U.S. patent application number 14/054623 was filed with the patent office on 2014-02-13 for capacity maximization for a unicast spot beam satellite system.
This patent application is currently assigned to ViaSat, Inc.. The applicant listed for this patent is ViaSat, Inc.. Invention is credited to Mark J. Miller.
Application Number | 20140045421 14/054623 |
Document ID | / |
Family ID | 40265230 |
Filed Date | 2014-02-13 |
United States Patent
Application |
20140045421 |
Kind Code |
A1 |
Miller; Mark J. |
February 13, 2014 |
CAPACITY MAXIMIZATION FOR A UNICAST SPOT BEAM SATELLITE SYSTEM
Abstract
Methods, systems, and apparatuses are presented for improved
satellite communications. The satellite system may comprises at
least one gateway, a satellite in orbit configured to communicate
with the at least one gateway and provide a plurality of spot
beams, and a plurality of subscriber terminals. The spot beams may
include a first spot beam to illuminate a first region and a second
spot beam to illuminate a second region adjacent to and overlapping
with the first region. The first spot beam as sent to at least one
subscriber terminal may be affected by (1) interference from other
signal sources including the second spot beam at a
signal-to-interference ratio C/I and (2) noise at a signal-to-noise
ratio C/N. Reception of signals from the first spot beam by the at
least one of the first plurality of subscriber terminals may be
interference-dominated such that C/I is less than C/N.
Inventors: |
Miller; Mark J.; (Vista,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ViaSat, Inc. |
Carlsbad |
CA |
US |
|
|
Assignee: |
ViaSat, Inc.
Carlsbad
CA
|
Family ID: |
40265230 |
Appl. No.: |
14/054623 |
Filed: |
October 15, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13591126 |
Aug 21, 2012 |
8600296 |
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14054623 |
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13218278 |
Aug 25, 2011 |
8285202 |
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13591126 |
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12176629 |
Jul 21, 2008 |
8010043 |
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13218278 |
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60951178 |
Jul 20, 2007 |
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Current U.S.
Class: |
455/12.1 |
Current CPC
Class: |
H04B 7/18513 20130101;
H04B 7/2041 20130101 |
Class at
Publication: |
455/12.1 |
International
Class: |
H04B 7/185 20060101
H04B007/185 |
Claims
1. A satellite for illuminating a geographic area with signals, the
satellite comprising: a transmission subsystem operable to generate
a plurality of service link signals; a spot beam antenna system
coupled to the transmission subsystem and operable to emit to
create a plurality of service link spot beam signals each in a
corresponding spot beam, wherein: each of the plurality of spot
beams is operable to illuminate a distinct geographic area when the
satellite is placed into orbit, and portions of a first one of the
plurality of spot beams overlaps a second one of the plurality of
spot beams so that a signal-to-interference ratio of a first one of
said service link signals in the first one of said plurality of
spot beams is interference-dominated by interference caused by a
second one of the service link signals in the second one of the
plurality of spot beams.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
to, U.S. patent application Ser. No. 13/591,126, entitled "Capacity
Maximization for a Unicast Spot Beam Satellite System," filed Aug.
21, 2012, which is a continuation of, and claims priority to, U.S.
patent application Ser. No. 13/218,278, entitled "Capacity
Maximization For A Unicast Spot Beam Satellite System," filed Aug.
25, 2011, which is a continuation of U.S. patent application Ser.
No. 12/176,629, entitled "Capacity Maximization For A Unicast Spot
Beam Satellite System," filed Jul. 21, 2008, which claims the
benefit of priority to U.S. Provisional Application No. 60/951,178,
entitled "Capacity Maximization for a Unicast Spot Beam Satellite
System," filed Jul. 20, 2007, the entire contents of each of which
are hereby incorporated by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to satellite communications
systems, and more particularly to radio frequency communications
between a gateway and a plurality of subscriber terminals via a
satellite.
[0003] The vast majority of subscribers in urban or suburban areas
are served by either hybrid fiber coaxial, cable, or ADSL networks.
Both cable and ADSL rely on physical wires to provide network
access. The capital expenditure depends on the geographic distance
between subscribers and access nodes. The infrastructure cost is
shared by all subscribers residing in the area. When the subscriber
density is low such as in the rural or remote areas, the wired
infrastructures are too costly to be deployed. An alternative
solution is providing services via satellite.
[0004] The satellite is conceptually similar to a base station in a
cellular communications network, where the base station is located
at a very high altitude above the earth. A geostationary (GEO)
satellite is in orbit about 36,000 km above the equator, and its
revolution around the earth is synchronized with the earth's
rotation. Therefore, the GEO satellite appears stationary, i.e.,
fixed on the earth's surface.
[0005] Like a cellular infrastructure, a satellite network can
divide the covered geography (footprint) into many smaller
footprints using multi-beam antennas. A gateway in the footprint of
one spot beam can communicate with subscriber terminals in the
footprint of other spot beams. The term spot beam refers to a
directional radiation pattern provided by a satellite antenna in
which the area of the geographical coverage is constrained to a
footprint having a direct line of sight to the satellite. The spot
beams can carry two-way communications, sent in packets at specific
time intervals and allotted frequencies. And all wireless
technologies for cellular communications such as CDMA, FDMA and
TDMA technologies and the combination thereof can also be applied
to the satellite communication. Similar to cellular communication
networks that employ frequency reuse to maximize bandwidth
efficiency, a satellite communication system has the additional
advantage of employing orthogonal polarization to increase the
bandwidth.
[0006] A satellite communications system has many parameters to
work with: (1) number of orthogonal time or frequency slots
(defined as color patterns hereinafter); (2) beam spacing
(characterized by the beam roll-off at the crossover point); (3)
frequency re-use patterns (the re-use patterns can be regular in
structures, where a uniformly distributed capacity is required);
and (4) number of beams (a satellite with more beams will provide
more system flexibility and better bandwidth efficiency, but
requires more transponders and amplifiers that are in general
traveling-wave tubes amplifiers (TWTAs). TWTAs are expensive and
consume power that must be supplied on-board the satellite.
[0007] The prior art satellite communications systems take the
approach of maximizing a symbol energy-to-noise-plus-interference
(SINR) to the worst-case location within a beam. This approach
leads to an increased cost in subscriber terminals (STs) because
the receiver at the STs will be over-designed to cope with the
worst-case condition. Another approach is to divide the available
bandwidth into multiple small frequency ranges (different color
patterns) and space them apart to reduce interference. This
approach will reduce the available frequency bandwidth for each
spot beam and require a large amount of TWTs and TWTAs, therefore
require a large power supply on-board the satellite.
[0008] Design approaches of prior satellite systems typically do
not take into account the effects that various system parameters
have on the data-carrying capacity of spot beams. Indeed, choices
made in the selection of particular system parameters may
significantly reduce capacity performance, especially in an
interference-dominated environment. Thus, there is a need for
techniques that allow system parameter adjustments to be found that
will improve data-carrying capacity.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention relates to a method, system, and
apparatus for improved satellite communications. In one embodiment
of the invention, a satellite communications system for
illuminating a geographic area with signals comprises at least one
gateway, a satellite in orbit configured to communicate with the at
least one gateway and provide a plurality of service beams to
illuminate a plurality of regions in the geographic area, and a
plurality of subscriber terminals located in the plurality of
regions. The spot beams may include a first spot beam and a second
spot beam. The first spot beam may illuminate a first region within
the geographic area, in order to send information to a first
plurality of subscriber terminals. The second spot beam may
illuminate a second region within the geographic area and adjacent
to the first region, in order to send information to a second
plurality of subscriber terminals. The first and second regions may
overlap.
[0010] The first spot beam as sent to at least one of the first
plurality of subscriber terminals may be affected by interference
from other signal sources, including the second spot beam, at a
signal-to-interference ratio C/I. The first spot beam as sent to
the at least one of the first plurality of subscriber terminals may
be further affected by noise at a signal-to-noise ratio C/N.
Reception of signals from the first spot beam by the at least one
of the first plurality of subscriber terminals may be
interference-dominated such that C/I is less than C/N.
