U.S. patent application number 09/726644 was filed with the patent office on 2001-04-05 for multiple satellite mobile communications method and apparatus for hand-held terminals.
This patent application is currently assigned to Hughes Electronics Corporation. Invention is credited to Chang, Donald C.D., Chang, Ming U., Hagen, Frank A., Mayfield, William, Novak, John I. III, Yung, Kar.
Application Number | 20010000167 09/726644 |
Document ID | / |
Family ID | 23037968 |
Filed Date | 2001-04-05 |
United States Patent
Application |
20010000167 |
Kind Code |
A1 |
Chang, Donald C.D. ; et
al. |
April 5, 2001 |
Multiple satellite mobile communications method and apparatus for
hand-held terminals
Abstract
A novel mobile satellite communications technique for hand-held
terminals includes a satellite system having a plurality of
individual satellites all in communication with a ground
telecommunications hub. A signal processed by the ground
telecommunications hub is radiated through multiple paths to a
plurality of the individual satellites in the satellite
constellation simultaneously. The radiated signal is then
re-radiated by the plurality of individual satellites to a mobile
satellite terminal that receives the re-radiated signal from the
plurality of individual satellites simultaneously such that the
same frequency spectrum may be re-used by another mobile user.
Inventors: |
Chang, Donald C.D.;
(Thousand Oaks, CA) ; Novak, John I. III; (West
Hills, CA) ; Yung, Kar; (Torrance, CA) ;
Hagen, Frank A.; (Palos Verdes Estates, CA) ; Chang,
Ming U.; (Rancho Palos Verdes, CA) ; Mayfield,
William; (Torrance, CA) |
Correspondence
Address: |
HUGHES ELECTRONICS CORPORATION
PATENT DOCKET ADMINISTRATION
BLDG 001 M/S A109
P O BOX 956
EL SEGUNDO
CA
902450956
|
Assignee: |
Hughes Electronics
Corporation
|
Family ID: |
23037968 |
Appl. No.: |
09/726644 |
Filed: |
November 30, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09726644 |
Nov 30, 2000 |
|
|
|
09271997 |
Mar 18, 1999 |
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Current U.S.
Class: |
455/13.1 ;
455/103; 455/12.1; 455/20; 455/427 |
Current CPC
Class: |
H04B 7/18534 20130101;
H04B 7/18506 20130101; H04B 7/18504 20130101; H04B 7/18532
20130101; H04B 7/212 20130101 |
Class at
Publication: |
455/13.1 ;
455/12.1; 455/427; 455/103; 455/20 |
International
Class: |
H04B 007/185 |
Claims
In the claims:
1. A mobile satellite communications system for mobile users,
comprising: a satellite constellation having a plurality of
individual satellites; a ground telecommunications hub in
communication with each of said plurality of individual satellites,
such that a signal processed by said ground telecommunications hub
is radiated through a plurality of paths to a plurality of said
individual satellites in said satellite constellation; and a mobile
terminal for receiving said signal radiated from said plurality of
individual satellites simultaneously.
2. The mobile satellite communications system of claim 1, wherein
said ground telecommunications hub includes a processing center for
receiving a plurality of signals and processing them to be
transmitted to said plurality of individual satellites.
3. The mobile satellite communications system of claim 2, wherein
said processing center includes apparatus for combining said
plurality of received signals for transmission to said plurality of
individual satellites.
4. The mobile satellite communications system of claim 3, wherein
said processing center includes apparatus for amplifying said
combined signals.
5. The mobile satellite communications system of claim 4, wherein
said processing center includes apparatus for filtering said
combined signals.
6. The mobile satellite communications system of claim 5, wherein
said processing center includes apparatus for up-converting said
combined signals.
7. The mobile satellite communications system of claim 6, wherein
said processing center includes a beam former for forming multiple
beams for transmitting signals to said plurality of satellites.
8. The mobile satellite communications system of claim 7, wherein
said processing center includes apparatus for predicting any time
variant differential among various paths between said hub and said
mobile terminal.
9. The mobile satellite communications system of claim 8, wherein
said apparatus for predicting said time variant utilizes two-way
ranging navigation.
