U.S. patent application number 12/037355 was filed with the patent office on 2008-10-30 for space-based network architectures for satellite radiotelephone systems.
Invention is credited to Carson E. Agnew, Peter D. Karabinis.
Application Number | 20080268836 12/037355 |
Document ID | / |
Family ID | 27372412 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080268836 |
Kind Code |
A1 |
Karabinis; Peter D. ; et
al. |
October 30, 2008 |
Space-Based Network Architectures for Satellite Radiotelephone
Systems
Abstract
A space-based network for a satellite radiotelephone system
includes at least one receive-only satellite and at least one
transmit satellite. The transmit satellite can be a transmit-only
satellite or a transmit and receive satellite. The receive-only
satellite(s) are configured to receive wireless communications from
a radiotelephone at a location over a satellite frequency band. The
transmit satellite(s) are configured to transmit wireless
communications to the radiotelephone at the location over the
satellite frequency band. By providing at least one receive-only
satellite and at least one transmit satellite, space-based networks
can offer a significant link margin, without the need to
undesirably burden the radiotelephones themselves to achieve this
link margin.
Inventors: |
Karabinis; Peter D.; (Cary,
NC) ; Agnew; Carson E.; (Vienna, VA) |
Correspondence
Address: |
THE LAW OFFICE OF JOHN T. WHELAN, L.L.C.
135 WEST DARES BEACH ROAD, SUITE 204
PRINCE FREDERICK
MD
20678
US
|
Family ID: |
27372412 |
Appl. No.: |
12/037355 |
Filed: |
February 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11560226 |
Nov 15, 2006 |
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12037355 |
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11241519 |
Sep 30, 2005 |
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11560226 |
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10225623 |
Aug 22, 2002 |
7006789 |
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11241519 |
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10074097 |
Feb 12, 2002 |
6684057 |
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10225623 |
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60322240 |
Sep 14, 2001 |
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60392771 |
Jul 1, 2002 |
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Current U.S.
Class: |
455/427 |
Current CPC
Class: |
H04B 7/216 20130101;
H04B 7/18539 20130101; H04B 7/18563 20130101; H04B 7/18534
20130101; H04B 7/18513 20130101 |
Class at
Publication: |
455/427 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
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48. A space-based network for a satellite radiotelephone system
comprising a first number of antennas each of which is configured
to receive information and a second number of antennas each of
which is configured to transmit information, wherein the first
number of antennas equals or exceeds the second number of antennas,
at least one antenna of the first number of antennas is configured
to receive information from a radiotelephone over at least two
substantially orthogonal polarizations and the radiotelephone is
configured to radiate electromagnetic energy substantially
concurrently over each one of the least two substantially
orthogonal polarizations and to radiate substantially the same
information over each one of the at least two substantially
orthogonal polarizations.
49. A space-based network according to claim 48 further comprising:
a gateway that is configured to process at least two signals
associated respectively with at least two antennas of the first
number of antennas in order to recover information transmitted to
the space-based network by a radiotelephone.
50. A space-based network according to claim 48 further comprising:
a gateway that is configured to process at least two signals
associated respectively with the at least two substantially
orthogonal polarizations in order to recover information
transmitted to the space-based network by a radiotelephone.
51. An antenna for a space-based network, the antenna comprising: a
plurality of elements configured to receive information from a
radiotelephone over at least two substantially orthogonal
polarizations, wherein the radiotelephone is configured to transmit
information to the antenna via at least one direct Line-of-Sight
(LOS) wireless link between the radiotelephone and the antenna and
the radiotelephone is further configured to radiate electromagnetic
energy substantially concurrently over each one of the at least two
substantially orthogonal polarizations and to radiate substantially
the same information over each one of the at least two
substantially orthogonal polarizations.
52. An antenna according to claim 51 further comprising: an
electronics subsystem that is configured to receive signals from
the antenna and to transmit signals to a gateway that is configured
to process signals associated respectively with the at least two
substantially orthogonal polarizations in order to recover
information transmitted to the space-based network by the
radiotelephone.
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Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/241,519, filed Sep. 30, 2005, entitled
Space-Based Network Architecture for Satellite Radiotelephone
Systems, which itself is a continuation of U.S. patent application
Ser. No. 10/225,623, filed Aug. 22, 2002, now U.S. Pat. No.
7,006,789, entitled Space-Based Network Architectures for Satellite
Radiotelephone Systems, which claims the benefit of provisional
Application No. 60/322,240, filed Sep. 14, 2001, entitled Systems
and Methods for Terrestrial Re-Use of Mobile Satellite Spectrum and
provisional Application No. 60/392,771, filed Jul. 1, 2002,
entitled Space-Based Network Architectures for Satellite
Radiotelephone Systems, all of which are assigned to the assignee
of the present application, the disclosures of all of which are
hereby incorporated herein by reference in their entirety as if set
forth fully herein. application Ser. No. 10/225,623 also is a
continuation-in-part (CIP) of application Ser. No. 10/074,097,
filed Feb. 12, 2002, entitled Systems and Methods for Terrestrial
Reuse of Cellular Satellite Frequency Spectrum, now U.S. Pat. No.
6,684,057, assigned to the assignee of the present application, the
disclosure of which is hereby incorporated herein by reference in
its entirety as if set forth fully herein.
FIELD OF THE INVENTION
[0002] This invention relates to radiotelephone communications
systems and methods, and more particularly to terrestrial cellular
and satellite cellular radiotelephone communications systems and
methods.
BACKGROUND OF THE INVENTION
[0003] Satellite radiotelephone communications systems and methods
are widely used for radiotelephone communications. Satellite
radiotelephone communications systems and methods generally employ
at least one space-based component, such as one or more satellites
that are configured to wirelessly communicate with a plurality of
satellite radiotelephones.
[0004] A satellite radiotelephone communications system or method
may utilize a single antenna beam covering an entire area served by
the system. Alternatively, in cellular satellite radiotelephone
communications systems and methods, multiple beams are provided,
each of which can serve distinct geographical areas in the overall
service region, to collectively serve an overall satellite
footprint. Thus, a cellular architecture similar to that used in
conventional terrestrial cellular radiotelephone systems and
methods can be implemented in cellular satellite-based systems and
methods. The satellite typically communicates with radiotelephones
over a bidirectional communications pathway, with radiotelephone
communication signals being communicated from the satellite to the
radiotelephone over a downlink or forward link, and from the
radiotelephone to the satellite over an uplink or return link.
[0005] The overall design and operation of cellular satellite
radiotelephone systems and methods are well known to those having
skill in the art, and need not be described further herein.
Moreover, as used herein, the term "radiotelephone" includes
cellular and/or satellite radiotelephones with or without a
multi-line display; Personal Communications System (PCS) terminals
that may combine a radiotelephone with data processing, facsimile
and/or data communications capabilities; Personal Digital
Assistants (PDA) that can include a radio frequency transceiver and
a pager, Internet/intranet access, Web browser, organizer, calendar
and/or a global positioning system (GPS) receiver; and/or
conventional laptop and/or palmtop computers or other appliances,
which include a radio frequency transceiver.
[0006] As is well known to those having skill in the art,
terrestrial networks can enhance cellular satellite radiotelephone
system availability, efficiency and/or economic viability by
terrestrially reusing at least some of the frequency bands that are
allocated to cellular satellite radiotelephone systems. In
particular, it is known that it may be difficult for cellular
satellite radiotelephone systems to reliably serve densely
populated areas, because the satellite signal may be blocked by
high-rise structures and/or may not penetrate into buildings. As a
result, the satellite spectrum may be underutilized or unutilized
in such areas. The use of terrestrial retransmission can reduce or
eliminate this problem.
[0007] Moreover, the capacity of the overall system can be
increased significantly by the introduction of terrestrial
retransmission, since terrestrial frequency reuse can be much
denser than that of a satellite-only system. In fact, capacity can
be enhanced where it may be mostly needed, i.e., densely populated
urban/industrial/commercial areas. As a result, the overall system
can become much more economically viable, as it may be able to
serve a much larger subscriber base. Finally, satellite
radiotelephones for a satellite radiotelephone system having a
terrestrial component within the same satellite frequency band and
using substantially the same air interface for both terrestrial and
satellite communications can be more cost effective and/or
aesthetically appealing. Conventional dual band/dual mode
alternatives, such as the well known Thuraya, Iridium and/or
Globalstar dual mode satellite/terrestrial radiotelephone systems,
may duplicate some components, which may lead to increased cost,
size and/or weight of the radiotelephone.
