W-CDMA

The choice of third generation radio technology employed by 3GPP^TM in implementing UTRA - Universal Terrestrial Radio Access - was decided upon after a long period of technical discussion in ETSI's SMG Technical Committee during the 1990s. These discussions involved interested parties from the wider cellular community and eventually resulted in the formation of the 3rd Generation Partnership Project (3GPP). The basic technology chosen was Wide-band Code-Division Multiple Access (W-CDMA), with several sub-flavours:

• Direct Sequence, Frequency-Division Duplex
• Direct Sequence, Time-Division Duplex
• Multi-Carrier.

Within the Time-Division Duplex category the initial specifications were augmented shortly afterwards by a low chip-rate version developed largely at the initiative of the Chinese partner. Subsequently, a high chip-rate version has also been specified.

Spread spectrum

W-CDMA is a spread-spectrum modulation technique; that is, one which uses channels whose bandwidth is much greater than that of the data to be transferred. Instead of each connection being granted a dedicated frequency band just wide enough to accommodate its envisaged maximum data rate, W-CDMA channels share a much larger band. The modulation technique encodes each channel in such a way that a decoder, knowing the code, can pick out the wanted signal from other signals using the same band, which simply appear as so much noise.

Using a wide frequency band makes the system inherently resistant to many of the aspects of radio communication which plague narrow band systems, such as bursty noise, multipath reflections, and other interfering transmissions. Since all information shares the same band, and channels can only be distinguished with a knowledge of the spreading codes used, W-CDMA is also very secure from eavesdropping, even before employing encryption algorithms. Also, unlike systems which use a fixed number of discrete channels in a given frequency band, with W-CDMA there is no fixed number of simultaneous communications links which can be supported. The effect of adding more active links to a given cell will eventually result in the need for data rates for all links to be reduced in order that the wanted signals can be picked out of the ever increasing noise, but this is generally preferable to allowing access of a first-come, first-served basis, with later users simply being denied access.

Further information

Many commercial publications exist which describe CDMA and W-CDMA in great technical detail. The various flavours of WCDMA employed in 3GPP^TM systems are defined and specified in the 25.-series of specifications.