UMTS / WCDMA basics tutorial
a tutorial about the basics of the 3G Wideband CDMA, WCDMA, or UMTS
cellular technology.
UMTS, the Universal Mobile Telecommunications System is the
third generation (3G) successor to the second generation GSM based cellular
technology including 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 does not require such a large instantaneous
investment.
UMTS uses Wideband CDMA (WCDMA or 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. Some are calling it 3GSM because it is a 3G migration for
GSM.
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).
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.
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Note on CDMA:
CDMA, Code Division Multiple Access, is a multiple
access scheme used by many 3G cellular technologies, and other forms of
wireless technology. It uses a process called Direct Sequence Spread
Spectrum where spreading codes are used to spread a signal out over a
given bandwidth and then reconstituting the data in the receiver by
using the same spreading code. By supplying different spreading codes to
different users, several users are able to utilises the same frequency
without mutual interference.
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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).
Summary
UMTS or as it is often termed, Wideband CDMA, WCDMA is being
widely deployed. It offers many advantages over GSM, GPRS, and EDGE in terms of
much higher data rates and greater flexibility. These basic technical abilities
reflect as a much richer number of applications and features that the 3G phones
can be used to perform. This not only gives the user a much more useful 'phone',
but this also translates into higher revenues for the operator.
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