[0008] One example of terrestrial reuse of satellite frequencies is
described in U.S. Pat. No. 5,937,332 to the present inventor
Karabinis entitled Satellite Telecommunications Repeaters and
Retransmission Methods, the disclosure of which is hereby
incorporated herein by reference in its entirety as if set forth
fully herein. As described therein, satellite telecommunications
repeaters are provided which receive, amplify, and locally
retransmit the downlink signal received from a satellite thereby
increasing the effective downlink margin in the vicinity of the
satellite telecommunications repeaters and allowing an increase in
the penetration of uplink and downlink signals into buildings,
foliage, transportation vehicles, and other objects which can
reduce link margin. Both portable and non-portable repeaters are
provided. See the abstract of U.S. Pat. No. 5,937,332.
[0009] In view of the above discussion, there continues to be a
need for systems and methods for terrestrial reuse of cellular
satellite frequencies that can allow improved reliability,
capacity, cost effectiveness and/or aesthetic appeal for cellular
satellite radiotelephone systems, methods and/or satellite
radiotelephones.
SUMMARY OF THE INVENTION
[0010] Some embodiments of the present invention provide a
space-based network for a satellite radiotelephone system that
includes at least one receive-only satellite and at least one
transmit satellite. In some embodiments, the transmit satellite is
a transmit-only satellite, whereas in other embodiments, the
transmit satellite is a transmit and receive satellite. The at
least one receive-only satellite is configured to receive wireless
communications from a radiotelephone at a predetermined location
over a satellite frequency band. The at least one transmit
satellite is configured to transmit wireless communications to the
radiotelephone at the predetermined location over the satellite
frequency band. By providing at least one receive-only satellite
and at least one transmit satellite, space-based networks according
to some embodiments of the present invention can offer a
significant link margin, without the need to undesirably burden the
radiotelephones themselves to achieve this link margin.
[0011] Accordingly, some embodiments of the invention provide a
space-based network for a satellite radiotelephone system that
comprises more receive satellites than transmit satellites. Other
embodiments of the invention provide a space-based network for a
satellite radiotelephone system comprising a plurality of
satellites that collectively provide greater uplink margin than
downlink margin.
[0012] In some embodiments of the invention, the at least one
receive-only satellite consists of two receive-only satellites. In
other embodiments, the at least one transmit satellite comprises at
least one transmit-only satellite. In other embodiments, the at
least one transmit-only satellite consists of a single
transmit-only satellite. In some embodiments, one of the two
receive-only satellites and a single transmit-only satellite are
collocated in an orbital slot.
[0013] In some embodiments, each of the receive-only satellites
comprises first and second receive antennas. In other embodiments,
the first and second receive-only antennas are about 24 meters in
diameter.
[0014] In other embodiments, the at least one transmit satellite
comprises at least one transmit and receive satellite. In other
embodiments, the at least one transmit and receive satellite
consists of a single transmit and receive satellite that is
collocated in an orbital slot with one of the two receive-only
satellites. In other embodiments, the at least one transmit and
receive satellite consists of two transmit and receive satellites,
a respective one of which is collocated in an orbital slot with a
respective one of the two receive-only satellites.
[0015] In some embodiments, the single transmit and receive
satellite includes a single transmit antenna and a single receive
antenna. In other embodiments, the single transmit and receive
satellite comprises a single transmit and receive antenna and a
single receive antenna. In yet other embodiments, the two transmit
and receive satellites each comprises a single transmit antenna and
a single receive antenna. In still other embodiments, the two
transmit and receive satellite antennas each comprises a single
transmit and receive antenna and a single receive antenna.
[0016] In other embodiments, each of the receive-only satellites
includes first through fourth processors. The first processor is
configured to process wireless communications that are received by
the first receive-only antenna in a first type of circular
polarization. The second processor is configured to process
wireless communications that are received by the first receive-only
antenna in a second type of circular polarization. The third
processor is configured to process wireless communications that are
received by the second receive-only antenna in the first
polarization, and the fourth processor is configured to process
wireless communications that are received by the second
receive-only antenna in the second polarization.
[0017] In other embodiments, each of the receive-only satellites
includes a feeder link signal generator. The feeder link signal
generator is configured to combine signals that are received by the
first and second receive-only antennas into at least one feeder
link signal, including a plurality of orthogonal dimensions and/or
polarizations, such as in-phase and quadrature dimensions,
horizontal and vertical polarizations, left hand circular and right
hand circular polarizations and/or other orthogonal dimensions
and/or polarizations.
[0018] Space-based networks according to other embodiments of the
invention also include a gateway that is configured to receive the
feeder link signal from each of the two receive-only satellites. In
other embodiments, the gateway may be configured to receive the
feeder link signal from each of the two receive-only satellites
when the feeder link signal has a bandwidth that is at least as
wide as the signals that are received by the first and second
receive-only antennas of one of the receive-only satellites. In
other embodiments, the space-based network includes a plurality of
gateways, a respective one of which is configured to receive a
feeder link signal from each of the two receive-only satellites. In
some embodiments, the signals that are received by the first and/or
second receive-only antennas of one of the receive-only satellites
have a bandwidth that is wider than the feeder link signal.
[0019] Still other embodiments of the present invention include a
combiner that is configured to combine the feeder link signals that
are received at least one of the plurality of gateways, in order to
reconstruct the wireless communications from the radiotelephone.
Still other embodiments of the present invention include an
ancillary terrestrial network that is configured to wirelessly
communicate with the radiotelephone at the predetermined location
over at least some of the satellite radiotelephone frequency band,
to thereby terrestrially reuse the at least some of the satellite
radiotelephone frequency band.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic diagram of cellular radiotelephone
systems and methods according to embodiments of the invention.
[0021] FIG. 2 is a block diagram of adaptive interference reducers
according to embodiments of the present invention.
[0022] FIG. 3 is a spectrum diagram that illustrates satellite
L-band frequency allocations.
[0023] FIG. 4 is a schematic diagram of cellular satellite systems
and methods according to other embodiments of the present
invention.
[0024] FIG. 5 illustrates time division duplex frame structures
according to embodiments of the present invention.
[0025] FIG. 6 is a block diagram of architectures of ancillary
terrestrial components according to embodiments of the
invention.
[0026] FIG. 7 is a block diagram of architectures of reconfigurable
radiotelephones according to embodiments of the invention.
[0027] FIG. 8 graphically illustrates mapping of monotonically
decreasing power levels to frequencies according to embodiments of
the present invention.
[0028] FIG. 9 illustrates an ideal cell that is mapped to three
power regions and three associated carrier frequencies according to
embodiments of the invention.
[0029] FIG. 10 depicts a realistic cell that is mapped to three
power regions and three associated carrier frequencies according to
embodiments of the invention.
[0030] FIG. 11 illustrates two or more contiguous slots in a frame
that are unoccupied according to embodiments of the present
invention.
[0031] FIG. 12 illustrates loading of two or more contiguous slots
with lower power transmissions according to embodiments of the
present invention.
[0032] FIG. 13 schematically illustrates the use of transmit-only
and receive-only satellites in a space-based network architecture
according to embodiments of the present invention.
[0033] FIG. 14 is a block diagram of architectures for space-based
networks according to embodiments of the present invention.
[0034] FIG. 15 schematically illustrates architectures for
space-based networks according to other embodiments of the present
invention.
DETAILED DESCRIPTION
[0035] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
embodiments of the invention are shown. This invention should not
be construed as limited to the embodiments set forth herein.
