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Claims  |
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What is claimed is:
1. A communication system, comprising:
at least first and second earth orbiting satellites individual ones of
which comprise means for transceiving first communication signals with
terrestrially located user terminals and for transceiving second
communication signals with terrestrially located stations, said first
communication signals being within a first band of frequencies and said
second communication signals being within a second band of frequencies,
said at least two earth orbiting satellites having first and second
terrestrial coverage areas, respectively;
at least one terrestrial repeater station that is located within an
overlapped region between said first and second terrestrial coverage
areas, said at least one terrestrial repeater station being comprised of a
first transceiver for receiving a first communication signal from a first
one of said satellites associated with said first satellite coverage area
and for transmitting said received first communication signal to a second
one of said satellites associated with said second satellite coverage
area, whereby a communication forward link is established between a first
station located within said first satellite coverage area and a second
station located within said second satellite coverage area;
wherein said at least one terrestrial repeater station is further comprised
of a second transceiver for receiving a first communication signal from
said second one of said satellites associated with said second satellite
coverage area and for transmitting said received first communication
signal to said first one of said satellites associated with said first
satellite coverage area, whereby a communication return link is
established between said first station and said second station.
2. A communication system, comprising:
at least first and second earth orbiting satellites individual ones of
which comprise means for transceiving first communication signals with
terrestrially located user terminals and for transceiving second
communication signals with terrestrially located stations, said first
communication signals being within a first band of frequencies and said
second communication signals being within a second band of frequencies,
said at least two earth orbiting satellites having first and second
terrestrial coverage areas, respectively;
at least one terrestrial repeater station that is located within an
overlapped region between said first and second terrestrial coverage
areas, said at least one terrestrial repeater station being comprised of a
first transceiver for receiving a first communication signal from a first
one of said satellites associated with said first satellite coverage area
and for transmitting said received first communication signal to a second
one of said satellites associated with said second satellite coverage
area, whereby a communication forward link is established between a first
station located within said first satellite coverage area and a second
station located within said second satellite coverage area;
wherein said at least one terrestrial repeater station is further comprised
of means for demodulating a call request transmission that is received
from said first satellite, and means for extracting call destination
information from said demodulated call request transmission.
3. A communication system as set forth in claim 2 wherein said at least one
terrestrial repeater station is further comprised of means for selecting a
satellite to receive said transmission from said terrestrial repeater
station.
4. A communication system as set forth in claim 2 wherein said demodulating
means includes means for despreading a spread spectrum signal that is
received from said first satellite.
5. A communication system, comprising:
at least first and second earth orbiting satellites individual ones of
which comprise means for transceiving first communication signals with
terrestrially located user terminals and for transceiving second
communication signals with terrestrially located stations, said first
communication signals being within a first band of frequencies and said
second communication signals being within a second band of frequencies,
said at least two earth orbiting satellites having first and second
terrestrial coverage areas, respectively;
at least one terrestrial repeater station that is located within an
overlapped region between said first and second terrestrial coverage
areas, said at least one terrestrial repeater station being comprised of a
first transceiver for receiving a first communication signal from a first
one of said satellites associated with said first satellite coverage area
and for transmitting said received first communication signal to a second
one of said satellites associated with said second satellite coverage
area, whereby a communication forward link is established between a first
station located within said first satellite coverage area and a second
station located within said second satellite coverage area;
wherein said first and second earth orbiting satellites are a portion of a
constellation of low earth orbit (LEO) repeater satellites.
6. A communication system as set forth in claim 5 wherein said
constellation is comprised of 48 satellites distributed in eight orbital
planes with six equally-spaced satellites per plane, said orbital planes
being inclined at 52 degrees with respect to the equator.
7. A method for operating a communication system having a constellation of
low earth orbit (LEO) repeater satellites each having an associated ground
coverage region for bidirectionally communicating with terrestrial
transceivers, including ground stations and user terminals that are
located within the associated ground coverage region, comprising the steps
of:
initiating a communication with a first ground station by forming a
communication request that includes information for specifying a
destination for the communication;
transmitting the communication request from the first ground station to a
first LEO repeater satellite;
repeating the communication request by receiving and transmitting the
communication request with the first LEO repeater satellite;
receiving the communication request with a ground repeater station that is
located within the ground coverage region of the first LEO repeater
satellite;
extracting the information that specifies the destination for the
communication from the received communication request;
selecting, at least partially in accordance with the extracted information,
at least one further LEO repeater satellite;
transmitting the communication request from the ground repeater station to
the at least one selected LEO repeater satellite, the at least one
selected LEO repeater satellite having a ground coverage region that
overlaps the ground coverage region of the first LEO repeater satellite;
repeating the communication request by receiving and transmitting the
communication request with the at least one selected LEO repeater
satellite;
receiving the repeated communication request with at least one further
ground station; and
establishing a communication link to a terrestrial communication network in
accordance with the destination specified by the call request.
