Abstract:
A method of initiating communication between a ground station and a mobile handset through a satellite beam includes the steps of transmitting a forward signal indicative of a fixed geographic area covered by each of a plurality of satellites on at least a periodic basis. The location of said mobile handset is registered by storing at least one variable geographic value associated with said forward control signal at said mobile handset. A reverse signal is transmitted from said mobile handset to said ground station which is indicative of a geographic area where said mobile handset was located during registration. At least one variable geographic value associated with said reverse signal is stored in a database accessible to said ground station. The mobile handset determines whether a currently transmitted forward signal matches said variable geographic value stored. If a match is not found, the current location of said mobile handset is re-registered. The ground station determines which of said satellite beams should initiate a call signal to said mobile handset based upon said variable geographic value stored in said database for said mobile handset.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to mobile satellite communications systems and, more particularly, to a mobile handset tracking and paging system for a mobile communications system in which the mobile handsets communicate directly with sub-geosynchronous satellites. 
     2. Discussion 
     Geosynchronous satellite constellations are less than ideal for use in mobile communications systems in which mobile handsets communicate directly with the orbiting satellites. Launching the satellites into geosynchronous orbit is more costly than lower orbits. Mobile handset transmitter power requirements become greater with increasing orbital distances from the earth. Transmission delays are prohibitively long, especially when the mobile handset communicates with another mobile handset, requiring transmission to geosynchronous orbit and back twice. 
     Proposed mobile satellite communications systems include mobile handsets which communicate directly with a sub-geosynchronous satellite constellation including a plurality of satellites. Such sub-geosynchronous satellites have lower launch costs, relatively low-power transmitters, and acceptable transmission delays. Each satellite handles a coverage region and divides the coverage region into adjacent subregions. The coverage region of one satellite slightly overlaps the coverage region of another satellite such that earth landmasses are completely covered by the sub-geosynchronous satellite constellation. Each satellite includes antennas which can generate multiple focused beams which may slightly overlap to handle the coverage region of the satellite. If the satellite is orbiting at sub-geosynchronous altitudes, areas covered by each satellite are changing, in contrast with geosynchronous satellite constellations in which coverage regions remain relatively fixed. In addition, mobile users are also changing position with time. 
     When a first fixed or mobile handset user wishes to initiate a call with a second mobile handset user, the mobile satellite communications system must page the second mobile handset using individual focused beams covering every subregion in which the second mobile handset user might be located. If the mobile communications system is ultimately to be global in scope, it must be assumed that the second mobile handset user might potentially be located anywhere on the earth. Accordingly, it may be possible that more than one satellite would need to be used to page a mobile user. 
     Paging the second mobile handset user with all of the individual focused beams for one or more satellites in the sub-geosynchronous satellite constellation would be extremely wasteful of both satellite power and bandwidth. Even in a more restricted mobile communications systems, paging the mobile user with most or all of the individual focused beams covering a large region, such as a country or state, would similarly be wasteful of both satellite power and bandwidth. Requiring the mobile user to transmit a signal to a satellite on a periodic basis to allow the communication system to readily locate the mobile handset is also wasteful. As battery-powered mobile handsets are quickly becoming the cellular phone of choice, unnecessary transmissions could substantially reduce available power which is needed by the user for normal telephone communication. 
     Therefore, a system which can track the mobile user with sufficient accuracy to reduce the number of individual focused beams used to page the mobile user, and consequently reducing bandwidth allocated to paging, is desirable. 
     SUMMARY OF THE INVENTION 
     A method of initiating communication between a ground station and a mobile handset through a satellite beam includes the steps of transmitting a forward signal indicative of a fixed geographic area covered by each of a plurality of satellite beams on at least a periodic basis. The location of said mobile handset is registered by storing at least one variable geographic value associated with said forward control signal at said mobile handset. A reverse signal is transmitted from said mobile handset to said ground station which is indicative of a geographic area where said mobile handset was located during registration. At least one variable geographic value associated with said reverse signal is stored in a database accessible to said ground station. Then, said mobile handset determines whether a currently transmitted forward signal matches said variable geographic value stored. If a match is not found, the current location of said mobile handset is re-registered. The ground station determines which of said satellite beams should initiate a call signal to said mobile handset based upon said variable geographic value stored in said database for said mobile handset. 