[0011] Furthermore, the satellite may be operated to maximize
data-carrying capacity of the plurality of spot beams as measured
in bits/Hz, by utilizing a beam pattern having a specific number of
color(s) of frequency and polarization and specific beam spacing
that results in higher data-carrying capacity of the plurality of
spot beams than achieved with other alternative numbers of color(s)
of frequency and polarization and beam spacings.
[0012] According to an embodiment of the invention, the plurality
of spot beams does not comprise adaptive coding and modulation
(ACM) signals, and the data-carrying capacity of the plurality of
spot beams is maximized by maximizing minimum data-carrying
capacity within the plurality of spot beams.
[0013] According to an alternative embodiment of the invention, the
plurality of spot beams comprise adaptive coding and modulation
(ACM) signals, and the data-carrying capacity of the plurality of
spot beams is maximized by maximizing average data-carrying
capacity within the plurality of spot beams.
[0014] In one specific embodiment, the beam pattern has a single
color of frequency and polarization, the beam pattern has a beam
spacing characterized by a cross-over point of less than -6 dB, and
the beam pattern has a regular frequency re-use pattern.
[0015] According to an embodiment of the invention, the first spot
beam includes at least a first portion sent to a first subscriber
terminal from the first plurality of subscriber terminals utilizing
a first coding and modulation combination, and the first spot beam
further includes a second portion sent to a second subscriber
terminal in the first plurality of subscriber terminals utilizing a
second coding and modulation combination, the first coding and
modulation combination being different from the second coding and
modulation combination. In one specific embodiment, the first
coding and modulation combination and second coding and modulation
combination are selected according to an adaptive coding and
modulation (ACM) scheme.
[0016] The following detailed description together with the
accompanying drawings will provide a better understanding of the
nature and advantages of the present invention.
BRIEF SUMMARY OF THE DRAWINGS
[0017] FIG. 1 shows a block diagram of an exemplary satellite
communications system according to one embodiment of the present
invention.
[0018] FIG. 2A shows a diagram of forward links according to one
embodiment of the present invention. FIG. 2B shows an exemplary
modcode table according to one embodiment of the present invention.
FIG. 2C shows an exemplary Address-SNR table according to one
embodiment of the present invention.
[0019] FIG. 3 shows a diagram of a prior art three-color spot beam
pattern.
[0020] FIG. 4 shows a diagram of a two-color spot beam pattern in
accordance with one embodiment of the present invention.
[0021] FIG. 5 shows a diagram of a one-color spot beam pattern in
accordance with one embodiment of the present invention.
[0022] FIG. 6 shows a diagram of a prior art three-color beam
overlap pattern.
[0023] FIG. 7 shows a diagram of a two-color beam overlap pattern
in accordance with one embodiment of the present invention.
[0024] FIG. 8 shows a diagram of a single-color beam overlap
pattern in accordance with one embodiment of the present
invention.
[0025] FIG. 9A shows a block diagram of a prior-art satellite
having a four-color beam pattern for the service link. FIG. 9B
shows a block diagram of the satellite 105 having a two-color beam
pattern for the service link according to one embodiment of the
present invention. FIG. 9C shows a block diagram of the satellite
105 having a one-color beam pattern according to another embodiment
of the present invention.
[0026] FIG. 10 shows a diagram of a forward channel of FIG. 2 in
accordance with one embodiment of the present invention.
[0027] FIG. 11 shows a diagram of a one-color beam pattern that
uses non-uniform beam dispersion in accordance with one embodiment
of the present invention.
[0028] FIG. 12A shows a spot beam that uses ACM in various regions
having circular shape of the spot beam in accordance with one
embodiment of the present invention. FIGS. 12B and 12C shows a spot
beam that use ACM in various regions having respective hexagon
shaped and irregular shape of the spot beam in accordance with one
embodiment of the present invention.
[0029] FIG. 13 shows a spot beam having individual subscriber
terminals distributed among vaguely defined coding and modulation
areas in accordance with one embodiment of the present
invention.
[0030] FIG. 14 shows a multi-beam forward channel having three
parallel data streams in accordance with one embodiment of the
present invention.
[0031] FIG. 15 shows a method of implementing adaptive coding and
modulation for maximizing a unicast spot beam capacity in
accordance with one embodiment of the present invention.
[0032] FIG. 16 graphically illustrates the Shannon capacity as well
as various waveform based capacities as a function of the
signal-to-noise ratio E.sub.s/N.sub.0.
[0033] FIG. 17 graphically illustrates an example of the normalized
gain |h.sub.j|.sup.2 as function of the location, r, within a
beam.
[0034] FIG. 18A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, at a "standardized" signal-to-noise (E.sub.s/N.sub.0)*=6
dB.
[0035] FIG. 18B shows the average capacity plotted against the
amount of beam spacing, for a satellite system operated with ACM,
at (E.sub.s/N.sub.0)*=6 dB.
[0036] FIG. 19A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, at (E.sub.s/N.sub.0)*=12 dB.
[0037] FIG. 19B shows the average capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated with
ACM, at (E.sub.s/N.sub.0)*=12 dB.
[0038] FIG. 20A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, with (E.sub.s/N.sub.0)*=0 dB.
[0039] FIG. 20B shows the average capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated with
ACM, at (E.sub.s/N.sub.0)*=0 dB.
[0040] FIG. 21A shows the capacity in bps/Hz per beam plotted
against the signal-to-noise ratio E.sub.s/N.sub.0, for a satellite
system with beam spacing characterized by a roll-off value of -3 dB
at the cross-over point.
[0041] FIG. 21B shows the capacity in bps/Hz per beam plotted
against E.sub.s/N.sub.0, for a satellite system with beam spacing
characterized by a roll-off value of -6 dB at the cross-over
point.
[0042] FIG. 22 illustrates the density (defined as average capacity
per unit area) for satellite systems having different selections of
the number of colors (L=1, 2, 3, 4, 7).
[0043] FIG. 23 presents a four-color system in accordance with one
embodiment of the invention.
[0044] FIG. 24 provides a summary of different systems having
different number of colors, bandwidth per beam, number of employed
gateways, TWT power per beam, number of TWTs per satellite, the
payload aperture, maximum PFD per pole, beam spacing, crossover
points, achieved beam capacity, and relative comparison to a
four-color baseline system.
DETAILED DESCRIPTION OF THE INVENTION
[0045] The ensuing description provides preferred exemplary
embodiments only, and is not intended to limit the scope,
applicability or configuration of the disclosure. Rather, the
ensuing description of the preferred exemplary embodiments will
provide those skilled in the art with an enabling description for
implementing a preferred exemplary embodiment. It is understood
that various changes may be made in the function and arrangement of
elements without departing from the spirit and scope as set forth
in the appended claims.
Satellite Communication System
[0046] Referring first to FIG. 1, a block diagram of an exemplary
satellite communications system 100 configured according to various
embodiments of the invention is shown. The satellite communications
system 100 includes a network 120, such as the Internet, interfaced
with one or more gateways 115 that is configured to communicate
with one or more subscriber terminals 130, via a satellite 105.
[0047] The gateway 115 is sometimes referred to as a hub or ground
station and services the feeder links 135, 140 to and from the
satellite 105. Although only one gateway 115 is shown, this
embodiment has a number of gateways all coupled to the network 120,
for example, twenty or forty gateways. The gateway 115 schedules
traffic to the subscriber terminals 130, although other embodiments
could perform scheduling in other parts of the satellite
communication system 100.
[0048] Subscriber or user terminals 130 include an outdoor unit
(ODU) 134, a satellite modem 132 and an antenna 125. Although the
satellite communications system 100 is illustrated as a
geostationary satellite based communication system, it should be
noted that various embodiments described herein are not limited to
use in geostationary satellite based systems, for example some
embodiments could be low earth orbit (LEO) satellite based systems.
Some embodiments could have one satellite 105, while others could
have more satellites working together in concert.
