Audible warning prior to losing cell call in progress

Details Audible warning prior to losing cell call in progress Audible warning prior to losing cell call in progress

1999-09-29

2001-12-11

Le, Thanh Cong (Department: 2684)

Telecommunications

Radiotelephone system

Out-of-range indication

C455S436000

Reexamination Certificate

active

06330438

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates generally to cellular telephone systems and is particularly directed to providing a cellular telephone user with an audible indication of low signal strength of a call in progress to allow the user to take precautionary steps to avoid loss of the call.
BACKGROUND OF THE INVENTION
Referring to
FIG. 1
, there is shown in simplified schematic diagram form a cellular telephone network
10
in which the present invention is intended for use. The three major components of the cellular telephone network
10
are a Mobile Switching Center (MSC)
16
, a plurality of cell sites, or base stations, and a mobile unit, or station,
18
.
FIG. 1
shows cell sites A, B and C in the cellular telephone network
10
. The MSC
16
is connected by wire to a Public Switched Telephone Network (PSTN)
14
shown in dotted line form in the figure. The PSTN
14
typically includes a large number of fixed telephones
12
for placing and receiving calls. The MSC
16
interfaces with the PSTN
14
and the cell sites via control links and voice trunks. The MSC
16
is also involved with switching calls between different cell sites in real time as the mobile unit
18
moves from the coverage area of one cell site to another. This process is known as a call “handoff” of the mobile unit
18
between adjacent cell sites.
A call handoff is performed in the following manner. Where the mobile unit
18
is served by cell site A, this cell site determines that the signal from the mobile unit is getting weak. Cell site A communicates the weakening of the received signal to the MSC
16
which then prompts neighboring cell sites B and C to measure the signal strength from the mobile unit and report their findings back to cell site A. Cell site A then selects the best set of neighbor cell sites that can accept the mobile unit
18
and requests a handoff from the MSC
16
. The MSC
16
then informs the new cell site B that it is to accept a handoff and directs cell site B to tune to a selected radio channel being used by the mobile unit
18
. The MSC
16
directs cell site A to direct the mobile unit
18
to tune to the new voice radio channel. The MSC
16
then switches the call from cell site A to cell site B, with the mobile unit
18
re-tuning to the new channel associated with cell site B.
The procedure described above is followed when a second cell site is available for handing off the mobile unit when the strength of the communications signal is too low to continue the call. In some cases, there may not be another cell site available for handing the mobile unit off to and the call is lost when the signal level is reduced to the point where the serving cell site can no longer maintain communications contact with the mobile unit. The present invention addresses this problem by alerting the mobile unit to a dangerously low signal level, allowing the user to take precautionary steps to avoid loss of the call.
SUMMARY OF THE INVENTION
This invention is intended for use in a cellular telephone system and is set into operation when the signal level to a mobile user from a serving cell site is reduced below a predetermined threshold signal level and there is no other cell site within the cellular network to hand the mobile user off to in order to continue the call in progress. This invention alerts the mobile user of the impending loss of the call because of the reduced signal level before it occurs and allows the user sufficient time to take precautionary steps to avoid loss of the call. Typical precautionary steps would include the user moving or re-orienting the hand-held mobile unit to compensate for the loss of signal strength by improving signal reception. An audio signal is provided during the call in progress via the hand-held mobile unit's earpiece to the user to alert the user to take appropriate precautionary steps for maintaining integrity of the communications link. The invention makes use of existing circuitry within the hand-held mobile unit to compare the received signal level with a predetermined threshold level and provide an appropriate drive signal to the unit's earpiece for providing an audio signal to the user either in the form of a continuous tone or as a pulsed signal. The invention also employs existing communications links between the mobile unit and the cell site.


REFERENCES:
patent: 5373548 (1994-12-01), McCarthy
patent: 5722068 (1998-02-01), Bartle et al.

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