The WCDMA Air Interface

As mentioned, WCDMA uses a chip rate of 3.84 Mcps. As also mentioned,
CDMA technology in general uses a spreading code to separate one user’s
transmissions from those of another. In reality, however, there will be multiple
simultaneous data streams from multiple users and multiple simultaneous
data streams from a singe base station. Therefore, not only is it
necessary to separate the transmissions of one user or base station from
those of another, it is also necessary to separate the various transmissions
that a single user might generate. In other words, if a single user (user A)
is transmitting both user data and control information, the base station
must first separate the set of transmissions from user A from the transmissions
of all other users. It must then separate the control information
from the user data.

In order to support this requirement, WCDMA takes a two-step
approach to the transmission from a single user, as shown in Figure 6-1.
First, each individual data stream is spread to the chip rate by the application
of a spreading code, also known as a channelization code, and which
operates at the chip rate of 3.84 Mcps. Then the combined set of spread signals
is scrambled by the application of a scrambling code, which also operates
at the chip rate. The channelization spreads the individual data
streams and hence increases the required bandwidth. Since the scrambling
code also operates at the chip rate, however, it does not further increase the
required bandwidth. At the receiving end, the combined signal is first
descrambled by application of the appropriate scrambling code. The individual
user data streams are then recovered through the application of the
appropriate channelization codes. Clearly, it is important that different
users use different scrambling codes. Multiple users, however, can use the
same channelization codes, provided, however, that no two transmissions
from the same user use the exact same channelization code.