     According to one feature of the invention, at least one satellite generating said beams is a sub-geosynchronous satellite. 
     According to another feature of the invention, a plurality of forward signals are transmitted using said at least one sub-geosynchronous satellite. 
     In yet another feature of the invention, each of said plurality of forward and reverse signals is associated with an individual focused beam. 
     In still another feature of the invention, a plurality of grid sections are defined and are fixed with respect to the earth. The forward signals include grid section identification data including grid sections completely or partially covered by said forward signal of said individual focused beam. 
     In another feature of the invention, the mobile handset selects a strongest individual focused beam received by said mobile handset. The mobile handset stores said grid section identification data of said strongest individual focused beam. 
     Other objects, features and advantages will be readily apparent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to those skilled in the art after studying the following specification and by reference to the drawings in which: 
     FIG. 1 is a schematic block diagram of a satellite-based mobile communication system incorporating a mobile handset tracking and paging system according to the invention; 
     FIGS. 2a, 2b, 2c and 2d illustrate a grid including a plurality of grid sections, fixed with respect to the earth, and a plurality of individual focused beams generated by one or more Sub-geosynchronous satellites; 
     FIG. 3 is a schematic block diagram of a mobile handset; and 
     FIG. 4 is a logic diagram of a registration operation performed by the mobile handset tracking and paging system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, a satellite-based mobile communications system 10 is shown. Communications system 10 tracks a mobile user to reduce individual focused beams, generated by one or more satellites, used to page the mobile user. Communications system 10 includes at least one mobile handset 16 which communicates with a satellite 20 in a sub-geosynchronous orbit. Satellite 20 includes one or more antennas 22 which relay data between mobile handset 16 and a ground station 26 (including an antenna 27). While the term &#34;mobile handset&#34; is used herein, any mobile telephone is contemplated. Similarly satellite 28 includes one or more antennas 29 which relay data between mobile handset 30 and ground station 32 (including an antenna 34). Ground station 26 is preferably connected to a communications switching network 36 which can connect mobile handset 16 with a plurality of fixed users 38 or a plurality of mobile handsets, such as mobile handset 30. A database 40 stores data required to track and page users of satellite-based mobile communications system 10 as will be described more fully below. It should also be understood that database 40 could reside in a single location, such as switching network 36, or it could be a distributed database as well. Satellites 20 and 28 form part of a multi-satellite sub-geosynchronous communications constellation orbiting the earth. For example, the sub-geosynchronous satellite constellation can be a medium orbit constellation disclosed in &#34;Cellular Telephone Satellite System,&#34; European Patent Application No. 92308604.5, filed on Sep. 22, 1992, and hereby incorporated by reference. 
     Referring to FIG. 2, one or more satellites generate a beam footprint including a plurality of individual focused beams 54. Each individual focused beam 54 includes order wire channels, which coordinate communication system operations, in addition to voice channels carrying two-way voice data. Order wire channels include forward control channels and reverse access channels. Forward control channels include communications from a ground station to a mobile handset, such as information from ground station 26 relating to access to the communications system by handset 16. 
     Reverse access channels include communications from a mobile handset to a ground station, such as responses to call requests. When a user initiates communications with mobile handset 16, handset 16 selects the strongest forward control channel received as is consistent with current cellular communications systems. As such, satellite-based mobile communications system 10 can be easily implemented to operate in conjunction with or as a replacement for the existing mobile cellular communications system. 
     According to the invention and referring to FIG. 2a, a grid 59 with a plurality of grid sections 60-1, 60-2, . . . , and 60-M is established using geographical coordinates. In other words, grid 59 is fixed with respect to earth. The position of mobile handsets 16 and 30 are tracked relative to the grid sections 60 of fixed grid 59, as will be described more fully below. The grid 59 can cover the entire surface of the earth, landmasses of the earth, or only part of the landmasses of the earth. The forward control channels in each individual beam identify, using unique grid identifications (for example, numbers), all grid sections completely or partially covered by the individual focused beam at the time of transmission. While only seven individual beams are shown (for purposes of clarity) in FIG. 2a, it can be appreciated that fixed grid 59 would be entirely covered by additional individual beams of one or more satellites. 
