Communication system employing spectrum reuse on a spherical surface

Orbiting satellites project footprints on the earth. Each footprint is divided into cells. Footprints of nearby satellites overlap one another, and the overlap increases as satellites approach a pole from the equator. The positions of all cells generated by all satellites are simulated at numerous points throughout an orbit. Cells are marked as being active or inactive to compensate for the overlap at each simulated point. Channel sets are assigned to cells within a reference footprint, and this assignment is propagated throughout all footprints in the referenced footprint's orbit. The channel set sequences are propagated to other orbits after taking into account inactive cells located near boundaries between the orbits. Each satellite stores channel set assignments for its cells at the various points throughout its orbit. All satellites switch their communication parameters at the same instant in response to the channel set assignments.

RELATED PATENT 
The present invention is related to "Satellite System Cell Management" by 
Pullman et al., Ser. No. 07/812,389, filed Dec. 23, 1991, assigned to the 
assignee of the present invention, and incorporated herein by reference. 
TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to communication systems. More 
specifically, the present invention relates to systems that divide an area 
within which communications are to take place into cells and which reuse 
spectrum among certain ones of the cells. 
BACKGROUND OF THE INVENTION 
Conventional cellular communication systems adopt a frequency reuse plan. 
Generally speaking, system antennas are erected at spaced apart locations. 
Each system antenna, along with transmitter power, receiver sensitivity, 
and geographical features, defines a cell. A cell is a geographical area 
on the surface of the earth within which communications may take place via 
a subscriber unit having predetermined operating characteristics and via 
the cell's antenna. In a cellular system that efficiently uses the 
spectrum allocated to it, system antennas are located to minimize overlap 
between their respective cells and to reduce gaps between the cells. 
The spectrum allocated to a conventional cellular system is divided into a 
few discrete portions, typically frequency bands. Each cell is allocated 
only one of the discrete portions of the spectrum, and each cell is 
preferably surrounded by cells that use other discrete portions of the 
spectrum. Communications within a cell use only the discrete portion of 
the spectrum allocated to the cell, and interference between 
communications taking place in other nearby cells is minimized because 
communications in such nearby cells use different portions of the 
spectrum. Co-channel cells are cells that reuse the same discrete portion 
of spectrum. To minimize interference, the frequency reuse plan spaces 
co-channel cells a predetermined distance apart. 
A cellular communication system which places antennas in moving orbits 
around the earth faces particular problems related to distributing 
discrete portions of the allocated spectrum to various cells. Due to the 
approximately spherical shape of the earth, cells which do not overlap in 
one region of the earth, such as the equator, may very well overlap in 
other regions, such as polar regions. When cells overlap, the co-channel 
cells that the overlapping cells are spacing apart reside closer together 
than permitted by the spectrum reuse plan. Interference between 
communications taking place in such closely spaced co-channel cells 
becomes more likely. 
In addition, when antennas move relative to each other, the overlap between 
cells changes as a function of time. Any allocation of discrete portions 
of the spectrum to the cells remains valid only until relative movement of 
the antennas causes the overlap between the cells to change. 
SUMMARY OF THE INVENTION 
Accordingly, it is an advantage of the present invention that an improved 
communication system is provided. 
Another advantage of the present invention is that a cellular communication 
system is provided which efficiently reuses spectrum throughout a 
spherical surface, such as the surface of the earth, to increase channel 
capacity given a fixed frequency spectrum. 
Yet another advantage is that the present invention operates a cellular 
communication system's antennas when the antennas project overlapping 
cells. 
Still another advantage is that the present invention provides a 
communication system that dynamically assigns discrete portions of 
spectrum to cells to compensate for varying overlap between the cells. 
The above and other advantages of the present invention are carried out in 
one form by an improved method of operating first and second spaced apart 
antennas. The first and second antennas project first and second 
footprints, respectively. These first and second footprints are each 
divided into a plurality of cells. The method calls for positioning the 
antennas so that the first and second footprints overlap. The cells of the 
footprints are defined as being active or inactive in response to the 
overlap. Channels are assigned to the active ones of the cells from the 
first and second footprints in accordance with a spectrum reuse plan that 
maintains a minimum predetermined separation distance between co-channel 
cells. 
The above and other advantages of the present invention are carried out in 
another form by an improved method of reusing spectrum on an approximately 
spherical surface. The method calls for simulating locations for first and 
second footprints projected on the surface from corresponding first and 
second antennas positioned outside the surface. Each footprint is divided 
into a plurality of cells, and the first and second footprints at least 
partially overlap one another. Channels of the spectrum are assigned to 
the first footprint cells in accordance with a spectrum assignment plan 
that spaces co-channel cells a predetermined minimum distance apart. 
