Title: Scalable satellite data communication system that provides incremental global broadband service using earth-fixed cells
Abstract: A scalable satellite data communication system that provides incremental global broadband services using Earth-fixed cells may begin with a limited satellite deployment that initially serves a limited number of Earth-fixed cells. The system has the flexibility to incrementally increase the number of Earth-fixed cells that are served, with minimal constraints on the relative locations of the cells on the Earth, by adding satellites of potentially greater complexity to the system. Backward compatibility with existing user terminals is achieved by maintaining the same satellite communication interface as with the already-deployed satellite constellation. Continuous and/or non-continuous service may be provided to selected Earth-fixed cells. Scheduled non-continuous service is particularly advantageous for bulk data transport services. Satellites may use simple mechanically-steered antennas. Communication links may be handed from one satellite to another when one satellite moves out of range and is no longer able to cover a selected Earth-fixed cell.
Patent Number: 6,850,732 Issued on 02/01/2005 to Patterson, et al.
| Inventors:
|
Patterson; David P. (Bellevue, WA);
Ghazvinian; Farzad (Mercer Island, WA);
Hinedi; Sami (Kirkland, WA);
Quadracci; Len (Seattle, WA);
Sturza; Mark A. (Encino, CA)
|
| Assignee:
|
Wengen Wireless LLC (Bellevue, WA)
|
| Appl. No.:
|
113840 |
| Filed:
|
March 29, 2002 |
| Current U.S. Class: |
455/12.1; 455/13.2; 455/427 |
| Intern'l Class: |
H04B 007/185 |
| Field of Search: |
455/12.1,13.2,13.3,561,427,430,525,554.2,575.7
|
References Cited [Referenced By]
U.S. Patent Documents
| 5559806 | Sep., 1996 | Kurby et al. | 370/325.
|
| 6370126 | Apr., 2002 | De Baere et al. | 370/316.
|
| 6542739 | Apr., 2003 | Garner | 455/427.
|
| 2002/0013149 | Jan., 2002 | Threadgill et al. | 455/427.
|
| 2002/0122408 | Sep., 2002 | Mullins | 370/347.
|
| 2003/0123481 | Jul., 2003 | Neale et al. | 370/466.
|
Primary Examiner: Corsaro; Nick
Assistant Examiner: Trinh; Tan
Attorney, Agent or Firm: Fulbright & Jaworski L.L.P.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of the filing date of U.S. Provisional
Patent Application No. 60/280,690, filed Mar. 30, 2001.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A scalable satellite data communication system for providing incremental
global data communication services, comprising:
(a) at least one satellite in non-geosynchronous orbit (NGSO) above the
Earth;
(b) at least one user terminal located in a first Earth-fixed cell and
having
(i) an antenna capable of tracking the NGSO satellite above a user
elevation mask;
(ii) a receiver capable of receiving a forward downlink signal from the
NGSO satellite; and
(iii) a transmitter capable of transmitting a reverse uplink signal to the
NGSO satellite; and
(c) at least one gateway located in a second Earth-fixed cell and having
(i) an antenna capable of tracking the NGSO satellite above a gateway
elevation mask;
(ii) a transmitter capable of transmitting a forward uplink signal to the
NGSO satellite;
(iii) a receiver capable of receiving a reverse downlink signal from the
NGSO satellite; and
(iv) a connection to a terrestrial data communication network, wherein the
gateway receives data from the terrestrial data communication network for
forward uplink transmission to the NGSO satellite, and transmits data
received from the NGSO satellite via reverse downlink transmission to the
terrestrial data communication network;
wherein the NGSO satellite includes
(i) a steerable forward link receive antenna capable of receiving the
forward uplink signal from the gateway in the second Earth-fixed cell;
(ii) a steerable forward link transmit antenna capable of transmitting the
forward downlink signal to the user terminal in the first Earth-fixed
cell;
(iii) a steerable reverse link receive antenna capable of receiving the
reverse uplink signal from the user terminal in the first Earth-fixed
cell; and
(iv) a steerable reverse link transmit antenna capable of transmitting the
reverse downlink signal to the gateway in the second Earth-fixed cell; and
wherein the satellite data communication system provides a communication
link between the user terminal and the terrestrial data communication
network via the NGSO satellite and the gateway; and
wherein the satellite data communication system is configured for
incremental expansion in multiple stages by adding additional NGSO
satellites that can be of different design or in different orbital
configuration, the additional satellites providing data communication
capabilities for additional user terminals and gateways located in
additional Earth-fixed cells, the additional satellites being further
configured to be backward compatible to communicate with existing user
terminals and gateways in the satellite data communication system.
Description
FIELD OF THE INVENTION
The present invention relates to data communication, and more particularly
to data communication systems and methods using satellites.
BACKGROUND OF THE INVENTION
Data communication networks that use satellite links to communicate data
are generally known. Such networks typically include a number of ground
terminals on the Earth and one or more satellites orbiting above the
Earth. The ground terminals communicate with one another by way of one or
more satellites. Prior U.S. patents assigned to the assignee of the
present invention have described satellite communication networks that
include hundreds of low-Earth orbit (LEO) satellites. See, e.g., U.S. Pat.
Nos. 5,408,237; 5,527,001; 5,548,294; 5,642,122; 5,650,788; 5,736,959; and
5,740,164. The satellites in these networks are capable of transmitting
data between them and to and from ground terminals on the Earth's surface.
