Title: Method and apparatus for reducing interference between terrestrially-based and space-based broadcast systems
Abstract: A system and method for reducing interference between communications systems sharing at a portion of at least some allocated frequency bands is described. The system comprises a first communication system broadcasting on a first set of broadcast bands having guard bands therebetween, and a second communications system broadcasting on a second set of broadcast bands substantially spanning the guard bands of the first communications system.
Patent Number: 6,975,837 Issued on 12/13/2005 to Santoru
| Inventors:
|
Santoru; Joseph (Agoura Hills, CA)
|
| Assignee:
|
The DIRECTV Group, Inc. (El Segundo, CA)
|
| Appl. No.:
|
348274 |
| Filed:
|
January 21, 2003 |
| Current U.S. Class: |
455/12.1; 455/427; 455/13.1; 725/69 |
| Intern'l Class: |
H04B 007/18.5 |
| Field of Search: |
455/121,311,427,131,430
725/69,49,70,63
|
References Cited [Referenced By]
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| 2004/0166801 | Aug., 2004 | Sharon et al.
| |
Primary Examiner: Trinh; Sonny
Assistant Examiner: Doan; Phuoc
Attorney, Agent or Firm: Grunebach; Georgann S.
Claims
1. A communications system, comprising
a first communications system having a plurality of substantially geosynchronous
satellites broadcasting a first set of information signals to a first plurality
of terrestrial receiver stations;
the first communication system satellites including first communication system
satellite having a first plurality of transponders transmitting a first subset
of information signals on a first subset of the first plurality of broadcast bands
at a first polarization and a second communication system satellite having a second
plurality of transponders transmitting a second subset of the first set of information
signals on a second subset of the first plurality of broadcast bands at a second
polarization substantially isolated from the first polarization;
a second communications system having a plurality of terrestrially-based transmitters
transmitting a second set of information signals to a second plurality of terrestrially-based
terrestrial receiver stations, wherein the first set of information signals each
have independent content from and are spatially and polarizationally diverse from
the second set of information signals;
wherein the first set of information signals are broadcast via one or more of
a first plurality of broadcast bands having guard bands therebetween; and
wherein the second set of information signals are broadcast on one or more of
a second plurality of broadcast bands, each of the second plurality of broadcast
bands associated with and substantially spanning one of the first information signal
guard bands.
2. The communications system of claim 1, wherein at least a portion of the broadcast
bands of the second plurality of broadcast bands are centered at the guard bands.
3. The communications system of claim 2, wherein a bandwidth of at least a subset
of the second plurality of broadcast bands is less than a bandwidth of the associated
band of the plurality of first broadcast bands.
4. The communications system of claim 1, wherein each of the plurality of satellites
are in a geosynchronous orbit.
5. The communications system of claim 1, wherein:
each of the first communication system satellites include a first plurality of
transponders transmitting on a first subset of the first plurality of broadcast
bands and a second plurality of transponders transmitting on a second subset of
the first plurality of broadcast bands, and wherein the guard bands of the first
subset of the first plurality of broadcast bands are substantially centered at
center frequencies of the second subset of the first plurality of broadcast bands.
6. The communications system of claim 1, wherein at least a portion of the second
set of information signals are turbo-coded.
7. The communications system of claim 1, wherein
the at least a portion of the first set of information signals are coded according
to a first coding scheme, and at least a portion of the second set of information
signals are coded according to a second coding scheme different from the first
coding scheme.
8. The communications system of claim 1, wherein the first set of information
signals are modulated according to a first modulation scheme and the second information
signals are modulated according to a second modulation scheme.
9. The communications system of claim 1, wherein a bandwidth of at least a subset
of the second plurality of broadcast bands is greater than a bandwidth of the first
information signal guard bands.