Active and inactive cells are defined. The inactive cells are located near 
a boundary between the first and second footprints to compensate for 
overlap therebetween. Channels of the spectrum are assigned to the active 
cells of the second footprint. The second footprint cells are determined 
in response to the active cells assigned to the first footprint and to the 
inactive cells defined for the first and second footprints to maintain 
approximately the minimum distance between active co-channel cells on 
opposing sides of the boundary. Communications take place through the 
antennas in accordance with the assignments of the channels to the active 
cells in the first and second footprints.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a satellite-based communication network 10. Network 10 
is dispersed over the earth through the use of several above-the-earth 
communication nodes, such as orbiting satellites 12. In the preferred 
embodiment, satellites 12 occupy polar, low-earth orbits 14. In 
particular, the preferred embodiment of network 10 uses seven polar 
orbits, with each orbit holding eleven satellites 12. For clarity, FIG. 1 
illustrates only a few of these satellites 12. 
Orbits 14 and satellites 12 are distributed around the earth. In the 
example depicted by the preferred embodiment, each orbit 14 encircles the 
earth at an altitude of around 765 km. Due to the relatively low orbits of 
satellites 12, substantially line-of-sight electromagnetic transmissions 
from any one satellite cover a relatively small area of the earth at any 
point in time. For example, when satellites 12 occupy orbits at around 765 
km above the earth, such transmissions cover "footprint" areas around 4075 
km in diameter. 
Due to the low-earth character of orbits 14, satellites 12 travel with 
respect to the earth at around 25,000 km/hr. This allows a satellite 12 to 
be within view of a point on the surface of the earth for a maximum period 
of around nine minutes. However, satellites 12 form a constellation in 
which satellites 12 remain relatively stationary with respect to one 
another, except for two modes of movement. 
One mode of movement results from orbits 14 converging and crossing over or 
intersecting each other in the polar regions. Due to this mode of 
movement, the distance between satellites 12 that reside in a common orbit 
14 remains substantially constant. However, the distance between 
satellites 12 that reside in adjacent orbits 14 varies with the latitudes 
of the satellites 12. The greatest distance between these cross-plane 
satellites 12 exists at the equator. This distance decreases as 
cross-plane satellites 12 approach the polar regions and increases as 
cross-plane satellites 12 approach the equator. 
A second mode of movement occurs at a constellation seam 16. Seam 16 
divides the earth into two hemispheres with respect to the constellation 
of satellites 12. In one hemisphere, satellites 12 move from 
south-to-north, as indicated by direction arrows 18 in FIG. 1. In the 
other hemisphere, satellites 12 move from north-to-south, as indicated by 
direction arrow 20 in FIG. 1. Seam 16 resides on opposing sides of the 
earth between a south-north orbit 14 and a north-south orbit 14. At seam 
16 satellites 12 approach and pass each other at a rate of around 50,000 
km/hr. 
Satellites 12 communicate with devices on the ground through many central 
switching offices (CSOs) 22, of which FIG. 1 shows only one, a few ground 
control stations (GCSs) 24, of which FIG. 1 shows only one, and any number 
of radiocommunication subscriber units 26, of which one is shown in FIG. 
1. Subscriber units 26 may be located anywhere on the surface of the earth 
or in the atmosphere above the earth. CSOs 22 are preferably distributed 
over the surface of the earth in accordance with geo-political boundaries. 
GCSs 24 preferably reside in extreme northern or southern latitudes, where 
the convergence of orbits 14 causes a greater number of satellites 12 to 
come within direct line-of-sight view of a single point on the surface of 
the earth when compared to more equatorial latitudes. Preferably, around 
four GCSs 24 are used so that all satellites 12 in the constellation may 
at some point in their orbits 14 come within direct view of their assigned 
GCS 24. 
Nothing prevents CSOs 22 and GCSs 24 from being located together on the 
ground. However, CSOs 22 serve a different function from that of GCSs 24. 
GCSs 24 preferably perform telemetry, tracking, and control (TT&C) 
functions for the constellation of satellites 12. Preferably, CSOs 22 
operate as communication nodes in network 10. Diverse terrestrial-based 
communications systems, such as the worldwide public switched 
telecommunications network (not shown), may access network 10 through CSOs 
22. Due to the configuration of the constellation of satellites 12, at 
least one of satellites 12 is within view of each point on the surface of 
the earth at all times. Accordingly, network 10 may establish a 
communication circuit through the constellation of satellites 12 between 
any two subscriber units 26, between any subscriber unit 26 and a CSO 22, 
or between any two CSOs 22. 