     For example, consider the situation where the user of mobile handset 16 initiates a call to one of the plurality of fixed users 38 or to a mobile handset such as mobile handset 30. Mobile handset 16 selects the strongest individual focused beam by sampling forward control channels to identify the strongest received forward control channel. Other methods of identifying the strongest individual focused beam will be readily apparent. 
     When mobile handset 16 is located at position &#34;X&#34; in FIG. 2a in grid section 60-28, individual focused beams 54-A1, 54-A6, and 54-A7 from one or more satellites could completely or partially cover grid section 62-28. Mobile handset 16 could identify individual focused beam 54-A7 as the strongest received beam. At the time of transmission, individual focused beam 54-A7 completely or partially covers grid sections 60-20, 60-21, 60-27, 60-28, 60-29, 60-35, 60-36, and 60-37. While each individual beam 54 in FIG. 2a completely or partially covers at most nine grid sections, it can be appreciated that additional or fewer grid sections can be covered. The forward control channel for individual focused beam 54-A7 will include grid section identification data identifying each of the partially or completely covered grid sections identified above. 
     FIG. 3 is a block diagram of mobile handset 16 which includes a microprocessor 80, memory 82, an input/output interface 88, an audio input 90, an audio output 92, a display 94, a keypad input 96, a data link device 98 providing transmit and receive functions, and an antenna 100. FIG. 4 is a logic diagram of a registration operation of mobile handset 16. When initially registering, mobile handset 16 selects the strongest received individual focused beam (block 110). Mobile handset 16 will receive and store the grid section identification data transmitted by the forward control channel of the strongest individual focused beam in memory 82 (blocks 114, 118, and 122). The mobile handset 16 will then transmit beam identification data over the reverse access channel to ground station 26 (block 126). 
     Ground stations 26 and 32 perform conventional modulation and demodulation functions, and other conventional downlink and uplink functions. Ground station 26 receives the beam identification data from mobile handset 16. Ground station 26 stores grid sections corresponding to the beam identification data in the database 40 coupled to ground stations 26 and 32. Since ground station 26 knows the approximate time that mobile handset received the grid section identification data and the beam identification data identifies the satellite, ground station 26 can determine and store the grid section identification data for mobile handset 16. Re-registration of mobile handset 16 is performed only when mobile handset 16 selects the strongest individual focused beam and none of the grid sections identified by the grid section identification data sent on the forward control channel of the selected individual focused beam match any of the grid sections previously stored in memory 82 of mobile handset 16. 
     Continuing with the example above and referring to FIG. 2b, some time later the user can transport mobile handset 16 to position &#34;Y&#34; in grid section 60-21. When the user initiates a call request, mobile handset 16 can search for and select individual focused beam 54-B5 as the strongest received beam (block 110). At the time beam 54-B5 was selected by mobile handset 16, beams 54-B3, also completely or partially cover grid section 60-21. The individual focused beams completely or partially covering grid section 60-21 at this time can be generated by one or more satellites. The individual focused beams may or may not be generated by the same satellite which generated beams 54-A1 through 54-A7. 
     Mobile handset 16 receives the grid section identification data by sampling the forward control channel of the individual focused beam 54-B5 (block 114). Re-registration of mobile handset 16 will not occur since individual focused beam 54-B5 partially or completely covers grid sections 60-20, 60-21, 60-28, and 60-29 which were also completely or partially covered by individual focused beam 54-A7 (block 140). When the grid section data in the forward control channel of individual focused beam 54-B5 is compared with grid sections stored in memory 82, a match occurs and re-registration is not required. 
