Abstract:
Advanced subscriber outage notification methods ( 100 ) notify a user of a subscriber unit ( 30 ) that its call is going to be dropped. The subscriber unit ( 30 ), the satellite ( 20 ) or the mobile telecommunication system ( 10 ) first predicts whether an outage is going to occur and how much time there is before the outage. Second, the subscriber unit ( 30 ) notifies the user of the outage and the time remaining before the outage. Third, the subscriber unit ( 30 ) waits for whether the user wants to end the call. Next, the subscriber unit ( 30 ), the satellite ( 20 ) or the mobile telecommunication system ( 10 ) terminates the call if the user wants to end the call. Lastly, the mobile telecommunication system ( 10 ) drops the call once the time remaining before the outage expires.

Description:
TECHNICAL FIELD 
     This invention relates generally to mobile telecommunication methods and, in particular, to space-based mobile telecommunication methods for notifying users in advance of dropping their calls. 
     BACKGROUND OF THE INVENTION 
     A space-based mobile telecommunication system is unique from ground-based or terrestrial cellular systems in at least three ways. First, the movement of subscribers from cell to cell is dictated by predictable motions of the satellites and not the movement of individual subscribers as in ground-based telecommunication systems. 
     Second, it is unique in the aspect of system outages and subsequent repair. A ground-based telecommunication system is accessible by field maintenance personnel who can fix problems as they arise. In contrast, a space-based mobile telecommunication system is prohibitively expensive and impractical to repair by traditional means. When a problem arises on a satellite, someone may have to decide whether the degree of the impairment is greater than the cost of decommissioning the entire satellite and replacing it with a new one. Even if another satellite is available, it may take a significant amount of time to maneuver the spare satellite into the orbit to replace the nonfunctioning satellite. Therefore, there will be coverage holes in the normally global communication coverage provided by the space-based mobile telecommunication system when a satellite becomes non-operational. 
     Third, space-based mobile telecommunication systems have frequent frequency replanning. In ground-based systems, frequency replanning is often measured on a scale of months. Space-based systems, however, are forced into frequency replanning after a few seconds because of the constantly changing relationship of the satellites to each other and to earth. One of the disadvantages of frequent frequency replanning is that cells undergoing frequency replanning may find that the new frequency plan does not support as much capacity as the old plan. In some cases, this may lead to some calls being terminated for lack of call capacity. 
     For these reasons, there is a significant need for methods which prewarn a subscriber that its call will shortly be dropped due either to a non-functioning satellite or a reduction in call capacity resources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a general view of a space-based mobile telecommunications system according to a preferred embodiment of the present invention; 
     FIG. 2 shows a flowchart of a method for advanced user outage notification according to a preferred embodiment of the present invention; and 
     FIG. 3 shows a flowchart of a process for predicting when an outage will occur according to a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A “satellite” as used throughout this description means a manmade object or vehicle intended to orbit earth and comprises both geostationary and orbiting satellites and/or combinations thereof including low-earth and medium-earth orbiting satellites. A “constellation” means an ensemble of satellites arranged in orbits for providing specified coverage (e.g., radio communication, photogrammetry, etc.) of portion(s) or all of earth. A constellation typically includes multiple rings (or planes) of satellites and may have an equal number of satellites in each plane, although this is not essential. The terms “cell,” “beam” and “antenna pattern” are not intended to be limited to any particular mode of generation and include those created by either terrestrial or spaced-based mobile telecommunication systems and/or combinations thereof. 
     FIG. 1 shows a general view of a space-based mobile telecommunication system  10  according to a preferred embodiment of the present invention. Although FIG. 1 illustrates a highly simplified diagram of mobile telecommunication system  10 , system  10  comprises at least one satellite  20 , any number of subscriber units  30  and at least one ground station  40 . 
     The present invention is applicable to mobile telecommunication systems  10  having at least one satellite  20  in low-earth, medium-earth or geosynchronous orbit. Satellite  20  is preferably a satellite in low-earth orbit around earth. Satellite  20  may be a single satellite or one of many satellites in a constellation of satellites orbiting earth, like the IRIDIUM® system which has a constellation of sixty-six satellites in low-earth orbits. The present invention is also applicable to mobile telecommunication systems  10  having satellites  20  which orbit earth at any angle of inclination (e.g., polar, equatorial or another orbital pattern). The present invention is applicable to systems  10  where full coverage of the earth is not achieved (i.e., where there are “holes” in the communication coverage provided by the constellation) and to systems  10  where plural coverage of portions of the earth occur (i.e., more than one satellite is in view of a particular point on earth&#39;s surface). 
     Satellites  20  communicate with other nearby or adjacent satellites  20  through cross-links. Thus, a call or communication from subscriber unit  30  located at any point on or near the surface of the earth may be routed through the constellation of satellites  20  to within range of substantially any other point on the surface of the earth. A communication may be routed down to subscriber unit  30  or near the surface of the earth from satellite  20 . Accordingly, mobile telecommunication system  10  may establish a communication path for relaying data through the constellation of satellites  20  between any two subscriber units  30  located almost anywhere on or near the surface of the earth. 
     Satellite  20  communicates with and are controlled by at least one ground station  40 . Ground station  40  provides satellite control commands to satellite  20  so that it maintains its proper position in its orbit and other house-keeping tasks. Moreover, ground station  40  may be connected to public switched telephone networks (PSTNs) and may provide a link between the PSTNs and satellite  20 . 
