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Claims  |
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What is claimed:
1. A method of conducting video communications between a first site and a
second site comprising the steps of:
(a) digitally compressing an input video signal, said video signal
containing the entirety of the contents of a video image having no
frequency information removed therefrom, to derive a compressed bandwidth
video signal;
(b) modulating a carrier signal, to be transmitted from said first site to
said second site, with said compressed bandwidth video signal;
(c) spreading the spectral density of the modulated carrier signal obtained
in step (b) and thereby causing the spectral density of the entirety of
the image information contents of said video image, as contained in said
compressed bandwidth video signal to be spread and transmitting the
resulting spread signal to said second site; and
(d) at said second site, receiving the spread signal transmitted in step
(c) by way of a phased array antenna.
2. A method according to claim 1, wherein step (d) comprises receiving the
spread signal transmitted in step (c) by way of an electronically steered,
circular aperture phased array antenna.
3. A method of transmitting video communication signals from a first
station by way of a satellite communication link to an airborne receiving
stations comprising the steps of:
(a) compressing input video communication signals, said video communication
signals containing the entirety of the contents of a video image having no
frequency information removed therefrom, to derive compressed video
communication signals;
(b) modulating a carrier signal, to be transmitted from said first station
to said airborne receiving station, with said compressed video
communication signals;
(c) spreading the spectral density of the modulated carrier signal obtained
in step (b) and thereby causing the spectral density of the entirety of
the image information contents of said video communication signals as
contained within said compressed video communication signals to be spread,
and transmitting the resulting spread signal to said airborne receiving
station by way of said satellite communications link; and
(d) at said airborne receiving station, receiving the spread signal
transmitted in step (c) by way of a phased array antenna.
4. A method according to claim 3, wherein step (d) comprises receiving the
spread signal transmitted in step (c) by way of an electronically steered,
circular aperture phased array antenna.
5. A method according to claim 4, wherein step (d) comprises receiving the
spread signal transmitted in step (c) by way of a phased array antenna,
the physical configuration of which is conformal with an airframe surface
of said airborne receiving station.
6. A method according to claim 5, wherein step (d) comprises controlling
the operation of said phased array antenna such that its polarization
response is effectively aligned with that of the spread signal received
thereby.
7. A method according to claim 6, wherein said step (d) of controlling the
operation of said phased array antenna such that its polarization response
is effectively aligned with that of the spread signal received thereby
includes adjusting respective weights of antenna elements of said array in
accordance with the geometry of the surface over which the antenna
elements of said phased array antenna are distributed.
8. A method of transmitting video communication signals from an airborne
transmitting station by way of a satellite communication link to a
receiving station comprising the steps of:
(a) compressing input video communication signals, said video communication
signals containing the entirety of the contents of a video image having no
frequency information removed therefrom, to derive compressed video
communication signals;
(b) modulating a carrier signal, to be transmitted from said airborne
station to said receiving station, with said compressed video
communication signals; and
(c) spreading the spectral density of the modulated carrier signal obtained
in step (b) and thereby causing the spectral density of the entirety of
the image information contents of said video communication signals as
contained within said compressed video communication signals to be spread,
and transmitting, by way of a phased array antenna, the resulting spread
signal from said airborne transmitting station by way of said satellite
communications link to said receiving station.
9. A method according to claim 8, wherein the physical configuration of
said phased array antenna is conformal with an airframe surface of said
airborne station.
10. A communication system for transmitting video communication signals
over a satellite communication link from a first station to an airborne
receiving station comprising:
at said first station,
a video signal compression unit which is operative to compress input video
communication signals supplied thereto, said video communication signals
containing the entirety of the contents of a video image having no
frequency information removed therefrom, and output therefrom compressed
video communication signals, and
a spread spectrum transmitter which is operative to modulate a carrier
signal, to be transmitted from said first station to said airborne
receiving station, with compressed video communication signals output from
said video compression unit and to spread the spectral density of the
modulated carrier signal and thereby causing the spectral density of the
entirety of the image information contents of said video communication
signals as contained within said compressed video communication signals to
be spread, for transmission to said airborne receiving station by way of
said satellite communications link; and
at said airborne receiving station,
an electronically steerable phased array antenna, the output of which is
coupled to a spread spectrum receiver.