[0049] A satellite communications system 100 applicable to various
embodiments of the invention is broadly set forth herein. In this
embodiment, there is a predetermined amount of frequency spectrum
available for transmission. The feeder links may use the same or
overlapping frequencies with the service links or could use
different frequencies. The gateways 115 could be placed outside the
service beams when frequencies are reused.
[0050] The network 120 may be any type of network and can include,
for example, the Internet, an IP network, an intranet, a wide-area
network (WAN), a local-area network (LAN), a virtual private
network (VPN), a virtual LAN (VLAN), a fiber optical network, a
hybrid fiber-coax network, a cable network, the Public Switched
Telephone Network (PSTN), the Public Switched Data Network (PSDN),
a public land mobile network, and/or any other type of network
supporting data communication between devices described herein, in
different embodiments. The network 120 may include both wired and
wireless connections, including optical links. As illustrated in a
number of embodiments, the network may connect the gateway 115 with
other gateways (not pictured), which are also in communication with
the satellite 105.
[0051] The gateway 115 provides an interface between the network
120 and the satellite 105. The gateway 115 may be configured to
receive data and information directed to one or more subscriber
terminals 130, and can format the data and information for delivery
to the respective destination device via the satellite 105.
Similarly, the gateway 115 may be configured to receive signals
from the satellite 105 (e.g., from one or more subscriber terminals
130) directed to a destination connected with the network 120, and
can format the received signals for transmission with the network
120. The gateway 115 may use a broadcast signal, with a modulation
and coding ("modcode") format adapted for each packet to the link
conditions of the terminal 130 or set of terminals 130 to which the
packet is directed (e.g., to account for the variable service link
150 conditions from the satellite 105 to each respective terminal
130).
[0052] A device (not shown) connected to the network 120 may
communicate with one or more subscriber terminals 130 and through
the gateway 115. Data and information, for example Internet
protocol (IP) datagrams, may be sent from the device in the network
120 to the gateway 115. The gateway 115 may format a Medium Access
Control (MAC) frame in accordance with a physical layer definition
for transmission to the satellite 130. A variety of physical layer
transmission modulation and coding techniques may be used with
certain embodiments of the invention, including those defined with
the DVB-S2 that is developed in 2003 and ratified by ETSI (EN 302
307), DOCSIS (Data Over Cable Service Interface Specification
developed by Cable Labs) and WiMAX (The Worldwide interoperability
for Microwave Access based on the IEEE802.16) standards. The link
135 from the gateway 115 to the satellite 105 is referred to
hereinafter as the downstream uplink 135.
[0053] The gateway 115 may use an antenna 110 to transmit the
downstream uplink signal to the satellite 105. In one embodiment,
the antenna 110 comprises a parabolic reflector with high
directivity in the direction of the satellite 105 and low
directivity in other directions. The antenna 110 may comprise a
variety of alternative configurations and include operating
features such as high isolation between orthogonal polarizations,
high efficiency in the operational frequency bands, and low
noise.
[0054] In one embodiment of the present invention, a geostationary
satellite 105 is configured to receive the signals from the
location of antenna 110 and within the frequency band and specific
polarization transmitted. The satellite 105 may, for example, use a
reflector antenna, lens antenna, phased array antenna, active
antenna, or other mechanism known in the art for reception of such
signals. The signals received from the gateway 115 are amplified
with a low-noise amplifier (LNA) and then frequency converted to a
transmit frequency. The satellite 105 may process the signals
received from the gateway 115 and forward the signal from the
gateway 115 to one or more subscriber terminals 130. In one
embodiment of the present invention, the frequency-converted
signals are passed through a demultiplexer that separate the
various received signals into their respective frequency bands. The
separate signals are amplified by TWTAs, one for each frequency
band and are combined in a multiplexer to form the high-power
transmission signals. The high-power transmission signal passed
through a transmit reflector antenna (e.g., a phased array antenna)
that forms the transmission radiation pattern (spot beam). In one
embodiment of the present invention, the satellite 105 may operate
in a multi-beam mode, transmitting a number of narrow beams each
directed at a different region of the earth, allowing for
segregating subscriber terminals 130 into the various narrow beams.
With such a multibeam satellite 105, there may be any number of
different signal switching configurations on the satellite 105,
allowing signals from a single gateway 115 to be switched between
different spot beams.
[0055] In another embodiment of the present invention, the
satellite 105 may be configured as a "bent pipe" satellite, wherein
the satellite 105 may frequency and polarization convert the
received carrier signals before retransmitting these signals to
their destination, but otherwise perform little or no other
processing on the contents of the signals. A spot beam may use a
single carrier, i.e., one frequency or a contiguous frequency range
per beam. A variety of physical layer transmission modulation and
coding techniques may be used by the satellite 105 in accordance
with certain embodiments of the invention. Adaptive coding and
modulation can be used in some embodiments of the present
invention.
[0056] For other embodiments of the present invention, a number of
network architectures consisting of space and ground segments may
be used, in which the space segment is one or more satellites while
the ground segment comprises of subscriber terminals, gateways,
network operations centers (NOCs) and a satellite management center
(SMC). The satellites can be GEO or LEO satellites. The gateways
and the satellites can be connected via a mesh network or a star
network, as evident to those skilled in the art.
[0057] The service link signals are transmitted from the satellite
105 to one or more subscriber terminals 130 and received with the
respective subscriber antenna 125. In one embodiment, the antenna
125 and terminal 130 together comprise a very small aperture
terminal (VSAT), with the antenna 125 measuring approximately 0.6
meter in diameter and having approximately 2 watts of power. In
other embodiments, a variety of other types of antennas 125 may be
used at the subscriber terminal 130 to receive the signal from the
satellite 105. The link 150 from the satellite 105 to the
subscriber terminals 130 may be referred to hereinafter as the
downstream downlink 150. Each of the subscriber terminals 130 may
comprise a single user terminal or, alternatively, comprise a hub
or router (not pictured) that is coupled to multiple user
terminals. Each subscriber terminal 130 may be connected to various
consumer premises equipment (CPE) 160 comprising, for example
computers, local area networks, Internet appliances, wireless
networks, etc.
[0058] In one embodiment, a Multi-Frequency Time-Division Multiple
Access (MF-TDMA) scheme is used for upstream links 140, 145,
allowing efficient streaming of traffic while maintaining
flexibility in allocating capacity among each of the subscriber
terminals 130. In this embodiment, a number of frequency channels
are allocated which may be fixed, or which may be allocated in a
more dynamic fashion. A Time Division Multiple Access (TDMA) scheme
is also employed in each frequency channel. In this scheme, each
frequency channel may be divided into several timeslots that can be
assigned to a connection (i.e., a subscriber terminal 130). In
other embodiments, one or more of the upstream links 140, 145 may
be configured with other schemes, such as Frequency Division
Multiple Access (FDMA), Orthogonal Frequency Division Multiple
Access (OFDMA), Code Division Multiple Access (CDMA), and/or any
number of hybrid or other schemes known in the art.
[0059] A subscriber terminal, for example 130-a, may transmit data
and information to a destination on the network 120 via the
satellite 105. The subscriber terminal 130 transmits the signals
via the upstream uplink 145-a to the satellite 105 using the
antenna 125-a. A subscriber terminal 130 may transmit the signals
according to a variety of physical layer transmission modulation
and coding techniques. In various embodiments of the present
invention, the physical layer techniques may be the same for each
of the links 135, 140, 145, 150, or may be different. The link from
the satellite 105 to the gateway 115 may be referred to hereinafter
as the upstream downlink 140.
[0060] Referring next to FIG. 2A, a diagram of an embodiment of a
forward link diagram 200 is shown. A number of gateway antennas 110
respectively have a forward channel 208 through the satellite 105
to a spot beam 204. A number of subscriber terminal (ST) antennas
125 are configured in the spot beam 204 to capture the forward
channel 208. The ST 130 are distributed among the n spot beams 204
based generally upon their presence within a particular spot beam
204. There are places where the spot beams 204 overlap such that a
particular subscriber terminal 130 could be allocated to one or
another spot beam 204.