FIG. 2 shows a static layout diagram of an exemplary cellular antenna 
pattern achieved by six of satellites 12, wherein three of the six 
satellites are sequentially positioned in one orbit 14 and another three 
of the six satellites 12 are sequentially positioned in an adjacent orbit 
14. For clarity, FIG. 2 depicts only the first three of satellites 12. 
Each satellite 12 includes an array (not shown) of directional antennas, 
which may also be viewed as a single multi-directional, multi-beam 
antenna. Each array projects numerous discrete antenna patterns on the 
earth's surface at numerous diverse angles away from its satellite 12. 
FIG. 2 shows a diagram of a resulting pattern of cells 28 that satellites 
12 collectively form on the surface of the earth. With satellites 12 
positioned at 765 km above the earth, cells 28 are around 690 km in 
diameter. With satellites 12 traveling at speeds of up to 25,000 km/hr 
with respect to the earth, cells 28 also travel over the earth at close to 
this speed, and any given point on the surface of the earth resides within 
a single cell 28 for no more than around one minute. 
The pattern of cells 28 which a single satellite 12 projects on the earth's 
surface is referred to as a footprint 30. FIG. 2 depicts footprints 30 as 
each having forty-eight cells 28. However, the precise number of cells 28 
included in a footprint 30 is unimportant for the purposes of the present 
invention. FIG. 2 further illustrates an overlap 32 which results from the 
above-discussed convergence of orbits 14. The size of overlap 32 varies in 
response to the location of the overlapping footprints 30. As can be 
determined by reference to FIGS. 1-2, the greatest amount of overlap 32 
occurs in the polar regions of the earth while little or no overlap occurs 
in the equatorial regions of the earth. Those skilled in the art will 
appreciate that FIG. 2 represents a static snap-shot of footprints 30, and 
that the portion of overlap 32 which is associated with any two adjacent 
cross planar footprints 30 changes as satellites 12 move within orbits 14. 
For convenience, FIG. 2 illustrates cells 28 and footprints 30 as being 
discrete, generally hexagonal shapes without overlap or gaps, other than 
those attributed to the convergence of orbits 14 near the polar regions of 
the earth and the divergence of orbits 14 near the equatorial regions of 
the earth. However, those skilled in the art will understand that in 
actual practice equal strength lines projected from the antennas of 
satellites 12 may be more circular or elliptic than hexagonal, that 
antenna side lobes may distort the pattern, and that some preferably minor 
overlap between adjacent cells may be expected. 
While FIGS. 1-2 and the above-presented discussion describe a preferred 
orbital geometry for network 10, those skilled in the art will appreciate 
that the communication nodes which satellites 12 provide need not be 
positioned precisely as described herein. For example, such nodes may be 
located on the surface of the earth or in orbits other than those 
described herein. Likewise, the precise number of nodes may vary from 
network to network. 
The constellation of satellites 12 communicates with all of subscriber 
units 26 using a limited amount of the electromagnetic spectrum. The 
precise parameters of this spectrum are unimportant to the present 
invention and may vary from network to network. The present invention 
divides this spectrum into discrete portions, hereinafter referred to as 
channel sets. The precise manner of dividing this spectrum is also 
unimportant to the present invention. For example, the spectrum may be 
divided into discrete frequency bands, discrete time slots, discrete 
coding techniques, or a combination of these. Desirably, each of these 
discrete channel sets is orthogonal to all other channel sets. In other 
words, simultaneous communication may take place at a common location over 
every channel set without significant interference between the channel 
sets. 
Likewise, the precise number of channel sets into which the spectrum is 
divided is not important to the present invention. FIG. 2 illustrates an 
exemplary assignment of channel sets to cells 28 in accordance with the 
present invention and in accordance with a division of the spectrum into 
twelve discrete channel sets. FIG. 2 references the twelve discrete 
channel sets through the use of the characters "A", "B", "C", "D", "E", 
"F", "G", "H", "I", "J", "K", and "L". Those skilled in the art will 
appreciate that a different number of channel sets may be used and that, 
if a different number is used, the resulting assignment of channel sets to 
cells 28 will differ from the assignment pattern depicted in FIG. 2. 
Likewise, those skilled in the art will appreciate that each channel set 
may include one channel or any number of orthogonal channels therein. 
FIG. 3 shows a flow chart of a procedure 34 used in one embodiment of the 
present invention to assign particular channel sets to particular cells 
28. The procedure depicted in FIG. 3 may be practiced on a general purpose 
computer or on-board one or more satellites 12, a block diagram of which 
is presented below in connection with FIG. 12. Those skilled in the art 
will appreciate that a general purpose computer may include one or more 
processors (not shown) which perform steps described in the flow chart of 
FIG. 3 in response to instructions stored in a memory (not shown) thereof. 
Procedure 34 performs a task 36 to simulate the position of all cells 28 
relative to one another. For a first iteration of procedure 34, any 
position for the constellation of satellites 12 will suffice. For 
subsequent iterations, the simulated positions are desirably the positions 
which result from the movement of satellites 12 within orbits 14 away from 
a position associated with the previous iteration of procedure 34 for a 
predetermined increment of time. In the preferred embodiment, this 
increment approximates a one cell diameter displacement from corresponding 
positions used in the previous iteration. 