11. A communication system according to claim 9, wherein said airborne
station comprises an aircraft and wherein said electronically steerable
phased array antenna is substantially conformal with an external body
portion of said aircraft.
12. A communication system according to claim 9, wherein said video
communication signals include teleconference communication signals and the
aircraft has an on-board video signal compression unit which is operative
to compress input video communication signals, supplied thereto from an
on-board video camera, output therefrom as compressed video communication
signals, and an airborne spread spectrum transmitter which is operative to
modulate a carrier signal, to be transmitted from said airborne station,
with compressed video communication signals output from said airborne
video compression unit and to spread the spectral density of the modulated
carrier signal for transmission to said first station by way of said
satellite communications link.
13. A communication system according to claim 11, including an antenna
controller which is operative to control the operation of said
electronically steerable phased array antenna such that its polarization
response is effectively aligned with that of the spread signal received
thereby.
14. A communication system according to claim 13, wherein said antenna
controller is operative to adjust respective weights of antenna elements
of said array in accordance with the geometry of a conformal surface over
which the antenna elements of said phased array antenna are distributed.
15. A communication system for conducting video communications between a
first site and a second site comprising:
at said first site,
a video signal compression unit which is operative to compress an input
video signal supplied thereto, said video signal containing the entirety
of the contents of a video image having no frequency information removed
therefrom, and to output a compressed video signal;
an encoding modulator which is operative to modulate said compressed video
signal in accordance with a prescribed modulation format and to spread the
spectral density of the modulated signal and thereby causing the spectral
density of the entirety of the image information contents of said video
image, as contained in said compressed bandwidth video signal to be
spread;
a transmitter which is operative to transmit the spread modulated signal
via a transmission channel to said second site; and
at said second site,
a receiver which is operative to receive the spread signal, transmitted
from said first site, by what of a phased array antenna.
16. A communication system according to claim 15, wherein said receiver is
operative to receive the spread signal, transmitted from said first site,
by way of an electronically steered, circular aperture phased array
antenna.
17. A communication system according to claim 15, wherein said receiver is
installed on board an aircraft for reception of said spread signal via a
satellite communications link.
18. A communication system according to claim 15, wherein said receiver is
installed aboard an aircraft and wherein said phased array antenna is
conformal with an airframe surface of said aircraft.
19. A communication system according to claim 18, including an antenna
controller which is operative to control the operation of said phased
array antenna such that its polarization response is effectively aligned
with that of the spread signal received thereby.
20. A communication system according to claim 19, wherein said antenna
controller is operative to adjust respective weights of antenna elements
of said array in accordance with the geometry of the airframe surface over
which the antenna elements of said phased array antenna are distributed.
21. A communication system according to claim 15, wherein said aircraft
further includes an airborne video signal compression unit which is
operative to compress input video communication signals, supplied thereto
from an on-board video camera, output therefrom as compressed video
communication signals, and an airborne spread spectrum transmitter which
is operative to modulate a carrier signal, to be transmitted from said
airborne station, with compressed video communication signals output from
said airborne video compression unit and to spread the spectral density of
the modulated carrier signal for transmission to said first station.
22. For use with a communication system having a phased array antenna that
is conformal with a non-planar surface, a method of controlling the
operation of said phased array antenna such that its polarization response
is effectively aligned with that of a signal received thereby, comprising
adjusting respective weights of antenna elements of said array in
accordance with the geometry of the non-linear surface over which the
antenna elements of said phased array antenna are distributed.
23. A method according to claim 22, wherein said non-linear surface
corresponds to a surface that is conformal with an aircraft surface.
24. A method according to claim 22, wherein outputs of said phased array
are coupled to a summation channel receiver and to an auxiliary
polarization channel receiver, and wherein outputs of said receivers are
combined to derive an error signal which is coupled in a feedback loop to
modify a geometry-based weighting mechanism through which the weights of
respective ones of said antenna elements are adjusted, whereby said error
signal is effectively driven to zero.
25. A method according to claim 1, wherein step (c) comprises transmitting
said spread signal over a common antenna beam to said second site, and
wherein step (d) comprises receiving the spread signal transmitted in step
(c) over said common antenna beam and detecting said spread signal by way
of said phased array antenna.
26. A method according to claim 3, wherein step (c) comprises transmitting
said spread signal over a common antenna beam to said airborne receiving
station, and wherein step (d) comprises receiving the spread signal
transmitted over said common antenna beam and deriving said spread signal
by way of said phased array antenna.