[0061] The upstream feeder link 140 is separated from the
downstream service link 150 using some sort of orthogonality, for
example, temporal, spatial, frequency, and/or polarization. In one
embodiment, the upstream feeder link 140 has a feeder spot beam
that is geographically separated from the service spot beams, but
any type of orthogonality could accomplish the separation.
[0062] Referring to FIG. 2B, an example of a modulation and coding
(modcode) table 202 is illustrated in the form of a block diagram.
This form of modcode table 202 may, for example, be used by a
gateway 115 to determine the modcode to be used for packets
destined for a subscriber terminal operating in a given signal
quality range. The table contains a column listing a number of
modcode formats 205. Each modcode format 205 corresponds to a
specified signal quality range 210. The signal quality range may
provide some knowledge on the channel for an associated region. For
example, the signal quality range 210 can be defined as the
signal-to-interference-plus-noise (SINR) ratio that may be measured
at the subscriber terminals for a predetermined bit error rates
(BER) and/or packet error rates (PER) and reported back to the
gateway. BER and/or PER can be extracted from a cyclic redundant
check (CRC) calculation with the gateway transmits data packets or
data frames containing a certain length of bits whose pattern are
known a prior by the subscriber terminal. Thus, using the signal
quality attributed to a destination link for a packet, a signal
quality range 210 encompassing the link may be identified, and the
appropriate modcode may be selected. For example, if a destination
link has a signal quality within Range 7, the modcode QPSK 3/4 may
be used. In some embodiments of the present invention, one or more
of the ranges may include a reliability margin (which may be
beneficial when channel conditions are changing rapidly, for
example). One or more of the ranges may be modified dynamically to
adjust this reliability margin as well.
[0063] In other embodiments of the present invention, other signal
quality indicators may be used, such as a measured signal to noise
ratio, an estimated signal to noise ratio, a bit error rate, a
received power level, or any other communication link quality
indicator. It is also worth noting that a number of other data
structures may also be used to relate signal quality ranges to
modcodes. In one embodiment, each signal quality is associated with
a different packet forwarding queue. In still other embodiments,
other information density parameters in addition to modcode changes
may be added to further adapt a signal to environmental or other
conditions.
Adaptive Code Modulation (ACM)
[0064] Turning to FIG. 2C, an example of an address/SNR table 250
is illustrated in the form of a block diagram. This form of
address/SNR table 250 may, for example, be used by a gateway 115 to
lookup the signal quality 260 of a subscriber terminal 130 to which
a packet is destined, based on the destination address 255. The
tables in FIGS. 2B and 2C may be embodied on one or more memories,
which may be either on or off chip, and may be used in conjunction
with one another to correlate a MAC address with a particular
modcode format.
[0065] Although a destination MAC address is used in this example,
other mechanisms may be used to identify particular subscriber
terminals, including destination VLAN-ID, a Destination Internet
Protocol (DIP) address, a private addressing ID, any other set of
data comprising or otherwise correlated with a destination address.
The data address may be parsed from a received data packet after
arrival at a device, or it may be received in any other manner
known in the art. It is also worth noting that a number of other
data structures may also be used to relate an address to signal
quality.
[0066] Once a modcode for a particular packet or packets is
identified, for example using the modcode table 202, it may then be
encapsulated, coded, mapped and transmitted in a variety of ways,
as known in the art. One way to implement an adaptive coding and
modulation (ACM) is via the DVB-S2 standard, which specifically
provides for its use. As noted above, ACM may change the modulation
format and Forward Error Correction (FEC) codes (modcodes) to best
match the current link conditions. This adaptation may occur on a
frame by frame basis. The discussion that follows assumes an IP
based packet network in the context of a DVB-S2 satellite
transmission system, but the concepts may be applied for a variety
of systems, including systems implementing DOCSIS, WiMAX, or any
wireless local loops (WLLs).
[0067] With reference to FIG. 3, a diagram of a prior art three
color spot beam pattern 300 is shown. These spot beams 304, 308,
312 could correspond to three different frequency groups, with one
group for each color. Patterns with even more colors are also
known. The pattern assures that no directly adjacent spot beams use
the same color. Orthogonality is achieved by the use of the
different colors. For example, the first color could correspond to
2.0 through 2.1 GHz, the second color could correspond to 2.1
through 2.2 GHz and the third color could correspond to 2.2 through
2.3 GHz. The spot beams are shown as hexagon shaped, but are more
circular or oval in shape such that there is overlap between the
spot beams 304, 308, 312.
[0068] Referring next to FIG. 4, a diagram of an embodiment of the
present invention having a two-color spot beam pattern 400 is
shown. With only two colors available, spot beams 404, 408 will
overlap. Here we have a row of spot beams 404 in a first color and
a row of spot beams 408 in a second color. For example, the first
color could be 2.0 through 2.15 GHz and the second color could be
2.15 through 2.3 GHz. Along the rows, there will be some confusion
between directly adjacent spot beams that have the same color. By
going from three to two colors, the available frequency bandwidth
for each spot beam 404, 408 increases by fifty percent.
[0069] With reference to FIG. 5, a diagram of an embodiment of the
present invention having a one-color spot beam pattern 500 is
shown. This embodiment uses the same or at least partially
overlapping frequencies in each spot beam 504. For example, the
spot beams 504 could each use 2.0 through 2.3 GHz. All immediately
adjacent spot beams 504 use the same frequency range. Other
embodiments of the present invention could have patches or portions
of the color pattern that have immediately adjacent spot beams that
use the same or overlapping frequency ranges.
[0070] Referring next to FIG. 6, a diagram of a prior art three
color beam overlap pattern 600 is shown. This diagram corresponds
to a portion of FIG. 3, but shows the spot beams 304, 308, 312 as
circles rather than hexagons. FIG. 6 is also idealized in that the
overlap could be of any size as the signal continues outside the
circle, but at a lower signal strength such that the diameter of
the circles are somewhat arbitrary as the radio signal strength
falls off quickly with distance relative to the center. Generally,
the STs 130 within the circle can receive information from the spot
beam corresponding to that circle.
[0071] Overlap occurs in various overlap regions 604, 608, 612. The
first type of overlap region 604 corresponds to an area where STs
130 can receive both from a first color beam 304 and a second color
beam 308. The second type of overlap region 608 corresponds to an
area where STs 130 can receive both from a second color beam 308
and a third color beam 312. The third type of overlap region 612
corresponds to an area where STs 130 can receive both from the
first color beam 304 and the third color beam 312. In the overlap
regions 604, 608, 612, STs 130 could optionally receive from either
spot beam causing the overlap.
[0072] With reference to FIG. 7, a diagram of an embodiment of the
present invention having a two-color beam overlap pattern 700 is
shown. There are three different types of overlap regions 704, 708,
712 in this embodiment also. The third overlap region 712
correspond to an area where STs 130 can receive from both a first
color beam 404 and a second color beam 408. In the first and second
types of overlap regions 704, 708, directly adjacent spot beams use
the same or overlapping frequencies such that STs 130 in the
overlap regions 704, 708 could become confused because of the
interference. This embodiment of the present invention uses
adaptive coding and modulation (ACM) to enable reception in the
presence of the interference in the overlap regions 704, 708.
Effectively, the coding and/or modulation are modified to slow the
data rate until an acceptable signal quality is achieved.
[0073] Using ACM, the modulation format and Forward Error
Correction (FEC) codes (modcodes) for a data frame may be adapted
to better match the link conditions for each user in a multi-user
system. ACM can be used in both directions. A return channel or
other means may be used to report the conditions of a receiving
terminal. These link conditions are often characterized by the
modem's 132 signal to noise ratio (SNR) or
signal-to-interference-plus-noise ratio (SINR) if the modem 132
resides in a color-beam overlap region. In a broadcast system, for
example, the data frame broadcasted to a number of users includes
data packets designated only for an individual modem or small group
of modems. A message transmitted to a user requires fewer symbols
and less time when a higher order modulation and higher code rate
is used. Lower order modulation and lower code rate are more
reliable but require more time to transmit the same size message.
Using ACM, each packet may be transmitted at an optimized
modulation and coding (modcode) level given the destination
terminal's link conditions.