Rather, these embodiments are provided so that this disclosure will
be thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0036] FIG. 1 is a schematic diagram of cellular satellite
radiotelephone systems and methods according to embodiments of the
invention. As shown in FIG. 1, these cellular satellite
radiotelephone systems and methods 100 include at least one
Space-Based Component (SBC) 110, such as a satellite. The
space-based component 110 is configured to transmit wireless
communications to a plurality of radiotelephones 120a, 120b in a
satellite footprint comprising one or more satellite radiotelephone
cells 130-130'''' over one or more satellite radiotelephone forward
link (downlink) frequencies f.sub.D. The space-based component 110
is configured to receive wireless communications from, for example,
a first radiotelephone 120a in the satellite radiotelephone cell
130 over a satellite radiotelephone return link (uplink) frequency
f.sub.U. An ancillary terrestrial network, comprising at least one
ancillary terrestrial component 140, which may include an antenna
140a and an electronics system 140b (for example, at least one
antenna 140a and at last one electronics system 140b), is
configured to receive wireless communications from, for example, a
second radiotelephone 120b in the radiotelephone cell 130 over the
satellite radiotelephone uplink frequency, denoted f'.sub.U, which
may be the same as f.sub.U. Thus, as illustrated in FIG. 1,
radiotelephone 120a may be communicating with the space-based
component 110 while radiotelephone 120b may be communicating with
the ancillary terrestrial component 140. As shown in FIG. 1, the
space-based component 110 also undesirably receives the wireless
communications from the second radiotelephone 120b in the satellite
radiotelephone cell 130 over the satellite radiotelephone frequency
f'.sub.U as interference. More specifically, a potential
interference path is shown at 150. In this potential interference
path 150, the return link signal of the second radiotelephone 120b
at carrier frequency f'.sub.U interferes with satellite
communications. This interference would generally be strongest when
f'.sub.U=f.sub.U, because, in that case, the same return link
frequency would be used for space-based component and ancillary
terrestrial component communications over the same satellite
radiotelephone cell, and no spatial discrimination between
satellite radiotelephone cells would appear to exist.
[0037] Still referring to FIG. 1, embodiments of satellite
radiotelephone systems/methods 100 can include at least one gateway
160 that can include an antenna 160a and an electronics system 160b
that can be connected to other networks 162 including terrestrial
and/or other radiotelephone networks. The gateway 160 also
communicates with the space-based component 110 over a satellite
feeder link 112. The gateway 160 also communicates with the
ancillary terrestrial component 140, generally over a terrestrial
link 142.
[0038] Still referring to FIG. 1, an Interference Reducer (IR) 170a
also may be provided at least partially in the ancillary
terrestrial component electronics system 140b. Alternatively or
additionally, an interference reducer 170b may be provided at least
partially in the gateway electronics system 160b. In yet other
alternatives, the interference reducer may be provided at least
partially in other components of the cellular satellite
system/method 100 instead of or in addition to the interference
reducer 170a and/or 170b. The interference reducer is responsive to
the space-based component 110 and to the ancillary terrestrial
component 140, and is configured to reduce the interference from
the wireless communications that are received by the space-based
component 110 and is at least partially generated by the second
radiotelephone 120b in the satellite radiotelephone cell 130 over
the satellite radiotelephone frequency f'.sub.U. The interference
reducer 170a and/or 170b uses the wireless communications f'.sub.U
that are intended for the ancillary terrestrial component 140 from
the second radiotelephone 120b in the satellite radiotelephone cell
130 using the satellite radiotelephone frequency f'.sub.U to
communicate with the ancillary terrestrial component 140.
[0039] In embodiments of the invention, as shown in FIG. 1, the
ancillary terrestrial component 140 generally is closer to the
first and second radiotelephones 120a and 120b, respectively, than
is the space-based component 110, such that the wireless
communications from the second radiotelephone 120b are received by
the ancillary terrestrial component 140 prior to being received by
the space-based component 110. The interference reducer 170a and/or
170b is configured to generate an interference cancellation signal
comprising, for example, at least one delayed replica of the
wireless communications from the second radiotelephone 120b that
are received by the ancillary terrestrial component 140, and to
subtract the delayed replica of the wireless communications from
the second radiotelephone 120b that are received by the ancillary
terrestrial component 140 from the wireless communications that are
received from the space-based component 110. The interference
reduction signal may be transmitted from the ancillary terrestrial
component 140 to the gateway 160 over link 142 and/or using other
conventional techniques.
[0040] Thus, adaptive interference reduction techniques may be used
to at least partially cancel the interfering signal, so that the
same, or other nearby, satellite radiotelephone uplink frequency
can be used in a given cell for communications by radiotelephones
120 with the satellite 110 and with the ancillary terrestrial
component 140. Accordingly, all frequencies that are assigned to a
given cell 130 may be used for both radiotelephone 120
communications with the space-based component 110 and with the
ancillary terrestrial component 140. Conventional systems may avoid
terrestrial reuse of frequencies within a given satellite cell that
are being used within the satellite cell for satellite
communications. Stated differently, conventionally, only
frequencies used by other satellite cells may be candidates for
terrestrial reuse within a given satellite cell. Beam-to-beam
spatial isolation that is provided by the satellite system was
relied upon to reduce or minimize the level of interference from
the terrestrial operations into the satellite operations. In sharp
contrast, embodiments of the invention can use an interference
reducer to allow all frequencies assigned to a satellite cell to be
used terrestrially and for satellite radiotelephone
communications.
[0041] Embodiments of the invention according to FIG. 1 may arise
from a realization that the return link signal from the second
radiotelephone 120b at f'.sub.U generally will be received and
processed by the ancillary terrestrial component 140 much earlier
relative to the time when it will arrive at the satellite gateway
160 from the space-based component 110 via the interference path
150. Accordingly, the interference signal at the satellite gateway
160b can be at least partially canceled. Thus, as shown in FIG. 1,
an interference cancellation signal, such as the demodulated
ancillary terrestrial component signal, can be sent to the
satellite gateway 160b by the interference reducer 170a in the
ancillary terrestrial component 140, for example using link 142. In
the interference reducer 170b at the gateway 160b, a weighted (in
amplitude and/or phase) replica of the signal may be formed using,
for example, adaptive transversal filter techniques that are well
known to those having skill in the art. Then, a transversal filter
output signal is subtracted from the aggregate received satellite
signal at frequency f'.sub.U that contains desired as well as
interference signals. Thus, the interference cancellation need not
degrade the signal-to-noise ratio of the desired signal at the
gateway 160, because a regenerated (noise-free) terrestrial signal,
for example as regenerated by the ancillary terrestrial component
140, can be used to perform interference suppression.
[0042] FIG. 2 is a block diagram of embodiments of adaptive
interference cancellers that may be located in the ancillary
terrestrial component 140, in the gateway 160, and/or in another
component of the cellular radiotelephone system 100. As shown in
FIG. 2, one or more control algorithms 204, known to those having
skill in the art, may be used to adaptively adjust the coefficients
of a plurality of transversal filters 202a-202n. Adaptive
algorithms, such as Least Mean Square Error (LMSE), Kalman, Fast
Kalman, Zero Forcing and/or various combinations thereof or other
techniques may be used. It will be understood by those having skill
in the art that the architecture of FIG. 2 may be used with an LMSE
algorithm. However, it also will be understood by those having
skill in the art that conventional architectural modifications may
be made to facilitate other control algorithms.
[0043] Additional embodiments of the invention now will be
described with reference to FIG. 3, which illustrates L-band
frequency allocations including cellular radiotelephone system
forward links and return links. As shown in FIG. 3, the
space-to-ground L-band forward link (downlink) frequencies are
assigned from 1525 MHz to 1559 MHz. The ground-to-space L-band
return link (uplink) frequencies occupy the band from 1626.5 MHz to
1660.5 MHz. Between the forward and return L-band links lie the
GPS/GLONASS radionavigation band (from 1559 MHz to 1605 MHz).
[0044] In the detailed description to follow, GPS/GLONASS will be
referred to simply as GPS for the sake of brevity. Moreover, the
acronyms ATC and SBC will be used for the ancillary terrestrial
component and the space-based component, respectively, for the sake
of brevity.