Task 36 may record the position of each cell 28 in a table similar to a 
position table 38, a block diagram of which is shown in FIG. 4. Each 
cell's position may be expressed by the cell's latitude and longitude, or 
in any other convenient form. The position data may be calculated by 
applying conventional trigonometric techniques to orbital and antenna 
geometries. In particular, the cells' positions may be determined from the 
orbits' positions, the satellites' speed, orbits' distance from the earth, 
and angles of displacement for various beams supported by the satellites' 
antennas away from the satellites' Nadir directions. The positions 
recorded in table 38 may desirably describe the location of the center of 
each cell 28 on the surface of the earth. If each footprint 30 includes 
forty-eight of cells 28 and the constellation includes seventy-seven of 
satellites 12, then task 36 describes 3696 positions. If each footprint 30 
includes thirty-seven of cells 28 and the constellation includes 
seventy-seven of satellites 12, then task 36 describes 2849 positions. 
With reference back to FIG. 3, after task 36 a task 40 defines each cell 
generated from the operation of the constellation of satellites 12 as 
being either active or inactive. Active cells may be viewed as being 
turned "on" while inactive cells may be viewed as being turned "off". 
Satellites 12 will refrain from broadcasting transmissions within inactive 
cells 28, and any signals received at satellites 12 from inactive cells 28 
will be ignored. Satellites 12 transmit/receive signals to/from active 
cells using channel sets assigned to the respective active cells. 
Generally speaking, task 40 may be performed by analyzing the positions 
recorded in table 38 (see FIG. 4) during task 36. The distances between 
the center of each cell and the centers of all other cells may be compared 
with a predetermined distance. When the distance between two of cells 28 
is less than this predetermined distance, an overlap is declared between 
the two cells. In the preferred embodiment, an overlap is declared when at 
least 70% of two cells 28 occupy the same area. Task 40 then determines 
which of the two overlapping cells 28 to define as being inactive to cure 
the overlap. Generally, task 40 defines a cell 28 located toward the outer 
region of its footprint 30 as being inactive rather than an overlapping 
cell 28 located closer to the center region of its footprint 30. Any cell 
which is not declared as being inactive is defined to be active. 
Additional details related to the operation of task 40 may be obtained 
from the above-referenced related patent. 
After task 40, a task 42 assigns channel sets to all active cells in the 
south-north hemisphere of the constellation of satellites 12. As discussed 
above in FIG. 1, this hemisphere is separated from a north-south 
hemisphere by seam 16. These assignments are preferably recorded in 
position table 38 (see FIG. 4) in association with each cell's identity. 
Details related to the assignment of channel sets to cells are discussed 
below in connection with FIGS. 5-11. After the completion of task 42, a 
task 44 repeats this assignment procedure, except that task 44 assigns 
channel sets for the north-south hemisphere of the constellation of 
satellites 12. 
After task 44, a query task 46 determines whether cell positions for an 
entire orbit have been simulated. If the entire orbit has not been 
simulated yet, program control loops back to task 36 to increment the 
cells' positions and repeat the assignment process. If task 46 determines 
that the entire orbit has been simulated, then procedure 34 may stop 
because channel set to cell assignments for additional positions will 
generally duplicate previous assignments recorded in table 38 (see FIG. 
4). 
Satellites 12 may then use this assignment data in controlling the 
operation of their transceivers and antennas, as discussed below in 
connection with FIGS. 12-14. Preferably, procedure 34 is performed 
"off-line" and only those channel set to cell assignments for an entire 
orbit which result from procedure 34 and relate to a given satellite 12 
are recorded in the memory of that satellite 12. However, in an alternate 
embodiment, satellites 12 may perform procedure 34, or portions of it, 
themselves and utilize the resulting channel set to cell assignments to 
control their transceiver's and antenna's operations. In this alternative 
embodiment, the loop depicted in FIG. 3 may repeat indefinitely, with each 
iteration of the loop simulating the positions of the satellites' cells 
just prior to the time when the cells actually reach the simulated 
positions. The resulting assignments will then be available when needed, 
and additional assignments will not be needed until the satellite movement 
causes cells to move to their incremented positions. 
FIG. 5 shows a flow chart of an Assign Channel Sets procedure 48 which is 
used by tasks 42 and 44 of procedure 34 (see FIG. 3) to assign channel 
sets to the cells generated by one hemisphere of the constellation of 
satellites 12. A task 50 in procedure 48 selects a reference footprint 30a 
in a reference orbital plane 14a, as shown in FIG. 6. 