A satellite's "communication footprint" defines a portion of the Earth's
surface over which the satellite can communicate with ground terminals on
the Earth. During the period of time that a ground terminal is within the
border of a satellite's communication footprint, the ground terminal may
transmit data signals to and receive data signals from the satellite. A
constellation of satellites may be configured to transfer, or "hand-off,"
communication links from one satellite to another satellite when the first
satellite's communication footprint no longer covers the ground terminal.
"Cells" may be defined within a satellite's communication footprint to
identify geographic areas for which the satellite provides data
communication service. A satellite data communication network of the type
described above may be configured to serve "Earth-fixed cells" or
"satellite-fixed cells." In a network that employs satellite-fixed cells,
the cells move in the same direction and velocity as the nadir of the
satellite projected on the Earth's surface as the satellite moves through
its orbit. In contrast, Earth-fixed cells are regions mapped onto the
surface of the Earth and have fixed boundaries for determining satellite
communication service. Although non-geosynchronous orbit (NGSO)
satellites, such as the LEO satellites referenced above, rapidly move
around the Earth, the moving footprint of the satellite does not change
the location of the Earth-fixed cells.
An advantage provided by using cells having boundaries that are fixed to an
Earth-based grid is realized when a ground terminal being served by a
communication beam in one satellite must switch to another communication
beam in the same satellite or to a second satellite because the first is
moving out of range. With satellite-fixed cells, this communication
"hand-off" involves assigning the ground terminal a new communication
channel with the new beam or new satellite. This assignment process takes
time and consumes processing capacity at both the terminal and the
satellites. It also places the current communication link in jeopardy of
blocking, call interruption, and call dropping if an idle communication
channel in the next servicing beam or satellite is not available.
Earth-fixed cell methods address the hand-off problems noted above by
allocating communication channels (frequency, code, and/or time slots) on
an Earth-fixed cell basis. Regardless of which satellite or beam is
currently serving a particular cell, the ground terminals in the cell
maintain the same channel assignments.
In any event, previous architectures for broadband NGSO satellite systems
required hundreds of satellites that collectively generated thousands of
beams to serve thousands of Earth-fixed cells. The satellites further
required electronically-steered phased array or multi-beam array antennas,
which are complex and expensive. The present invention is directed to
solving the foregoing problems and other shortcomings in the prior art by
providing a satellite data communication system having a scalable
architecture that provides incremental global broadband services.
SUMMARY OF THE INVENTION
The present invention provides a scalable satellite data communication
system that provides incremental global broadband services using
Earth-fixed cells. For example, in one implementation of the invention, a
suitable system may begin with a deployment of less than 20 satellites
that initially serve 10 Earth-fixed cells. The present invention has the
flexibility to incrementally increase the number of Earth-fixed cells that
are served, with minimal constraints on the relative locations of the
cells on the Earth, by adding satellites of potentially greater complexity
to the system.
Where previous satellite communication system architectures required
continuous service to Earth-fixed cells, a system constructed according to
the present invention may also provide non-continuous service to selected
Earth-fixed cells. The non-continuous service may be provided based on
user demand and satellite availability. Scheduled non-continuous service
is particularly advantageous for bulk data transport services. The present
invention provides the capability of scheduling service in particular
Earth-fixed cells for durations up to a limit depending on the satellite
constellation parameters and constraints on the allocation of
communication resources.
Furthermore, where previous satellite system architectures required complex
electronically-steered phased array or multi-beam array satellite antennas
(which the present invention may use), the present invention may also be
implemented using simple mechanically-steered satellite antennas. In a
basic form, satellites may be constructed with a single
mechanically-steered transmit/receive antenna. The beam formed by the
antenna is pointed to cover a selected Earth-fixed cell with a user
terminal and a gateway. The pointing can be either continues or discrete.
Another advantage of the present invention is that satellites may be added
to the satellite data communication system to incrementally increase the
service coverage provided by the system. The satellites added to the
system do not need to be identical to the satellites already in orbit.
More complex satellites with multiple antennas may be added to increase
the service coverage at a greater rate. Backward compatibility with
existing user terminals is achieved by maintaining the same satellite
communication interface as the already-deployed satellite constellation.
When a sufficient number of satellites are properly arranged in the
satellite constellation, a system of the present invention may provide
communication hand-off from one satellite to another when one satellite is
no longer able to cover a selected Earth-fixed cell. The ability of a
satellite to cover a selected Earth-fixed cell is determined by the
elevation mask angle for the ground terminal, the location of the cell on
the Earth's surface, and the location of the satellite in orbit.
Scheduled service may also be provided in circumstances where the number of
available satellite antenna beams exceeds the number of Earth-fixed cells
that can be continuously covered by the satellite constellation. These
"temporarily uncommitted" beams can be advantageously used to provide
communication coverage to selected Earth-fixed cells on a temporary
scheduled basis.
A system constructed according to the present invention has many benefits.
