User Guide
PAMS
Technical Documentation
NHD–4
System Overview
Page 3–14
Original 11/97
Reused Voice Channels
Channel Reuse and SAT Frequencies
Different SAT freq
prevents hook–up
to other cell
333
101–150
A4D7
5970 Hz
319
101–150
B8C3
6030 Hz
320
201–250
8FB2
313
1–50
B9CE
315
151–200
BC43
327
51–100
796F
AMPS_13
Control Ch #
Traffic Ch’s
Base Station
ID
Figure 13. AMPS: Ch reuse & SAT Freq
In this example Base Stations A4D7 and B8C3 are using the same traffic channels.
Base Stations are normally spaced far enough apart so that two stations using the
same traffic channels will not interfere with each other. However!!! Under some
conditions it is possible for two stations like A4D7 and B8C3 to be received by one
mobile phone. The cure for this problem is different SAT frequencies. When a
mobile is ordered to a traffic channel it is given a traffic channel number and an SAT
frequency. If the mobile should be getting its traffic from A4D7 it expects to see an
SAT frequency of 5970 Hz. If an SAT frequency of 6030 were received, the mobile
would drop the call and try listening for the traffic channel that had 5970 Hz for SAT.
When an AMPS mobile station (MS) travels from one Base Station (BS) cell to
another a “Hand–off” must occur. As the mobile travels from “A” to “B” the signal
strength will decrease in “A” and increase in “B”. This information is sent from the
BS to the Mobile Telephone Exchange (MTX). Each BS has a receiver that can
scan all channels. When a hand–off is imminent the MTX instructs BS “B” to listen
for the MS about to be handed–off in order to insure that signal strength is
adequate. When the MTX decides its time for a hand–off, the traffic channel from
BS “A” to the mobile is interrupted and hand–off instructions are sent to the mobile.
The mobile then sends a 50 msec burst of Signalling Tone to confirm receipt of
instructions, then changes to the new Base Station, traffic channel and ST. The
user does not hear any data because the ear piece is muted when the phone is not
receiving SAT.
Handoff
X
X
A
B
C
D
MTX
AMPS_14
Figure 14. AMPS: Hand off