FIG. 6 shows an exploded layout diagram for six of footprints 30. FIG. 6 is 
similar to the layout diagram shown in FIG. 2. However, FIG. 6 illustrates 
a system which uses thirty-seven cells 28 per footprint 30 rather than the 
forty-eight cells 28. In addition, FIG. 6 is an exploded diagram because 
the footprints 30 produced by satellites 12 in one orbit 14 are spaced 
apart from the footprints 30 produced by satellites 12 in an adjacent 
orbit 14. FIG. 6 illustrates actually overlapping footprints 30 as being 
separated to clarify the below-discussed procedure for assigning channel 
sets to cells. 
While any footprint 30 in the selected hemisphere may serve as reference 
footprint 30a, task 50 (see FIG. 5) in the preferred embodiment utilizes 
an equatorially located footprint 30 in an orbital plane 14 adjacent to 
seam 16 (see FIG. 1). After task 50, a task 52 follows a channel set 
assignment plan to assign channel sets to the cells 28 located within 
reference footprint 30a. FIG. 6 shows an exemplary twelve cell reuse 
pattern that task 52 assigns to cells 28 of reference footprint 30a. In 
other words, twelve discrete channel sets are distributed among cells 28 
of footprint 30a so that co-channel cells are spaced a minimum 
predetermined distance apart from one another. 
Those skilled in the art will appreciate that footprints 30 located in the 
two hemispheres may use different channel sets to prevent interference at 
seam 16. 
Task 52 may assign channel sets using the plan or formula: 
EQU N=i.sup.2 +j.sup.2 +i*j 
where, 
N=the number of discrete channel sets available in the spectrum being 
assigned; 
i=a shift parameter for a first direction; 
j=a shift parameter for a second direction; and 
i.gtoreq.j. 
Using this formula, co-channel cells are determined by moving "i" cells in 
a first direction away from a source cell, as shown by line segment 54 in 
FIG. 6, then rotating clockwise (or counter-clockwise) 60.degree. and 
moving "j" cells, as shown by line segment 56 in FIG. 6. The results from 
applying this assignment plan for all cells 28 within footprint 30a are 
then recorded in a memory structure, such as position table 38 (see FIG. 
4). 
In an alternate embodiment, task 52 (see FIG. 5) may use a table (not 
shown) which associates channel set assignments with cell numbers. In 
connection with another one of footprints 30, FIG. 6 shows an exemplary 
assignment of cell numbers to cells 28. These cell numbers are indicated 
by the numerals 1-37 in FIG. 6. Each footprint 30 preferably uses the same 
cell numbering scheme. The cell numbers are used to identify cells 28 in 
position table 38 (see FIG. 4) and other memory structures used by 
procedures 34 (see FIG. 3) and 48. Those skilled in the art will 
appreciate that the particular assignment of cell numbers to cells 28 is 
arbitrary and that, so long as each cell 28 within a footprint 30 has its 
own unique identifier, other cell numbering schemes may be used as well. 
In this alternate embodiment, task 52 may assign channel sets to the cells 
28 of reference footprint 30a by copying channel set assignments to an 
appropriate section of a memory structure, such as position table 38. 
In yet another embodiment of the present invention, task 52 may utilize one 
table (not shown) to assign a channel set to one predetermined cell in 
each row 60 of reference footprint 30a. Then, this embodiment may use a 
row sequence table 62, such as that illustrated in FIG. 7, to propagate 
the assignments left and/or right, with respect to the direction of 
movement of satellite 12, within each row 60 until the boundaries of 
footprint 30a have been reached. The locations of each cell 28 in a 
footprint 30 may be determined from any subject cell in the footprint 30 
by reference to a cell location table 64, such as that shown in FIG. 8. 
Using table 64, a table look-up operation may be performed to determine 
which cell is located immediately in left, right, up left, up right, down 
left, or down right directions from any other cell within a footprint 30. 
Those skilled in the art will appreciate that tables 62 and 64 shown in 
FIGS. 7-8 illustrate only one exemplary situation. Row sequence table 62 
is applicable to a twelve cell reuse assignment plan and cell location 
table 64 is applicable to a thirty seven cell footprint 30 having cell 
number assignments as depicted in FIG. 6. Other similar tables or memory 
structures may be fashioned to achieve an acceptable assignment plan that 
spaces co-channel cells a predetermined distance apart from one another 
for a different number of discrete channel sets in a spectrum, for a 
different number of cells per footprint, and/or a different channel 
numbering scheme. 
Regardless of the particular assignment plan process used, task 52 assigns 
channel sets to cells 28 within footprint 30a in alternating rows 60. For 
the example shown in FIG. 6, channel sets A-F are confined to rows 60a and 
channel sets G-L are confined to rows 60b. Rows 60a and 60b are 
interleaved with one another. In addition, a constant pattern of channel 
set assignments results from progressing through cells 28 up-right 
(A-H-E-L-C-J, B-I-F-G-D-K) or up-left (A-G-C-I-E-K, B-H-D-J-F-L). Of 
course, the opposite sequences result from progressing through cells 28 
down-left or down-right, respectively. 