[0045] As is known to those skilled in the art, GPS receivers may
be extremely sensitive since they are designed to operate on very
weak spread-spectrum radionavigation signals that arrive on the
earth from a GPS satellite constellation. As a result, GPS
receivers may to be highly susceptible to in-band interference.
ATCs that are configured to radiate L-band frequencies in the
forward satellite band (1525 to 1559 MHz) can be designed with very
sharp out-of-band emissions filters to satisfy the stringent
out-of-band spurious emissions desires of GPS.
[0046] Referring again to FIG. 1, some embodiments of the invention
can provide systems and methods that can allow an ATC 140 to
configure itself in one of at least two modes. In accordance with a
first mode, which may be a standard mode and may provide highest
capacity, the ATC 140 transmits to the radiotelephones 120 over the
frequency range from 1525 MHz to 1559 MHz, and receives
transmissions from the radiotelephones 120 in the frequency range
from 1626.5 MHz to 1660.5 MHz, as illustrated in FIG. 3. In
contrast, in a second mode of operation, the ATC 140 transmits
wireless communications to the radiotelephones 120 over a modified
range of satellite band forward link (downlink) frequencies. The
modified range of satellite band forward link frequencies may be
selected to reduce, compared to the unmodified range of satellite
band forward link frequencies, interference with wireless receivers
such as GPS receivers that operate outside the range of satellite
band forward link frequencies.
[0047] Many modified ranges of satellite band forward link
frequencies may be provided according to embodiments of the present
invention. In some embodiments, the modified range of satellite
band forward link frequencies can be limited to a subset of the
original range of satellite band forward link frequencies, so as to
provide a guard band of unused satellite band forward link
frequencies. In other embodiments, all of the satellite band
forward link frequencies are used, but the wireless communications
to the radiotelephones are modified in a manner to reduce
interference with wireless receivers that operate outside the range
of satellite band forward link frequencies. Combinations and
subcombinations of these and/or other techniques also may be used,
as will be described below.
[0048] It also will be understood that embodiments of the invention
that will now be described in connection with FIGS. 4-12 will be
described in terms of multiple mode ATCs 140 that can operate in a
first standard mode using the standard forward and return links of
FIG. 3, and in a second or alternate mode that uses a modified
range of satellite band forward link frequencies and/or a modified
range of satellite band return link frequencies. These multiple
mode ATCs can operate in the second, non-standard mode, as long as
desirable, and can be switched to standard mode otherwise. However,
other embodiments of the present invention need not provide
multiple mode ATCs but, rather, can provide ATCs that operate using
the modified range of satellite band forward link and/or return
link frequencies.
[0049] Embodiments of the invention now will be described, wherein
an ATC operates with an SBC that is configured to receive wireless
communications from radiotelephones over a first range of satellite
band return link frequencies and to transmit wireless
communications to the radiotelephones over a second range of
satellite band forward link frequencies that is spaced apart from
the first range. According to these embodiments, the ATC is
configured to use at least one time division duplex frequency to
transmit wireless communications to the radiotelephones and to
receive wireless communications from the radiotelephones at
different times. In particular, in some embodiments, the at least
one time division duplex frequency that is used to transmit
wireless communications to the radiotelephones and to receive
wireless communications from the radiotelephones at different
times, comprises a frame including a plurality of slots. At least a
first one of the slots is used to transmit wireless communications
to the radiotelephones and at least a second one of the slots is
used to receive wireless communications from the radiotelephones.
Thus, in some embodiments, the ATC transmits and receives, in Time
Division Duplex (TDD) mode, using frequencies from 1626.5 MHz to
1660.5 MHz. In some embodiments, all ATCs across the entire network
may have the stated configuration/reconfiguration flexibility. In
other embodiments, only some ATCs may be reconfigurable.
[0050] FIG. 4 illustrates satellite systems and methods 400
according to some embodiments of the invention, including an ATC
140 communicating with a radiotelephone 120b using a carrier
frequency f'.sub.U in TDD mode. FIG. 5 illustrates an embodiment of
a TDD frame structure. Assuming full-rate GSM (eight time slots per
frame), up to four full-duplex voice circuits can be supported by
one TDD carrier. As shown in FIG. 5, the ATC 140 transmits to the
radiotelephone 120b over, for example, time slot number 0. The
radiotelephone 120b receives and replies back to the ATC 140 over,
for example, time slot number 4. Time slots number 1 and 5 may be
used to establish communications with another radiotelephone, and
so on.
[0051] A Broadcast Control CHannel (BCCH) is preferably transmitted
from the ATC 140 in standard mode, using a carrier frequency from
below any guard band exclusion region. In other embodiments, a BCCH
also can be defined using a TDD carrier. In any of these
embodiments, radiotelephones in idle mode can, per established GSM
methodology, monitor the BCCH and receive system-level and paging
information. When a radiotelephone is paged, the system decides
what type of resource to allocate to the radiotelephone in order to
establish the communications link. Whatever type of resource is
allocated for the radiotelephone communications channel (TDD mode
or standard mode), the information is communicated to the
radiotelephone, for example as part of the call initialization
routine, and the radiotelephone configures itself
appropriately.
[0052] It may be difficult for the TDD mode to co-exist with the
standard mode over the same ATC, due, for example, to the ATC
receiver LNA stage. In particular, assuming a mixture of standard
and TDD mode GSM carriers over the same ATC, during the part of the
frame when the TDD carriers are used to serve the forward link
(when the ATC is transmitting TDD) enough energy may leak into the
receiver front end of the same ATC to desensitize its LNA
stage.
[0053] Techniques can be used to suppress the transmitted ATC
energy over the 1600 MHz portion of the band from desensitizing the
ATC's receiver LNA, and thereby allow mixed standard mode and TDD
frames. For example, isolation between outbound and inbound ATC
front ends and/or antenna system return loss may be increased or
maximized. A switchable band-reject filter may be placed in front
of the LNA stage. This filter would be switched in the receiver
chain (prior to the LNA) during the part of the frame when the ATC
is transmitting TDD, and switched out during the rest of the time.
An adaptive interference canceller can be configured at RF (prior
to the LNA stage). If such techniques are used, suppression of the
order of 70 dB can be attained, which may allow mixed standard mode
and TDD frames. However, the ATC complexity and/or cost may
increase.
[0054] Thus, even though ATC LNA desensitization may be reduced or
eliminated, it may use significant special engineering and
attention and may not be economically worth the effort. Other
embodiments, therefore, may keep TDD ATCs pure TDD, with the
exception, perhaps, of the BCCH carrier which may not be used for
traffic but only for broadcasting over the first part of the frame,
consistent with TDD protocol. Moreover, Random Access CHannel
(RACH) bursts may be timed so that they arrive at the ATC during
the second half of the TDD frame. In some embodiments, all TDD ATCs
may be equipped to enable reconfiguration in response to a
command.
[0055] It is well recognized that during data communications or
other applications, the forward link may use transmissions at
higher rates than the return link. For example, in web browsing
with a radiotelephone, mouse clicks and/or other user selections
typically are transmitted from the radiotelephone to the system.
The system, however, in response to a user selection, may have to
send large data files to the radiotelephone. Hence, other
embodiments of the invention may be configured to enable use of an
increased or maximum number of time slots per forward GSM carrier
frame, to provide a higher downlink data rate to the
radiotelephones.
[0056] Thus, when a carrier frequency is configured to provide
service in TDD mode, a decision may be made as to how many slots
will be allocated to serving the forward link, and how many will be
dedicated to the return link. Whatever the decision is, it may be
desirable that it be adhered to by all TDD carriers used by the
ATC, in order to reduce or avoid the LNA desensitization problem
described earlier. In voice communications, the partition between
forward and return link slots may be made in the middle of the
frame as voice activity typically is statistically bidirectionally
symmetrical. Hence, driven by voice, the center of the frame may be
where the TDD partition is drawn.
[0057] To increase or maximize forward link throughput in data
mode, data mode TDD carriers according to embodiments of the
invention may use a more spectrally efficient modulation and/or
protocol, such as the EDGE modulation and/or protocol, on the
forward link slots. The return link slots may be based on a less
spectrally efficient modulation and/or protocol such as the GPRS
(GMSK) modulation and/or protocol. The EDGE modulation/protocol and
the GPRS modulation/protocol are well known to those having skill
in the art, and need not be described further herein. Given an EDGE
forward/GPRS return TDD carrier strategy, up to (384/2)=192 kbps
may be supported on the forward link while on the return link the
radiotelephone may transmit at up to (115/2).apprxeq.64 kbps.