8. A method as set forth in claim 7 wherein the step of extracting includes
a step of despreading and demodulating a spread spectrum communication
signal.
9. A method as set forth in claim 7 wherein the constellation is comprised
of 48 satellites distributed in eight orbital planes with six
equally-spaced satellites per plane, and wherein the orbital planes are
inclined at 52 degrees with respect to the equator.
10. A ground-based repeater station for use with a plurality of low earth
orbit (LEO) communication satellites individual ones of which have an
associated ground coverage area, comprising:
a first transceiver for receiving a downlink transmission from a first one
of said LEO communication satellites associated with a first coverage area
and for transmitting said received transmission on an uplink to a second
one of said LEO communication satellites associated with a second coverage
area that overlaps the first coverage area;
a second transceiver for receiving a downlink transmission from said second
one of said LEO communication satellites associated with said second
coverage area and for transmitting said received transmission on an uplink
to said first one of said LEO communication satellites associated with
said first coverage area;
means for demodulating a call request transmission that is received from
said first LEO communication satellite;
means for extracting call destination information from said demodulated
call request transmission; and
means for selecting a LEO communication satellite to receive said uplink
transmission.
11. A ground-based repeater station as set forth in claim 10 wherein said
demodulating means includes means for despreading a spread spectrum signal
that is received from said first LEO communication satellite.
12. A method for operating a communication system having a constellation of
low earth orbit (LEO) repeater satellites each having an associated ground
coverage region for bidirectionally communicating with terrestrial
transceivers, including ground stations and user terminals that are
located within the associated ground coverage region, comprising the steps
of:
initiating a communication by sending a transmission from a user terminal
to a first LEO repeater satellite;
repeating the transmission through the first LEO repeater satellite to a
first ground station;
forming a communication request with the first ground station, the
communication request including information for specifying a destination
for the communication;
transmitting the communication request from the first ground station to the
first LEO repeater satellite;
repeating the communication request by receiving and transmitting the
communication request with the first LEO repeater satellite;
receiving the communication request with a ground repeater station that is
located within the ground coverage region of the first LEO repeater
satellite;
transmitting the communication request from the ground repeater station to
at least one further LEO repeater satellite, the at least one further LEO
repeater satellite having a ground coverage region that overlaps the
ground coverage region of the first LEO repeater satellite;
repeating the communication request by receiving and transmitting the
communication request with the at least one further LEO repeater
satellite;
receiving the repeated communication request with at least one further
ground station; and
establishing a communication link to a terrestrial communication network in
accordance with the destination specified by the call request.
13. A method as set forth in claim 12 wherein the steps of transmitting and
receiving each include a step of amplifying a spread spectrum
communication signal.
14. A method as set forth in claim 12 wherein the constellation is
comprised of 48 satellites distributed in eight orbital planes with six
equally-spaced satellites per plane, and wherein the orbital planes are
inclined at 52 degrees with respect to the equator. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates generally to communications systems and, in
particular, to a low earth orbit (LEO) satellite-based communications
system.
BACKGROUND OF THE INVENTION
Satellite-based communications systems are well are represented in the
prior art. By example, reference is made to U.S. Pat. No. 5,303,286,
issued on Apr. 12, 1994 to Robert A. Wiedeman, and which is entitled
"Wireless Telephone/Satellite Roaming System". Reference is also made to
the numerous U.S. Patents, foreign patents, and other publications that
are of record in U.S. Pat. No. 5,303,286.
Of particular interest herein is a class of satellite-based communications
systems that employs multiple satellites in a low earth orbit, referred to
as a `LEO` system or LEOS. LEOS are characterized by moving patterns of
signal `footprints` on the ground, where each footprint corresponds to the
coverage area of one or more beams that are transmitted and received by a
given satellite as it orbits the earth. The satellites communicate with
terrestrial stations which may be referred to as `gateways`.