With reference back to FIG. 5, after task 52, a task 66 propagates the 
assignment plan across northern and southern boundaries for all footprints 
30 within the reference plane 14a. Propagation of the assignment pattern 
across a northern boundary may be achieved by following the 
above-discussed up-right and up-left sequences or the above-discussed 
assignment formula. Propagation of the assignment pattern across a 
southern boundary may be achieved by following the above-discussed 
down-right and down-left sequences or the above-discussed assignment 
formula. Cell location table 64 (see FIG. 8) may be consulted to determine 
which cells 28 of an unassigned footprint 30 located immediately to the 
north or south of an assigned footprint 30 are the initial targets in 
following these sequences. 
Those skilled in the art will appreciate that cell location table 64 (see 
FIG. 8) records an additional 30.degree. of rotation across a footprint's 
northern and southern boundaries in addition to the 60.degree. discussed 
above in connection with the assignment formula. This additional 
30.degree. is accounted for in the identity of cells located across 
northern or southern boundaries from a source cell. This additional 
30.degree. is needed because thirty-seven cell footprints, and other 
footprints which have a single centrally located cell 28, do not smoothly 
mate together at these boundaries, as illustrated in FIG. 6. In contrast, 
forty-eight cell footprints, and other footprints which do not have a 
single centrally located cell 28, smoothly mate together at these 
boundaries, as illustrated in FIG. 2. When a network employs footprints 
not having a single centrally located cell 28, the additional 30.degree. 
of rotation is not needed. 
FIG. 9 shows an assignment of channel sets to cells in footprints 30 
located in the reference plane 14a after the performance of task 66. Of 
course, task 66 also assigns channel sets to cells 28 of footprints 30 
located to the south of reference footprint 30a even though such southern 
footprints 30 are not depicted in FIG. 9. FIG. 9 also indicates by 
cross-hatching that certain ones of cells 28 have been defined as being 
inactive through the operation of task 40 (see FIG. 3). 
After task 66 (see FIG. 5), a task 68 selects a first east/west boundary 70 
between orbital planes 14. Preferably, the first boundary 70 is located on 
the opposing side of reference plane 14a from seam 16. Some of the 
inactive cells 28 are located near boundary 70, and more inactive cells 
are located in polar regions than in equatorial regions. FIG. 9 
illustrates an assignment of channel sets to inactive cells in reference 
plane 14a. However, it is unimportant whether channel sets are actually 
assigned to inactive cells. 
After task 68, a task 72 (see FIG. 5) identifies aligned source and target 
rows 60 on opposing sides of boundary 70. Referring to FIG. 9, source rows 
60 are confined to a single footprint 30 that is located on a previously 
assigned side of boundary 70. Target rows 60 are confined to a single 
footprint 30 located on an unassigned side of boundary 70. Aligned source 
and target rows 60 are indicated in FIG. 9 by dotted lines. Cell location 
table 64 (see FIG. 8) may be used to identify a specific cell 28 in a 
specific footprint 30 which is aligned with a target row. For the example 
depicted in FIGS. 8-9, cells 28 located to the right of the right-most 
cells 28 in a previously assigned footprint 30 reside in the target row. 
For the first iteration of task 72, any of source and target rows 60 that 
are aligned may be selected. 
With reference to FIGS. 5 and 9, after task 72 a task 74 determines the 
first active cell 28 encountered while moving away from boundary 70 in the 
source row 60 selected above in task 72. After identifying this first 
active cell 28, task 74 records the identity of the channel set assigned 
to it. After task 74, a task 76 identifies the first active cell 
encountered while moving away from boundary 70 in the target row 60 
selected above in task 72. 
After tasks 74 and 76, a task 78 assigns a channel set to this first active 
cell in the target row 60. The assignment is made by following the row 
sequence from the channel set recorded above in task 74. The row sequence 
may be determined, for example, by referring to row sequence table 62 (see 
FIG. 7). Starting from this first active cell in the target row and 
proceeding away from boundary 70, a task 80 follows the row sequence to 
assign channel sets to the remaining cells in the target row of the target 
footprint 30. Row sequence table 62 (see FIG. 7) and cell location table 
64 (see FIG. 8) may be used in making these channel set assignments. 
After tasks 72-80, a query task 82 determines whether the previously 
assigned target row 60 is the last row to be assigned on boundary 70. So 
long as additional rows 60 remain to be assigned, program control repeats 
tasks 72-82 to assign channel sets to other rows along boundary 70. 
