User Guide

Technical Documentation
NHP–4
System Overview
PAMS
Page 3–29
Issue 1 04/99
Mobile Phone Operation
When a CDMA mobile scans for the strongest Pilot Channel signal, the scanning is
done in time rather than frequency scanning like an analog phone does. Once the
strongest Pilot channel has been located, Sync Channel information is demodulated.
The sync channel contains information the mobile needs in order to decode the
Paging Channel. The Paging channel’s use can be compared to a digital control
channel for DAMPS phones When the mobile goes into a call a Traffic Channel is
used.
Pilot Channel
The Pilot Channel is transmitted continuously by the base station to provide mobiles
with pilot and sync channel timing. The only modulation on the Pilot Channel is
Walsh code zero XOR’ed with the Short Code The Short Code is 215, (32768) bits
long and at 1.2288 Mbps takes 26.67 msec before repeating its self. The start time
of any base station pilot channel is always an exact multiple of 64 system clock
cycles (called chips) offset in time from any other base station. The mobile checks
all 215 short code offsets to find the strongest pilot signal using the “searcher”
special hardware dedicated to doing pilot correlations. After checking all chip
offsets the mobile stores signal strengths of any Pilot Channel it hears. When the
strongest pilot signal is found the Rake demodulator aligns its self to the short code
offset, then applies Walsh code 32 in order to demodulate the Synch Channel. The
mobile knows when this Pilot channel and Synch Channel starts but it does not
know if it is time slot 1, 45, 248 or what. Figure 36 “CDMA Pilot & Synch Channel
Timing” will help you understand how the mobile gets timing and other information
from these two channels.
Time
Master
Start
Time
Time
SYNC CHANNEL
Master
Start
Time
Received base
station pilot
channels
PILOT CHANNEL
Figure 21. CDMA Pilot & Synch Channel Timing
The Pilot channel circle has small tick marks sticking outside the circle. These tick
marks represent signal strengths of the received Pilot channels from surrounding
base stations. Each base station’s pilot channel is separated from the next by 64
clock chips for a total of 512 different pilot channels. The longest tick on the right
side represents the strongest Pilot channel.