10. A method of broadcasting a first set of information signals from a first
communications system having a plurality of geosynchronous satellites to a first
plurality of terrestrial receiver stations and a second set of information signals,
independent in content and spatially diverse from the first set of information
signals, from a second communications system having a plurality of terrestrially-based
transmitters to a second set of terrestrial receiver stations, comprising the steps of:
broadcasting the first set of information signals via one or more of a first
plurality of broadcast bands having guard bands therebetween having substantially
no information signal; and
broadcasting the second set of information signals via one or more of a second
plurality of broadcast bands, each of the second plurality of broadcast bands associated
with and substantially spanning at least one of the first information signal guard bands.
11. The method of claim 10, wherein each of the second plurality of broadcast
bands are centered at the guard bands.
12. The method of claim 11, wherein a bandwidth of at least a subset of the second
plurality of broadcast bands is less than a bandwidth of the associated guard band
of the plurality of first broadcast bands.
13. The method of claim 10, wherein each of the plurality of satellites are in
a geosynchronous orbit.
14. The method of claim 10, wherein:
each of the first communication system satellites include a first plurality of
transponders transmitting on a first subset of the first plurality of broadcast
bands and a second plurality of transponders transmitting on a second subset of
the first plurality of broadcast bands, and wherein the guard bands of the first
subset of the first plurality of broadcast bands are substantially centered at
center frequencies of the second subset of the first plurality of broadcast bands.
15. The method of claim 10, wherein at least a portion of the second set of information
signals are turbo-coded.
16. The method of claim 10, wherein
the at least a portion of the first set of information signals are coded according
to a first coding scheme, and at least a portion of the second set of information
signals are coded according to a second coding scheme different from the first
coding scheme.
17. The method of claim 10, wherein the first set of information signals are
modulated according to a first modulation scheme and the second information signals
are modulated according to a second modulation scheme.
18. A communications system, usable with a first communications system having
a plurality of substantially geosynchronous satellites broadcasting a first set
of information signals to a first plurality of terrestrial receiver stations wherein
the first set of information signals are broadcast via one or more of a first plurality
of broadcast bands having guard bands therebetween, comprising:
a plurality of terrestrially-based transmitters transmitting a second set of
information signals to a second plurality of terrestrial receiver stations;
wherein the first set of information signals each have independent content from
and ate spatially diverse from the second set of information signals; and
wherein the second set of information signals are broadcast on a second plurality
of broadcast bands, each of the second plurality of broadcast bands associated
with and substantially spanning one of the first information signal guard bands.
19. The communications system of claim 18, wherein at least a portion of the
broadcast bands of the second plurality of broadcast bands are centered at the
guard bands.
20. The communications system of claim 19, wherein a bandwidth of at least a
subset of the second plurality of broadcast bands is less than a bandwidth of the
associated guard band of the plurality of first broadcast bands.
21. The communications system of claim 18, wherein each of the plurality of satellites
are in a geosynchronous orbit.
22. The communications system of claim 18, wherein:
each of the first communication system satellites include a first plurality of
transponders transmitting on a first subset of the first plurality of broadcast
bands and a second plurality of transponders transmitting on a second subset of
the first plurality of broadcast bands, and wherein the guard bands of the first
subset of the first plurality of broadcast bands are substantially centered at
center frequencies of the second subset of the first plurality of broadcast bands.
23. The communications system of claim 18, wherein at least a portion of the
second set of information signals are turbo-coded.
24. The communications system of claim 18, wherein
the at least a portion of the first set of information signals are coded according
to a first coding scheme, and at least a portion of the second set of information
signals are coded according to a second coding scheme different from the first
coding scheme.
25. The communications system of claim 18, wherein the first set of information
signals are modulated according to a first modulation scheme and the second information
signals are modulated according to a second modulation scheme.