When all rows on boundary 70 have been assigned, the resulting assignment 
resembles the situation depicted in FIG. 10. As discussed above, it is 
unimportant whether channel sets are assigned to inactive cells. When all 
rows on boundary 70 have been assigned, a task 84 (see FIG. 5) selects 
another boundary 86 which is preferably on the opposing side of the target 
footprints 30 from boundary 70. Next, a query task 88 (see FIG. 5) 
determines whether the selected boundary 86 is located on a seam 16 (see 
FIG. 1). So long as the new boundary 86 is not located on a seam 16, tasks 
72-84 are repeated to propagate the channel set assignments across the 
selected hemisphere. When channel sets have been assigned to cells 28 
located on seam 16, procedure 48 stops. 
FIG. 11 illustrates an exemplary assignment pattern that results from the 
performance of tasks 72-84 and accounts for overlap 32. As a result of 
performing tasks 72-84, channel sets are assigned to cells in target rows 
of a footprint 30 by following the row sequence and keying off of the 
channel set assigned to the last active cell in the source row. 
Consequently, the assignment of channel sets in the target row is 
responsive to the assignments made in the active cells of a corresponding 
source row and to any inactive cells residing in either or both of the 
source and target rows. By following the row sequence but skipping the 
inactive cells, a minimum separation distance between co-channel cells is 
maintained across boundary 70. 
Those skilled in the art will appreciate that the occasional skipping of 
cells in the channel assignment plan and the above-discussed 30.degree. 
rotation will cause the up-right, up-left, down-right, and down-left 
sequences discussed above to be altered in various locations after 
crossing a boundary 70. This alteration will have only a small impact on 
the minimum separation between co-channel cells because it occurs in a 
north-south direction between rows 60 and not in an east-west direction 
within any single row 60. As discussed above, channel set assignments 
follow sequences which repeat within interleaved rows. Consequently, all 
channel sets assigned to rows located immediately to the north and south 
of a subject row are orthogonal to each channel set in the subject row. In 
accordance with the procedure discussed herein, co-channel cells maintain 
a minimum separation distance which is at least as great as the diameter 
of a single cell 28. 
FIG. 12 shows a block diagram of a satellite 12 used by network 10 (see 
FIG. 1). In the preferred embodiment, all satellites 12 within network 10 
have substantially the same structure for the purposes of the present 
invention. Thus, FIG. 12 depicts each and every one of satellites 12. 
Satellite 12 includes any number of transceivers. For example, cross link 
transceivers 90 serve communication links between a satellite 12 and other 
nearby satellites 12 (see FIG. 1). In addition, satellite 12 includes one 
or more earth-link transceivers 92 which support communication links to 
CSOs 22 and GCSs 24. Transceivers 90 and 92 communicate via their 
respective antennas 94 and 96. Satellite 12 additionally includes a 
subscriber unit transceiver 98. Transceiver 98 communicates with 
subscriber units 26 through a multi-beam, multi-directional antenna 100. 
Transceiver 98 and antenna 100 may be divided into any number of 
independent channels, segments, or groups so that discrete cells 28 are 
formed and discrete channel sets are supported. 
Each of transceivers 90, 92, and 98, along with various memory components 
and a timer 102 couple to a controller 104. Controller 104 may be 
implemented using a single processor or multiple processors operated in a 
parallel architecture. Generally speaking, controller 104 coordinates and 
controls transceivers 90, 92, and 98 along with their associated antennas 
so that satellite 12 receives data communications from receivers of the 
various communication links and appropriately distributes the received 
communications among transmitters for the various communication links. 
Timer 102 is utilized to synchronize controller 104 and satellite 12 with 
timing constraints imposed by network 10 (see FIG. 1). 
The memory components include a cell assignment table 106. Table 106 
associates channel set assignments with cell identities in a one to one 
correspondence. Table 106 may also record which, if any, of the cells 28 
in the footprint 30 generated by satellite 12 are to be inactive. Thus, by 
supplying a cell number to table 106, a channel set assigned to the 
identified cell or data defining whether the cell is active or inactive 
may be obtained. The channel set to cell assignments may be determined as 
described above in connection with FIGS. 3-11. Moreover, as discussed 
above each cell may have numerous channel sets assigned thereto, with 
particular ones of the numerous channel sets being responsive to 
corresponding positions of satellite 12 within its orbit 14. 
The memory components also include other memory 108. Memory 108 includes 
data which serve as instructions to controller 104 and which, when 
executed by processor(s) within controller 104, cause satellite 12 to 
carry out procedures that are discussed below. Memory 108 also includes 
other variables, tables, and databases that are manipulated due to the 
operation of satellite 12. 