It is often the case that two or more satellites of a constellation of LEO
satellites will have overlapping footprints or coverage areas. The
presence of overlapping coverage areas enables a ground-based receiver to
simultaneously receive a communication signal from and transmit a
communication signal through a plurality of satellites whose coverage
areas overlap. For a receiver that receives multiple copies of the same
signal through a plurality of satellites the effects of multi-path fading
and signal blockage can be greatly reduced. Reference in this regard can
be had to U.S. Pat. No. 5,233,626, issued Aug. 3, 1993 to Stephen A. Ames
and entitled "Repeater Diversity Spread Spectrum Communication System",
the disclosure of which is incorporated by reference herein in its'
entirety.
Communication systems that make use of repeater diversity generally use
spread spectrum (SS) techniques, and possibly also code division multiple
access (CDMA) as the modulation scheme in order to maximize the
communications capability. In such systems there is a desire to cause the
satellite footprints and any interior beams generated to have the maximum
overlap possible to maximize the use of diversity techniques to combat
fading and blockage.
SUMMARY OF THE INVENTION
This invention is directed to a unique use of overlapping footprints in a
LEO satellite communications system to increase the overall connectivity
of the system, thus providing a wide service availability. In particular,
this invention teaches the use of at least one terrestrial LEOS relay
station that is positioned within an overlap of at least two satellite
coverage areas for relaying a communication from a gateway associated with
a first coverage area to a gateway associated with a second coverage area.
A plurality of LEOS relay stations can be so provided to enable a
communication, such as a voice communication, to be routed through a
plurality of coverage areas and gateways, thereby bypassing a substantial
portion of an underlying terrestrial communication system.
More particularly, this invention teaches a ground-based repeater station
for use with a plurality of low earth orbit (LEO) communication satellites
individual ones of which have an associated ground coverage area. The
repeater station includes a first transceiver for receiving a downlink
transmission from a first one of the LEO communication satellites
associated with a first coverage area and for transmitting the received
transmission on an uplink to a second one of the LEO communication
satellites associated with a second coverage area that overlaps the first
coverage area. The repeater station further includes a second transceiver
for receiving a downlink transmission from the second one of the LEO
communication satellites associated with the second coverage area and for
transmitting the received transmission on an uplink to the first one of
the LEO communication satellites associated with the first coverage area.
The repeater station further includes a demodulator for demodulating a
call request transmission that is received from the first LEO
communication satellite; a controller for extracting call destination
information from the demodulated call request transmission and for
selecting a LEO communication satellite to receive the uplink
transmission.
In a preferred embodiment of this invention the demodulator includes
circuitry for despreading and tracking a spread spectrum signal that is
received from the first LEO communication satellite.
BRIEF DESCRIPTION OF THE DRAWINGS
The above set forth and other features of the invention are made more
apparent in the ensuing Detailed Description of the Invention when read in
conjunction with the attached Drawings, wherein:
FIG. 1 is block diagram of a satellite communication system that is
constructed and operated in accordance with a presently preferred
embodiment of this invention;
FIG. 2 is a block diagram of one of the gateways of FIG. 1;
FIG. 3A is a block diagram of the communications payload of one of the
satellites of FIG. 1;
FIG. 3B illustrates a portion of a beam pattern that is associated with one
of the satellites of FIG. 1;
FIG. 4 is a block diagram that depicts the ground equipment support of
satellite telemetry and control functions;
FIG. 5 is block diagram of the CDMA sub-system of FIG. 2;
FIG. 6 is a block diagram of a LEO gateway-to-gateway relay system showing
a communication path for a forward link;
FIG. 7 is a block diagram of the LEO gateway-to-gateway relay system
showing the communication path for a reverse link;
FIG. 8 is a flow chart depicting the sequence of steps that are executed by
a method of this invention;
FIG. 9 is block diagram that illustrates a first example of the use of this
invention; and
FIG. 10 is block diagram that illustrates a second example of the use of
this invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a presently preferred embodiment of a satellite
communication system 10 that is suitable for use with the presently
preferred embodiment of this invention. Before describing this invention
in detail, a description will first be made of the communication system 10
so that a more complete understanding may be had of the present invention.
The communications system 10 may be conceptually subdivided into a
plurality of segments 1, 2, 3 and 4. Segment 1 is referred to herein as a
space segment, segment 2 as a user segment, segment 3 as a ground
(terrestrial) segment, and segment 4 as a telephone system infrastructure
segment.
