User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–48
Original 11/97
The pilot strength message starts a soft hand–off. When the pilot strength
message is received; base station A passes this request to the MTSO (Mobile
Telephone Switching Office). The MTSO passes the request to station B to see if a
traffic channel is available for the soft hand–off request.
CDMA Soft Hand–off
If a channel is available, cell site B sends the Walsh Code that will be assigned for
the soft hand–off to the MTSO. At this point base station A orders the soft hand–off
by sending a hand–off direction message to the mobile using the traffic channel.
When the hand–off message is acknowledged, the MTSO sends the land link to
base station B who then begins to send information on the assigned Walsh code
traffic channel to the mobile. The mobile then receives both signals from the two
cell sites, each operating on different PN offsets and Walsh coded traffic channels.
The two signals are then combined by using the two pilot signals as coherent phase
references. In a two way soft hand–off, two of the mobile’s rakes are used: one for
each received base station At the same time both base stations are independently
receiving the mobile’s signal. The demodulated signal is sent to the MTSO where
the two signal are compared on a frame–by–frame bases. The MTSO selects the
best of the two signals and sends that signal to the CODEC where it is passed to the
public telephone network.
CDMA Hand–off Completion
When the signal from station A degrades and goes below “Drop Threshold” the
mobile sends another pilot strength message to base station B indicating that base
station A’s link should be terminated. At this point the mobile is being power
controlled by base station B. The mobiles request is passed by the MTSO to cell
site A to terminate transmission and reception of the mobile’s signal. The mobile is
now exclusively terminated with base station B.
If the hand–offs are between sectors on a base station the same routine applies. It
makes no difference to the mobile whether the hand–off is between sectors or cell
sites.