Downlink Scrambling

The downlink scrambling codes are used
to separate the transmissions of one cell from those of another. The downlink
scrambling codes are Gold codes similar to the long scrambling codes
used in the uplink. As is the case for the long codes used on the uplink, the
codes used on the downlink are limited to a 10-ms duration. There is a total
of 218  1 (262,143) downlink scrambling codes. Not all of these are used,
however. If all possible codes were to be useable, then one could find a situation
where a terminal would have to check a received signal against all
262,143 codes. This could occur, for example, during cell selection. Clearly,
checking against so many scrambling codes is impractical.

Therefore, the available downlink scrambling codes are separated into
512 groups. Each group contains one primary scrambling code and 15 secondary
scrambling codes. Thus, 512 primary scrambling codes exist and
7,680 secondary scrambling codes exist, for a total of 8,192 downlink scrambling
codes. Table 6-1 shows the allocation of secondary downlink scrambling
codes to primary downlink scrambling codes.
A cell is allocated one, and only one, primary scrambling code, which, of
course, has 15 secondary scrambling codes associated with it. A given base
station will use its primary scrambling code for the transmission of channels
that need to be heard by all terminals in the cell. Thus, paging messages
need to be scrambled by the cell’s primary scrambling code. For that
matter, all transmissions from the base station can simply use the cell’s primary
scrambling code. After all, it is the scrambling code that identifies the
cell, while the various channelization codes are used to separate the various
transmissions (physical channels) within the cell.
A cell can, however, choose to use a secondary scrambling code for
channels that are directed to a specific user and do not need to be
decoded by other users. In general, it is a good idea for all transmissions
from a cell to use the cell’s primary scrambling code, as this helps to minimize
interference.

As described, 512 primary scrambling codes are available. These are
divided into 64 groups, each consisting of 8 scrambling codes, as shown in
Table 6-2.

As mentioned, downlink spreading and scrambling are applied to all
downlink physical channels transmitted on a cell, with the exception of the
synchronization channel (SCH). This channel is added to the downlink
stream, as shown in Figure 6-7. In fact, as explained later in this chapter,
the SCH contains two subchannels—the primary SCH and secondary SCH.
The reason why these are transmitted without scrambling is the fact that
they are the first channels decoded by a terminal. If they were scrambled,
then the terminal would first have to know the scrambling code of the base
station just to synchronize.