In the presently preferred embodiment of this invention there are a total
of 48 satellites in, by example, a 1414 km Low Earth Orbit (LEO). The
satellites 12 are distributed in eight orbital planes with six
equally-spaced satellites per plane (Walker constellation). The orbital
planes are inclined at 52 degrees with respect to the equator and each
satellite completes an orbit once every 114 minutes. This approach
provides approximately full-earth coverage with, preferably, at least two
satellites in view at any given time from a particular user location
between about 70 degree south latitude and about 70 degree north latitude.
As such, a user is enabled to communicate to or from nearly any point on
the earth's surface within a gateway (GW) 18 coverage area to or from
other points on the earth's surface (by way of the PSTN), via one or more
gateways 18 and one or more of the satellites 12, possibly also using a
portion of the telephone infrastructure segment 4.
It is noted at this point that the foregoing and ensuing description of the
system 10 represents but one suitable embodiment of a communication system
within which the teaching of this invention may find use. That is, the
specific details of the communication system are not to be read or
construed in a limiting sense upon the practice of this invention.
Continuing now with a description of the system 10, a soft transfer
(handoff) process between satellites 12, and also between individual ones
of 16 spot beams transmitted by each satellite (FIG. 3B), provides
unbroken communications via a spread spectrum (SS), code division multiple
access (CDMA) technique. The presently preferred SS-CDMA technique is
similar to the TIA/EIA Interim Standard, "Mobile Station-Base Station
Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular
System" TIA/EIA/IS-95, July 1993, although other spread spectrum and CDMA
techniques and protocols can be employed.
The low earth orbits permit low-powered fixed or mobile user terminals 13
to communicate via the satellites 12, each of which functions, in a
presently preferred embodiment of this invention, solely as a "bent pipe"
repeater to receive a communications traffic signal (such as speech and/or
data) from a user terminal 13 or from a gateway 18, convert the received
communications traffic signal to another frequency band, and to then
re-transmit the converted signal. That is, no on-board signal processing
of a received communications traffic signal occurs, and the satellite 12
does not become aware of any intelligence that a received or transmitted
communications traffic signal may be conveying.
Furthermore, there need be no direct communication link or links between
the satellites 12. That is, each of the satellites 12 receives a signal
only from a transmitter located in the user segment 2 or from a
transmitter located in the ground segment 3, and transmits a signal only
to a receiver located in the user segment 2 or to a receiver located in
the ground segment 3.
The user segment 2 may include a plurality of types of user terminals 13
that are adapted for communication with the satellites 12. The user
terminals 13 include, by example, a plurality of different types of fixed
and mobile user terminals including, but not limited to, handheld mobile
radio-telephones 14, vehicle mounted mobile radio-telephones 15,
paging/messaging-type devices 16, and fixed radio-telephones 14a. The user
terminals 13 are preferably provided with omnidirectional antennas 13a for
bidirectional communication via one or more of the satellites 12.
It is noted that the fixed radio-telephones 14a may employ a directional
antenna. This is advantageous in that it enables a reduction in
interference with a consequent increase in the number of users that can be
simultaneously serviced with one or more of the satellites 12.
It is further noted that the user terminals 13 may be dual use devices that
include circuitry for also communicating in a conventional manner with a
terrestrial cellular system.
Referring also to FIG. 3A, the user terminals 13 may be capable of
operating in a full duplex mode and communicate via, by example, L-band RF
links (uplink or return link 17b) and S-band RF links (downlink or forward
link 17a) through return and forward satellite transponders 12a and 12b,
respectively. The return L band RF links 17b may operate within a
frequency range of 1.61 GHz to 1.625 GHz, a bandwidth of 16.5 MHz, and are
modulated with packetized digital voice signals and/or data signals in
accordance with the preferred spread spectrum technique. The forward S
band RF links 17a may operate within a frequency range of 2.485 GHz to 2.5
GHz, a bandwidth of 16.5 MHz. The forward RF links 17a are also modulated
at a gateway 18 with packetized digital voice signals and/or data signals
in accordance with the spread spectrum technique.
The 16.5 MHz bandwidth of the forward link is partitioned into 13 channels
with up to, by example, 128 users being assigned per channel. The return
link may have various bandwidths, and a given user terminal 13 may or may
not be assigned a different channel than the channel assigned on the
forward link. However, when operating in the diversity reception mode on
the return link (receiving from two or more satellites 12), the user is
assigned the same forward and return link RF channel for each of the
satellites.
