User paging for mobile satellite communications

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.

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.

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 "mobile handset" 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 "Cellular Telephone Satellite 
System," 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 "X" 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 "Y" 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 
"Z" 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 "X" 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.