26. An apparatus for broadcasting a first set of information signals from a first
communications system having a plurality of geosynchronous satellites to a first
plurality of terrestrial receiver stations and a second set of information signals,
independent in content and spatially diverse from the first set of information
signals, from a second communications system having a plurality of terrestrially-based
transmitters to a second set of terrestrial receiver stations, comprising:
means for broadcasting the first set of information signals via one or more of
a first plurality of broadcast bands having guard bands therebetween having substantially
no information signal; and
means for broadcasting the second set of information signals via one or more
of a second plurality of broadcast bands, each of the second plurality of broadcast
bands associated with and substantially spanning one of the first information signal
guard bands.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to the following co-pending and commonly assigned
patent application(s), all of which applications are incorporated by reference herein:
- application Ser. No. 09/480,089, entitled "METHOD AND APPARATUS FOR
MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH
SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES," filed on Jan.
10, 2000, by Paul R. Anderson, which application claims priority to U.S. Provisional
Application No. 60/169,005, filed Dec. 3, 1999 by Paul R. Anderson, and entitled
"METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING
THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES"; and
- application Ser. No. 09/992,992, entitled "METHOD AND APPARATUS FOR
REDUCING EARTH STATION INTERFERENCE FROM NON-GSO AND TERRESTRIAL SOURCES," by Joseph
Santoru and Ernest C. Chen, filed Nov. 6, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods for broadcasting information
signals, and in particular to a system and method for reducing interference between
independent terrestrially-based and space-based broadcasting systems.
2. Description of the Related Art
It has been proposed to cooperatively share the current Broadcasting-Satellite
Service (BSS) frequency bands to allow additional programming material to be transmitted
to subscribers of other broadcast systems using the same frequency bands as currently
used by BSS satellites. The other systems may be implemented through the use of
non-geostationary orbit (GSO) and/or terrestrially-based transmitters to transmit
the additional programming. Such systems typically rely on spatial diversity to
minimize the probability of interference. This usually requires a BSS satellite
ground antenna having highly directional, monocular sensitivity characteristics
in order to realize low interference levels.
Unfortunately, existing BSS antennae do not exhibit a highly directional
sensitivity characteristic. Instead, as described in application Ser. No. 09/480,089,
entitled "METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS
SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR
SIDELOBES," which application is hereby incorporated by reference, existing BSS
antennae exhibit a sensitivity characteristic that includes substantial sensitivity
in a rearward direction. They also exhibit sensitivity characteristics in the sideward
and upward directions. This sensitivity can result in substantial interference
between transmissions from BSS satellites and transmissions from non-GSO or terrestrial sources.
Solutions have been proposed to reduce interference, including those described
in the related applications described above. However, even when using the methods
described in the above-referenced patent applications, excessive interference with
existing BSS system broadcasts may result.
Two-way communication systems has been proposed which would allow consumers
to transmit narrowband interstitial return path signals within the 12.2-12.7 GHz
frequency bands already used by digital broadcast service providers. However, these
systems limit the interstitial transmission to reverse link transmissions (from
subscribers to terrestrially based antennae). One of the reasons that such systems
have been limited in the past to reverse link transmissions is because it has been
assumed that the limited bandwidth available on the interstitial signals is suitable
only for low data rate transmissions.
Interstitial return link transmissions are typically made possible by
the use of high-directivity ground antennae at the subscriber's location. Such
antennae make it feasible for each subscriber antenna to direct energy in a narrow
beam to the terrestrial antenna, thereby spatial diversity can be used to minimize
transmission interference. However, it is not economically feasible for the terrestrial
antenna to direct an individual beam to each subscriber. Hence, such methods are
generally inapplicable to forward link communications.
What is needed is a further method for isolating current BSS transmissions from
proposed forward link transmissions from terrestrially based transmitters. The
present invention satisfies that need at least in part by taking advantage of the
spatial diversity provided by multiple and spot-beam capable broadcast satellites,
and the use of spatial and polarization diversity to minimize interference while
maximizing forward path transmission bandwidth.
