What is 3G UMTS: WCDMA Tutorial

3G UMTS includes:
What is UMTS     Network architecture     Radio access     Radio interface     Frequency bands     UMTS CDMA     Modulation     Data channels     UMTS TDD     TD-SDCDMA     Handover    

UMTS - Universal Mobile Telecommunications System, is the 3G successor to the GSM family of standards including GPRS and EDGE.

3G UMTS uses a totally different radio interface based around the use of Direct Sequence Spread Spectrum as CDMA or Code Division Multiple Access.

Although 3G UMTS uses a totally different radio access standard, the core network is the same as that used for GPRS and EDGE to carry separate circuit switched voice and packet data.

UMTS uses a wideband version of CDMA occupying a 5 MHz wide channel. Being wider than its competition CDMA2000 which only used a 1.25MHz channel, the modulation scheme was known as wideband CDMA, or WCDMA / W-CDMA. This name was often used to refer to the whole system.

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.

UMTS WCDMA specification summary

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

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
• 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
• 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.
• UMTS modulation schemes:   Within the CDMA signal format, a variety of forms of modulation are used. These are typically forms of phase shift keying.
• 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 accomplished.
• 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
• 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.

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.

The basic 3G UMTS was able to provide a good increase in data performance over that of GPRS and EDGE, although the expected use of video did not occur when the basic form of UMTS was introduced. It was too slow especially when many users were connected. However the enhancements in terms of HSPA helped increase the performance and data usage increased at a much greater rate.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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