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UMTS CDMA technology
- tutorial, or overview of the basics of CDMA, code division multiple access
scheme used within UMTS, or Wideband CDMA, WCDMA.
The use of CDMA, code division multiple access, in the form of Wideband CDMA, WCDMA
for use with the 3G UMTS telecommunications system marked a distinct change in the type oftechnology used for the multiple access scheme for a telecommunications system. However it
offered many advantages for both users and operators and as a result, it has provided many
benefits.
The use of CDMA for UMTS and other cellular formats was made possible by the fact that
semiconductor technology had moved forward sufficiently. At the time the first cellulartechnologies were introduced sufficient processing power could be provided to enable the coding
and decoding of CDMA to be accomplished within a mobile handset.
CDMA as a form of multiple access scheme was first used on the cdmaOne system that was first
deployed in the USA in 1995, and has successors that were marketed under the CDMA2000
banner. The use of a CDMA based technology for UMTS represented a further step forward in
the use of CDMA.
CDMA for UMTS
The choice of CDMA for use with the third generation, 3G UMTS telecommunications system
arose from a variety of technical reasons. It offers significant advantages over the schemes usedin the previous 2G systems that were predominantly TDMA based schemes.
The main benefits of the use of CDMA as a multiple access scheme are:
Improved spectral efficiency: The use of CDMA as the multiple access technology, combinedwith the QPSK modulation format used provides significant improvements in terms of the
spectral efficiency. Figures for the performance improvements gained vary considerably
dependent upon the conditions, but the scheme gives some significant benefits. Some
calculated estimates give figures as high as three or four times that of technologies such as
GSM, although in reality the benefits may be a bit less.
Adjacent cells may use the same channel frequency: As a result of the way in which spreadspectrum signals such as CDMA operate. Improved handover: Within CDMA it is possible to do what is termed a "soft handover" where
the UE communicates with two base stations at the same time. This significantly improves
handover reliability.
Enhanced security: The use of spread spectrum and the multiple spreading codes for CDMAsignificantly reduces the possibility of eavesdropping, although within GSM eavesdropping of
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the transmitted signal was not the problem it was for the original analogue systems where
anyone with a scanner radio receiver could listen to telephone conversations.
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 bandwidthand 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.
Click on the link for aCDMA tutorial
UMTS CDMA format
The data to be transmitted is encoded using a spreading code particular to a given user. In this
way only the desired recipient is able to correlate and decode the signal, all other signalsappearing as noise. This allows the physical RF channel to be used by several users
simultaneously.
The data of a CDMA signal is multiplied with a chip or spreading code to increase the bandwidth
of the signal. For WCDMA, each physical channel is spread with a unique and variable
spreading sequence. The overall degree of spreading varies to enable the final signal to fill therequired channel bandwidth. As the input data rate may vary from one application to the next, so
the degree of spreading needs to be varied accordingly.
For the downlink the transmitted symbol rate is 3.84 M symbols per second. As the form of
modulation used is QPSK this enables two bits of information to be transmitted for everysymbol, thereby enabling a maximum data rate of twice the symbol rate or 7.68 Mbps. Therefore
if the actual rate of the data to be transmitted is 15 kbps then a spreading factor of 512 is required
to bring the signal up to the required chip rate for transmission in the required bandwidth. If thedata to be carried has a higher data rate then a lower spreading rate is required to balance this
out. It is worth remembering that altering the chip rate does alter the processing gain of the
overall system and this needs to be accommodated in the signal processing as well. Higherspreading factors are more easily correlated by the receiver and therefore a lower transmit powercan be used for the same symbol error rate.
The codes required to spread the signal must be orthogonal if they are to enable multiple usersand channels to operate without mutual interference. The codes used in W-CDMA are
Orthogonal Variable Spreading Factor (OVSF) codes, and they must remain synchronous to
operate. As it is not possible to retain exact synchronisation for this, a second set of scrambling
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codes is used to ensure that interference does not result. This scrambling code is a pseudo
random number (PN) code. Thus there are two stages of spreading. The first using the OSVF
code and the second using a scrambling PN code. These codes are used to provide different
levels of separation. The OVSF spreading codes are used to identify the user services in theuplink and user channels in the downlink whereas the PN code is used to identify the individual
node B or UE.
On the uplink there is a choice of millions of different PN codes. These are processed to include
a masked individual code to identify the UE. As a result there are more than sufficient codes toaccommodate the number of different UEs likely to access a network. For the downlink a short
code is used. There are a total of 512 different codes that can be used, one of which will be
assigned to each node B.
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