Channelization Codes

As mentioned, the channelization codes
are used to separate multiple streams of data from a given user, whereas the
scrambling codes are used to separate transmissions from different users.
The channelization codes are known as Orthogonal Variable Spreading
Factor (OVSF) codes. They are taken from the code tree shown in Figure
6-3. The generation of channelization codes is given by the following
equations:

3Cch,2 1n12 ,3 4  3Cch,2 1n2 ,1 Cch,2 1n2 ,1 4 , etc.
3Cch,2 1n12 ,2 4  3Cch,2 1n2 ,1 Cch,2 1n2 ,1 4
3Cch,2 1n12 ,1 4  3Cch,2 1n2 ,0 Cch,2 1n2 ,0 4
3Cch,2 1n12 ,0 4  3Cch,2 1n2 ,0 Cch,2 1n2 ,0 4
3Cch,2,1 4  3Cch,1,0 Cch,1,0 4  11, 12
3Cch,2,0 4  3Cch,1,0 Cch,1,0 4  11, 12
Cch,1,0  112


In general, a given physical channel uses a channelization code that is
related to the spreading factor being used for the channel. When only one
DPDCH is to be transmitted, then the channelization code is Cch,SF,k, where
SF is the spreading factor and K  SF/4. Therefore, if the spreading factor
is 128 (as determined by the user data rate plus coding overhead), then the
code to be used shall be Cch,128,32. The spreading factor for the DPCCH is
always 256 and the channelization code is Cch,256,0.

When more than one DPDCH is to be transmitted (greater than 960
Kbps of combined user data and coding overhead), then each DPDCH shall
have a spreading factor of 4 and the channelization code for each DPDCH
shall be Cch,4,k. K  1 for DPDCH1 and DPDCH2, K2 for DPDCH3 and
DPDCH4, and K  3 for DPDCH5 and DPDCH6. For example, DPDCH3 and
DPDCH4 would both use the channelization code Cch,4,2  (1, 1, 1, 1).
Given that channelization codes are used to separate different transmissions
from a single user, the fact that certain codes can be simultaneously
used on two channels is, at first glance, troubling. The fact, however, that
those two channels will always be on separate I and Q branches means that
they can still be separated.

Some important restrictions apply to the use of channelization codes.
That is because, in the case where more than one channel is being transmitted,
the chosen channelization codes must be orthogonal. For example,
consider the channelization code Cch,4,0. This code is simply the sequence
1,1,1,1 repeated over and over, with each sequence of four bits repeated
960,000 times per second. Consider the channelization code Cch,8,0. This is
simply the sequence 1,1,1,1,1,1,1,1 repeated over and over, with each
sequence of eight bits repeated 480,000 times per second. Clearly, if one
data stream from a given user is spread with the code Cch,4,0, and a second
data stream from the same user is spread with the code Cch,8,0, the net effect
is that they are spread in the same way and cannot be distinguished at the
receiver. Consequently, channelization codes must be selected in a manner
that ensures that each channel is spread differently.