For example, satellites can be kept technologically simple and lower in
weight, reducing the overall cost of the system. Marketing, service
providers and user terminal manufacturers may benefit from a smaller and
more focused satellite deployment. Furthermore, the service offerings
provided by the system may be tailored to meet the needs of each
Earth-fixed cell that is served.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same become better
understood by reference to the following detailed description, when taken
in conjunction with the accompanying drawings, wherein:
FIG. 1 is a pictorial diagram showing an area covered by an Earth-fixed
cell situated over the Los Angeles Basin;
FIG. 2 is a pictorial diagram of a satellite data communication system of
the present invention;
FIG. 3 is a pictorial diagram of a coverage map for one exemplary
embodiment of the invention with ten service cell/gateway pairs located
worldwide;
FIG. 4 illustrates major components of a gateway that may be used in the
present invention;
FIG. 5 is a pictorial diagram of satellite coverage for both a user
terminal elevation mask and gateway elevation mask having different mask
angles;
FIG. 6 is a diagram illustrating spectrum use in an exemplary embodiment of
the invention with heavy data traffic;
FIG. 7 is a diagram illustrating spectrum use in an exemplary embodiment of
the invention with light data traffic;
FIG. 8 is a diagram illustrating spectrum use in an exemplary embodiment of
the invention wherein the spectrum is shared between a user terminal and a
co-located gateway;
FIG. 9 is a conceptual block diagram of major components of a satellite
that may be used in the invention;
FIG. 10 depicts an exemplary uplink frequency and channelization plan; and
FIG. 11 depicts an exemplary downlink frequency and channelization plan.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A satellite data communication system of the present invention may be
viewed as having three principal physical segments: a user terminal
segment, a space segment, and gateway segment. The user terminal and
gateway segments are located in Earth-fixed cells defined on the Earth's
surface. Satellites in the space segment facilitate communication of data
between user terminals and their associated gateway terminals in the user
terminal and gateway segments. Prior to discussing further details of
these segments and the operation of a satellite data communication system
constructed according to the present invention, a brief description of
partitioning processes suitable for use in defining Earth-fixed cells is
warranted.
Partitioning of the Earth's surface into Earth-fixed cells may be complete
or incomplete. Complete partitioning means that the entire Earth's surface
has been partitioned into cells. Incomplete partitioning results when
Earth-fixed cells are defined for only a fraction of the Earth's surface.
An incomplete partitioning can be connected or non-connected. A connected
partitioning permits one to move along the Earth's surface from an
Earth-fixed cell to any other Earth-fixed cell without moving through a
geographic location that is not contained within an Earth-fixed cell. A
non-connected partitioning is one in which it is impossible to move along
the Earth's surface from an Earth-fixed cell to another Earth-fixed cell
without moving through a geographic location not contained within an
Earth-fixed cell. Further, a sparse partition is one in which no
Earth-fixed cell is adjacent to any other Earth-fixed cell.
One example of a complete partitioning of the Earth's surface into
Earth-fixed cells is obtained by first tessellating a sphere inscribed
within the Earth's surface with rectangular hexagons. The cell boundaries
of the hexagons are projected to the Earth's surface using a line from the
center of the Earth. Another example of complete partitioning is obtained
by dividing the Earth's surface into an even number of bands of latitude
and dividing each band into a number of trapezoids. The number of
Earth-fixed cells in each band is approximately proportional to the cosine
of latitude at the band center.
An example of an incomplete connected partitioning is obtained by selecting
a latitude X, where X is greater than 0 degrees and less than 90 degrees.
An integer N greater than 1 is also selected. The lines of latitude at X
degrees North and X degrees South are divided into N equal-aligned
segments. The cell boundaries are finally constructed by connecting the
North and South segments with lines of longitude.
An incomplete non-connected partitioning may also be obtained by defining
Earth-fixed cells that encompass only particular geographical locations,
such as cities. For example, Earth-fixed cells may be centered on cities,
such as London, New York, Moscow, and Bombay, with the cell boundaries
being defined as the locally legally-recognized borders of the cities.
Another example of an incomplete non-connected partitioning is obtained by
centering an Earth-fixed cell on a specific geographic location. The
Earth-fixed cell may be uniform in shape, e.g., circular, with a specified
radius, e.g., 200 kilometer radius.
FIG. 1 illustrates one exemplary embodiment of an Earth-fixed cell having a
defined shape and centered on a specific location. In FIG. 1, an
Earth-fixed cell 10 having a circular border 12 is shown situated over the
Los Angeles Basin. As described in greater detail below, a suitable
satellite data communication system according to the present invention may
provide 500 MHz of Ka-band uplink spectrum and 500 MHz of Ka-band downlink
spectrum to be shared by user terminals located within the boundaries of
the Earth-fixed cell 10. Also as described below, the service coverage of
the Earth-fixed cell 10 may be continuous or non-continuous.
As noted above, a satellite data communication system of the invention may
be conceptually divided into three segments. FIG. 2 illustrates a user
terminal segment 20, a space segment 22, and a gateway segment 24. The
user terminal segment 20 includes one or more user terminals 26 that are
located on the Earth's surface. The user terminals 26 are located within
an Earth-fixed cell 28. Earth-fixed cells that include user terminals 26
are also referred to herein as "service cells." The service cell 28 may,
for example, have the properties of the Earth-fixed cell 10 shown in FIG.
1.
The space segment 22 is comprised of one or more satellites 36 in
non-geosynchronous orbit (NGSO), such as medium Earth orbit (MEO) and/or
low Earth orbit (LEO) satellites. The one or more satellites 36 are
organized in a constellation of satellites that provides two-way data
communication between user terminals 26 in the user terminal segment 20
and gateway terminals 30 in the gateway segment 24.
The gateway segment 24 includes one or more gateway terminals 30 (also
referred to herein as "gateways"). The gateways 30 are located on the
Earth's surface within an Earth-fixed cell 32. As shown in FIG. 2,
gateways 30 are connected to a terrestrial data communication network 34,
such as the Internet. The terrestrial data communication network 34 may
include one or more public or private networks. The satellite data
communication system thus provides user terminal access to the terrestrial
data communication network 34 and vice-versa by way of the space segment
22 and gateway segment 24.
