User Guide

Technical Documentation
NHP–4
System Overview
PAMS
Page 3–33
Issue 1 04/99
Each frame is
divided into 16
Power Control
Groups
CDMA Frame = 20 ms
Each Power
Control Group
contains 1536
chips (represents
12 encoded voice
data bits
Average power is lowered 3 dB for
each lower data rate
Half Rate 4.8kbps
8 Power Control
Groups
Quarter Rate 2.4kbps
4 Power Control
Groups
Eighth Rate 1.2kbps
2 Power Control
Groups
Full Rate 9.6 kbps
16 Power Control
groups
Figure 22. Mobile Power Bursting
Here is how that breaks down: when the VOCODER is running at the full rate of
9600 bps, each 1.25 ms power control group represents 12 encoded voice data bits
(0.00125 seconds X 9600 bps = 12 bits). The 1536 number is the number of bits in
a 1.25 ms period at a rate of 1.2288 Mbps which is the final spread data rate. The
VOCODER can run at 9.6 kbps, 4.8 kbps, 2.4 kbps, and 1.2 kbps for rate set one.
When the VOCODER data rate drops below 9.6 kbps the CDMA mobile starts
transmitting in bursts. Not only does the mobile save power by turning off the
transmitter, each decrease in data rate lowers the average power output by 3 dB, a
50% reduction in radiated power. Average power decrease will result in lower
interference to other CDMA signals which will result in capacity increase.
The Rake Receiver
When AMPS and DAMPS cellular phones encounter multipath signal problems, the
cure is a very strong signal–to–noise ratio. Remember a CDMA phone receives a
“channel” by correlating (matching) the received spread code with an unmodulated
internal copy. Mobile CDMA phones have three correlation receivers called a rake
receiver. When a CDMA mobile receives signals with different delay times the
phone will synchronize to the strongest signal. Usually the strongest signal has
arrived via the most direct route. One of the other two receivers will synchronize
with the reflected signal, then combine this signal with the direct signal for a much
stronger totally combined signal.