-FT (Frequency Time) (Digital European Cordless Telephone [DECT]).

3G is not a technology per se but a term encompassing all aspects of future wireless networks. 3G adjoins high-speed radio access and IP-based services to enable subscribers to be “always on,” that is, “always be online.” This new technology is created to support a large numbers of users within one network. The 3G standards support different types of user traffic:

• Constant-bit-rate traffic, such as high-quality audio speech, video telephony, and video, that need QoS since they are sensitive to delays and delay variation.

• Real-time, variable-bit-rate traffic, such as variable-bit-rate audio, Moving Pictures Expert Group (MPEG)/International Organization for Standardization (ISO) video, and so on. This type of traffic also requires QoS, being sensitive to delays and delay variation.

• Non-real-time, variable-bit-rate traffic that can tolerate delays or delay variations.

Increased speeds may not be achieved in one shot in 3G: the data rate supported may initially be only 144 Kbps, it may be 384 Kbps in the second phase, and it may reach 2.048 Mbps in the final phase; phases are designed to be backwards compatible.

A number of national standards bodies have developed next-generation wireless standards. Eventually, four systems for 3G mobile communications materialized: CDMA2000, W-CDMA Universal Mobile Telecommunications System (UMTS) Frequency Division Duplex (FDD), W-CDMA UMTS Time Division Duplexing (TDD), and UWC-136. Recommendations on these systems were published by ITU-R as a harmonized standard with four modes in 1999. CDMA2000 must comply with Electronics Industry Association (EIA)/Telecommunications Industry Association (TIA) IS-41, and W-CDMA UMTS must comply with GSM Manufacturing Automation Protocol (MAP) intersystem networking standards. UWC-136 is based on Time Division Multiple Access (TDMA), while the other three use direct-sequence code division multiple access (DS-CDMA).

CDMA2000 is a multicarrier, direct-sequence CDMA FDD system with a single carrier that has a bandwidth of 1.25 MHz (later on it may have as many as three carriers.) CDMA2000 is an evolution of the existing North American CDMA system cdmaOne (IS-95 standards). CDMA2000 supports packet mode data services. UMTS W-CDMA FDD is a direct-sequence CDMA system with a nominal bandwidth of 5 MHz. UMTS W-CDMA TDD also uses CDMA with a bandwidth of 5 MHz; however, the frequency band is time-shared in both directions (half the time it is used for transmission in the forward direction, and the other half it’s used in the reverse direction.)

The TDMA version of the 3G system for use in North America is known as UWC-136. UWC-136 is a TDMA scheme where each physical channel is partitioned into a number of fixed time slots; each user is assigned one or more slots. The UWC-136 system is planned to be introduced in three stages:

• IS-136+ with a bandwidth of 30 kHz (provides voice and up to 64 Kbps of data)

• IS-136 HS (vehicular/outdoor) with a bandwidth of 200 kHz (provides data rates up to 384 Kbps for outdoor/vehicular operations)

• IS-136 HS (indoor) with a bandwidth of 1.6 MHz (users can get a data rate of up to 2 Mbps)

IS-136+ is an enhancement of the existing IS-136 and uses improved modulation techniques. IS-136 HS (vehicular/outdoor) supports data rates up to 384 Kbps and has parameters similar to those of EDGE. IS-136 HS (indoor) provides data rates up to about 2 Mbps.^[2]

All the various technologies build on a so-called core network. The technologies in the GSM evolution are based on a so-called GSM MAP core network and the others on the IS-41 core network. With the new IMT-2000 standards, all radio options (such as CDMA) should work on all core networks (such as GSM MAP — currently CDMA does not work over GSM MAP). This might make later changes in the network radio interfaces easier.^[3][4]

As we have seen, several evolutionary paths to 3G exist. GSM operators can enhance their networks with GPRS and later deploy EDGE, which is already defined as a 3G technology by the IMT-2000. These networks will then evolve to future 3G networks based on W-CDMA, the standard technology for the UTMS band. TDMA operators can either switch to GSM and continue with that approach or go on to CDMA2000. The Japanese Personal Digital Cellular (PDC) standard will evolve to W-CDMA.^[3] Specifications for 3G technologies are maintained by the Third Generation Partnership Project (3GPP) and the IMT-2000.

Several prototypes showing possible 3G terminal designs have appeared, but the first commercial introduction of 3G terminals has yet to take place. One important aspect is that terminals need to be dual-mode (3G and 2G) to enable efficient network usage.

^[2]Dr. Al Javed, CTO, Wireless Internet, Nortel Networks.

^[3]M.R. Karim, and Mohsen Sarraf, W-CDMA and cdma2000 for 3G Mobile Network, New York: McGraw-Hill, 2002.

^[4]www.imt-2000.org

^[3]M.R. Karim, and Mohsen Sarraf, W-CDMA and cdma2000 for 3G Mobile Network, New York: McGraw-Hill, 2002.