The location of each gateway on the Earth's surface may depend on a number
of factors, including (1) proximity to network backbones to guarantee data
speed and integrity, and (2) proximity to one of the Earth-fixed cells
being served by the gateway 30, such as the service cell 28. For each
service cell 28, there is typically one active satellite 36 and one active
gateway 30, though multiple satellites and multiple gateways may be
associated with and serve the service cell 28. As a result, the gateway 30
may be located near the service cell 28. Nevertheless, the following
considerations should be taken into account when determining how close to
locate the gateways and the user terminals. One implementation of the
invention described herein uses 500 MHz of Ka-band spectrum for each of
the uplink and downlink data communication bands. If the full 500 MHz of
spectrum is utilized in each band, the associated gateway 30 should not be
located closer than 750 kilometers to avoid interference. If the user
terminals 26 and gateway 30 are co-located (i.e., in the same Earth-fixed
cell), they must share the same 500 MHz of spectrum. In that circumstance,
the separation requirement is no longer needed.
If the service cell 28 requires less than 250 MHz of spectrum, then its
associated gateway 30 can be located closer to, or possibly within, the
service cell 28. The 500 MHz band is split so the user terminals 26 and
gateway 30 communicate via the satellite 36 using different frequencies.
In this example, the satellite 36 may generate two antenna beams: one
antenna beam directed towards the service cell 28 and the other antenna
beam directed toward the gateway 30 (i.e., to encompass the cell 32 in
which the gateway 30 is located). Control commands from ground stations,
such as the gateways 30, may be communicated to the satellite 36 to direct
the manner in which the antenna beams are pointed and the number of
transponders that are allocated between the antenna beams. The satellites
36 may be constructed to provide continuous service to the user terminals
26 by providing communication hand-off from one satellite to another.
One initial satellite constellation suitable for the space segment 22 may
have the characteristics shown in Table 1.
TABLE 1
Constellation Parameters
Altitude 10,930 km
Inclination 46.7.degree.
Total Number of Satellites 18
Number of Planes 3
Number of Satellites per Plane 6
Satellite Spares 3 (1 in each plane)
Walker Delta Phase Factor 0
Minimum user terminal elevation angle 35.degree.-40.degree.
Minimum gateway elevation angle 25.degree.
An initial deployment of a satellite constellation for use in the present
invention may be configured to serve a determined number of cells, e.g.,
10 Earth-fixed cells, worldwide. FIG. 3 illustrates a coverage map 40 with
exemplary locations for ten service cells and associated gateway cells.
The circular spots denote service cells that contain the user terminals
while the square spots represent cells containing the gateways. In FIG. 3,
four service cell/gateway pairs in the Americas are illustrated at
reference numerals 42, 44, 46 and 48. Service cell/gateway locations in
Africa and Europe are shown at reference numerals 50 and 52. In the Middle
East, Asia, and Australia, service cell/gateway locations are as indicated
by reference numerals 54, 56, 58, and 60. Depending on the number of
satellites in the satellite data communication system, the elevation masks
required for communication with the user terminals and the gateways, and
the communication frequencies used, the initial locations of the service
cell/gateway pairs may be separated by a certain distance, e.g., 2000
kilometers, from each other.
It should be understood that a system of the present invention with even a
single satellite can provide coverage to some, or all, of the Earth-fixed
cells. When fewer satellites are included in the satellite constellation,
communication coverage for each service cell is generally provided on a
non-continuous basis. In that regard, communication service may be
scheduled for each service cell, which is particularly advantageous for
bulk data transport services described in greater detail below. Moreover,
bulk data transport services may provide early entry revenue generation
and incremental revenue growth for the satellite data communication
system.
The total coverage capacity for the satellite data communication system may
increase proportionally as additional satellites are placed in orbit.
Depending on the orbital relationship of these satellites, this capacity
can be used to increase the scheduled coverage duration in some
Earth-fixed cells, increase the number of Earth-fixed cells that receive
scheduled coverage, or some combination of the two. Furthermore, the
present invention advantageously accommodates the addition of higher
complexity satellites (e.g., with multiple beam coverage) while retaining
backward compatibility with the existing satellite constellation.
Multi-beam satellites added to the system increase the coverage capacity
of the satellite data communication system.
Once the satellite constellation of the satellite data communication system
reaches a critical capacity, a subset of Earth-fixed cells may receive
service coverage on a continuous basis. These cells are also referred to
herein as "active" Earth-fixed cells or the "active subset" of Earth-fixed
cells. The active subset need not remain static. It can be modified in
response to market, economic, or regulatory forces. The ability to change
the active subset of cells can be used to respond to unexpected demand,
such when natural disasters or military conflicts occur.
As the capacity of the satellite constellation increases beyond a critical
size, the number of Earth-fixed cells that may be covered on a continuous
basis also increases. Alternatively, some Earth-fixed cells may be covered
by multiple satellites. Multiple coverage can be used to increase the
communication capacity of the system where needed, provide path diversity,
provide enhanced reliability, or any combination thereof.
Because NGSO satellites move rapidly around the Earth, each satellite
serving an Earth-fixed cell will eventually orbit out of range of the
Earth-fixed cell. Where a system of the present invention includes a
constellation of multiple satellites, the system is preferably configured
to hand-off communication from one satellite to another as one satellite
moves out of range and another enters into range of an Earth-fixed cell.
The hand-off process may operate on a regular basis and be configured to
provide one or more Earth-fixed cells with continuous converge.
To preclude the need for additional components in user terminals, such as
dual receivers and power amplifiers, the satellite data communication
system is preferably designed to operate in a "break before make" mode for
communication hand-off. In a break before make mode, a user terminal ends
data communication with the first satellite and breaks contact with the
satellite. The user terminal then acquires the second satellite coming
into range and begins data communication with the second satellite. The
total time for a break before make operation is preferably less than 10
milliseconds so as to appear transparent to the user.
