Code Properties

Direct-Sequence Spread Spectrum (DSSS) techniques create a wide RF bandwidth signal
by multiplying the user data and control data with digital spreading codes. The wideband
characteristics are used in 3G systems to help overcome propagation distortions.
As all users share the same frequency, it is necessary to create individual user discrimination
by using unique code sequences. Whether a terminal has a dedicated communication
link or is idle in a cell, it will require a number of defined parameters from
the base station. For this reason, a number of parallel, or overlaying, codes are used
(see Figure 3.3):
 Codes that are run at a higher clock, or chip, rate than the user or control data
will expand the bandwidth as a function of the higher rate signal (code). These
are spreading codes.
 Codes that run at the same rate as the spread signal are scrambling codes. They
do not spread the bandwidth further.

The scrambling codes divide into long codes and short codes. Long codes are 38,400
chip length codes truncated to fit a 10-ms frame length. Short codes are 256 chips long
and span one symbol period. On the downlink, long codes are used to separate cell
energy of interest. Each Node B has a specific long code (one of 512). The handset uses
the same long code to decorrelate the wanted signal (that is, the signal from the serving
Node B). Scrambling codes are designed to have known and uniform limits to their
mutual cross correlation; their distance from one another is known and should remain
constant. 60