10. A method for transmitting a communications signal to a mobile
hand-held terminal, comprising the steps of: providing a ground
telecommunications hub; processing a received signal at said ground
telecommunications hub; radiating said signal through multiple
paths to a plurality of satellites in a satellite constellation;
re-radiating said signal from said plurality of satellites to the
mobile hand-held terminal; and combining said re-radiated signal
received from said plurality of satellites simultaneously at the
mobile hand-held terminal.
11. The method of claim 10, further comprising: processing a
plurality of received user signals at said ground
telecommunications hub.
12. The method of claim 11, wherein said step of processing, said
plurality of received user signals further comprises: combining
said plurality of received signals for transmission to said
plurality of individual satellites.
13. The method of claim 12, wherein said step of processing said
plurality of received user signals further comprises: filtering
said combined signals; and up-converting said combined signals.
14. The method of claim 13, wherein said step of processing said
plurality of received user signals further comprises: forming
multiple beams for transmitting signals to said plurality of
satellites.
15. A method for transmitting a communications signal to a mobile
hand-held terminal, comprising the steps of: providing a ground
telecommunications hub; processing a received signal at said ground
telecommunications hub; predicting any time variant differential
among multiple paths between said hub and said mobile terminal;
radiating said signal through said multiple paths to a plurality of
satellites in a satellite constellation; and re-radiating said
signal from said plurality of satellites through multiple paths to
the mobile hand-held terminal such that the multiple paths are
received at the mobile hand-held terminal simultaneously.
16. The method of claim 15, wherein said step of processing a
received signal at said ground telecommunications hub further
comprises: filtering said signal prior to said step of
radiating.
17. The method of claim 16, wherein said step of processing a
received signal at said ground telecommunications hub further
comprises: up-converting said signal prior to said step of
radiating.
18. The method of claim 17, wherein said step of processing said
plurality of received user signals further comprises: forming
multiple beams for transmission to said plurality of
satellites.
19. The method of claim 15, further comprising: processing a
plurality of received user signals at said ground
telecommunications hub.
20. The method of claim 19, wherein said step of processing said
plurality of received user signals further comprises: combining
said plurality of received signals for transmission to said
plurality of satellites.
Description
TECHNICAL FIELD
1. The present invention relates generally to a mobile satellite
communication system. More specifically, the present invention
relates to a mobile satellite communication system with increased
user capacity by allowing frequency re-use through the use of
multiple satellites to radiate one signal.
BACKGROUND ART
2. Current mobile satellite communication systems, such as Iridium,
Globalstar, and ICO, utilize low-cost user terminals as one of
their key system features. To maintain communications linkage with
these current mobile systems, the system satellites provide
multiple beam and high-gain services to the subscribers. The
low-cost and low-gain hand-held terminals utilized by the users of
these systems, transmit and receive signals to and from high
performance satellites which populate almost the entire hemisphere.
Some of these current systems require the usage of at least two
satellites to assure a soft hand-over process as the satellites
progress from horizon to horizon. As a result, as more satellites
come into a user's field of view (FOV), the satellite system
becomes more reliable and available. The satellite constellations
provided by these current systems are thus sized to guarantee a
minimum number of satellites within a user's FOV over large
coverage areas at all times.
3. All of these current mobile satellite communication systems,
however, suffer from a variety of disadvantages. First, they all
have limited frequency resources. Any given frequency over a given
ground position can only be utilized by one user at a time. This is
true regardless of the sophistication of the system, including
systems that utilize multiple beam satellite designs. Even when
multiple satellites are available at a given geographic location,
the same frequency spectrum cannot be used by more than one nearby
user. The availability of multiple satellites merely serves to
increase the availability of the system to that user who is
assigned the specific frequency spectrum. However, the total
capacity of these mobile communication satellite systems is still
limited by the inefficient usage of the frequency spectrum. Thus,
the potential growth of these current satellite communication
systems is inherently limited.
4. Additionally, current telecommunications systems only allow
mobile-to-hub and hub-to-mobile communications in most of the low
earth orbit and medium earth orbit mobile satellite constellations.