[0058] In other embodiments, it also is possible to allocate six
time slots of an eight-slot frame for the forward link and only two
for the return link. In these embodiments, for voice services,
given the statistically symmetric nature of voice, the return link
vocoder may need to be comparable with quarter-rate GSM, while the
forward link vocoder can operate at full-rate GSM, to yield six
full-duplex voice circuits per GSM TDD-mode carrier (a voice
capacity penalty of 25%). Subject to this non-symmetrical
partitioning strategy, data rates of up to (384)(6/8)=288 kbps may
be achieved on the forward link, with up to (115)(2/8).apprxeq.32
kbps on the return link.
[0059] FIG. 6 depicts an ATC architecture according to embodiments
of the invention, which can lend itself to automatic configuration
between the two modes of standard GSM and TDD GSM on command, for
example, from a Network Operations Center (NOC) via a Base Station
Controller (BSC). It will be understood that in these embodiments,
an antenna 620 can correspond to the antenna 140a of FIGS. 1 and 4,
and the remainder of FIG. 6 can correspond to the electronics
system 140b of FIGS. 1 and 4. If a reconfiguration command for a
particular carrier, or set of carriers, occurs while the carrier(s)
are active and are supporting traffic, then, via the in-band
signaling Fast Associated Control CHannel (FACCH), all affected
radiotelephones may be notified to also reconfigure themselves
and/or switch over to new resources. If carrier(s) are reconfigured
from TDD mode to standard mode, automatic reassignment of the
carrier(s) to the appropriate standard-mode ATCs, based, for
example, on capacity demand and/or reuse pattern can be initiated
by the NOC. If, on the other hand, carrier(s) are reconfigured from
standard mode to TDD mode, automatic reassignment to the
appropriate TDD-mode ATCs can take place on command from the
NOC.
[0060] Still referring to FIG. 6, a switch 610 may remain closed
when carriers are to be demodulated in the standard mode. In TDD
mode, this switch 610 may be open during the first half of the
frame, when the ATC is transmitting, and closed during the second
half of the frame, when the ATC is receiving. Other embodiments
also may be provided.
[0061] FIG. 6 assumes N transceivers per ATC sector, where N can be
as small as one, since a minimum of one carrier per sector
generally is desired. Each transceiver is assumed to operate over
one GSM carrier pair (when in standard mode) and can thus support
up to eight full-duplex voice circuits, neglecting BCCH channel
overhead. Moreover, a standard GSM carrier pair can support sixteen
full-duplex voice circuits when in half-rate GSM mode, and up to
thirty two full-duplex voice circuits when in quarter-rate GSM
mode.
[0062] When in TDD mode, the number of full duplex voice circuits
may be reduced by a factor of two, assuming the same vocoder.
However, in TDD mode, voice service can be offered via the
half-rate GSM vocoder with almost imperceptible quality
degradation, in order to maintain invariant voice capacity. FIG. 7
is a block diagram of a reconfigurable radiotelephone architecture
that can communicate with a reconfigurable ATC architecture of FIG.
6. In FIG. 7, an antenna 720 is provided, and the remainder of FIG.
7 can provide embodiments of an electronics system for the
radiotelephone.
[0063] It will be understood that the ability to reconfigure ATCs
and radiotelephones according to embodiments of the invention may
be obtained at a relatively small increase in cost. The cost may be
mostly in Non-Recurring Engineering (NRE) cost to develop software.
Some recurring cost may also be incurred, however, in that at least
an additional RF filter and a few electronically controlled
switches may be used per ATC and radiotelephone. All other
hardware/software can be common to standard-mode and TDD-mode
GSM.
[0064] Referring now to FIG. 8, other radiotelephone systems and
methods according to embodiments of the invention now will be
described. In these embodiments, the modified second range of
satellite band forward link frequencies includes a plurality of
frequencies in the second range of satellite band forward link
frequencies that are transmitted by the ATCs to the radiotelephones
at a power level, such as maximum power level, that monotonically
decreases as a function of (increasing) frequency. More
specifically, as will be described below, in some embodiments, the
modified second range of satellite band forward link frequencies
includes a subset of frequencies proximate to a first or second end
of the range of satellite band forward link frequencies that are
transmitted by the ATC to the radiotelephones at a power level,
such as a maximum power level, that monotonically decreases toward
the first or second end of the second range of satellite band
forward link frequencies. In still other embodiments, the first
range of satellite band return link frequencies is contained in an
L-band of satellite frequencies above GPS frequencies and the
second range of satellite band forward link frequencies is
contained in the L-band of satellite frequencies below the GPS
frequencies. The modified second range of satellite band forward
link frequencies includes a subset of frequencies proximate to an
end of the second range of satellite band forward link frequencies
adjacent the GPS frequencies that are transmitted by the ATC to the
radiotelephones at a power level, such as a maximum power level,
that monotonically decreases toward the end of the second range of
satellite band forward link frequencies adjacent the GPS
frequencies.
[0065] Without being bound by any theory of operation, a
theoretical discussion of the mapping of ATC maximum power levels
to carrier frequencies according to embodiments of the present
invention now will be described. Referring to FIG. 8, let
.nu.=(.rho.) represent a mapping from the power (.rho.) domain to
the frequency (.nu.) range. The power (.rho.) is the power that an
ATC uses or should transmit in order to reliably communicate with a
given radiotelephone. This power may depend on many factors such as
the radiotelephone's distance from the ATC, the blockage between
the radiotelephone and the ATC, the level of multipath fading in
the channel, etc., and as a result, will, in general, change as a
function of time. Hence, the power used generally is determined
adaptively (iteratively) via closed-loop power control, between the
radiotelephone and ATC.
[0066] The frequency (.nu.) is the satellite carrier frequency that
the ATC uses to communicate with the radiotelephone. According to
embodiments of the invention, the mapping is a monotonically
decreasing function of the independent variable .rho..
Consequently, in some embodiments, as the maximum ATC power
increases, the carrier frequency that the ATC uses to establish
and/or maintain the communications link decreases. FIG. 8
illustrates an embodiment of a piece-wise continuous monotonically
decreasing (stair-case) function. Other monotonic functions may be
used, including linear and/or nonlinear, constant and/or variable
decreases. FACCH or Slow Associated Control CHannel (SACCH)
messaging may be used in embodiments of the invention to facilitate
the mapping adaptively and in substantially real time.
[0067] FIG. 9 depicts an ideal cell according to embodiments of the
invention, where, for illustration purposes, three power regions
and three associated carrier frequencies (or carrier frequency
sets) are being used to partition a cell. For simplicity, one ATC
transmitter at the center of the idealized cell is assumed with no
sectorization. In embodiments of FIG. 9, the frequency (or
frequency set) f.sub.1 is taken from substantially the upper-most
portion of the L-band forward link frequency set, for example from
substantially close to 1559 MHz (see FIG. 3). Correspondingly, the
frequency (or frequency set) f.sub.M is taken from substantially
the central portion of the L-band forward link frequency set (see
FIG. 3). In concert with the above, the frequency (or frequency
set) f.sub.O is taken from substantially the lowest portion of the
L-band forward link frequencies, for example close to 1525 MHz (see
FIG. 3).
[0068] Thus, according to embodiments of FIG. 9, if a
radiotelephone is being served within the outer-most ring of the
cell, that radiotelephone is being served via frequency f.sub.O.
This radiotelephone, being within the furthest area from the ATC,
has (presumably) requested maximum (or near maximum) power output
from the ATC. In response to the maximum (or near maximum) output
power request, the ATC uses its a priori knowledge of
power-to-frequency mapping, such as a three-step staircase function
of FIG. 9. Thus, the ATC serves the radiotelephone with a low-value
frequency taken from the lowest portion of the mobile L-band
forward link frequency set, for example, from as close to 1525 MHz
as possible. This, then, can provide additional safeguard to any
GPS receiver unit that may be in the vicinity of the ATC.