27. A method according to claim 8, wherein step (c) comprises transmitting
said resulting spread signal by way of a common antenna pattern derived
from said phased array antenna.
28. A communication system, according to claim 10, wherein said spread
spectrum transmitter is operative to transmit a spectral density-spread
modulated carrier signal over a common antenna beam to said airborne
receiving station, and wherein said electronically steerable phased array
antenna at said airborne receiving station is operative to receive said
common antenna beam into derived said spectral density-spread modulated
carrier signal therefrom for application to said spread spectrum receiver.
29. A communication system according to claim 15, wherein said first site
further includes an antenna, coupled to said transmitter and being
operative to transmit said spread modulated signal over a common antenna
beam to said second site, and wherein said phased array antenna at said
second site is operative to receive said common antenna beam transmitted
from said first site, and to derive therefrom said spread signal for
application to said receiver. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates in general to communication systems and is
particularly directed to system architecture and communication methodology
that significantly reduces the size of the aircraft antenna required to
provide full broadcast quality video to (or from) an aircraft via a
satellite communications link.
BACKGROUND OF THE INVENTION
Conventional schemes for conducting video communications by way of a
satellite link use analog modulation formats, which require a very large
information bandwidth in order to achieve the full motion and resolution
that is characteristic of `broadcast quality` video. Due to international
restrictions and FCC regulations placed upon satellite transmission power
spectral density, it is necessary to use a physically large receive
antenna with these traditional wideband analog modulation formats, in
order to achieve the high signal-to-noise ratio associated with broadcast
quality video.
Another factor that mandates the use of a physically large video receive
antenna is the need to reject (interfering) transmissions from other
satellites which are near the satellite sourcing the video. Because
commercial satellites can be spaced as closely as 2.degree. (in longitude)
from one another, the antennas utilized in the receive link from these
satellites are typically designed to have a null-to-null beamwidth of less
than 4.degree. (+/-2.degree.). Such a narrow beamwidth requires a
considerably large antenna aperture at the allocated commercial satellite
operating frequencies.
Unfortunately, the need to install a large geometry antenna on the aircraft
is one of the greatest obstacles incurred to date in attempting to receive
broadcast quality video from satellites. This has generally rendered the
antenna, and therefore the communication system, to be impractical,
because of size, cost, power and/or weight constraints associated with the
aircraft. Indeed, the use of a purely mechanically steered aircraft
antenna for this application is generally precluded, since most aircraft
have limited space on board and the fact that a mechanically steered
antenna requires a volume larger than that of the antenna itself, in order
to accommodate steering over the range of pointing angles required to
maintain communications during normal aircraft flight maneuvers.
To significantly reduce the volume required and to allow placement of the
antenna on or near the aircraft's skin, an electronically steered (phased
array) antenna (or one that is at least partially electronically steered)
is preferred. Electronic scanning, however, affects the antenna aperture
area required, since the gain of a phased array antenna configuration
decreases (the beamwidth widens) as the antenna is electronically scanned
off-boresight. For example, at a scan angle of 60.degree., the gain may
drop by approximately 5 dB from what is achievable at boresight. This
reduction in gain must generally be compensated by an increase in antenna
aperture area (e.g. by a factor of more than three to recoup the five dB
loss). Hence, although the phased array antenna occupies a much smaller
volume than a mechanically steered antenna, there is still a strong
incentive to reduce the required antenna aperture.
SUMMARY OF THE INVENTION
In accordance with the present, the size of the antenna can be
significantly reduced, thereby greatly increasing the practicality of
conducting satellite-linked broadcast quality video communications with an
aircraft, by means of a combination of video bandwidth compression, spread
spectrum waveform processing, forward error correction coding and circular
aperture phased array antenna technology. By combining the signal
processing methodologies with a phased array antenna, there is realized a
communication which ensures that sufficient signal power can be received
at the aircraft, interference from other satellites can be rejected and
the power spectral density of the satellite's video transmission can be
kept within FCC requirements while, at the same time, using a
significantly smaller aircraft antenna aperture than would otherwise be
possible.