FIG. 13 shows a flow chart of a Control procedure 110 performed by a single 
satellite 12 within network 10. Procedure 110 causes satellite 12 to 
become synchronized to an external timing signal. Those skilled in the art 
will appreciate that, while procedure 110 is described for a single 
satellite 12, each of satellites 12 desirably performs substantially the 
same procedure. Generally speaking, Control procedure 110 is invoked when 
a TT&C command is received from a GCS 24 (see FIG. 1). TT&C may involve 
numerous diverse commands, such as orbit control commands, diagnostics 
commands, and programming commands, to name a few. 
Such TT&C commands may additionally include a synchronizing command. When 
the synchronizing command is detected, as indicated at a query task 112, 
program control retrieves data from the synchronizing command and programs 
timer 102 (see FIG. 12) in response to synchronization data carried by the 
command, as shown in a task 114. In the preferred embodiment, satellites 
12 come within view of a GCS 24 every few orbits. Thus, they may 
synchronize their internal time to the system time for network 10 every 
few hours. With every satellite 12 performing substantially the same 
procedure, the internal timers 102 of all satellites 12 recognize a given 
point in time at substantially the same instant. In the preferred 
embodiment, timers 102 for all of satellites 12 remain synchronized to 
within 50 microseconds of one another. After synchronization, procedure 
110 may engage in other TT&C activities not related to the present 
invention. Due to the operation of procedure 110, all satellites 12 within 
network 10 recognize a given instant at substantially the same actual 
absolute point in time. 
FIG. 14 shows a flow chart of a Timer Interrupt procedure 116 which is 
performed by a single satellite 12 within network 10. Procedure 116 causes 
satellite 12 to change the assignments of channel sets to cells with which 
it is operating into a new set of assignments. While procedure 116 is 
described for a single satellite 12, each of satellites 12 desirably 
performs substantially the same procedure. Generally speaking, Timer 
Interrupt procedure 116 is invoked in response to a signal supplied from 
timer 102 (see FIG. 12). Upon entry into procedure 116, a task 118 obtains 
the next segment of cell assignment table 106. This next segment includes 
either the identity of channel sets to be assigned to each of the 
satellite's cells upon the occurrence of an upcoming event or the identity 
of those cells and channel sets that will change as a result of the 
upcoming event. 
After task 118, a task 120 causes satellite 12 to communicate with those 
subscriber units 26 (see FIG. 1) currently supported by satellite 12. In 
particular, satellite 12 informs such units 26 of an upcoming change in 
the identity of the channels over which communications are taking place. 
Such communications concerning upcoming changes in channel identities are 
called "hand-off" communications. After task 120, a task 122 waits until a 
particular point in time occurs. This particular point in time represents 
the upcoming event discussed above in connection with task 118. It is a 
predetermined point in time at which all satellites 12 within network 10 
will change their channel set to cell assignments to compensate for 
overlap in their footprints 30 as they orbit the earth. The occurrence of 
this point in time may be determined from timer 102. As discussed above, 
procedure 110 (see FIG. 13) synchronizes all satellites 12 so that each 
satellite 12 recognizes this point in time at substantially the same 
instant. 
After task 122, a task 124 programs subscriber unit transceiver 98 (see 
FIG. 12) so that its operational programming switches in accordance with 
the active/inactive status and channel set assignment data obtained above 
in connection with task 118. After task 124, program control leaves 
procedure 116. Procedure 116 will be executed again when satellite 12 
reaches the point in its orbit that corresponds to the next segment of 
assignment table 106 (see FIG. 12). Until procedure is executed again, 
satellite 12 will communicate through antenna 100 in accordance with these 
updated channel set to cell assignments. 
In summary, the present invention provides an improved communication 
system. The present invention relates to a cellular communication system 
that efficiently reuses spectrum throughout a spherical surface, such as 
the surface of the earth. The spectrum is efficiently used because channel 
set to cell assignments are made in a manner which recognizes overlap 
between footprints, marks certain overlapped cells as being inactive, and 
assigns channel sets to active cells in a manner which is responsive to 
the inactive cells. Furthermore, in accordance with the present invention 
the marking of certain overlapped cells as being inactive and the 
assignment of channel sets to active cells are repeated from time to time 
to compensate for dynamic variance occurring in the overlap. 
The present invention has been described above with reference to preferred 
embodiments. However, those skilled in the art will recognize that changes 
and modifications may be made in these preferred embodiments without 
departing from the scope of the present invention. For example, others may 
devise alternate procedures to accomplish substantially the same functions 
as those described herein. Memory structures other than those depicted 
herein may by employed. Moreover, while the preferred embodiments 
described herein relate to a particular orbital geometry, footprint 
geometry, and channel set size, those skilled in the art will appreciate 
that the present invention may be applied to different geometries and 
channel set sizes. These and other changes and modifications which are 
obvious to those skilled in the art are intended to be included within the 
scope of the present invention.