[0069] Embodiments of FIG. 9 may be regarded as idealized because
they associate concentric ring areas with carrier frequencies (or
carrier frequency sets) used by an ATC to serve its area. In
reality, concentric ring areas generally will not be the case. For
example, a radiotelephone can be close to the ATC that is serving
it, but with significant blockage between the radiotelephone and
the ATC due to a building. This radiotelephone, even though
relatively close to the ATC, may also request maximum (or near
maximum) output power from the ATC. With this in mind, FIG. 10 may
depict a more realistic set of area contours that may be associated
with the frequencies being used by the ATC to serve its territory,
according to embodiments of the invention. The frequency (or
frequency set) f.sub.1 may be reused in the immediately adjacent
ATC cells owing to the limited geographical span associated with
f.sub.1 relative to the distance between cell centers. This may
also hold for f.sub.M.
[0070] Referring now to FIG. 11, other modified second ranges of
satellite band forward link frequencies that can be used by ATCs
according to embodiments of the present invention now will be
described. In these embodiments, at least one frequency in the
modified second range of satellite band forward link frequencies
that is transmitted by the ATC to the radiotelephones comprises a
frame including a plurality of slots. In these embodiments, at
least two contiguous slots in the frame that is transmitted by the
ATC to the radiotelephones are left unoccupied. In other
embodiments, three contiguous slots in the frame that is
transmitted by the ATC to the radiotelephones are left unoccupied.
In yet other embodiments, at least two contiguous slots in the
frame that is transmitted by the ATC to the radiotelephones are
transmitted at lower power than remaining slots in the frame. In
still other embodiments, three contiguous slots in the frame that
is transmitted by the ATC to the radiotelephones are transmitted at
lower power than remaining slots in the frame. In yet other
embodiments, the lower power slots may be used with first selected
ones of the radiotelephones that are relatively close to the ATC
and/or are experiencing relatively small signal blockage, and the
remaining slots are transmitted at higher power to second selected
ones of the radiotelephones that are relatively far from the ATC
and/or are experiencing relatively high signal blockage.
[0071] Stated differently, in accordance with some embodiments of
the invention, only a portion of the TDMA frame is utilized. For
example, only the first four (or last four, or any contiguous four)
time slots of a full-rate GSM frame are used to support traffic.
The remaining slots are left unoccupied (empty). In these
embodiments, capacity may be lost. However, as has been described
previously, for voice services, half-rate and even quarter-rate GSM
may be invoked to gain capacity back, with some potential
degradation in voice quality. The slots that are not utilized
preferably are contiguous, such as slots 0 through 3 or 4 through 7
(or 2 through 5, etc.). The use of non-contiguous slots such as 0,
2, 4, and 6, for example, may be less desirable. FIG. 11
illustrates four slots (4-7) being used and four contiguous slots
(0-3) being empty in a GSM frame.
[0072] It has been found experimentally, according to these
embodiments of the invention, that GPS receivers can perform
significantly better when the interval between interference bursts
is increased or maximized. Without being bound by any theory of
operation, this effect may be due to the relationship between the
code repetition period of the GPS C/A code (1 msec.) and the GSM
burst duration (about 0.577 msec.). With a GSM frame occupancy
comprising alternate slots, each GPS signal code period can
experience at least one "hit", whereas a GSM frame occupancy
comprising four to five contiguous slots allows the GPS receiver to
derive sufficient clean information so as to "flywheel" through the
error events.
[0073] According to other embodiments of the invention, embodiments
of FIGS. 8-10 can be combined with embodiments of FIG. 11.
Furthermore, according to other embodiments of the invention, if an
f.sub.1 carrier of FIG. 9 or 10 is underutilized, because of the
relatively small footprint of the inner-most region of the cell, it
may be used to support additional traffic over the much larger
outermost region of the cell.
[0074] Thus, for example, assume that only the first four slots in
each frame of f.sub.1 are being used for inner region traffic. In
embodiments of FIGS. 8-10, these four f.sub.1 slots are carrying
relatively low power bursts, for example of the order of 100 mW or
less, and may, therefore, appear as (almost) unoccupied from an
interference point of view. Loading the remaining four (contiguous)
time slots of f.sub.1 with relatively high-power bursts may have
negligible effect on a GPS receiver because the GPS receiver would
continue to operate reliably based on the benign contiguous time
interval occupied by the four low-power GSM bursts. FIG. 12
illustrates embodiments of a frame at carrier f.sub.1 supporting
four low-power (inner interval) users and four high-power (outer
interval) users. In fact, embodiments illustrated in FIG. 12 may be
a preferred strategy for the set of available carrier frequencies
that are closest to the GPS band. These embodiments may avoid undue
capacity loss by more fully loading the carrier frequencies.
[0075] The experimental finding that interference from GSM carriers
can be relatively benign to GPS receivers provided that no more
than, for example, 5 slots per 8 slot GSM frame are used in a
contiguous fashion can be very useful. It can be particularly
useful since this experimental finding may hold even when the GSM
carrier frequency is brought very close to the GPS band (as close
as 1558.5 MHz) and the power level is set relatively high. For
example, with five contiguous time slots per frame populated, the
worst-case measured GPS receiver may attain at least 30 dB of
desensitization margin, over the entire ATC service area, even when
the ATC is radiating at 1558.5 MHz. With four contiguous time slots
per frame populated, an additional 10 dB desensitization margin may
be gained for a total of 40 dB for the worst-case measured GPS
receiver, even when the ATC is radiating at 1558.5 MHz.
[0076] There still may be concern about the potential loss in
network capacity (especially in data mode) that may be incurred
over the frequency interval where embodiments of FIG. 11 are used
to underpopulate the frame. Moreover, even though embodiments of
FIG. 12 can avoid capacity loss by fully loading the carrier, they
may do so subject to the constraint of filling up the frame with
both low-power and high-power users. Moreover, if forward link
carriers are limited to 5 contiguous high power slots per frame,
the maximum forward link data rate per carrier that may be aimed at
a particular user, may become proportionately less.
[0077] Therefore, in other embodiments, carriers which are subject
to contiguous empty/low power slots are not used for the forward
link. Instead, they are used for the return link. Consequently, in
some embodiments, at least part of the ATC is configured in reverse
frequency mode compared to the SBC in order to allow maximum data
rates over the forward link throughout the entire network. On the
reverse frequency return link, a radiotelephone may be limited to a
maximum of 5 slots per frame, which can be adequate for the return
link. Whether the five available time slots per frame, on a reverse
frequency return link carrier, are assigned to one radiotelephone
or to five different radiotelephones, they can be assigned
contiguously in these embodiments. As was described in connection
with FIG. 12, these five contiguous slots can be assigned to
high-power users while the remaining three slots may be used to
serve low-power users.
[0078] Other embodiments may be based on operating the ATC entirely
in reverse frequency mode compared to the SBC. In these
embodiments, an ATC transmits over the satellite return link
frequencies while radiotelephones respond over the satellite
forward link frequencies. If sufficient contiguous spectrum exists
to support CDMA technologies, and in particular the emerging
Wideband-CDMA 3G standard, the ATC forward link can be based on
Wideband-CDMA to increase or maximize data throughput capabilities.
Interference with GPS may not be an issue since the ATCs transmit
over the satellite return link in these embodiments. Instead,
interference may become a concern for the radiotelephones. Based,
however, on embodiments of FIGS. 11-12, the radiotelephones can be
configured to transmit GSM since ATC return link rates are
expected, in any event, to be lower than those of the forward link.
Accordingly, the ATC return link may employ GPRS-based data modes,
possibly even EDGE. Thus, return link carriers that fall within a
predetermined frequency interval from the GPS band-edge of 1559
MHz, can be under loaded, per embodiments of FIG. 11 or 12, to
satisfy GPS interference concerns.