SUMMARY OF THE INVENTION
To address the requirements described above, the present invention discloses a
system and method for reducing interference between terrestrially-based and space-based
communications systems. The system comprises a first communications system having
a plurality of substantially geosynchronous satellites broadcasting a first set
of information signals to a first plurality of terrestrial receiver stations and
a second communications system having a plurality of terrestrially-based transmitters
transmitting a second set of information signals to a second plurality of terrestrially-based
terrestrial receiver stations, wherein the first set of information signals each
have independent content from and are spatially diverse from the second set of
information signals. The first set of information signals are broadcast via one
or more of a first plurality of broadcast bands having guard bands therebetween,
and the second set of information signals are broadcast on one or more of a second
plurality of broadcast bands, each of the second plurality of broadcast bands associated
with and substantially spanning one of the first information signal bands.
The method comprises the steps of broadcasting the first set of information signals
via one or more of a first plurality of broadcast bands having guard bands therebetween
having substantially no information signal; and broadcasting the second set of
information signals via one or more of a second plurality of broadcast bands, each
of the second plurality of broadcast bands associated with and substantially spanning
one of the first information signal guard bands.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings in which like reference numbers represent
corresponding parts throughout:
FIG. 1 is a diagram illustrating an overview of a satellite communication system 100;
FIG. 2 is a diagram illustrating a representative transmission spectrum;
FIGS. 3A-3D are diagrams illustrating different embodiments of a composite
communication system;
FIG. 4 is a diagram illustrating broadcast and guard bands of a composite communication
system; and
FIG. 5 is a diagram illustrating another embodiment of broadcast and guard bands
of a composite communications system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the following description, reference is made to the accompanying drawings
which
form a part hereof, and which show, by way of illustration, several embodiments
of the present invention. It is understood that other embodiments may be utilized
and structural changes may be made without departing from the scope of the present invention.
SATELLITE COMMUNICATION SYSTEM
FIG. 1 is a diagram illustrating an overview of a satellite communication system
100, which can be used to broadcast a first set of information signals having
broadcast video programs, data, or other information to a plurality of subscribers.
The satellite communication system 100 comprises a control center 102
in communication with an uplink center 104 via a ground or other link 114
and one or more integrated receiver/decoders (IRDs) 132A and 132B
(hereinafter alternatively referred to as IRD(s)
- 132) at a plurality of receiver stations 130A and 130B (hereinafter
alternatively referred to as receiver station(s) 130) via a public switched telephone
network (PSTN) or other link 120. The control center 102 provides one or more of
the first set of information signals to the uplink center 104, and coordinates
with the receiver station(s) 130 to offer subscribers 110 pay-per-view (PPV) program
services, including billing and associated decryption of video programs.
The uplink center 104 receives data and/or program control information
from the control center 102, and using an uplink antenna 106, transmits
a first set of information signals via a first set of broadcast bands having guard
bands therebetween to one or more satellites 108A and 108B (hereinafter
alternatively referred to as satellite(s) 108). The satellite(s) 108
receive and process this information, and transmit the data received from the uplink
center 104 to the IRD(s) 132 at the receiver station(s) 130
via downlinks 118A and 118B (hereinafter alternatively referred to
as downlink(s) 118). The IRD(s) 132 receive this information using
the subscriber antennae 112A and 112B (hereinafter alternatively
referred to as subscriber antenna(e) 112), to which each of the IRD(s) 132
are communicatively coupled.
As shown in FIG. 1, satellite 108A includes a first (odd) transponder bank
107A, which includes a plurality of transponders T1, T3, T5,
and T7. Also, satellite 108B includes a second (even) transponder
bank 107B, which includes a second plurality of transponders T2,
T4, and T6. The transponders T1-T7 each accept a signal
from the uplink center 104 and retransmit (e.g. broadcast) the received
signal to the receiver stations 110.
The transponders T1-T7 typically implement a "bent-pipe" communication
system. That is, transponders T1-T7 typically receive and downconvert
the signal from the uplink frequency to a downlink frequency, and retransmit that
downconverted signal to the receiver stations 110. In one embodiment, the
uplink signals are downconverted from the 17.3-17.8 GHz frequency band to the 12.2-12.7
GHz frequency band. Non-bent-pipe communication systems can also be used. For example,
if desired, the satellites 108 may include modules for demodulating the
uplink signal and remodulating the demodulated uplink signal to generate the downlink
signal. Additional processing can also be performed using the received signal.