The present invention also extends the Earth-fixed cell concept by enabling
the location of one or more Earth-fixed cells to be dynamically changed as
needed. For example, an Earth-fixed cell may be centered on one or more
user terminals that are affixed to ships moving across the ocean. The
satellite data communication system may dynamically relocate the
Earth-fixed cell according to the motion of the user terminals that are
moving across the Earth's surface. When an Earth-fixed cell is relocated,
the location information may be communicated through the satellite
communication system so that the user terminal and satellite antennas are
steered appropriately.
A significant number of terrestrial data communication networks, such as
the Internet, use Internet Protocol (IP) for data transport. In reference
to the configuration shown in FIG. 2, a user terminal 26 may receive IP
packets from a user's computer via a network interface (e.g., Ethernet).
An indoor portion of the user terminal then reformats the IP packets for
satellite transmission, performs packet segmentation, encapsulation,
encryption, coding, and modulation to produce an intermediate frequency
(IF) signal. The IF signal is transmitted, e.g., via a cable, to an
outdoor portion of the user terminal that includes the antenna. The IF
signal is translated to the Ka frequency band and transmitted to the
steerable antenna that is tracking the satellite 36 currently serving the
user's service cell. Known technology for generating and transmitting data
signals from a ground terminal to a satellite may be used in this regard.
The satellite 36 is preferably configured to have a "bent-pipe"
configuration in which the uplink signal received from the user terminal
26 is redirected back to the Earth toward the associated gateway 30. The
serving satellite receives the uplink signal via the satellite's steerable
antenna that is tracking the user's service cell. A transponder converts
the signal's uplink frequency to the downlink frequency, amplifies it and
sends the signal to the service cell via the steerable antenna that is
tracking the gateway assigned to the service cell.
At the gateway 30, the signal carrying the IP packets is received at the
gateway antenna tracking the serving satellite, demodulated, decoded,
decrypted, and reassembled as necessary into IP packets. The IP packets
are then routed by the gateway 30 to the terrestrial data communication
network 34 for communication to the final destination.
Data transmission from the user terminal 26 to the terrestrial data
communication network 34 via the gateway 30 is referred to herein as a
reverse communication link. Data received by the gateway 30 from the
terrestrial data communication network 34 for transmission to the user
terminal 26 is transmitted via a forward communication link. Forward link
transmission (gateway to user terminal) roughly follows a path opposite to
that described above for reverse link transmission.
Various classes of user terminals 26 may be used to support the diversified
needs of users in each service cell. In one exemplary implementation of
the invention, four classes of user terminals are defined, e.g., Class I,
Class II, Class III and Class IV user terminals. A Class I user terminal,
for example, may constitute a basic form of user terminal that is most
commonly used. A Class I user terminal may use a low-cost, low profile
antenna that is easy to install and is unobtrusive.
In this exemplary implementation, Class I user terminals are preferably
constructed so that they can be sited anywhere having clear view of the
sky above a specified elevation. Different user terminal elevations may be
required for different latitudes in which the user terminals are located.
After initial installation, the user terminal antenna may direct itself
using a satellite beacon/control signal and point itself to the serving
satellite. The user terminals 26 may interface with a user's hardware
(i.e., computer or computer network) via standard communication
interfaces, such as a 10Base-T, 100Base-T, USB, Ethernet or other type
connection.
Other classes of user terminals, e.g., Classes II, III, and IV, may be
defined to provide equipment capable of higher transmission rates and/or
better link availability. Exemplary parameters for the four classes of
user terminal in this example are provided below in Table 2.
TABLE 2
User Terminal Performance Parameters
Minimum Data Rate Maximum Data NYC
Availability
EIRP [dBW] G/T [dB/K] [Mbps] Rate [Mbps] [%]
Class I 31.8 2.9 0.256 0.512 99.5
Class II 37.8 5.9 1.024 2.048 99.5
Class III 46.9 9.2 2.048 8.192 99.7
Class IV 59.9 15.6 10.24 51.2 99.9
FIG. 4 illustrates major components of one example of a suitable gateway
70. The gateway 70 as shown includes multiple antennas 72, multiple modems
74, and multiple routers 76. Downlink signals received at the antennas 72
from a satellite are demodulated and decoded by the modems 74 and switched
and routed to the terrestrial data communication network 78, via the
routers 76. The terrestrial data communication network, in this example,
is an Internet backbone. The antennas 72 may be constructed relatively
small (e.g., 1.2 meters in diameter) and can be located in cities.
Moreover, to reduce latency, the gateway 70 may be located in places, such
as major cities, in which access to the Internet backbone is available.
In circumstances where a gateway is not co-located with the user terminals
in the service cell, the service cell may require more than one associated
gateway to insure that there is always at least one gateway within the
same satellite footprint as the service cell. In this case, as the serving
gateway passes out of the satellite footprint, the service cell is
handed-off to another gateway in the satellite footprint. For these
multi-homed service cells, the two (or more) associated gateways are
preferably interconnected by terrestrial facilities. Gateways may operate
down to a lower elevation angle than user terminals, which increases the
size of the satellite's gateway coverage footprint.
