User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–45
Original 11/97
The second base station is a mini–cell and therefore transmits at a lower power.
The mobile would transmit a higher power than necessary to the mini–cell because
the weaker signal would be interrupted as a distant station. This problem is taken
care of after the mobile has located the strongest base station. Information
contained in the Sync Channel of each cell site transmits its characteristics for
power control.
CDMA Call
After each access attempt, the mobile listens to the Paging Channel for a response
from the base station. When the base station detects the mobiles access probe, it
responds with a channel assignment message. This message contains all of the
information required to get the mobile onto a traffic channel. Information required
for the mobile to start using a traffic channel includes, Walsh code channel to be
used for the forward traffic channel, the frequency being used, and the frame offset
to indicate the delay between the forward and reverse links. Once this information
has been acknowledged by the mobile a move to the designated traffic channel is
accomplished. At this point conversations can began. To accommodate traffic
other than voice data, two methods of temporarily seizing the traffic channel are
used: blank and burst signaling and dim and burst signaling. Blank and burst
signaling seizes several blocks of data frames, removes the voice data and replaces
it with house keeping data. Dim and burst reduces the VOCODER rate and then
uses the remaining traffic channel time to more slowly send house keeping
messages.
Reverse Link Closed Loop Power Control
Because of multipath, atmospheric conditions, and the number of CDMA users
among other reasons the Open Loop Power Control method is not precise enough.
Remember to optimize capacity all CDMA mobile signals should arrive at the base
station at the same strength. The base station monitors each mobile’s receive
signal strength and directs the mobile to raise or lower it’s power in 1 dB steps until
the signal level is just adequate. One side benefit from lower power output is longer
battery life for the mobile.
CDMA Variable Rate Speech Coder
The VOCODER takes advantage of quiet times and less complex parts of speech to
raise capacity. An oooooo vowel sound is less complex than a word like fat or cat
with consonants in it. It takes more coded samples to reproduce consonants than
vowels. During speech activity the VOCODER operates at 9.6 kbps and during
pauses the rate will drop to 1.2 kbps. The data rate is based on speech activity and
a decision is made every 20 msec as to the rate. The variable rate speech coder
saves a great deal of power because the mobile goes to pulsed operation at 4.8
kbps and below. The section on Mobile Power Bursting will explain pulsed
operation further.
Mobile Power Bursting
Each 20 millisecond CDMA data frame is divided into sixteen “power control
groups”. Each power control group contains 1536 data symbols (chips) at a data
rate of 1.2288 Mbps which represent 12 encoded voice data bits. Figure 40, Mobile
Power Bursting shows the relationship between the four VOCODER data rates.