Motorola M3097 Service Manual download pdf (Page 24)

©1999 Motorola, Inc.

Cellular OverviewService Manual

13

Power Steps

As a call progresses, the cell site continuously

monitors the reverse channel for signal

strength.

Every cellular telephone has a number of

power steps ranging from full power (3 watts

in a mobile and .6 watts in a portable) down

to as low as about half a milliwatt. In real-

ity all cellular telephones have eight power

steps, but portable models are prevented from

using the two highest power steps by the cell

site. Transmit power level commands are

sent to the cellular telephone as required to

maintain the received signal strength within

prescribed limits.

This is done to minimize interference possi-

bilities within the frequency re-use scheme.

If the signal received from the cellular tele-

phone is higher than the prescribed limit

(such as when the unit is very near the cell

site), the subscriber unit will be instructed

to step down to a lower level.

Hand-offs

If the cellular telephone is at its maximum

allowed power for the cell site it is using and

the received signal at the cell site is approach-

ing the minimum allowable (typically -100

dBm), the cell site will signal the switch to

consider the subscriber unit for a hand-off.

The central controller (switch) will in turn

have a scanning receiver at each of the sur-

rounding cell sites measure the cellular

telephone’s signal strength. The site with

the strongest signal will be the site to which

the call will be handed to if there are avail-

able voice channels.

On an AMPS channel the hand-off is executed

by interrupting the conversation with a burst

of data (called blank and burst) containing

the new voice channel assignment. The tele-

phone acknowledges the order by a 50 milli-

second burst of 10 kHz signaling tone on the

originally assigned voice channel. The mo-

bile telephone then drops the original voice

channel and tunes to the newly assigned voice

channel, keying up on that channel and

transponding the assigned SAT. But on a

NAMPS channel the hand-off is executed

with a low speed data transmission that does

not interrupt the voice. The telephone ac-

knowledges the order in this case by a DST

message. In either case, once the hand-off

has been accomplished, the newly assigned

cell site then alerts the switch that the hand-

off has been completed, and the old voice

channel is dropped.

It should be noted that this data exchange

happens very quickly, lasting only as long as

260 milliseconds. However, when data or

signaling tones are transmitted, audio is

muted for the duration of that transmission

and a syllable or two may be dropped from

conversation. This is normally not a prob-

lem, but during data signaling, such as that

employed for telefacsimile, answering ma-

chine, and computer communications, signifi-

cant amounts of information may be lost. For

this reason it is recommended that when

THE Cellular Connection™ equipment is

used the vehicle should be stationary to avoid

data loss during hand-offs and other data

transmissions. Otherwise the equipment

should employ an error correction protocol.

Analog Cellular

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 132 133

Comments to this Manuals

No comments