Mobile-to-mobile linkages require multiple hops between hubs. Thus,
one user utilizes a satellite at a frequency slot to communicate to
his counterpart on the network. Other satellites on or in the same
region cannot reuse the same frequency slot for other nearby users.
Thus, if a secondary user nearby has a handset that requires a
particular frequency, which is being utilized by the first user
nearby, the second user is unable to access the system through the
same frequency via different satellites. It is therefore desirable
to provide a mobile communication satellite system that relaxes
these constraints and more efficiently utilizes current mobile
satellite communication system resources, while also providing much
greater opportunity for system growth.
SUMMARY OF THE INVENTION
5. It is an object of the present invention to provide a mobile
satellite communication system with no limitation on frequency
re-use for point-to-point communications.
6. It is another object of the present invention to provide a
mobile satellite communication system that utilizes simple and low
cost satellite designs.
7. It is a further object of the present invention to provide a
mobile satellite communication system with high system reliability
through graceful degradation.
8. It is still another object of the present invention to provide a
mobile satellite communication system wherein the individual
satellites and the mobile terminals are of low complexity with the
complexity of the system concentrated at the ground hub
terminal.
9. It is yet another object of the present invention to provide a
mobile satellite communication system with more accurate
capabilities for satellite and user positioning.
10. In accordance with the objects of the present invention, a
novel mobile satellite communications technique for hand-held
terminals is provided. The mobile satellite communications system
includes a satellite system having a plurality of individual
satellites. The plurality of individual satellites are each in
communication with a ground telecommunications hub such that a
signal processed by the ground telecommunications hub is radiated
through multiple paths to a plurality of the individual satellites
in the satellite constellation. The radiated signal is then
re-radiated by the plurality of individual satellites to a mobile
satellite germinal which receives the re-radiated signal from the
plurality of individual satellites simultaneously such that the
same frequency spectrum may be re-used by another mobile user.
11. These and other features of the present invention will become
apparent from the following description of the invention, when
viewed in accordance with the accompanied drawings and appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
12. FIG. 1 is a perspective view illustrating the forward link
geometry of a mobile satellite communications system in accordance
with a preferred embodiment of the present invention;
13. FIG. 2 is a schematic block diagram illustrating the signal
transmission function of a ground telecommunications hub for a
mobile satellite communications system in accordance with a
preferred embodiment of the present invention;
14. FIG. 3 is a perspective view illustrating the return link
geometry of a mobile satellite communications system in accordance
with a preferred embodiment of the present invention;
15. FIG. 4 is a schematic block diagram illustrating the signal
receive function of a ground telecommunications hub for a mobile
satellite communications system in accordance with a preferred
embodiment of the present invention; and
16. FIG. 5 is a schematic flow diagram illustrating the overall
architecture for a multiple satellite mobile communications system
in accordance with a preferred embodiment of the present
invention.
BEST MODE(S) FOR CARRYING OUT THE INVENTION
17. Referring now to the figures, the disclosed mobile
communication system can be utilized to break away from the
frequency spectrum limitation discussed above and provide much more
efficient means to re-use the allocated mobile satellite spectrum
multiple times. By eliminating this frequency spectrum limitation,
the overall capacity of existing mobile satellite communication
systems will be allowed to grow.
18. Referring now to FIG. 1, a mobile satellite communication
system 10 in accordance with a preferred embodiment of the present
invention is illustrated. In FIG. 1, the mobile satellite
communications system 10 is illustrated in a forward link mode. The
mobile satellite communications system 10 includes a ground
telecommunications hub 12, a satellite constellation 14 including a
plurality of individual satellites 16, and a plurality of hand-held
user terminals 18 such as mobile phones. As discussed in more
detail below, the user terminals 18 can receive signals 20
simultaneously from multiple satellites 16 via their broad beam
antennas 22. The ground telecommunications hub 12 connects to all
of the satellites 16 in the satellite constellation 14 individually
and simultaneously. The hub 12 also pre-processes received signals
to compensate for path differentials before sending radiating
signals 24 to the satellites 16 as discussed in more detail
below.