In one embodiment, the satellites 108A and 108B are disposed in
a substantially geosynchronous (GEO) or geostationary (GSO) orbit. That is, the
satellites are deployed so that their apparent position in the sky, when viewed
by the receiver stations 110 either remains in one place does not move beyond
the effective beamwidth of the receiving antenna 112.
The plurality of satellites 108A and 108B can be used to provide
wider terrestrial coverage, to provide additional channels, or to provide additional
bandwidth per channel to subscribers. Although six transponders (T1-T7)
are shown, a greater or lesser number of transponders may be used. In one embodiment
of the invention, each of the satellite(s) 108 comprise 16 transponders
to receive and transmit program material and other control data from the uplink
center 104 and provide it to the subscribers 110. However, the transmission
capacity of each transponder T1-T7 can be increased using advanced
modulation and coding, data compression, and multiplexing techniques. For example,
two-satellites 108 working together can receive and broadcast over 150 conventional
(non-HDTV) audio and video channels via 32 transponders.
The uplink station 104 can transmit the same uplink information to both
satellites 108A and 108B, or different information (e.g. only the
information that is retransmitted by the satellite 108) to each satellite
108. This can be accomplished using spatial diversity (e.g. a directional
antenna 106 which is directed only at the selected satellite (108A
or 108B, but not both) and additional uplink stations 104, if desired.
FIG. 2 is a diagram illustrating a representative radio frequency (RF) transmission
spectrum for transponders T1-T7.
Each transponder T1-T7 transmits in an assigned broadcast band
202(1)-202(7). The broadcast bands 202(1)-202(7)
(hereinafter alternatively referred to as broadcast band(s) 202) are separated
from adjacent broadcast bands by guard bands 204(1-3)-204(5-7)
(hereinafter alternatively referred to as broadcast band(s) 204). For example,
broadcast bands 202(1) and 202(3) (used by transponders
T1 and T3, respectively) are separated by guard band 204(1-3).
The guard bands 204(1-3)-204(5-7) prevent
signals from the broadcast bands 202(1)-202(7) from
interfering with one another. The width of the guard bands 204(1-3)-204(5-7)
is selected to provide adequate isolation between broadcast bands 202(1)-202(7)
to prevent unacceptable interference. This is typically a function of the technique
used to modulate the signal on broadcast bands 202(1)-202(7),
the quality of the transponders T1-T7 and receiver stations 130,
as well as other factors. In one embodiment, the broadcast bands are 24 MHz wide,
whereas the guard bands are 5.16 MHz wide.
In the illustrated example, guard bands for the odd transponders T1, T3,
T5, and T7 are centered at the center of the broadcast bands for
the even transponders T2, T4, and T6. Hence, guard band 204(1-3)
is centered at the center of broadcast band 202(2), guard band 204B(2-3)
is centered at the center of broadcast band 202(3) and so on.
It is noted that in the spectrum shown in FIG. 2, each of the broadcast bands
202(1)-202(7) share a portion of a frequency spectrum
with another of the broadcast bands 202(1)-202(7).
For example, broadcast band 202(1) shares a spectrum band 206
with broadcast band 202(2). With such a system, techniques are employed
to prevent interference between the broadcast bands. This can be accomplished by
via selection of appropriate modulation or multiplexing techniques, spatial diversity,
and polarization diversity, or a combination of such techniques. For example, if
antenna 112B is directed at satellite 108B and is sufficiently directional
to exclude significant energy from the first satellite 108A, broadcast band
202(1), used by transponder T1 on satellite 108A, will
not interfere with broadcast band 201(2), which is used by transponder
T2 on satellite 108B. In another embodiment broadcast bands 202(1)-202(7)
can be distinguished from one another via polarization or modulation technique
(e.g. QPSK, 8PSK, CDMA).
