User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–38
Original 11/97
The “Pilot Channel” can be compared with the control channels used in analog.
The “Pilot Channel” is unmodulated Walsh code zero spread with Short Code that
has a unique mask applied in order for mobiles to identify cells from each other.
Pilot channel power is the strongest channel from the base station, with about 20%
of the total output power. The Pilot Channel provides the mobile with an easy to
demodulate strong signal that is used as a time reference.
The “Sync Channel” transmits timing information and always uses Walsh code 32
which is half zero’s and half one’s. The most important timing information contains
the state of the long code feedback shift registers 320 milli–seconds in the future.
The mobile can load this information into its long code generator, and start the
generator at the proper time. Long code state information is not all that is sent by
the sync channel but is one of the more important data.
The “Paging Channel” is the forward links digital control channel. Quite a lot of
information is sent to the mobile on this channel, a more complete discussion of all
four channels will be given in the section on how a CDMA mobile operates. The
first paging channel is always Walsh code one. If more paging channels are
needed Walsh codes 2 through 7 can be used.
The “Traffic Channel” is the same thing as an analog voice channel. This is where
conversations take place. At least 55 Walsh codes are available for use as traffic
channels but the actual number that can be used is around 30 at present.
When all the various channels have been Walsh modulated they are split into I and
Q channels which are re–spread with the short code to provide base station identity,
filtered to reduce bandwidth and converted to analog signals. The analog I and Q
signals from all the channels are summed together and sent to the I/Q modulator for
modulation onto an RF carrier.
Figure 34, “Forward Link Channel Format “shows how the various channels are
made up.