     Subscriber units  30  may be located anywhere on the surface of earth or in the atmosphere above earth (e.g., in an jet). Mobile telecommunication system  10  may accommodate any number of subscriber units  30 . Subscriber units  30  are preferably communication devices capable of transmitting voice and/or data to and receiving voice and/or data from satellites  20 . By way of example, subscriber units  30  may be hand-held, portable cellular telephones adapted to communicate with satellites  20 , or it may be a facsimile device. Subscriber units  30  may also be a dual mode cellular telephone which can transmit to and receive from satellite  20  and terrestrial cellular systems or equipment. Ordinarily, subscriber units  30  need not perform any control functions for mobile telecommunication system  10 . Subscriber units  30  comprise hardware which is well known to those of ordinary skill in the art for communicating with satellite  20 . 
     How subscriber units  30  physically transmit data (or calls) to and receive data (or calls) from satellites  20  is well known to those of ordinary skill in the art. In the preferred embodiment of the present invention, subscriber units  30  communicate with nearby satellites  20  using a limited portion of the electromagnetic spectrum that is divided into numerous channels (e.g., ring-alert channels, broadcast channels, acquisition channels and traffic channels). The channels are preferably combinations of L-Band and/or K-Band frequency channels but may encompass Frequency Division Multiple Access (FDMA) and/or Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDMA) communication or any combination thereof. Other methods may be used as known to those of ordinary skill in the art. 
     FIG. 2 shows a flowchart of method  100  for advanced user outage notification according to a preferred embodiment of the present invention. Generally, method  100  waits in step  102  until an outage is predicted and notifies a user through subscriber unit  30  in step  104  that an outage is imminent. The user of subscriber unit  30  then chooses in step  106  whether to end the call or not. If subscriber unit  30  ends the call, the call terminates in step  108 . Otherwise the call continues until it is dropped in step  110 . A detailed description of each of the steps is discussed below. 
     According to FIG. 2, method  100  waits in step  102  until an outage is going to occur (or predicted to occur). An outage is predicted to occur either by satellite  20  or subscriber unit  30  in a variety of ways. One way is for subscriber unit  30  to determine whether there are no other available cells to which it can handoff. For example, satellite  20  may provide subscriber unit  30  with a list of upcoming candidate handoff cells that are currently out-of-service. Subscriber unit  30  then scans the list to determine if all up-coming candidates are out-of-service such that an outage will occur. 
     Another way to determine whether an outage is going to occur is shown in FIG.  3 . As shown in FIG. 3, subscriber unit  30  determines in step  120  to which cell it is going to handoff. This determination is based on its current position on earth, the relative position of the satellite cells and their size, shape and location. This information is provided from satellite  20  to subscriber unit  30  so that subscriber unit  30  can calculate to which cell it has to handoff. Subscriber unit  30  then determines in step  122  whether the cell it is going to handoff to is a dead cell (i.e., a cell that cannot service the call of subscriber unit  30  because it is overloaded, out-of-service or out of radio contact). If the cell is not dead, the process returns to step  120 . 
     If the cell is dead as determined in step  122  of FIG. 3, subscriber unit  30  then determines in step  124  how much time there is before the outage or the handoff to the dead cell is going to occur. This time can be used to inform the user of subscriber unit  30  of how long before an outage is going to occur. The process ends after step  124 . 
     There are other ways for determining whether an outage is going to occur that are well known to those of ordinary skill in the art other than the processes described above. Returning to FIG. 2, once an outage is going to occur, subscriber unit  30  notifies the user. The user may either be human or a machine (e.g., modem used in data transfer). If the user is a human, a message may be played to indicate that an outage is imminent. The message may also state the time remaining so that the user knows how much time remains before the call is dropped. The message may be also broadcasted, emitted, flashed or vibrated to the user. Moreover, the message may be a pre-recorded message, a sound or sounds, vibration, a light or other ways known to those of ordinary skill in the art. 
     In step  106  of FIG. 2, the user is given the option to end the call before it is dropped. This takes place by the user finishing talking (i.e., saying good-bye) and pressing the call release button (or similar button) on subscriber unit  30  to end the call, for example. There are other ways than pressing the call release button to terminate the call on subscriber unit  30  that are well known to those of ordinary skill in the art. If the user chooses to end the call in step  106 , the call is gracefully terminated in step  108  using call termination procedures well known to those of ordinary skill in the art. If the user does not choose to end the call in step  106 , the call will eventually be dropped in step  110  when the handoff to a dead cell occurs. How a call is dropped is well known to those of ordinary skill in the art. Method  100  then ends after either step  108  or step  110 . 
     An alternative embodiment of FIG. 2 is when satellite  20  finds that it does not have enough call capacity resources to keep all calls active due to frequency replanning or other resource adjustments, satellite  20  sends the affected subscriber units  30  a warning message to give its users some advance notice before dropping their calls. Subscriber units  30  will attempt to handoff to another beam if possible. If the handoff request is rejected, subscriber unit  30  may warn the user that it appears the call will be dropped. If a subscriber unit&#39;s  30  call is dropped, satellite  20  may send a message to a ground station to credit those users some amount for the system dropping their calls. 
     It is preferred that the warning messages generated when an outage is predicted to occur, a handoff is rejected and other situations be different or unique from each other. This would help the user understand the source of the interruption and deal with whatever situation arises. 
     It will be appreciated by those skilled in the art that the present invention provides a way of indicating to a user that an upcoming outage is imminent (whether due to a system or satellite failure or a lack of call capacity resources). Another advantage of the present invention is that a user has time to end a call gracefully rather than be unexpectantly dropped. 
     Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention. For example, these methods are applicable to terrestrial cellular systems, although the methods are described in relation to space-based mobile telecommunication systems  10 .