3G UMTS / WCDMA Basics Tutorial

- a tutorial, or overview of the basics of UMTS, Universal Mobile Telecommunications System, using WCDMA technology run under the auspices of 3GPP.

3GPP UMTS, the Universal Mobile Telecommunications System is the third generation (3G) successor to the second generation GSM based cellular technologies which also include GPRS, and EDGE. Although UMTS uses a totally different air interface, the core network elements have been migrating towards the UMTS requirements with the introduction of GPRS and EDGE. In this way the transition from GSM to the 3G UMTS architecture did not require such a large instantaneous investment.

UMTS uses Wideband CDMA (WCDMA / W-CDMA) to carry the radio transmissions, and often the system is referred to by the name WCDMA. It is also gaining a third name.

A typical 3G UMTS UE cellphone handset

3GPP UMTS Specifications and Management

In order to create and manage a system as complicated as UMTS or WCDMA it is necessary to develop and maintain a large number of documents and specifications. For UMTS or WCDMA, these are now managed by a group known as 3GPP - the Third Generation Partnership Programme. This is a global co-operation between six organisational partners - ARIB, CCSA, ETSI, ATIS, TTA and TTC.

The scope of 3GPP was to produce globally applicable Technical Specifications and Technical Reports for a 3rd Generation Mobile Telecommunications System. This would be based upon the GSM core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).

Since it was originally formed, 3GPP has also taken over responsibility for the GSM standards as well as looking at future developments including LTE (Long Term Evolution) and the 4G technology known as LTE Advanced.

3G UMTS capabilities

UMTS uses Wideband CDMA - WCDMA - as the radio transmission standard. It employs a 5 MHz channel bandwidth. Using this bandwidth it has the capacity to carry over 100 simultaneous voice calls, or it is able to carry data at speeds up to 2 Mbps in its original format. However with the later enhancements of HSDPA and HSUPA (described in other articles accessible from the cellular telecommunications menu page ) included in later releases of the standard the data transmission speeds have been increased to 14.4 Mbps.

Many of the ideas that were incorporated into GSM have been carried over and enhanced for UMTS. Elements such as the SIM have been transformed into a far more powerful USIM (Universal SIM). In addition to this, the network has been designed so that the enhancements employed for GPRS and EDGE can be used for UMTS. In this way the investment required is kept to a minimum.

A new introduction for UMTS is that there are specifications that allow both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes. The first modes to be employed are FDD modes where the uplink and downlink are on different frequencies. The spacing between them is 190 MHz for Band 1 networks being currently used and rolled out.

However the TDD mode where the uplink and downlink are split in time with the base stations and then the mobiles transmitting alternately on the same frequency is particularly suited to a variety of applications. Obviously where spectrum is limited and paired bands suitably spaced are not available. It also performs well where small cells are to be used. As a guard time is required between transmit and receive, this will be smaller when transit times are smaller as a result of the shorter distances being covered. A further advantage arises from the fact that it is found that far more data is carried in the downlink as a result of internet surfing, video downloads and the like. This means that it is often better to allocate more capacity to the downlink. Where paired spectrum is used this is not possible. However when a TDD system is used it is possible to alter the balance between downlink and uplink transmissions to accommodate this imbalance and thereby improve the efficiency. In this way TDD systems can be highly efficient when used in picocells for carrying Internet data. The TDD systems have not been widely deployed, but this may occur more in the future. In view of its character, it is often referred to as TD-CDMA (Time Division CDMA).

3G UMTS / WCDMA technologies

There are several key areas of 3G UMTS / WCDMA. Within these there are several key technologies that have been employed to enable UMTS / WCDMA to provide a leap in performance over its 2G predecessors.

Some of these key areas include:

  • Radio interface:   The UMTS radio interface provides the basic definition of the radio signal. W-CDMA occupies 5 MHz channels and has defined formats for elements such as synchronisation, power control and the like Read more about the UMTS / W-CDMA radio interface.
  • CDMA technology :   3G UMTS relies on a scheme known as CDMA or code divison multiple access to enable multiple handsets or user equipments to have access to the base station. Using a scheme known as direct sequence spread spectrum, different UEs have different codes and can all talk to the base station even though they are all on the same frequency Read more about the code division multiple access.
  • UMTS network architecture:   The architecture for a UMTS network was designed to enable packet data to be carried over the network, whilst still enabling it to support circuit switched voice. All the usual functions enabling access toth e network, roaming and the like are also supported. Read more about the UMTS network architecture.
  • UMTS modulation schemes:   Within the CDMA signal format, a variety of forms of modulation are used. These are typically forms of phase shift keying. Read more about the modulation schemes.
  • UMTS channels:   As with any cellular system, different data channels are required for passing payload data as well as control information and for enabling the required resources to be allocated. A variety of different data channels are used to enable these facilities to be accomplishedRead more about the physical & logical channels.
  • UMTS TDD:   There are two methods of providing duplex for 3G UMTS. One is what is termed frequency division duplex, FDD. This uses two channels spaced sufficiently apart so that the receiver can receive whilst the transmitter is also operating. Another method is to use time vision duplex, TDD where short time blocks are allocated to transmissions in both directions. Using this method, only a single channel is required Read more about the TDD system.
  • Handover:   One key area of any cellular telecommunications system is the handover (handoff) from one cell to the next. Using CDMA there are several forms of handover that are implemented within the system. Read more about the Handover.

3G UMTS enhancements

The basic 3G UMTS cellular system enabled data rates up to 2048kbps to be achieved. However as the use of data rapidly increased, these figures were no longer sufficient and further data rate increases were required.

A scheme known as HSDPA, high speed packet download access was first introduced to enable the downlink speed to be increased. This was followed with HSUPA, high speed packet uplink access was introduced. The combined suite was then known as HSPA, high speed packet access.

Note on HSPA, High Speed Packet Access:

High speed packet access added additional channels and signalling to the downlink (HSDPA) and in the Uplink (HSUPA). By adding these channels, significant increases in data rates and capacity have been achieved.

Click on the link for further information about High Speed Packet Access, HSPA

UMTS WCDMA specification summary

The UMTS WCDMA system offered a significant improvement in capability over the previous 2G services.

3G UMTS Specification Summary
Parameter Specification
Maximum data rate
2048 kbps low range
384 kbps urban and outdoor
RF channel bandwidth
5 MHz
Multiple access scheme
Duplex schemes
FDD and also TDD

The basic 3G UMTS is able to provide a reasonable data transfer rate, although by the latest standards it is relatively slow. Nevertheless 3G UMTS paved the way for mobile broadband, providing data rates that were unequalled at the time.

By Ian Poole

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