[0074] With reference to FIG. 8, a diagram of an embodiment of the
present invention of a single color beam overlap pattern 800 is
shown. Here, all beams use the same or overlapping frequencies in
this embodiment. The overlap regions 804 receive interference from
adjacent beams as frequencies used are common among beams. Once
again, ACM is used to mitigate the effect of interference. STs 130
proximate to the overlap regions may see signals from two or more
beams 504. The ST 130 may sample the signal from each beam 504 and
use the one that provides the most reliable signal reception. In
this way, the system 100 can assign STs 130 among the beams.
[0075] Referring next to FIG. 9A, a block diagram of a prior art
satellite 105 is shown in block diagram form. The satellite 105 in
this embodiment communicates with twenty gateways 115 and all STs
130 using twenty feeder and eighty service spot beams. Each feeder
link spot beam feeds four service link spot beams in this
embodiment. Other embodiments could use more or less gateways/spot
beams. There are likely to be thousands or millions of STs 130
divided by geography between the service link spot beams 204. Buss
power 912 is supplied using a power source such as chemical fuel,
nuclear fuel and/or solar energy. A satellite controller 916 is
used to maintain attitude and otherwise control the satellite 105.
Software updates to the satellite 105 can be uploaded from the
gateway 115 and performed by the satellite controller 916.
[0076] Information passes in two directions through the satellite
105. A downstream translator 908 receives information from the
twenty gateways 115 for relay to subscriber terminals 130 using
eighty service spot beams. An upstream translator 904 receives
information from the subscriber terminals 130 occupying the eighty
spot beam areas and relays that information to the twenty gateways
115. This embodiment of the satellite only translates carrier
frequencies in the downstream and upstream links from the spot
beams 308, 304 in a "bent-pipe" fashion, i.e., the only processing
is frequency translation and retransmission, but other embodiments
could do baseband switching between the various forward and return
channels. The frequencies and polarization for each spot beam could
be programmable or preconfigured.
[0077] With reference to FIG. 9B, a block diagram of satellite 105
according to one embodiment of the present invention is shown. This
embodiment uses two colors on the service link spot beams 404, 408.
There are eighty service link spot beams 404, 408. The gateways 115
support the service link spot beams with forty gateways 115. With
the two colors, each gateway 115 can support two service link spot
beams 404, 408.
[0078] Referring next to FIG. 9C, a block diagram of satellite 105
according to another embodiment of the present invention is shown.
This embodiment uses one color on the service link spot beams 504.
There are eighty service link spot beams 504. The gateways 115
support the service link spot beams with eighty gateways 115. With
the one color, each gateway 115 can support one service link spot
beams 504.
[0079] With reference to FIG. 10, a diagram of forward channel 208
according to an embodiment of the present invention is shown. In
this simplified example, a superframe 1004 is divided between two
modcodes 1008. A first modcode 1008-1 is used for the STs 130
largely outside the overlapping regions, and a second modcode is
used for the STs 130 inside the overlapping regions. For example,
the first modcode 1008-1 could be 32 APSK rate 5/6 and the second
modcode could be QPSK rate 1/2. To deliver the same amount of data,
the second modcode 1008-2 uses a larger time slice of the
superframe 1004. The division of time slices between the two groups
is also affected by the number of group members and the bandwidth
requirements of the groups.
[0080] Other embodiments of the present invention could have more
than two modcode schemes that divide the data stream (e.g., three,
four, five, eight, twelve, sixteen, etc.). The relative size of the
modcode schemes in the superframe 1004 can remain static or change
over time in various embodiments. Further, some embodiments of the
present invention may not use a superframe structure and change the
coding and modulation as needed. STs 130 can be moved between the
various modcode 1008 as a function of their bit error rate (BER) or
other factors.
[0081] Referring to FIG. 11, a diagram of a one-color beam pattern
that uses non-uniform beam dispersion according to one embodiment
of the present invention is shown. Population gradients 1104
indicate where the population is most dense relative to other
gradients in a topographic manner. In this embodiment, the spot
beams 504 can be moved to get more the central region of each spot
beam 504 over the population. For example, spot beam 504-1 was
moved away from a uniform grid spacing to sit over population
1104-1 more squarely. Other spot beams 504 may also be moved. In
some cases, there may be two spot beams that overlap to a
substantial degree, for example, at least 80%, 70%, 60%, 50%, 40%,
30%, or 10% overlap.
[0082] With reference to FIG. 12A, an embodiment of a spot beam
1200-1 is shown that uses ACM in various regions of the spot beam.
Not in strict adherence to the geometric shapes shown in the
figure, the subscriber terminals 130 in the spot beam 1200 are
divided among several coding and modulation (CM) areas 1204. The
various CM areas are shown in an idealized shape, but are not
completely geometric as STs 130 may be irregularly distributed
according to these general areas. As STs 130 report higher or lower
error rates, they can be moved from one CM area to another. For
example, a ST 130 may be assigned to a third CM area because of a
location adjacent to a neighboring beam causing interference, but
the ST 130 may have a very low bit-error rate (BER). The system may
temporarily assign the ST 130 to a second CM area with a higher
data rate and observe the BER. If the BER is acceptable, the ST 130
will remain in the second CM area until the BER becomes
unacceptable.
[0083] The first CM area 1204-1 is generally circular and located
near the center of the spot beam. A location near the center makes
it less likely neighboring beams in a one- or two-color scheme will
overlap. The first CM area 1204-1 would have the highest data rate
by selecting an appropriate coding and modulation, the second CM
area 1204-2 would have a lower data rate and the third CM area
1204-3 would have the lowest data rate. Generally, lower data rates
and their corresponding coding and modulation will produce higher
link margin or gain. Those areas at the periphery of the spot beam
1200 are more likely to have interference from neighboring spot
beams that can be compensated with the higher link margin a lower
data rate affords.
[0084] Referring next to FIGS. 12B and 12C, two additional
embodiments of a spot beam 1200 are shown that use ACM in various
regions of the spot beam 1200. These embodiments demonstrate that
other geometries for the CM areas 1204 are possible. The embodiment
of FIG. 12B has two hexagon shaped CM areas 1204-4, 1204-5
surrounded by a circular shaped CM area 1204-3. In FIG. 12C, the
shape of the interior two CM areas 1204-6, 1204-7 are irregular and
may change over time. Obstructions, geography, weather and other
factors may change the geometry.
[0085] With reference to FIG. 13, an embodiment of a spot beam 1300
is shown with individual STs 1308 shown distributed among vaguely
defined CM areas 1204. The circular-shaped STs 1308-3 are generally
located in an area defined between an outside the larger hexagon CM
area 1204-5 and an inside the circular CM area 1204-3, but there
are a few STs that do not fall in that area. As particular STs 1308
no longer need a CM combination with higher link margin, they are
moved between CM areas 1204 regardless of the general shape of the
CM areas 1204.
[0086] Referring next to FIG. 14, a multi-beam forward channel 1400
according to an embodiment of the present invention is shown as
three parallel data streams. Each data stream uses a superframe
1004 structure, but other embodiments may not use superframes. Each
superframe 1004 is shown with a first, second and third modcode
schemes 1008. These are generally equal in temporal size in this
embodiment, but other embodiments could vary the size according to
the bandwidth usage of the STs 130 that are part of each modcode
scheme 1008. For example, the first modcode scheme 1008-1 could be
32APSK 8/9, the second modcode scheme 1008-2 could be 16APSK 3/4,
and the third modcode scheme 1008-3 could be 8APSK 2/3. Other
modcode schemes other than in FIG. 2B can also be used.
[0087] The three spot beams corresponding to these parallel data
streams may be adjacent to each other. A particular ST 130 assigned
with a particular modcode scheme 1008 will not see the same modcode
scheme 1008 used on an adjacent beam in an overlapping way. Other
embodiments could have partial overlap between adjacent beams. Some
embodiments could avoid overlap between some modcode schemes 1008
while allowing more overlap on others. For example, the center of
the spot beam would likely use the highest data rate, but is also
least likely to have any overlap. The CM scheme corresponding to
the center of the spot beam could tolerate more overlap. The first
data packet and at least part of the second data packet are
encapsulated in the same frame. The first and second data packets
use different modcodes because of varying signal qualities. For
example, the signal quality is likely to be better near the center
of a beam as there would likely be less overlap with adjacent beams
using the same frequency.