Preferred embodiments of a gateway have at least three satellite
communication antennas with associated RF equipment: one in use with the
current serving satellite, one acquiring or tracking the next satellite,
and one standby spare. In some climates, antenna site diversity can be
used to improve availability. One exemplary configuration uses two antenna
sites, separated by a distance, e.g., 30 km, each with at least two
antennas for use to communicate with the current satellite and to track
the next satellite. The output power and aperture diameter (and therefore
the EIRP and G/T) of the gateway antennas can vary to compensate for rain
attenuation in different climate zones. One exemplary gateway
configuration uses a 1.8 m antenna with 40-watts output power per channel.
FIG. 5 illustrates an exemplary satellite coverage map 80 for both a user
terminal mask angle (inner ovoid 84) and gateway mask angle (outer ovoid
86). A mask angle is the minimum elevation above the horizon which a
ground terminal is able to communicate with a satellite. A user terminal
mask angle in one example of the invention is 40.degree., while a gateway
elevation mask is 25.degree.. Other implementations of the invention may
employ other mask angles. A currently preferred embodiment of the
invention uses a user terminal mask angle of 20.degree. and a gateway mask
angle of 15.degree..
For a user terminal and a gateway to communicate, both must be in the
communication footprint of the same serving satellite. The satellite
altitude and the mask angle affect the size of the satellite communication
footprint. To insure that a user terminal and gateway share the same
satellite footprint for a satellite 82, the gateway preferably operates
with a lower gateway elevation angle. A lower gateway elevation mask
results in a coverage footprint being larger than the coverage footprint
for a user terminal having a higher elevation mask, as shown in FIG. 5. A
user terminal and a gateway are guaranteed to be within the coverage area
of the same satellite if the distance between them does not exceed the
difference between these footprint radii. If the separation between a user
terminal in a service cell and a gateway does exceed this distance, it may
be necessary to associate the service cell with more than one gateway to
insure continuous coverage.
Further, gateways 30 (FIG. 2) may be connected to each other and to the
terrestrial data communication network 34 by way of known data
communication conduits, including dedicated fiber optic cables, public
switched telephone network links, private or leased lines, virtual private
networks (VPNs), terrestrial-based microwave links, GEO VSATs, Internet
links, or by multiple hops through the satellite network. A single gateway
can also be associated with multiple Earth-fixed cells and provide service
to user terminals located in those Earth-fixed cells.
One significant feature of the present invention is that it provides a
scalable system that enables incremental growth in global broadband
services. By increasing the number of satellites in the satellite
communication system, as well as the performance characteristics of the
satellites (e.g., beam count), the number of Earth-fixed cells served by
the satellite communication system can be dramatically increased. One
possible growth scenario is described below in Table 3.
TABLE 3
Space Segment Phases
Beams Per Total Active
Satellites Satellite Served Cells
Initial Rollout 18 1 10
Additional Dual-Beam Satellites 18 2 30
Additional Multi-Beam Satellites 18 16 200+
The initial space segment in Table 3 includes eighteen satellites, each
with a single service beam. This initial deployment is configured to serve
a total of 10 Earth-fixed cells. The second set of satellites added to the
system in this example include two service beams per satellite and are
each capable of serving two Earth-fixed cells at a time. As a result, the
total number of service cells grows to 30 (i.e., the initial 10 cells plus
20 cells serviceable by the newly-added satellites). At a further stage,
multi-beam satellites capable of serving 16 cells each may be added to the
system, providing service to hundreds of cells in which millions of users
may be located.
Two broad categories of service that are advantageously provided by a
satellite data communication system of the present invention are (1)
interactive broadband communications and (2) data transport services.
Interactive broadband communications, which are likely to constitute the
primary service of the system, are provided to user terminals inside
designated service cells. Data transport services, which also may be
provided to user terminals inside the designated service cells, may
further be provided to user terminals outside the designated service cells
on a scheduled basis. These service offerings may, and probably will,
change over time. As such, the flexibility provided by the system of the
invention is particularly desirable. Additionally, this flexibility
enables different service offerings to be provided to different service
cells.
Another strength of a system constructed according to the present invention
is that it provides high quality, cost competitive broadband access to the
Internet or other wide area network to users in remote locations. The
system's broadband connectivity characteristics compare favorably with
DSL, cable modem, and geosynchronous orbit satellite services. Moreover,
time delay, or latency, through the system is much lower when compared
with more traditional broadband satellite communication systems that use
geosynchronous satellites. This is largely in part because the satellites
in the space segment 22 (FIG. 2) operate at about 1/3 the altitude of a
geosynchronous satellite. Table 4 below displays typical round-trip
communication time, in milliseconds, to and from a user terminal for a
system of the present invention and for a geosynchronous satellite system.
TABLE 4
Round-Trip System Latency
Present System GEO System
User terminal to satellite (uplink) 40 ms 120 ms
Satellite to gateway (downlink) 40 ms 120 ms
Gateway to Internet 0 ms 0 ms
Total Round-Trip Latency 160 ms 480 ms
As a general rule, data transmission to and from individual users is
typically bursty, asymmetric, and unpredictable. However, in the
aggregate, data transmissions generated in each service cell may be
projected in advance and used to plan satellite transponder activation
times and power allocation. Reference data transmission profiles for a
service area and for servicing satellites are also used to estimate solar
array and battery requirements for the satellites.
In one suitable embodiment of the invention, a satellite payload includes
two antennas, each of which may have an equivalent aperture (e.g., 71 cm).
Preferably, the two antennas are independently steerable and capable of
simultaneously transmitting data to and receiving data from Earth-based
terminals. The satellite antenna for uplink data transmission may
generate, for example, a communication footprint of approximately 200 km
diameter at nadir. In order to minimize network interruptions, the antenna
beam is preferably constructed so that it can be repointed to a new
location, preferably within a short amount of time. Moreover, it can
preferably be pointed a determined number of degrees from boresight to
meet a minimum user terminal elevation for bulk data delivery service,
where applicable.