     However, if the user subsequently transports mobile handset 16 to position &#34;Z&#34; in grid section 60-15 (FIG. 2c), mobile handset 16 will select individual focused beam 54-C2 as the strongest received beam (block 110). The individual focused beams 54-C1 through 54-C4 may or may not be generated by the same satellite which generated beams 54-A1 through 54-A6 or 54-B1 through 54-B6. Mobile handset 16 samples the forward control channel of the strongest received beam (block 114) and compares the grid section identification data contained therein with the grid section identification stored in memory 82 (block 118). None of the grid sections previously stored in memory 82 of mobile handset 16 match the grid sections completely or partially covered by selected beam 54-C2 (block 140). Therefore, mobile handset 16 re-registers by storing (block 122) all of the grid sections contained in the grid section identification data on the forward control channel of the strongest received beam 54-C2 (including grid sections 60-6, 60-7, 60-8, 60-14, 60-15, 60-16, 60-22, 60-23, and 60-24). The previously stored grid section identification data is deleted from memory 82. Then mobile handset 16 transmits the beam identification data on the reverse access channel to ground station 26 which stores the grid section identification data in paging database 114 for mobile handset 16. It should also be appreciated that mobile handset 16 could also transmit grid section identification data instead of the beam identification data. However, such grid section identification data would require more bandwidth and transmission power than shorter-length beam identification data. 
     Ground station 26 knows the approximate time that mobile handset 16 selected the strongest individual focused beam 54-C2, and received the beam identification data identifying the strongest individual focused beam 54-C2. Database 40 or ground stations 26 and 32 could include a look-up table storing grid section numbers covered by each individual focused beam at a given time. Using the look-up tables or database 40, in addition to the beam identification data, ground station 26 could determine grid sections completely or partially covered at the time of selection of the strongest beam and could then determine and store grid sections for mobile handset 16 in database 40. 
     When mobile handset 16 is not initiating a call request or completing a call, mobile handset 16 can be placed in a standby mode waiting for incoming calls. When ground station 26 needs to complete a call to mobile handset 16, ground station 26 initiates paging to locate mobile handset 16. 
     Ground station 26 initiates a paging message for mobile handset 16 over forward control channels of every individual focused beam which completely or partially covers a grid section identified by the grid section identification data stored in database 40. In other words, in the example registration described with respect to FIG. 2a, mobile handset 16 initially registered grid sections 60-20, 60-21, 60-27, 60-28, 60-29, 60-35, 60-36, and 60-37. Ground station 26 pages mobile handset 16 using individual focused beams completely or partially covering each of the registered grid sections 60-20, 60-21, 60-27, 60-28, 60-29, 60-35, 60-36, and 60-37. Therefore, referring to FIG. 2d, and assuming the re-registration described in conjunction with FIG. 2c has not occurred (in other words, mobile handset has not left grid sections 60-20, 60-21, 60-27, 60-28, 60-29, 60-35, 60-36 and 60-37), ground station pages mobile handset on the forward control channels of individual focused beams 54-D1 through 54-D7. Accordingly, even if the mobile handset 16 does not move from position &#34;X&#34; in grid section 60-28 of FIG. 2a, different satellite beams will pass over the initially registered grid sections, and it will be these subsequently arriving beams which will be used to page the mobile handset 16. 
     While FIGS. 2a through 2d illustrate individual focused beams 54 as circular, one skilled in the art can appreciate that the shape of individual focused beams 54 on a grid section 60 will be circular when the satellite is directly above the grid section 64. Otherwise, individual focused beams 54 are ellipses and other distorted circular shapes. Dimensions of grid sections can be selected by optimizing two competing concerns. Larger grid size increases the number of beams over which the paging message must be transmitted. Smaller grid sizes increase the length of the paging message because of the large number of grid sections covered by each beam. Different grid shapes can also be used. 
     As can be appreciated from the foregoing, satellite-based mobile communications system 10 significantly reduces the number of forward control channels (each associated with an individual focused beam) used to transmit the paging message required to page a mobile handset user. Satellite-based mobile communications system 10 does not require determination of user position based upon properties of signals transmitted by either the satellites or by the mobile handset. Satellite-based communications system significantly reduces satellite power and bandwidth requirements without requiring determination of user position precisely. 
     The various advantages of the present invention will become apparent to those skilled in the art after a study of the foregoing specification and following claims.