IMT 2000 has following key characteristics:-

• IMT 2000 includes different environments, such as Indoor use, Vehicles, Satellite and Pedestrians.
• It offers value added services and applications on the basis of single standard globally over a number of media fixed and satellite.
• It provides a platform for fixed, mobile, voice, data, internet, multimedia and converged services over one connection.
• It provides seamless service delivery, global roaming, enables the users to move across boundaries with the same number and hand set.

• In IMT 2000, three different data rates are 144 Kbps, 384 Kbps and 2000 Kbps. Note that the data transmission rates are dependent upon the environment in which a call is being made.

  (i) 144 Kbps is the absolute minimum acceptable transmission data rates for mobile users with high speed

  (ii) 384 Kbps is the ideal achievable data rate for pedestrians and slow moving vehicles.

  (iii) Speeds up to 2 Mbps are achievable for indoors and in stationary environments or walking users.

• It provides highly flexible system to cater to the need of wide variety of services and applications by proposing five air (radio)interfaces based on three multiple access technologies (FDMA, TDMA and CDMA ). These are:

  (i) Wideband CDMA (WCDMA): It is variation of CDMAOne technology and based on FDD.

  (ii) Code Division Multiple Access 2000 (CDMA 2000): This interface also falls under CDMAOne technology and based on FDD.

  (iii) Time Division Synchronous Code Division Multiple Access (TD-SCDMA): It is proposed by China and based on TDMA/TDD and CDMA techniques for high speed data transfer up to 384 Kbps.

  (iv) Universal Wireless Communications (UWC-136): It is based on TDMA and also known as EDGE.

  (v) Digital Enhanced Cordless Telephone (DECT): It falls under FD-TDMA category, mainly used in indoor environments.

  Among the five air interfaces, first three air interfaces (WCDMA, CDMA 2000 and TDSCDMA) are the main interfaces and became popular.

• The services provided by IMT 2000 are compatible with the existing systems like 2G with GSM standards and assure effective and seamless delivery.

• These systems are easily expandable depending upon growth in users, coverage area and new services.
• The channel spacing (bandwidths) needed for accommodating wide variety of services in 3G is 15 - 20 MHz as compared to 30 KHz (1G) and 30 - 200 KHz (2G), which is 500 times increase in bandwidth requirements. To cater to the need of huge bandwidth, new frequency spectrum for 3G was identified in the band 1885 - 2025 MHz (140 MHz and 2110 - 2200 (90 MHz) MHz bands, makes 230 MHz available for various services as illustrated in following Figure 1. Out of these 230 MHz band, 2$\times$30 MHz is allocated to satellite network of IMT 2000 and the rest for terrestrial networks.

• 3G also defines the paired and unpaired parts of the spectrum. Paired spectrum means a bit of spectrum is in lower frequency band and a bit of spectrum is in upper frequency band. For the paired spectrum, the bands for uplink (mobile transmit) and downlink (Base Station transmit) are defined for Frequency Division Duplex (FDD) operation. The two users communicating with each other on two separate frequencies is called FDD. WCDMA uses paired frequency band.
• The unpaired spectrum is used for Time Division Duplex (TDD) operation. In this, both uplink and downlink transmissions are carried out on the same frequency band in synchronized time intervals. Thus, time slots in a physical channel are divided into transmission and reception part. TD SCDMA uses unpaired spectrum.

• For Satellite services :

  1980 - 2010 MHz for uplink (30MHz) 2170 - 2200 MHz for downlink (30MHz)

  And For Terrestrial services:

1900 - 1980 MHz, 2010 - 2015 MHz and 2110 - 2170 MHz

Figure 2 illustrates the UMTS frequency plan in uplink & downlink.

Across the world, the following two major organizations are formed for the research and development of the air interface (WCDMA and CDMA 2000) technologies.

• 3G Project Partnership (3GPP) and
• 3G Project Partnership 2 (3GPP2).

The main aim of these two groups is to meet the standards defined by the International Mobile Telecommunication (IMT) 2000. WCDMA system specifications were developed by 3GPP and CDMA 2000 system specifications by 3GPP2. Europe and parts of Asian countries are using WCDMA for its 3G networks and United States is using the CDMA 2000. Universal mobile Telecommunication System (UMTS) of Europe is using the DS-CDMA as a multiple access technology and Japan has been at the forefront in the research. UMTS promises a data rate of 384 Kbps in the microcellular environment and 2 Mbps for indoor environment. It operates in the frequency band of around 2 GHz. CDMA 2000 of United States is a backward compatible upgrade, whereas WCDMA is not backward compatible with CDMA. But both systems use many of the same concepts and features. This chapter presents the principles of WCDMA air interface, also referred as UMTS terrestrial radio access (UTRA) and CDMA 2000 air interface.