[0088] Referring next to FIG. 15, a process for implementing
adaptive coding and modulation in accordance with an embodiment of
the present invention is shown. At process step 1510, a first
subscriber terminal (ST) determines a first signal quality range,
and a second ST determines a second signal quality range. The first
and second STs may be located within a first spot beam, or they may
be located at the intersection of the first spot beam with other
adjacent beams. In one embodiment, the signal quality range may be
associated with a signal to noise ratio (SNR) and/or a carrier to
interference ratio (C/I) at the VSAT input of the subscriber
terminal residing at the location r for a predetermined BER or
PER.
[0089] At process step 1520, the SNR and/or C/I obtained from the
location r is sent to gateway 115 via satellite 105 using the
return downlink 140. At process 1530, gateway 115 associates a
first modcode to the first signal quality range and a second
modcode to the second signal quality range.
[0090] At process step 1540, gateway 115 receives a first data
packet destined for the first ST within the first signal quality
range and assigns the first modcode to the first data packet. At
process step 1540, the gateway 115 may also receive a second data
packet destined for the second ST within the second signal quality
range and assign the second modcode to the second data packet. At
process step 1550, gateway 115 encapsulates the first data packet
encoded and modulated with the first modcode and at least a part of
the second data packet encoded and modulated with the second
modcode in a first frame. At process step 1560, gateway 115
transmits the first frame to the first spot beam via satellite
105.
[0091] A number of variations and modifications of the disclosed
embodiments can also be used. For example, the gateways could
support multiple colors to reduce the number of feeder links. LEO,
GEO satellite orbits, or cellular towers could be used. Further,
the invention is not meant to be limited to only the forward link
or only the return link. Some embodiments may use overlapping
frequencies on one or both of the forward and return link.
Maximizing Outbound Capacity
[0092] According to at least one embodiment of the present
invention, the data-carrying capacity of the service spot beams
(referred to as outbound capacity) as measured in bits-per-second
per Hertz (bps/Hz) may be altered by adjusting certain system
parameters. Indeed, the outbound capacity can be maximized by
selecting system parameters appropriately. Such system parameters
include the number of "colors" of frequency and polarization
combinations, as well as the amount of beam spacing.
[0093] Just as a specific example, a satellite system may have a
"bent pipe" feeder link, a single carrier per traveling wave tube
amplifier (TWT), 417 Mega symbols per second (Msps) time division
multiplexing (TDM) outbound, operating at 0 dB output back off
(OBO). Such a satellite system may specify a beam pattern that uses
(1) either 4-color or 7-color frequency and polarization
combination and (2) an amount of beam spacing defined according to
a particular roll-off value at the cross-over point (also referred
to as the triple point). These system parameters may be altered to
increase the outbound capacity of the system. The frequency re-use
pattern may be assumed to be uniformly distributed. That is, the
re-use pattern may be regular in structure. Other system parameters
such as number of beams, while they can affect performance, may not
represent a design tradeoff specific to data-carrying capacity. In
this example, use of either a 4-color or 7-color frequency and
polarization combination may result in an outbound capacity that is
not optimized and can be greatly improved. For instance, a 1-color
frequency and polarization beam pattern may result in a higher
outbound capacity.
[0094] Maximization of outbound capacity may be defined in
different ways. According to one embodiment of the invention, for a
satellite system that does not utilize adaptive coding and
modulation (ACM), what is maximized may be the outbound capacity as
experienced at the worst-case location within a spot beam. This is
because in such a non-ACM system, there is only one outbound
capacity, and it is determined by a particular selection of
modulation rate and coding that accommodates the signal quality
experienced at the worst-case location within a spot beam. Here, it
is this worst-case outbound capacity that is maximized. For
example, this maximization may be achieved by maximizing the ratio
of symbol energy to noise and interference,
E.sub.s/(N.sub.0+I.sub.0), where E.sub.s/N.sub.0 represents the
thermal symbol energy to thermal noise ratio, and C/I represents
the spot beam carrier to interference ratio.
[0095] According to another embodiment of the invention, for a
satellite system that utilizes adaptive modulation and coding
(ACM), what is maximized may be the average outbound capacity as
experienced at all subscriber terminal locations within a spot
beam. This is because in such an ACM system, there may be different
outbound capacities experienced by different subscriber terminals
within the spot beam, resulting from different selections of
modulation rate and coding used at different subscriber locations
within the spot beam. Here, it is the average of these different
outbound capacities that is maximized. For example, this
maximization may be achieved by finding the symbol energy to
thermal noise ratio E.sub.s/N.sub.0 and E.sub.s/I.sub.0 for every
subscriber terminal location within the spot beam, then maximizing
the capacity averaged over the entire beam. Furthermore, if the
user distribution over the service spot beam is known, it may be
worthwhile to maximize a weighted average capacity that takes into
account the user distribution.
[0096] Specific calculations for such "worst-case" and "average"
capacity are described in illustrative equations presented below.
First, a "standardized" signal-to-noise ratio referred to as
(E.sub.s/N.sub.0)* is defined as the signal-to-noise ratio
experienced by a subscriber terminal at the beam center, assuming a
single-color frequency re-use beam pattern (L=1):
( Es No ) * = SNR to a terminal at beam center assuming L = 1 ( 1 )
##EQU00001##
[0097] This may be a constant value for a particular system. For
example, in one embodiment of the present invention, the standard
signal-to-noise ratio (E.sub.s/N.sub.0)* to a terminal at the beam
center for a one-color pattern (L=1) may be about 6 dB with a 67 cm
VSAT.
[0098] Next, the signal-to-noise ratio
Es No ( r ) ##EQU00002##
and the signal-to-interference ratio
C I ( r ) ##EQU00003##
are each defined as function of the location, r, within a beam
j:
Es No ( r ) = L h j 2 ( Es No ) * ( 2 ) C I ( r ) = h j 2 i h j 2 (
3 ) ##EQU00004##
where |h.sub.j|.sup.2 is the "normalized gain," which is defined as
the beam gain relative to the maximum gain. L is the number of
color patterns. Here, the signal-to-noise ratio varies linearly
with the number of colors (L) because it is assumed that the
satellite is power limited and the total power radiated per beam is
constant, regardless of the bandwidth of the beam (which is related
to the number of colors employed). Under this assumption, a system
employing 4 colors has a bandwidth per beam that is 25% of the
bandwidth per beam of a system employing 1 color. Hence the EIRP
density, dBW/Hz, is 4 times as large for a 4 color system than a 1
color system which results in the signal-to-noise ratio being 4
times as large for the 4 color system than the 1 color system.
[0099] Location-specific capacity may be expressed as either the
Shannon capacity or the Waveform based capacity. These two types of
capacity can each be defined as a function of r and can be
expressed in terms of an intermediate expression .gamma.(r):
[ .gamma. ( r ) ] - 1 = [ Es No ( r ) ] - 1 + [ C I ( r ) ] - 1 ( 4
) ##EQU00005## Capacity(r)=log.sub.2(1+.gamma.(r)) (Shannon
Capacity) (5)
Capacity(r)=f(.gamma.(r)) (Waveform based capacity) (6)
where f(.gamma.) is a function that maps signal-to-noise ratio
(.gamma.) into capacity in bps/Hz and is related to the library of
waveforms (modulation and codepoint) used in the forward link.
[0100] Average capacity can be found by integrating the
location-specific capacity over locations within a beam. This may
be defined as:
C avg ( j ) = 1 A j .intg. r Capacity ( r ) r ( 7 )
##EQU00006##
where is the area covered by beam j. This represents the average
capacity over the area covered by beam j.
