Abstract:
In a cellular telephone system, as the strength of the signal between a cell site and a mobile unit weakens in the absence of another cell site to hand off the mobile unit to, the mobile unit user is warned of the impending loss of the call before it occurs. This allows the user sufficient time to take precautionary steps such as moving or re-orienting the hand-held mobile unit to compensate for the loss of signal strength and maintain integrity of the communications link. The mobile unit user is warned of a signal level reduction below a predetermined threshold by means of an audible tone emitted by the mobile unit&#39;s earpiece prior to loss of a call in progress.

Description:
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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which: 
     FIG. 1 is a simplified schematic diagram of a cellular telephone network in which the present invention is intended for use; 
     FIG. 2 is a block diagram showing the communication links between a mobile unit, a cell site, and a mobile switching center in a typical cellular telephone network; 
     FIG. 3 is a block diagram of circuitry employed in a mobile unit with which the present invention is intended for use; 
     FIG. 4 is a simplified flow diagram illustrating the signals communicated between various elements of a cellular telephone system in accordance with the present invention; 
     FIG. 5 is a combined block diagram and flow chart illustrating the flow of signals between various components of a cellular telephone system and the steps involved in providing an audible warning to a cellular telephone system user prior to losing a cellular telephone call in progress in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, there is shown in simplified block diagram form the communication links in a typical cellular telephone network. The mobile switching center (MSC)  16  is connected to the public switched telephone network (PSTN)  14  by means of a plurality of land lines  22   a ,  22   b  and  22   c . Communications between the MSC  16  and cell site A, or base station, is by means of a pair of voice circuits  24  and a pair of data links (shown in dotted line form)  26 . The voice circuits  24  allow voice communication between the MSC  16  and cell site A in both directions, while the data links  26  allow for control signals to pass in both directions between the MSC and cell site A. Cell site A is in communication with the mobile unit  18  via a forward voice channel  28  and a reverse voice channel  30 . Communications on the voice channels  28 , 30  are both analog and digital. Communications between cell site A and mobile unit  18  is also by means of forward and reverse setup channels  32  and  34  (shown in the figure in dotted line form). Communications on the setup channels is in digital form. The setup channels allow the mobile unit  18  to access the cellular telephone network. The voice channels  28 , 30  are used for voice communication, for handoff of a mobile unit between a pair of cell sites, dynamic power control, and other functions carried out between a cell site and a mobile unit. 
     Referring to FIG. 3, there is shown a simplified combined schematic and block diagram of a hand-held mobile unit  40  with which the present invention is intended for use. Mobile unit  40  is conventional in design and operation and includes (as shown in dotted line form), a transmitter  42 , a user interface  44 , and a receiver  46 . A tone request signal described below is received by the antenna  48  of the mobile unit  40  from the cell site with which the mobile unit is in communication. The tone request signal is processed by the mobile unit&#39;s receiver  46  for providing an audio tone at the mobile unit&#39;s earpiece  50 . The audio tone emitted by earpiece  50  alerts a user of the mobile unit  40  that the cellular telephone signal level is reduced and that the communications link will shortly be lost because of the absence of another cell site to hand the call off to. This audio signal alerts the user to take precautionary measures to avoid interruption of the cellular call such as by reorienting or repositioning the mobile unit to increase the received signal strength. 
     Referring to FIG. 4, there is shown in simplified schematic diagram form a procedure carried out in a cellular telephone network for providing an audible warning prior to losing a cell call in progress in accordance with the present invention. At the top of FIG. 4 are listed four elements of the cellular telephone network, i.e., cell sites  1  and  2 , an MSC, and a mobile station. Arrows in the figure represent the transmission of signals between the various elements of the network, with the process starting at the top of the figure and continuing over time in proceeding downwardly in the figure. Initially, cell site  1  determines that the strength of the reverse Traffic Channel (TCH) signal from the mobile station requires the consideration of a handoff of a call to the mobile station from cell site  1  to another cell site. Cell site  1  then sends a handoff request to the MSC containing information relevant to the mobile station such as power level, station class mark (SCM), and current reverse TCH signal strength. The MSC receives the handoff request and determines which cells are adjacent to cell site  1 . In the present example, cell site  2  is adjacent the MSC. Thus, the MSC sends a handoff measurement request to cell site  2  as shown in the figure. The handoff measurement request includes essentially the same information as that received by the MSC from cell site  1 . Cell site  2  receives the handoff measurement request and uses its locating receiver to determine the suitability of handoff. The locating receiver in cell site  2  tunes to the reverse TCH frequency the mobile station is currently using, makes signal strength measurements on all (or some) antennas, and determines whether or not the Supervisory Audio Tone (SAT) color code the mobile station is using can be detected. Cell site  2  then provides a handoff measurement response and a power level indication to the MSC. In the present invention, cell site  2  provides a hand off response to the MSC indicating that a hand off of the call is not possible. 
     Referring to FIG. 5, there is shown a simplified flowchart of the flow of signals and the steps carried out at a cell site for providing an audio indication to a mobile unit user that the received signal level is below a predetermined threshold level in accordance with the present invention. As described earlier, each cell site  64  is connected to a PTSN  60  by means of a mobile switching center  66 . Details of the communications links between the MSC  66  and the PTSN  60  and cell site  64  as well as with the mobile unit  62  are described with reference to FIG. 2. A cell site controller  64   b  compares the power level of the received voice signal at step  68  with a predetermined threshold power level. If the voice receiver power level is not less than the predetermined threshold power level, the program stored in the cell site&#39;s controller  64   b  proceeds to step  82  and enables the mobile unit  62  to communicate with the cell site  64  via the cell site&#39;s transmitter/receiver  64   a.    
     If at step  68 , it is determined that the voice receiver power level is less than or equal to the alert threshold power level, the program proceeds to step  70  to determine if there is another cell site available to handoff the mobile unit to. If the program determines at step  70  that there is another available cell site for handling the communications of the mobile unit, the program proceeds to step  80  and processes a hand off request message from the mobile unit. If at step  70  it is determined that there is not another cell site available to hand off the mobile unit to, the program proceeds to step  72  and sends a flash intercept message with tone and text to the mobile unit. This signal is provided to the primary radio controller at step  74  and thence to the primary radio cluster controller at step  76 . The signal is then provided from the primary radio cluster controller  76  to the mobile unit at step  78  for activating the mobile unit&#39;s earpiece for providing an audio tone to the user indicating that the communications signal is below a predetermined alert threshold power level and that communications will be lost unless the user undertakes precautionary steps to either maintain or increase the signal level. 
     If the handoff measurement response provided by cell site  2  to the MSC indicates that cell site  2  is not a good candidate for call handoff, e.g., the signal strength is too low, the MSC sends a Tone Request to cell site  1  which, in turn, relays the Tone Request to the mobile station. Typically, a database in the cell site is used to determine whether or not a response to the handoff measurement request is warranted. An algorithm is used to detect the need for handoff and to detect the probability that the new cell site will be able to sustain the call. The database as well as the algorithm used in the cell site for call handoff purposes are not part of the present invention and thus not described further herein. The tone request signal provided to the mobile station is processed by the mobile station to provide an audio tone from the mobile station&#39;s earpiece as previously described. Tone request signals are currently provided to telephone receivers, including mobile telephone receivers, for such purposes as providing an audio indication of call waiting. Current applications of the use of tone signals provided to a mobile station do not include the use of these types of signals for indicating to a user of the mobile unit of an impending loss of a cellular telephone call as contemplated by the present invention. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawing is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.