In one embodiment, the center frequency of the broadcast band for transponder
T1 is 12.224 GHz and the guard band between adjacent transponders is 5.16 MHz.
FIG. 3A is a diagram illustrating a top-view of a composite communication system
300, which includes a first, the satellite-based communication system 100
described above and a second, terrestrially-based communication system 320.
The second communications system 320 includes a first terrestrially-based
transmitter 302A and a second terrestrially based transmitter 302A
(hereinafter alternatively referred to collectively as terrestrial transmitter(s)
302). The terrestrial transmitter(s) transmit a second set of information
signals to a plurality of second subscribers 310 via second communication
system subscriber antennae 306 and links 308A and 308B. The
terrestrially-based communication system 320 transmits a second set of information
signals on one or more of a second plurality of broadcast bands.
Spatial diversity can be used to reduce interference between the first communication
system 100 and the second communications system 320. This is accomplished
by selecting the transmission characteristic 304A and 304B of the
transmitter(s) 302 to direct the second set of information signals in a
direction away from the sensitive axis of first communication system subscriber
antennae 112 and in a direction towards the sensitive axis of the second
communication system subscriber antennae 306 sensitive axis.
FIG. 3B is a side view of the composite communication system illustrated in
FIG. 3A.
FIG. 3C is a top view of a second embodiment of the composite communication
system. In this embodiment, spatial diversity is also used to reduce interference
between the first communication system 100 and the second communication
system 320, however, this spatial diversity is accomplished by unique sensitivity
characteristics of the subscriber antennae 112 which have a "hole" (reduced
sensitivity) along vectors from the subscriber antennae to the other communication
system (e.g. subscriber antenna 112 has reduced sensitivity along a vector
from the subscriber antenna 112 to the transmitter 302 of the second
communication system 320, and/or the subscriber antenna 306 has a
reduced sensitivity along a vector from the subscriber antenna 306 to the
first communication system 100.
FIG. 3D is a side view of the composite communication system illustrated in
FIG. 3C.
FIG. 4 is a diagram illustrating a relationship between the first set of broadcast
bands 202(1), 202(3), 202(5), and 202(7)
second plurality of broadcast bands 320(1)-320(4) that
are used by the second communication system 320 to transmit the second set
of information signals. Since the terrestrial broadcast bands 320(1)-320(4)
coincide with those of the first communications system 100 (in particular,
with bands 202(1), 202(3), 202(5), and
202(7) associated with the odd transponders 1, 3, 5,
and 7), the frequency diversity is not assured, and the first set of information
signals may interfere with the second set of information signals and vice versa.
Interference is also possible between bands 202(2), 202(4),
and 202(6) and terrestrial broadcast bands 320(1)-320(4).
However, since there is less overlap of the broadcast bands (due to the guard bands),
interference between these bands will generally be less.
The second plurality of broadcast bands of the second communications system 320
correspond to the odd numbered transponders of the first communications system
100. That is, the first broadcast band 320(1) (e.g. channel
1) would completely overlap broadcast band 202(1) (24 MHz
total bandwidth). Although spatial diversity can somewhat ameliorate the problem,
if the polarization of the second communications system 320 were linear
and the polarization of channels one and two of the first communications system
100 were left and right circularly polarized, respectively, channel one
of the second communications system 320 may introduce interference into
both channel one and channel two of the first communications system 100.
FIG. 5 is a diagram illustrating another relationship between the first set
of broadcast bands 202(1)-202(7) and a second set of
broadcast bands 502(1)-502(8). Each pair of the second
set of broadcast bands 502(1)-502(8) (hereinafter alternatively
referred to as broadcast band(s) 502) includes a guard bands 504(1-2),
504(2-3), 504(3-4), 504(4-5),
504(5-6), 504(6-7), 504(7-8),
and 504(8-9), (hereinafter alternatively referred to as guard
band(s) 504) disposed therebetween. For example, guard band 502(1-2)
is disposed between broadcast bands 502(1) and 502(2)
and guard band 502(2-3) is disposed between broadcast bands
502(2) and 502(3).
