User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–47
Original 11/97
One more advantage of CDMA mobiles is utilized when a hand–off to another base
station is necessary, a make–before–break soft hand–off is used. The rake
receiver constantly searches for and measures multi–path and neighboring signals.
The multi–path signals are time adjusted then combined for a stronger total signal.
The neighboring cell site signals are used to determine the best choice when a
handoff when necessary.
CDMA Hand–offs
Normally CDMA hand–offs are make–before–break and either “Soft” or “Softer”.
A Soft hand–off is between base stations at two different locations.
A Softer hand–off is between two sectors at the same base station
Figure 38, CDMA Hand–off will help explain how soft, make–before–break
hand–offs are accomplished.
Add Threshold
Drop Threshold
E
/N
C
O
Signal
Margin
Time
CDMA20.DRW
Signal A
Signal B
Signal C
Figure 38. CDMA Hand–off
Once a call is established, the mobile is constantly searching for other possible cell
sites that might be good candidate for soft hand–offs. A search list of neighboring
base stations from the base station in use is used to look for hand–off candidates.
CDMA Soft Hand–off Initiation
The following scenario describes what has to happen to get a soft hand–off. A
mobile with an established call using signal A starts receiving signal B. When signal
B exceeds the Add Threshold level as defined by B’s cell site, a pilot strength
message is sent to cell site A from the mobile. The pilot strength message is sent
on the traffic channel using either dim and burst or blank and burst signaling.