[0079] Finally, other embodiments may use a partial or total
reverse frequency mode and may use CDMA on both forward and return
links. In these embodiments, the ATC forward link to the
radiotelephones utilizes the frequencies of the satellite return
link (1626.5 MHz to 1660.5 MHz) whereas the ATC return link from
the radiotelephones uses the frequencies of the satellite forward
link (1525 MHz to 1559 MHz). The ATC forward link can be based on
an existing or developing CDMA technology (e.g., IS-95,
Wideband-CDMA, etc.). The ATC network return link can also be based
on an existing or developing CDMA technology provided that the
radiotelephone's output is gated to cease transmissions for
approximately 3 msec once every T msec. In some embodiments, T will
be greater than or equal to 6 msec.
[0080] This gating may not be needed for ATC return link carriers
at approximately 1550 MHz or below. This gating can reduce or
minimize out-of-band interference (desensitization) effects for GPS
receivers in the vicinity of an ATC. To increase the benefit to
GPS, the gating between all radiotelephones over an entire ATC
service area can be substantially synchronized. Additional benefit
to GPS may be derived from system-wide synchronization of gating.
The ATCs can instruct all active radiotelephones regarding the
gating epoch. All ATCs can be mutually synchronized via GPS.
Space-Based Network (SBN) Architectures
[0081] As was described above, some embodiments of the present
invention may employ a Space-Based Network (SBN) and an Ancillary
Terrestrial Network (ATN) that both communicate with a plurality of
radiotelephones using satellite radiotelephone frequencies. The SBN
may include one or more Space-Based Components (SBC) and one or
more satellite gateways. The ATN may include a plurality of
Ancillary Terrestrial Components (ATC). In some embodiments, the
SBN and the ATN may operate at L-band (1525-1559 MHz forward
service link, and 1626.5-1660.5 MHz return service link). Moreover,
in some embodiments, the radiotelephones may be similar to
conventional handheld cellular/PCS-type terminals that are capable
of voice and/or packet data services. In some embodiments,
terrestrial reuse of at least some of the mobile satellite
frequency spectrum can allow the SBN to serve low density areas
that may be impractical and/or uneconomical to serve via
conventional terrestrial networks, while allowing the ATN to serve
pockets of densely populated areas that may only be effectively
served terrestrially. The radiotelephones can be attractive,
feature-rich and/or low cost, similar to conventional
cellular/PCS-type terminals that are offered by terrestrial-only
operators. Moreover, by operating the SBN and ATN modes over the
same frequency band, component count in the radiotelephones, for
example in the front end radio frequency (RF) section, may be
reduced. In particular, in some embodiments, the same frequency
synthesizer, RF filters, low noise amplifiers, power amplifiers and
antenna elements may be used for terrestrial and satellite
communications.
[0082] Some embodiments of space-based network architectures
according to embodiments of the present invention can offer
significant link margin over and above the clear sky conditions,
represented by an Additive White Gaussian Noise (AWGN) channel,
without the need to undesirably burden the radiotelephones
themselves to achieve this link margin. In some embodiments, the
SBN may employ relatively large reflectors, for example on the
order of about 24 meters in diameter, that can produce relatively
small, high gain, agile spot beams. Digital processors in the
space-based component and/or at the satellite gateways can be used
to improve or optimize performance with respect to each individual
user.
[0083] In general, space-based networks for a satellite
radiotelephone system according to some embodiments of the
invention include at least one receive-only satellite and at least
one transmit satellite. In some embodiments, the transmit satellite
is a transmit-only satellite, whereas in other embodiments, the
transmit satellite is a transmit and receive satellite. It will be
understood that the terms "receive" and "transmit" are used
relative to ground based radiotelephones and that a receive-only
satellite and a transmit-only satellite also may transmit to and
receive from a gateway or other ground station. The at least one
receive-only satellite is configured to receive wireless
communications from a radiotelephone at a predetermined location
over a satellite frequency band. The at least one transmit
satellite is configured to transmit wireless communications to the
radiotelephone at the predetermined location over the satellite
frequency band. By providing at least one receive-only satellite,
link margins may be improved compared to the use of a conventional
transmit and receive satellite of comparable antenna sizes,
according to some embodiments of the present invention.
Accordingly, some embodiments of the invention provide a
space-based network for a satellite radiotelephone system that
comprises more receive satellites than transmit satellites.
[0084] FIG. 13 conceptually illustrates space-based network
architectures according to some embodiments of the present
invention. As shown in FIG. 13, at least one transmit-only
(TX-only) satellite 1310 and at least one receive-only (RX-only)
satellite 1320a, 1320b, are used to communicate with
radiotelephones such as the radiotelephone 1330. As also shown in
FIG. 13, a space-based network according to some embodiments of the
invention may include a single TX-only satellite 1310 and first and
second RX-only satellites 1320a, 1320b, also referred to as RX-only
satellite 1 and RX-only satellite 2, respectively. Finally, as also
shown in FIG. 13, in some embodiments of the present invention, the
first RX-only satellite 1320a may be co-located with the TX-only
satellite 1310, and the second RX-only satellite 1320b may be
located at a different orbital slot.
[0085] Referring again to FIG. 13, in some embodiments, each
RX-only satellite antenna 1340a-1340d may be approximately 24
meters in diameter. This can provide a return link aggregate
space-based aperture with an equivalent diameter of about 40
meters. The RX-only satellite antennas 1340a-1340d may be of same
size or different sizes. This relatively large, effective return
link aperture may be used to allow the SBN to accommodate a
relatively low Effective Isotropic Radiated Power (EIRP) on the
radiotelephones 1330, for example about -6 dBW.
[0086] The TX-only satellite 1310 may contain an on-board digital
processor that can perform various functions, such as feeder-link
channelization, filtering, beam routing and/or digital beam
forming. Such functions have already been implemented in the
Thuraya satellite that is currently providing service in the Middle
East, and are well known to those having skill in the art. These
functions therefore need not be described in further detail
herein.
[0087] Referring again to FIG. 13, in some embodiments of the
present invention, each receive antenna 1340a-1340d of each RX-only
satellite 1320a, 1320b receives Left-Hand Circular Polarization
(LHCP) energy and Right-Hand Circular Polarization (RHCP) energy.
This may be received, since the radiotelephone 1330 may radiate
linearly polarized energy, which contains half of its energy in
LHCP and the remaining half in RHCP.
[0088] In some embodiments, each RX-only satellite 1320a, 1320b may
contain up to four digital processors. In each satellite 1320a or
1320b, a first digital processor may be configured to operate on
the aggregate signal received by the first antenna, for example
antenna 1340a or 1340c, in LHCP, and perform the functions of
signal channelization, filtering, beam forming and/or routing of
signals to the feeder link. A second processor may be configured to
perform the identical functions as the first, but on the RHCP
signal received by the first antenna, such as antenna 1340a or
1340c. The remaining two processors may be configured to repeat
these functions on the RHCP and LHCP signals of the second RX-only
antenna, such as antenna 1340b or 1340d. All eight sets of received
signals, from both RX-only satellites 1320a and 1320b, may be sent
via one or more feeder links to one or more gateways for combining,
as will now be described.
[0089] FIG. 14 is a block diagram of portions of the space-based
network that illustrates how the signals from the RX-only satellite
1 1320a and RX-only satellite 2 1320b may be combined according to
some embodiments of the present invention. Embodiments of FIG. 14
assume that the available feeder link bandwidth, from an RX-only
satellite 1320a, 1320b to a gateway is X MHz, but that Y MHz is
desired to transport the signals to the gateway, where Y is greater
than X.
[0090] As shown in FIG. 14, a first X MHz of LHCP signal spectrum
1410a, received from RX-only satellite 1, antenna 1 1340a via the
first processor, and a first corresponding X MHz of RHCP signal
spectrum 1410b also received by RX-only satellite 1, antenna 1
1340a via the second processor, are mapped into in-phase (I) and
quadrature (Q) dimensions of a first carrier. In other embodiments,
the X MHz of signal spectrum that is mapped into the I and Q
dimensions of the carrier need not be an RHCP signal received by
satellite 1, antenna 1. Instead, it may be a corresponding X MHz of
signal spectrum (LHCP or RHCP) from satellite 1, antenna 2 1340b.