The ground segment 3 includes at least one but generally a plurality of the
gateways 18 that communicate with the satellites 12 via, by example, a
full duplex C band RF link 19 (forward link 19a (to the satellite), return
link 19b (from the satellite)) that operates within a range of frequencies
generally above 3 GHz and preferably in the C-band. The C-band RF links
bi-directionally convey the communication feeder links, and also convey
satellite commands to the satellites and telemetry information from the
satellites. The forward feeder link 19a may operate in the band of 5 GHz
to 5.25 GHz, while the return feeder link 19b may operate in the band of
6.875 GHz to 7.075 GHz.
The satellite feeder link antennas 12g and 12h are preferably wide coverage
antennas that subtend a maximum earth coverage area as seen from the LEO
satellite 12. In the presently preferred embodiment of the communication
system 10 the angle subtended from a given LEO satellite 12 (assuming
10.degree. elevation angles from the earth's surface) is approximately
110.degree.. This yields a coverage zone that is approximately 3600 miles
in diameter.
The L-band and the S-band antennas are multiple beam antennas that provide
coverage within an associated terrestrial service region. The L-band and
S-band antennas 12d and 12c, respectively, are preferably congruent with
one another, as depicted in FIG. 3B. That is, the transmit and receive
beams from the spacecraft cover the same area on the earth's surface,
although this feature is not critical to the operation of the system 10.
As an example, several thousand full duplex communications may occur
through a given one of the satellites 12. In accordance with a feature of
the system 10, two or more satellites 12 may each convey the same
communication between a given user terminal 13 and one of the gateways 18.
This mode of operation, as described in detail below, thus provides for
diversity combining at the respective receivers, leading to an increased
resistance to fading and facilitating the implementation of a soft handoff
procedure.
It is pointed out that all of the frequencies, bandwidths and the like that
are described herein are representative of but one particular system.
Other frequencies and bands of frequencies may be used with no change in
the principles being discussed. As but one example, the feeder links
between the gateways and the satellites may use frequencies in a band
other than the C-band (approximately 3 GHz to approximately 7 GHz), for
example the Ku band (approximately 10 GHz to approximately 15 GHz) or the
Ka band (above approximately 15 GHz).
The gateways 18 function to couple the communications payload or
transponders 12a and 12b (FIG. 3A) of the satellites 12 to the telephone
infrastructure segment 4. The transponders 12a and 12b include an L-band
receive antenna 12c, S-band transmit antenna 12d, C-band power amplifier
12e, C-band low noise amplifier 12f, C-band antennas 12g and 12h, L band
to C band frequency conversion section 12i, and C band to S band frequency
conversion section 12j. The satellite 12 also includes a master frequency
generator 12k and command and telemetry equipment 121.
Reference in this regard may also be had to U.S. Pat. No. 5,422,647, by E.
Hirshfield and C. A. Tsao, entitled "Mobile Communications Satellite
Payload" (U.S. Ser. No. 08/060,207).
The telephone infrastructure segment 4 is comprised of existing telephone
systems and includes Public Land Mobile Network (PLMN) gateways 20, local
telephone exchanges such as regional public telephone networks (RPTN) 22
or other local telephone service providers, domestic long distance
networks 24, international networks 26, private networks 28 and other
RPTNs 30. The communication system 10 operates to provide bidirectional
voice and/or data communication between the user segment 2 and Public
Switched Telephone Network (PSTN) telephones 32 and non-PSTN telephones 32
of the telephone infrastructure segment 4, or other user terminals of
various types, which may be private networks.
Also shown in FIG. 1 (and also in FIG. 4), as a portion of the ground
segment 3, is a Satellite Operations Control Center (SOCC) 36, and a
Ground Operations Control Center (GOCC) 38. A communication path, which
includes a Ground Data Network (GDN) 39 (see FIG. 2), is provided for
interconnecting the gateways 18 and TCUs 18a, SOCC 36 and GOCC 38 of the
ground segment 3. This portion of the communications system 10 provides
overall system control functions.
FIG. 2 shows one of the gateways 18 in greater detail. Each gateway 18
includes up to four dual polarization RF C-band sub-systems each
comprising a dish antenna 40, antenna driver 42 and pedestal 42a, low
noise receivers 44, and high power amplifiers 46. All of these components
may be located within a radome structure to provide environmental
protection.
The gateway 18 further includes down converters 48 and up converters 50 for
processing the received and transmitted RF carrier signals, respectively.
The down converters 48 and the up converters 50 are connected to a CDMA
sub-system 52 which, in turn, is coupled to the Public Switched Telephone
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