Understanding UMTS-WCDMA

W-CDMA applied to UMTS PDF

W-CDMA is a multiple access technique using a concept known as Spread Spectrum.
In Spread Spectrum systems, the information bandwidth is “spread” across a wider
transmission bandwidth. This transmission bandwidth is determined by a function
which is independent of the information being sent.
So the information from both simple speech and complex video services, which
clearly have very different “information bandwidth”, will both be converted to be sent
over the full frequency band defined for the system.
Application of the same spreading codes once again at the receiver returns the
transmitted signals back to their original bandwidths.
In order to allow them to be distinguished at a receiver, signals which coexist within
the transmission frequency band must be separated using codes which have a low
cross-correlation with the other codes. In other words decoding signals using the
wrong code will simply produce “noise”, since only the signal with the desired code
will be strongly recognised.
Being able to distinguish signals from different users using codes gives rise to the
term Code Division Multiple Access.