In some embodiments, any appropriate mapping of signals from the
RX-only satellite antennas 1340a-1340b may be used, for example, by
utilizing as many orthogonal polarizations and/or dimensions as
possible, over the same available feeder bandwidth, so as to reduce
or minimize the number of gateways or diversity sites that are used
on the ground to transport the desired signals for processing
thereof.
[0091] Returning again to FIG. 14, the X MHz bandwidth quadrature
carrier may be transported to a first gateway 1440a over the X MHz
of available feeder link bandwidth using a vertically (V) polarized
orientation. Concurrently, a first X MHz of LHCP signal spectrum
1410c from RX-only satellite 1, antenna 2 1340b via the third
processor, and a first RHCP signal spectrum 1410d from RX-only
satellite 1, antenna 2 1340b via the fourth processor, are mapped
onto the I and Q dimensions of a second carrier, at the same
frequency as the first carrier, and are concurrently transported to
the first gateway 1440a over the X MHz of available feeder link
bandwidth using a horizontally (H) polarized orientation. The
transmission medium is indicated schematically by summing node 1430
to indicate a concurrence of the horizontally and vertically
polarized signals in the transmission medium.
[0092] This mapping onto X MHz bandwidth carriers in the I and Q
dimensions may be repeated up to n times, as shown in FIG. 14 by
the summing nodes 1420a, 1420b, in order to transmit the entire
signal bandwidth received by the RX-only satellite 1320a
corresponding to all satellite cells of each polarization (LHCP and
RHCP) of each antenna. Accordingly, the processors and summing
nodes 1420a, 1420b, along with other conventional components such
as frequency translators, phase shifters, and/or filters, may
comprise a feeder link signal generator according to some
embodiments of the invention, which is configured to combine
signals that are received by the first and second receive only
antennas 1340a, 1340b into the feeder link signal 1490 that is
transmitted on at least one carrier in a plurality of orthogonal
dimensions.
[0093] Still referring to FIG. 14, similar operations may take
place with respect to the second RX-only satellite 1320b. This
mapping only is shown generally in FIG. 14 at 1450, for the sake of
clarity. A plurality of gateways 1440a-1440n may be provided to
spatially reuse the same available feeder link spectrum, up to n
times in FIG. 14, and thus transport all the satellite receive
signals to the ground, for demodulation and combining. Thus, the
gateways 1440a-1440n can function as frequency reuse sites, as well
as providing for diversity combining according to some embodiments
of the present invention, as will be described below. It will be
understood that if Y is less than or equal to X, only one gateway
location 1440 may need to be used. Moreover, it also will be
understood that other polarization schemes may be used at the
various stages of FIG. 14, instead of the LHCP/RHCP and/or V/H
polarization.
[0094] Demodulation and combining of the received signals for each
user, according to some embodiments of the present invention, now
will be described. In particular, in some embodiments, a given user
signal will reach the ground via the plurality of polarizations
(LHCP and RHCP) of each satellite antenna, via the plurality of
satellite antennas 1340a-1340d of each RX-only satellite 1320a,
1320b, and via the plurality of RX-only satellites 1320a, 1320b.
Furthermore, a plurality of satellite beams (cells) of each
polarization, of each antenna, and of each RX-only satellite, may
be contributing a desired signal component relative to the given
user, particularly when the user is geographically close to the
intersection of two or more of the satellite beams. Thus,
embodiments of demodulation and combining may include processing of
multiple signal components that are received by the various RX-only
satellite antennas 1340a-1340d from a given radiotelephone 1330, in
order to reconstruct the wireless communications from the
radiotelephone.
[0095] In one example, up to three cells may be receiving useful
signal contributions in a seven-cell frequency reuse plan.
Moreover, in embodiments of FIGS. 13 and 14, there are two
polarizations per cell, two antennas per satellite, and a total of
two RX-only satellites. Thus, there may be
3.times.2.times.2.times.2 or 24 signal components per user that may
be combined in some embodiments. In some embodiments, each of the
plurality of signal components may be weighted in accordance with,
for example, a least mean squared error performance index, and then
summed, for example, in a combiner such as an optimum combiner
1460, to yield the received signal output S, shown in FIG. 14. A
receiver decision stage 1470 then may be used to generate symbol
estimates S.
[0096] Finally, as was described above, in transporting a plurality
of X MHz signal segments to the ground, each gateway site
1440a-1440n may also receive interference between the I and Q
dimensions (also referred to as cross-rail interference) and/or
cross-polarization interference between the vertical and horizontal
polarizations, for example due to the non-ideal passband
characteristics of the channel and/or the system.
[0097] In order to reduce or minimize these interferences, some
known symbols may be transmitted over at least some of the
orthogonal dimensions that were described above, to enable an
adaptive receiver at a gateway site, to compensate at least in part
for any such effect. In other embodiments, precompensation may be
performed for the channel and/or system non-ideal passband
characteristics at the satellite, prior to transmission over a
feeder link. When using precompensation, error information may be
sent back to the satellite from a processing gateway site.
[0098] In still other embodiments, the overhead of the known
symbols, as was described above, may be avoided by relying on the
decisions of the receiver. However, the reliability of the
receiver's demodulation process may be increased by transporting
the known symbols. Moreover, the overhead due to a known symbol
sequence can be small, since the feeder link channel generally is
quasi-static.
[0099] FIG. 15 conceptually illustrates space-based network
architectures according to other embodiments of the present
invention. As shown in FIG. 15, these embodiments of the present
invention include at least one receive-only satellite and at least
one transmit and receive satellite. In particular, in some
embodiments, a first receive-only satellite 1510a and a first
transmit and receive satellite 1520a are co-located, for example at
orbital slot 101.degree. W. A second receive-only satellite 1510b
and a second transmit and receive satellite 1520b also are
co-located, for example at orbital slot 107.3.degree. W.
[0100] Moreover, in still other embodiments of the invention, as
also illustrated in FIG. 15, the transmit and receive satellites
1520a, 1520b can each include a respective first antenna 1540a,
1540c that is configured as a receive-only antenna, and a
respective second antenna 1540b, 1540d that is configured to
perform both transmit and receive functions. In still other
embodiments of the invention, the second antenna 1540b, 1540d may
be configured to perform transmit-only functions. In yet other
embodiments, the first antenna 1540a, 1540c also may be configured
to perform transmit and receive functions. In all embodiments, the
antennas may be of same and/or different sizes.
[0101] Embodiments of FIG. 15 also can be used to obtain a
relatively high return link (uplink) margin. For example, a
comparison will be made relative to the Thuraya satellite. It will
now be shown that a return link margin of approximately 13 dB
higher may be obtained using space-based architectures according to
some embodiments of the present invention.
[0102] In particular, assuming a single satellite with a single 24
meter diameter antenna, about 4 dB of additional margin may
practically be obtained relative to the Thuraya 12 meter antenna.
However, as shown in FIG. 15, if the satellite 1520a has two
receive antennas 1540a, 1540b, the return link margin may be
increased by an additional 3 dB, for a total of 7 dB over Thuraya,
assuming that both antennas 1540a, 1540b on the satellite 1520a are
of the same size and that combining of their outputs is performed.
Thus, using only a single satellite 1520a of FIG. 15, with one dual
purpose 24 meter transmit and receive antenna 1540b, and one
receive-only 24 meter antenna 1540a, embodiments of the present
invention can obtain 7 dB more return link margin than may be
obtained in the Thuraya system.
[0103] The addition of the first receive-only satellite 1510a can
add 3 dB more to the above link margin, since it includes two
additional 24 meter receive-only L band antennas. Finally,
satellites 1520b and 1510b can add 3 dB more to the above, for a
total of 13 dB over and above that which may be obtained with
Thuraya without even having considered diversity gains.
[0104] As was described above in connection with FIG. 14, each
satellite receive antenna may be assumed to be receiving both RHCP
and LHCP. The polarizations may be combined in a manner similar to
that described in FIG. 14.
[0105] In the drawings and specification, there have been disclosed
embodiments of the invention and, although specific terms are
employed, they are used in a generic and descriptive sense only and
not for purposes of limitation, the scope of the invention being
set forth in the following claims.
* * * * *