[0101] FIG. 16 graphically illustrates the Shannon capacity as well
as various waveform based capacities as a function of the
signal-to-noise ratio E.sub.s/N.sub.0. As depicted in FIG. 16,
architectures requiring low FEC code rates generally perform closer
to theoretical limits as represented by the Shannon capacity. For
example, the capacity of a waveform utilizing QPSK modulation and
DVB-S2 coding is 1.4 dB from the theoretical limit (at
E.sub.s/N.sub.0 around 0 dB). By contrast, the capacity of a
waveform utilizing 8-PSK modulation and DVB-S2 coding is 3 dB away
from the theoretical limit (at E.sub.s/N.sub.0 around 7 dB).
[0102] FIG. 17 graphically illustrates an example of the normalized
gain |h.sub.j|.sup.2 as function of the location, r, within a beam.
Here, the normalized gain is calculated for a system employing a
circular aperture with 10 dB taper in accordance with one
embodiment of the present invention.
[0103] According to an embodiment of the present invention, the
capacity of the system may be systematically calculated for
different choices of system parameter settings. Both types of
capacity may be calculated--"worst-case" and "average" capacity. As
discussed previously, "worst-case" capacity may be a more
appropriate measure of capacity for non-ACM systems. "Average"
capacity may be a more appropriate measure of capacity for ACM
systems. The systems parameters that may be varied include: (1) the
number of "colors" (L) used in the frequency and polarization beam
pattern, (2) the amount of beam spacing, as measured by the
roll-off value at the cross-over point (also referred to as the
triple point). These capacity calculations may also be performed at
various levels of signal-to-noise (E.sub.s/N.sub.0) ratio. By
plotting the capacity of the system as these system parameters are
varied, a picture begins to emerge to indicate how capacity is
affected by the choices made in different parameter settings.
Figures described below represent such pictures for selected
scenarios of system parameter choices.
[0104] FIG. 18A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, at a "standardized" signal-to-noise (E.sub.s/N.sub.0)*=6 dB.
The plot is repeated for different selections of the number of
colors (L=1, 2, 3, 4, 7). Here, the minimum capacity corresponds to
the "worst case" capacity referred to previously. For comparison,
FIG. 18B shows the average capacity plotted against the amount of
beam spacing, for a satellite system operated with ACM, at
(E.sub.s/N.sub.0)*=6 dB. The plot is again repeated for different
selections of the number of colors (L=1, 2, 3, 4, 7). The beam
capacity is obtained by multiplying the average capacity (bps/Hz)
by the total system bandwidth. For a system bandwidth of 2 GHz, the
baseline capacity is about 1 Gbps per beam. In FIG. 18 B, a
baseline is chosen as a four-color system. As it can be seen in
this figure, a one-color satellite system may have 60 percent more
average capacity than a four-color satellite system, up to the
crossover point of about -7 dB.
[0105] FIG. 19A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, at (E.sub.s/N.sub.0)*=12 dB. The plot is repeated for
different selections of the number of colors (L=1, 2, 3, 4, 7). For
comparison, FIG. 19B shows the average capacity (bps/Hz) plotted
against the amount of beam spacing, for a satellite system operated
with ACM, at (E.sub.s/N.sub.0)*=12 dB. The plot is repeated for
different selections of the number of colors (L=1, 2, 3, 4, 7).
[0106] FIG. 20A shows the minimum capacity (bps/Hz) plotted against
the amount of beam spacing, for a satellite system operated without
ACM, with (E.sub.s/N.sub.0)*=0 dB. The plot is repeated for
different selections of the number of colors (L=1, 2, 3, 4, 7). For
comparison, FIG. 20B shows the average capacity (bps/Hz) plotted
against the amount of beam spacing, for a satellite system operated
with ACM, at (E.sub.s/N.sub.0)*=0 dB. The plot is repeated for
different selections of the number of colors (L=1, 2, 3, 4, 7).
[0107] FIG. 21A shows the capacity in bps/Hz per beam plotted
against the signal-to-noise ratio E.sub.s/N.sub.0, for a satellite
system with beam spacing characterized by a roll-off value of -3 dB
at the cross-over point. FIG. 21B shows the capacity in bps/Hz per
beam plotted against E.sub.s/N.sub.0, for a satellite system with
beam spacing characterized by a roll-off value of -6 dB at the
cross-over point.
[0108] FIG. 22 illustrates the density (defined as average capacity
per unit area) for satellite systems having different selections of
the number of colors (L=1, 2, 3, 4, 7). Maximizing the density
requires the use of smaller beam spacing. One way of achieving
smaller beam spacing is the use of more colors, which results in
the reduction of the C/I. As it can be seen, maximizing density
will maximize the capacity into a geographic hot spot, which is
bigger than 1 spot beam, but provides less overall capacity.
[0109] In one embodiment of the present invention, the total
capacity may be optimized by adopting a single-color frequency
re-use beam pattern, along with beam spacing characterized by a
roll-off value of less than -6 dB at the cross-over point. Such a
single-color system may achieve an outbound capacity increase of 60
percent over the four-color baseline system without an increase in
bus power or an increase in bandwidth. A 60-beam single-color
system with beam spacing of 0.47.degree. (vs. 0.32.degree.) can
cover twice the area of the full continental United States of
America (FULL CONUS). And the capacity can be increased further by
reducing coverage areas (the capacity can reach about 2 Gbps per
beam with the same coverage area and bus power as the four-color
system). However, the single-color system does need to increase the
payload TWTs from 60.times.90 Watts to 240.times.23 Watts and
requires 40 gateways for a total bandwidth of 120 GHz (60
beams.times.2 GHz).
[0110] In some other embodiments, the four-color system may get a
better capacity density for hot spots when the crossover point is
about -2 dB. The four-color system is not very sensitive to
crossover points and has essentially the same capacity for
crossover points from -1.5 dB to -7 dB (FIG. 22).
[0111] FIG. 23 presents a four-color system in accordance with one
embodiment of the invention. This system employs two 45 Watt TWTs
per beam for a total of 90 Watt, the same power TWT used in a known
four-color baseline system. The four-color baseline system may use
two frequency ranges 18.3 to 18.8 GHz and 19.7 to 20.3 GHz, each
having a 500 MHz bandwidth. The baseline system may further
polarize the two frequency ranges with a left and right
polarization to achieve the four colors. Due to channel locations,
intermodulation products fall out of band. The orthomode transducer
(OMT) combines the signals from the two 45 W TWTs into s single
waveguide port with the proper waveguide modes at the two frequency
ranges so that a single feed horn may be used to illuminate the
spot beam. The OMT may contribute an 1 dB additional suppression
loss at 0 dB OBO, which is defined as the measured power ratio in
dB between the unmodulated carrier at saturation and the modulated
carrier.
[0112] In one embodiment of the present invention, the four-color
system is used as the baseline with the following parameters: 2.8 m
aperture, beam spacing=0.32.degree., crossover point=-3.5 dB, and a
power flux density (PFD) less than or equal -119 dBW/MHz-m.sup.2.
The single-color system may have the following parameters to cover
the same area: 4.1 m aperture, beam spacing=0.32.degree., crossover
point=-8 dB; the bigger reflector will provide about 3.3 dB more
gain, sufficiently more than needed to compensate for the 1 dB
additional suppression loss due to the OBO. There may be two
options for utilizing this additional gain. Under the first option,
the additional gain is taken as a payload power reduction; for
example, reduce TWT power by 2.3 dB. The transmission power will be
2.times.26 W TWTs per beam (or 3.18 kW vs. 5.4 kW for the
four-color system). The beam capacity is about 1.6 Gbps, and the
PFD per pole is less than -125 dBW/MHz-m.sup.2. Under the second
option, the additional gain is taken as more capacity due to the
increase of E.sub.s/N.sub.0 by 2.3 dB. The transmission power is
2.times.45 W TWTs per beam, the same as the four-color system; but
the achieved capacity is now 1.96 Gbps, and the PFD per pole is
less than -122.7 dBW/MHz-m.sup.2.
[0113] In another embodiment of the present invention, the
single-color system uses a 3.6 m aperture, beam
spacing=0.32.degree., crossover point=-6 dB; the bigger reflector
provides about 2.2 dB more gain, sufficiently more than needed to
compensate for the 1 dB additional suppression loss due to the OBO.