19. In accordance with the preferred embodiment, the design of the
individual satellites 14 can be significantly simplified over those
utilized in prior mobile systems because the satellite
constellation 14 functions as a sparse radiating array. It is known
that the more satellites 16 that are included in a satellite
constellation 14, the better the performance the mobile satellite
communications system 10 will achieve. Satellites that are simple,
small, and provide high performance are preferable. This is because
the performance of the system 10 depends more heavily on the
satellite constellation 14 than on the individual satellites
16.
20. In a transmit mode, shown in FIG. 1, the individual satellites
16 radiate modulated RF power to a chosen FOV. The system 10 is
still operable with reduced capacity and no reconfiguration even if
one individual satellite 16 is lost for any reason. As a result,
the system 10 features graceful degradation characteristics and
provides very high reliability and availability. Most of the
complexity of the system 10 is located in the ground hubs 12, which
locate and track the potential users and perform the major
functions of beamforming and filtering, as discussed below.
21. As shown in FIG. 2, the processing performed at the ground
telecommunications hub 12 is diagrammatically illustrated. The hub
12 tracks, updates, and forward predicts the time variant
differential information among various paths between the hub 12 and
the intended user terminals 18. The accuracy of this information
must be within a tenth of an RF wavelength. For UHF satellite
systems, the required path differential accuracy must be about ten
(10) centimeters. For L and S band mobile satellite constellations,
the accuracy must be on the order of one (1) centimeter.
Unfortunately, the conventional or GPS techniques will not provide
the required accuracy.
22. In accordance with the present invention, the required accuracy
of equivalent path differentials, including all propagation
distortion, can be provided using two-way active calibration and
R2N (two-way ranging navigation) techniques. An R2N technique is
just one technique for obtaining positioning information by which
to locate the positioning of the satellites and users precisely
using multiple calibration cites and is described in co-pending
U.S. patent application Ser. No. 09/209,062, entitled "Method and
System for Determining a Position of a Transceiver Unit
Incorporating Two-Way Ranging Navigation as a Calibration Reference
for GPS," and filed on Dec. 10, 1998. Other known techniques may
also be utilized.
23. The ground telecommunications hub 12 has a processing center 26
that processes each signal and is shown in a transmit mode in FIG.
2. The hub 12 has the capability to address the plurality of
satellites 16 individually through the use of antenna spatial
discrimination to separate signals to different satellites.
Alternatively, code identification can also be used to address
different satellites independently.
24. As shown in FIG. 2, assuming that there are "H" users, the
signals from user 1 to user H, identified generally by reference
number 28, are input into the processing center 26. The position of
the various users (1 to H), is determined generally by the
circuitry from the various user signals 28, designated by reference
number 30. The various user signals 28 for user 1 to user H are
then combined for transmission to the different satellites 16, as
generally indicated by reference number 32. In this case, the
signal is sent to N satellites, assuming N satellites in the
constellation. The combined signals are then amplified, filtered,
up converted, and then further amplified, as generally indicated by
reference number 36. These signals are then delivered to a multiple
beam antenna 38 where beamforming processing is done so that the
signals can be transmitted to the N satellites via radiating
signals 24. The beam-forming process can be done in baseband or a
low IF frequency band by either digital or analog means. For a low
bandwidth(less than a few MHz signals), digital implementation can
provide cost advantages. The processed signal 24, radiated from the
ground hub 12 via multiple paths, is amplified, filtered, and then
re-radiated by each of the multiple satellites 16 to arrive at a
designated user location simultaneously. Consequently, the radiated
signals from the multiple satellites will be combined coherently
via a hand held terminal 22.
25. Equivalently, the effect of the spatial processing performed by
the processing center 26 is to focus signal strength on the user
for multiple satellites 16, which act like sparsely separated
portions of a large active reflector. Therefore, the processing on
the ground will insert different time delays into the signals 24
which are radiated via various paths. The time delays will be
inserted into the signals 24 as if the satellites were located on
an ellipsoidal surface, of which the two foci are located exactly
a: the hub 12 and the designated user 18 positions respectively. In
low and middle earth orbit constellations, the users 18 and the hub
12 will always be in the near field of the sparse array.