As noted earlier, the satellite antenna payload preferably uses a
"bent-pipe" configuration, which generally minimizes the cost and
complexity of the satellite. The satellite antenna components include low
noise amplifiers, block down converters to convert signals to an
intermediate frequency (IF), block up converters to convert the signals to
radio frequency (RF), and a switchable bank of filters. The filter bank
should be constructed to satisfy regulatory and national sovereignty
issues with respect to communication signals, and also prevent renegade
user terminals from using unallocated communication spectrum without
authorization.
A satellite with two antennas as described above may point one antenna
toward a gateway and the other antenna toward a service cell.
Alternatively, the gateway and the user terminal may be co-located in the
same service cell. In the latter circumstance, the same satellite antenna
may serve both the gateway and the user terminals in the service cell by
sharing the available spectrum. A switch matrix is used by the satellite
to manage cross coupling of the two antennas.
To conserve power, one exemplary implementation of the invention uses a
bank of five 50-MHz traveling wave tube amplifiers (TWTAs) covering a
250-MHz bandwidth. This bank of TWTAs enables the satellite to scale its
power usage according to the current data traffic profile. The system
takes advantage of traffic profiles in order to reduce the required power
that the satellite data bus must deliver. Lower power requirements
translate into lower satellite mass, which translates into lower overall
system cost. The average satellite duty cycle is 55% compared to an
average relative duty cycle of 25%. Reducing the required power during low
periods of demand may result in a power reduction of over 50%.
In order to maximize frequency reuse, a user terminal and its associated
gateway may share the same spectrum. In embodiments described above, the
user terminal and the gateway may jointly use a 500 MHz spectrum. An
exemplary embodiment locates the uplink and downlink spectrums at
28.6-29.1 GHz and 18.8-19.3 GHz, respectively. In Earth-fixed cells where
less than 500 MHz is available, the signal transmissions may be tailored
according to the available spectrum.
In order to transmit or receive uplink and downlink signals across a full
500 MHz of bandwidth in this example, sufficient angular separation
between the two satellite antenna beams may be necessary. Furthermore, in
order to minimize interference, the user terminals and the gateway
preferably transmit uplink and downlink signals having orthogonal
polarization. In one exemplary embodiment, the user terminals and gateway
use right-hand circular polarization (RHCP) and left-hand circular
polarization (LHCP), respectively, for uplink and downlink transmission.
See FIGS. 6 and 7. In the event that it is not possible to locate the
gateway sufficiently apart from the user terminal, the user terminal and
the gateway may share the same spectrum. See FIG. 8.
Each 250 MHz channel shown in FIGS. 6-8 may be divided into five 50-MHz
channels. Each 50 MHz channel may be powered by a separate TWTA that can
be turned on and off depending on data traffic demand. As shown in FIG. 6,
when the service area and the gateway are heavily loaded with data
traffic, all 50 MHz channels are used to serve the traffic. Inside each 50
MHz channel, the user terminals on the reverse data communication link can
burst at various rates depending on environmental considerations, such as
rain fade, and the class of user terminal involved. FIG. 7 illustrates an
example of when a service cell and the gateway are lightly loaded. In this
case, only certain 50 MHz channels are needed to carry the data traffic.
Since only certain 50 MHz channels are activated, satellite power is
conserved.
Data communication parameters and operational aspects of the satellite may
be controlled by control signals generated within the satellite or within
Earth-based terminals. For example, the gateways in the present invention
may be assigned with the task of controlling the allocation of data
communication resources. Depending on the communication protocols of the
satellite data communication system, gateways may control resources in the
frequency domain (i.e., assign communication channels in the frequency
spectrum), control resources in the time domain (i.e., assign time slots
for data communication), and/or control resources in the code domain
(i.e., assign spreading codes).
Examples of a priority-based system and method of allocating resources for
transmission of one or more data packets from a ground terminal to an NGSO
satellite are disclosed in U.S. Pat. No. 6,366,761 entitled PRIORITY-BASED
BANDWIDTH ALLOCATION AND BANDWIDTH-ON-DEMAND IN A LOW-EARTH ORBIT
SATELLITE DATA COMMUNICATION NETWORK, assigned to the assignee of the
present invention and incorporated by reference herein. Uplink bandwidth
may be allocated based on a priority status assigned to the data packets
to be transmitted to enable the data transmission to meet or exceed a
user-selected quality of service. Different data packets in a data
transmission may be assigned different levels of priority status. The '761
patent referenced above describes dividing uplink bandwidth into slots
representing time and frequency which are allocated for transmission of
data packets in accordance with the assigned priority status of the data
packets. The patent also describes a bandwidth on-demand feature in which
bandwidth can be allocated on request.
Where an Earth-based terminal, such as a gateway, is employed to control
data transmission parameters and satellite operation, the Earth-based
terminal may communicate with the satellite by either an "in-band"
transmission or an "out-of-band" transmission. An in-band data
transmission is generally more efficient because the control message is
essentially piggy-backed with data communications already taking place
(i.e., additional bandwidth allocation is not required). An out-of-band
transmission may be necessary when the ground terminal does not have an
existing bandwidth allocation to communicate with the satellite. An
out-of-band transmission may, for example, involve transmitting data over
a contention channel comprised of a communication frequency that remains
open for unscheduled data transmissions. Of course, when using a
contention channel, there is risk that the satellite or gateway will not
successfully receive control messages if, for example, two or more control
messages are simultaneously transmited on the contention channel by
different satellites or gateways. For that reason, it is preferred that
the satellite or gateway confirm receipt of control messages by
transmitting an acknowledgement signal.