Band-assignment parameters (e.g. number, distribution, and bandwidth
of broadcast bands as well as the number, distribution, and bandwidth of the guard
bands) can be chosen to minimize disruption of the data transmission and interference
objectives of the first communications system 100 while simultaneously providing
for data communications using the second communications system 320. In one
embodiment, the bandwidth of the broadcast bands of the second communication system
320 is selected as substantially the same bandwidth as the guard bands of
the first communication system 100, and the number and distribution of the
broadcast bands of the second communications system 320 are selected to
coincide with the guard bands of the first communications system 100. Preferably,
at least one of the broadcast bands 502 of the second communications system
320 substantially span the associated guard band 204 of the first
communications system 100. In one exemplary embodiment, the guard bands
204 of the first communications system 100 and the broadcast bands
502 of the second communications system 320 are both 5 MHz wide,
and centered at the same frequencies. The frequency overlap between the second
broadcast band 502(2) of the second communications system 320
and the first broadcast band 202(1) (transponder T1) of the
first communications system 100 is 5 MHz. In this situation, the overlap
between the broadcast band 502(2) and 202(1) is 5 MHz/24
MHz or 0.208 (or about -6.8 dB). Hence, if the power per Hz first communications
system broadcast band 202(1) and the second communications system
broadcast bands 502(2) are equal, the power of the signal from the
second communications system 302 (broadcast using broadcast band 502(2))
within the first communication system broadcast band 202(1) is about
6.8 dB less than that of the signal from the first communication system 100.
Further, since second communication system broadcast bands 502(1)
and 502(3) are within the guard bands (e.g. 204(0-1)
and 204(1-3), the interference contribution from the channels
using broadcast bands 502(1) and 502(3) will be much
smaller than the contribution from channels using broadcast band 502(2).
A reduction of -6.8 dB is significant, and has the effect of reducing the distance
at which any specified interference threshold is found by more than one half. For
example, if it is found that the threshold for harmful interference from the second
communications system 320 on the first communications system 100
is at a distance of 4 kilometers from the transmission source 302 of the
second communications system 320, the foregoing technique would reduce this
distance to approximately
##EQU1##
thus permitting service to more customers and/or reducing interference in existing
broadcast satellite services. Similarly, the level of separation may be maintained,
yet interference with the first communication system 100 reduced accordingly.
The bandwidth of the broadcast bands 502 of the second communications
system 320 can be selected to be greater or less than that of the guard
bands 204 of the first communications system 100. All other things
equal, increasing the bandwidth of the broadcast bands 502 relative to the
width of the guard bands 204 will increase interference between the first
communications system 100 and the second communications system 320,
and decreasing the bandwidth of the broadcast bands 502 relative to the
width of the guard bands 204 will decrease interference, but will also reduce
data throughput in the second communications system 320 channels. It should
also be noted that while the frequency bands are presented with infinite rising
and declining slopes (e.g. as if implemented with infinite pole filters), the actual
delineation between broadcast bands and guard bands is not typically as well-defined,
but rather, defined in terms of thresholded amplitude (e.g. a broadcast band may
be defined as the band in which the signal magnitude exceeds -20 dB or other value
from the amplitude at the center of the broadcast band). Indeed, one of the reasons
for using the guard bands 204 of the first communication system 100
between broadcast bands 202 is to account for this characteristic and assure
that there is adequate separation between broadcast bands so as to minimize interference.