26. In a receive mode, shown in FIG. 3, the individual satellites
16 collect the RF signals from the same FOV. FIG. 3 thus
illustrates the return link geometry for receiving signals sent
from the user terminals 18 to the ground telecommunications hub 12.
As shown in FIG. 3, there are two groups of links involved: the
links between users 18 and the satellites 16, generally indicated
by reference number 40, and those between the satellites 16 and the
hub 12, as generally indicated by reference number 42. The user
antennas 22 must be able to illuminate all the satellites 16
involved. There will also be a constraint on the variation and gain
of the user antenna 22 over the cluster.
27. As with the forward link geometry, the satellites 16 will
amplify the signals 40 received from the users 18 and re-radiate
the signals 42 toward the hub 12. The hub 12 can receive signals 42
independently, but simultaneously from the satellites 16, and will
add the signals 42 from different satellites coherently in the
post-processor 44 as illustrated in FIG. 4.
28. The signal flows on the block diagram shown in FIG. 4
illustrate the receive function of the post-processor 40 and the
hub 12. The signal flows are reversed from the corresponding ones
in FIG. 2. Therefore the receive process will not be reiterated in
detail. However, the links 42 from the satellites 16 to the hub 12
are received at the beamformer 38 and then transferred to the
receiver and down converters 46 before the signals are separated.
The signals are separated depending upon the user to which they are
to be transmitted, as generally indicated by reference number 48,
and then sent to the specific user 1 through H, as generally
indicated by reference number 50. It should be understood that both
the receive and transmit function are a necessary part of pathlink
calibration and user positioning.
29. The technique of the present invention has been demonstrated to
significantly reduce the average side loeb levels. It has been
determined that this is due to three factors. First, the proposed
architecture is not a periodic array, but rather a randomly spaced
sparse array, which has no grating lobes. Although the average side
lobe at a single frequency is relatively high, the level decreases
with increasing bandwidth. Second, the large sparsely filled array
formed by moving satellites is a large extended aperture size.
Thus, all of the users on the ground are in the near field of the
extended aperture and the wave fronts received by all users are
spherical instead of planar. Consequently, dispersion effects
become much more pronounced than would be the case in the far
field. The dispersion grows very fast as a probe is scanned away
from the main beam and the dispersion smears the power distribution
very effectively over a finite signal bandwidth. Third, the
communication system is preferably designed with a finite frequency
spectrum. The information signal will therefore be spread over a
finite bandwidth via CDMA or through short duration waveforms for
TDMA schemes.
30. FIG. 5 illustrates diagrammatically the operation of the
invention, which allows for the increased re-use of precious
frequency spectrum. The advantages provided by this system include
no limitation on frequency re-use for point-to-point
communications. Rather, the capacity of this system is only limited
by total satellite RF power. Further, the preferred embodiment
allows for the use of simple and low cost satellite designs,
because the more satellites included in the constellation, the
better the performance of the overall system. The system also
provides high system reliability through graceful degradation, as
well as concentrating complex processing at the hubs. The preferred
embodiment creates demand for a large number of low cost satellites
and also uses R2N techniques to perform satellite and user
positioning. The more users using this system, the more accurately
the satellite and user positions can be determined. However, even
more important than the actual positions of one users and
satellites are the path lengths traversed by the signals.
Therefore, periodic calibration techniques applied directly to
those path lengths may be much simpler and more cost effective.
Further, the system also provides advantages of CDMA and TDMA to
the system performance through large percentage bandwidth.
31. As shown in FIG. 5, the present invention is divided up into
three segments; a hub segment 52 containing the ground
telecommunications hub 12, a space segment 54 containing a
plurality of individual satellites 16, and a user segment 56,
having a plurality of user terminals 18. The hub segment also has a
processing center 26 and a post-processor 44 for processing the
signals during the transmission and receive modes.
32. Having now fully described the invention, it will be apparent
to one of ordinary skill in the art that many changes and
modifications can be made thereto without departing from the spirit
or scope of the invention as set forth herein.
* * * * *