Here again there may be two options for utilizing this additional
gain. Under the first option, the additional gain of 1.2 dB is
taken as a payload power reduction; for example, reduce TWT power
by 1 dB. The total transmission power of the system is then
60.times.68 W TWTs. The beam capacity is about 1.28 Gbps, and the
PFD per pole is less than -122 dBW/MHz-m.sup.2. Under the second
option, the additional gain is taken as more capacity due to the
increase of E.sub.s/N.sub.0 by 1 dB. The transmission power is
60.times.90 W TWTs, the achieved beam capacity is 1.40 Gbps, and
the PFD per pole is less than -120.8 dBW/MHz-m.sup.2.
[0114] In another embodiment of the present invention, a two-color
system uses a 3.6 m aperture, beam spacing=0.32.degree., crossover
point=-6 dB, and a service link bandwidth of 500 MHz. The power is
60.times.54 W TWTs due to the 2.2 dB gain of the bigger reflector.
The E.sub.s/N.sub.0 is 9 dB due to reduced system bandwidth; and
the PFD is less than -119 dBW/MHz-m.sup.2. The beam capacity is
about 0.77 Gbps (about 77 percent of the four-color system). One
way to increase beam capacity is to increase PFD to -118
dBW/MHz-m.sup.2 as extra power is available (60.times.68 W TWTs);
the E.sub.s/N.sub.0 is 10 dB; and the beam capacity is 0.82 Gbps
(82 percent of the four-color baseline system).
[0115] In another embodiment of the present invention, the system
uses a 4.1 m aperture, beam spacing=0.32.degree., crossover
point=-8 dB for a high capacity design. The 4.1 m aperture provides
3.3 dB more gain with no additional suppression loss. The power is
120.times.45 W TWTs (the TWT has the same power as those used in
the four-color baseline system). The E.sub.s/N.sub.0 is 6 dB (the
baseline)+3 dB (bandwidth)+3.3 dB (antenna gain) for a total of
12.3 dB. The PFD is less than -118.7 dBW/MHz-m.sup.2, and the beam
capacity is about 1.24 Gbps (24 percent more than the baseline
system). Even with a 50 percent power reduction of TWTs (22.5 W
TWTs), the achieved beam capacity is still 1.05 Gbps.
[0116] In yet another embodiment of the present invention, the
system uses a 2.8 m aperture, beam spacing=0.47.degree., crossover
point=-8 dB for a high coverage area design (to cover the full
continental United States. The power is 120.times.45 W TWTs (same
as the baseline system). The obtained PFD per pole is less than
dBW/MHz-m.sup.2; the E.sub.s/N.sub.0 is 6 dB (the baseline)+3 dB
(bandwidth) for a total of 9 dB. The achieved beam capacity is 1.04
Gbps. Even with a 50 percent power reduction of TWTs (22.5 W TWTs),
the achieved beam capacity of this system is still 0.8 Gbps.
[0117] FIG. 24 provides a summary of different systems 2410 having
different number of colors 2420, bandwidth per beam 2425, number of
employed gateways 2430, TWT power per beam 2435, number of TWTs per
satellite 2440, the payload aperture 2445, maximum PFD per pole
2450, beam spacing 2455, crossover points 2460, achieved beam
capacity 2470, and relative comparison to a four-color baseline
system LF4 2480. The relative comparison is further divided into
capacity 2485, TWT power 2486, satellite link bandwidth 2487, and
coverage area 2488.
Interference-Dominated Environment
[0118] The present invention makes it possible for satellite
communications to be effectively carried out in an
interference-dominated environment. Here, the term
"interference-dominated" refers to a situation where reception of
signals from a spot beam at a subscriber terminal is affected by
interference from sources that collectively result in a
signal-to-interference ratio C/I, as well as noise at a
signal-to-noise ratio C/N, such that the signal-to-interference
ratio C/I is less than the signal-to-noise ratio C/N. That is,
C/I<C/N. In more serious cases, C/I may be 3 dB or more below
C/N. That is, C/I<C/N-3 dB. Numerous factors may compound to
create such an interference-dominated environment, which has not
confronted previous satellite systems. One factor may be the high
number of service spot beams in the system. From the perspective of
a subscriber terminal receiving signals from a desired spot beam,
interference may come from not only immediately adjacent spot
beams, but also from spot beams beyond the immediately adjacent
spot beams.
[0119] For example, referring to the two-color beam pattern shown
in FIG. 4, a subscriber terminal in one particular spot beam may
receive interference from the six immediately adjacent spot beams
in the beam pattern. Variations in interference may exist, as some
of these six immediately adjacent spot beams may have the same
color as the desired spot beam and therefore cause more
interference than those that have different color as the desired
spot beam. In any case, these six immediately adjacent spot beams
may not represent the only sources of interference. The twelve spot
beams just beyond the six immediately adjacent spot beams may also
introduce interference. These twelve spot beams are farther away
from the subscriber terminal, but they also contribute to the total
interference experienced by the subscriber terminal (although to a
lesser extent). Furthermore, the eighteen spot beams just beyond
the twelve spot beams mentioned above are yet further away but may
nevertheless also introduce interference (although to an even
lesser extent). In this manner, all spot beams in the system other
than the desired spot beam can potentially contribute, in varying
degrees, to the total interference received by the subscriber
terminal--and together they increase the overall interference and
thus lower the signal-to-interference ratio C/I.
[0120] Another factor that may contribute to the presence of an
interference-dominated environment is frequency re-use by service
spot beams. For instance, even if two adjacent service spot beams
utilize different "colors," such two colors may operate in the same
frequency range and only differ by polarization. While appropriate
equipment is used to isolate signals of different polarizations,
perfect isolation may not be achieved. As such, to the extent
isolation is not complete, interference from service spot beams
having a different polarization but the same frequency range as the
desired spot beam may also contribute to further increase the
overall interference and thus lower the signal-to-interference
ratio C/I.
[0121] Yet another factor that may contribute to the presence of an
interference-dominated environment is frequency re-use between
feeder spot beams and service spot beams. As mentioned previously
with reference to FIG. 2, in at least one embodiment of the
invention, a feeder link 140 may have a feeder spot beam that is
geographically separated from the service spot beams. Such
geographic separation theoretically provides the orthogonality
needed to isolate the feeder spot beam from the service spot beams.
As such, a system according to an embodiment of the present
invention may allow the feeder spot beam to re-use the same
frequency and polarization as one or more of the service spot
beams. This facilitates more efficient frequency utilization.
However, as a consequence of this frequency re-use between feeder
spot beams and service spot beams, a subscriber terminal receiving
signals from a desired service spot beam may be affected by
interference coming from a feeder spot beam found at a
geographically separated location from the desired service spot
beam. The severity of this interference varies depending on how far
away the feeder spot beam is located. Such interference from feeder
spot beams may also contribute to further increase the overall
interference and thus lower the signal-to-interference ratio
C/I.
[0122] Techniques described above in various embodiments of the
invention allow the satellite communications system to operate in
such an interference-dominated environment. The novel design of
satellite communication system such as those presented here leads
to a significant amount of interference from signal sources within
the system. Embodiments of the present invention effectively handle
such interference levels among spot beams and accomplish efficient
communication of data within the interference-dominated
environment.
[0123] Specific details are given in the above description to
provide a thorough understanding of the embodiments. However, it is
understood that the embodiments may be practiced without these
specific details. For example, circuits may be shown in block
diagrams in order not to obscure the embodiments in unnecessary
detail. In other instances, well-known circuits, processes,
algorithms, structures, and techniques may be shown without
unnecessary detail in order to avoid obscuring the embodiments.
[0124] While the invention has been described with respect to
exemplary embodiments, one skilled in the art will recognize that
numerous modifications are possible. It will, however, be evident
that various modifications and changes may be made thereunto
without departing from the broader spirit and scope of the
invention as set forth in the claims and that the invention is
intended to cover all modifications and equivalents within the
scope of the following claims.
* * * * *