Hence, the phrase "substantially span" as it is used above, refers to a relationship
between the bandwidth and center frequency of each of the broadcast band(s) 502
of the second communications system 320 to the bandwidth and center frequency
of the related guard bands 204 of the first communications system 100
such that the transmission capacity of the second communications system 320
on such broadcast bands 502 meets design objectives, while also minimizing
interference to the broadcast bands 202 of the first communication system
100 to an acceptable degree. Thus, the bandwidth of the broadcast bands
502 of the second communications system 320 may be less than that
of the bandwidth of the guard bands 204 of the first communication system
100 (e.g. to account for the non-ideal behavior described above), or may
be greater than the bandwidth of the guard bands 204 of the first communication
system (e.g. allowing some frequency overlap-induced interference that can be either
accepted or ameliorated by other techniques, including spatial diversity, polarization
diversity, or modulation techniques).
The data transmission capacity of the second communications system 320
can be selected to maximize data throughput and minimize interference. For example,
if the channel allocation for the second communication system 320 shown
in FIG. 4 included 16 channels with 24 MHz bandwidth and quadrature phase-shift
keying (QPSK) modulation, it could achieve a payload data rate of approximately
30.3 Mbits/sec. With a 6/7 code rate, a total payload data rate would be approximately
16×30.3 Mbits/sec=48.4 Mbits/sec. However, if the channel allocation shown
in FIG. 5 is used instead, the data rate (assuming the same modulation technique
and code rate) would be approximately 30.3*(5/24)=6.3 Mbits/sec per channel. Using
34 channels would provide a total payload data rate of approximately 6.3×34=214.2
Mbits/sec. This is about 44% of the total payload data rate that is obtainable
with the system shown in FIG. 4.
However, the data throughput of the second communication system 320
can be improved through judicious choice of channel center frequency and bandwidth
allocation, in order to account for different modulation techniques and/or signal
characteristics between the first communication system 100 and the second
communications system 320. Further, the modulation and/or coding of the
signal from the second communications system 320 and the first communications
system 100 can be selected to further minimize interference and/or provide
additional data throughput. For example, a data rate of about 38 Mbits/sec could
be realized using the same carrier to noise (C/N) ratio needed for QPSK 6/7 modulation
and coding. Hence, a channel on a 5 MHz bandwidth broadcast band of the second
communication system 320 could have a payload data rate of about 38*(5/24)=7.9
Mbits/sec, thus providing about 268.6 Mbits/sec on 34 channels. This is approximately
55% of the total payload rate that is obtainable with the system shown in FIG. 4.
Further techniques can be employed to maximize the data transmission capacity
of the second communications system 320, including, for example, turbo-coding
techniques. Such techniques are described in C. Berrou, A. Glavieux, and P. Thitimajshima,
"Near Shannon limit error-correcting coding and decoding: Turbo-codes(1),", in
Proc. ICC'93, Geneva, Switzerland, May 1993, pp. 1064-1070", which is hereby incorporated
in reference.
Although the foregoing has been described with respect to an embodiment
in which the program material delivered to the subscriber is video (and audio)
program material such as a movie, the foregoing method can be used to deliver program
material comprising purely audio information or data as well.
Also, the present invention can be used to prevent communication interference
between existing satellite subscription services and other services that use space-based
(e.g. Middle Earth Orbit or Lower Earth Orbit), or suborbital (e.g. atmospherically
based) subscription services.
CONCLUSION
The foregoing description of the preferred embodiment of the invention has been
presented for the purposes of illustration and description. It is not intended
to be exhaustive or to limit the invention to the precise form disclosed. Many
modifications and variations are possible in light of the above teaching. For example,
while the invention disclosed herein will be described with reference to a satellite
based satellite communication system 100, the present invention may also
be practiced with terrestrial-based transmission of program information, whether
by traditional broadcasting means, cable, or other means. Further, the different
functions collectively allocated among the control center 102 and the uplink
center 104 as described above can be reallocated as desired without departing
from the intended scope of the present invention.
It is intended that the scope of the invention be limited not by this detailed
description, but rather by the claims appended hereto. The above specification,
examples and data provide a complete description of the manufacture and use of
the composition of the invention. Since many embodiments of the invention can be
made without departing from the spirit and scope of the invention, the invention
resides in the claims hereinafter appended.
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