Preferably, communication resource allocation also takes into account the
hand-off of communication links from one satellite to another.
Communication hand-off may be accomplished using a "break before make"
process as described earlier. It may also be accomplished by directing
data packets in a particular communication link to different queues for
transmission to the different satellites.
In embodiments of the invention where resource allocation is performed in
the satellites, the user terminals and gateways may transmit resource
request signals to the satellite by either in-band or out-of-band
transmission as described above. Resource allocation algorithms operating
in the satellite may allocate time, frequency, and/or codes in the form of
resource assignment data that is transmitted back to the requesting user
terminal or gateway. Data structures and processes for managing the
allocation of resources, particularly bandwidth on-demand, are described
in the '761 patent referenced above.
In a currently preferred embodiment of the invention, the satellites are
capable of autonomously performing some low-level operations such as
attitude control and maintaining antennas pointed to designated locations.
However, satellite and constellation operation is primarily controlled
from a satellite control system (SCS) operating in connection with or
within the satellite data communication system. For each satellite, the
SCS generates a set of time-tagged commands that are transmitted to that
satellite, which then executes each command at its designated time. These
commands control the payload configuration, antenna pointing/tracking
locations, etc. The satellite also generates data such as telemetry data
and alarms that it transmits to the SCS. Telemetry, tracking and control
systems (TTC) in the satellite data communication system provide the
communication paths between the satellites and the SCS. During normal
operations, a small portion of the Ka-band uplink and downlink spectrum is
used to transmit TTC data on the satellite-gateway link. When the
satellite is not in its normal operating mode, such as during satellite
launch and deployment, and certain abnormal conditions, TTC data may be
transmitted over a separate path using S-band spectrum and a separate set
of ground stations. Communication between the SCS and the TTC systems in
the gateways and/or other terrestrial facilities may use a terrestrial
VPN.
Other system elements may include a network operations center (NOC) and
business operation support systems. These systems support high-level
network management and operations functions, including billing,
provisioning, fault detection and reconfiguration, security management,
service quality management, etc.
For maximum reliability, two physically separate NOCs may operate in an
active-standby mode. Each NOC interfaces with the SCS and with the
gateways. The active NOC provides the SCS with service locations and
traffic demand profiles for the service cells, gateways and bulk data
delivery terminals. The SCS generates satellite configuration commands,
which it may send to the satellites via TTC systems. The SCS also sends to
the NOC the satellite ephemeris updates and a schedule of satellite and
transponder assignments for the gateways, service cells, and bulk data
delivery operations. The NOC in turn passes the required information to
the gateways. Network management responsibilities include remote
monitoring and control of gateway and network systems, maintaining gateway
and network status, performing fault detection, reconfiguration and
recovery, monitoring network service quality, and configuration
management.
In a currently preferred embodiment of the invention, the satellites use
functionally equivalent bent-pipe communication payloads to relay
communication channels with apparent transparency between user terminals
and their associated gateways. Uplink transmissions operate within a
frequency band of 28.6 GHz to 29.1 GHz, and downlink transmissions operate
within 18.8 GHz to 19.3 GHz. The uplink and downlink bands are divided
into four channels each, with a center-frequency separation of 125 MHz and
a usable bandwidth of 115 MHz. Each uplink channel has a corresponding
downlink channel separated by 9.8 GHz.
FIG. 9 depicts a conceptual satellite component block diagram. In this
example, each satellite has four independently steerable antennas, each of
which can point to and track any location in the satellite's footprint.
Each antenna supports simultaneous transmit and receive functions on
orthogonal polarizations. Two antennas may serve areas up to 800 km
diameter on the Earth's surface. Each of these antennas can receive up to
four 115 MHz RHCP channels and can transmit up to two fixed-frequency 115
MHz LHCP carriers. Channels 1 and 2 are fixed-assigned to one antenna and
channels 3 and 4 to the other. The remaining two antennas may serve areas
up to 200 km diameter. Each of these antennas can receive up to four 115
MHz LHCP channels and can transmit up to two fixed-frequency 115 MHz RHCP.
Channels 1 and 2 are fixed-assigned to one antenna and channels 3 and 4 to
the other.
The satellite provides interconnectivity between an uplink channel received
on any antenna and the corresponding downlink carrier on either of the two
antennas that can transmit that carrier. Each received uplink channel is
down-converted by a fixed frequency offset of 9.8 GHz to a corresponding
downlink channel. The down-converted signal is routed via RF switches and
bandpass filters to one of the two satellite-service cell antenna chains
equipped for the downlink channel. Each channel is amplified by a TWTA,
and the two channels are combined and transmitted via the antenna. This
interconnectivity supports a number of operating modes as described below.
Transponders in the satellites can be operated in either a fixed-gain mode
or an automatic level control (ALC) mode. The gateway-to-user terminal
links (forward link) and bulk-data-delivery links normally operate single
channel per carrier (SCPC), using ALC to maintain the downlink power
amplifier at saturation. The user terminal-to-gateway links (reverse link)
typically carry multiple user terminal subchannels and are normally
operated in the fixed-gain mode, with the downlink operating at an
appropriate back-off from saturation.
The satellite control system (SCS) generates the time-tagged commands that
control satellite operation. The SCS may be connected to TTC systems that
transmit these commands to, and receive telemetry from, the satellites.
Processors on-board the satellites execute these commands to reconfigure
transponders, steer antennas to new locations, track Earth-fixed locations
wit