In accordance with the communication mechanism employed by the present
invention, video signals to be transmitted to the aircraft, which can
originate on the ground from any of a number of potential sources, such as
a TV-receive only satellite receiver, cable, etc. are initially digitized
and compressed. The compression operation reduces the data rate of the
digitized video (which, for example, may be on the order of 100 Mb/s) by
nearly two orders of magnitude (with present day technology), while
maintaining the full motion and resolution associated with broadcast
quality video. The video compression reduces the information bandwidth
which, in turn, reduces the receive aperture size required to maintain a
given bit error rate (assuming all other factors remain the same). The
compressed information bandwidth also improves spread spectrum processing
gain.
The digitized compressed video signal can be encoded for forward error
correction and then spread spectrum-modulated onto a carrier. The use of
error correction coding in conjunction with efficient (e.g. coherent
PSK-type or MSK) data modulation further reduces the aperture size for a
given bit error rate.
The power spectral density of the modulated signal is reduced via the
spread spectrum processing. Spread spectrum processing provides several
benefits: reduced power spectral density (for FCC compliance), privacy (to
prevent unauthorized users from demodulating the video signal) and it
enables the receiver on the aircraft to reject interfering transmissions
from other satellites. Spread spectrum processing can take the form of
direct PN sequence modulation and/or frequency hopping, for example.
The spread signal is then transmitted from the ground to a relay satellite.
The relay satellite retransmits the spread signal through a transmission
zone (e.g. continental U.S. conical coverage) within which the aircraft is
travelling. The aircraft receives the satellite's transmission via a
compact phased array antenna which is preferably conformally configured so
that it may be mounted on the fuselage of the aircraft. The phased array
antenna provides the required amount of antenna gain, while occupying less
volume than would a purely mechanically steered antenna. The phased array
antenna may be totally electronically scanned or it may only be partially
electronically scanned. An example of a phased array which is only
partially electronically scanned is one which scans electronically in one
dimension (e.g. elevation) and mechanically (e.g. rotational) in the
other. The face (aperture) of such an antenna may be parallel to the plane
of rotation or may be tilted. This architecture still provides significant
volume reduction as compared with a purely mechanically scanned antenna.
In a preferred embodiment where the antenna can be mounted conformal with
the aircraft surface, the antenna is mounted on the top of the fuselage as
two phased arrays, one on the port side and one on the starboard side of
the aircraft, so as to provide maximal spatial coverage with the satellite
regardless of the attitude of the aircraft. A single antenna could be used
in place of the port-starboard pair in situations where a more restricted
beam scanning volume is acceptable. Conformal mounting provides additional
benefits, such as minimal visibility of the antenna, no consumption of
cabin space, minimal aerodynamic drag, etc. The antenna aperture is
approximately circular so as to reduce the antenna sidelobe levels. This
minimizes interference with respect to satellites that are neighbors to
the satellite being used.
A monopulse comparator difference channel is employed to control antenna
aiming so as to keep the phased array pointed at the satellite regardless
of the attitude of the aircraft. The output of the antenna is despread,
demodulated, (optionally) decoded and decompressed for use on board the
aircraft.
Where the aircraft has an on board video source, such as a video
teleconference system, the same basic communication techniques employed
for ground-to-air video transmissions are employed for the transmission of
video from the aircraft. Compression of the video on the aircraft reduces
the required e.i.r.p. from the aircraft and increases attainable spread
spectrum processing gain. Spectrum spreading reduces the spectral density
of the transmitted signal, which reduces the transmit antenna's aperture
size required to allow the transmitted signal to remain within FCC
requirements, so as not to interfere with other satellites.
In addition to maintaining the phased array antenna on board the aircraft
pointed at the satellite, it is necessary to maintain the polarization of
the receive and transmit arrays aligned with those of the relay satellite.
For this purpose the output of each antenna element preferably drives a
polarizing network containing respective vertical and horizontal
polarization associated 90.degree. hybrids and two phase shifters. The
phase shift elements are operative to rotate the polarizations of the
input waveforms output by the antenna elements, so that any linear
polarization can be obtained at the hybrid outputs. Corresponding ports of
each 90.degree. hybrid are summed together. The resulting amplitude and
phase of the summation output is proportional to the sine and cosine of
the angular error between the phase shifter settings and the angular
offset of the phased arrays relative to the polarization. Other acceptable
means of varying the antenna polarization include mechanically adjustable
polarizers (which are especially applicable for the hybrid
electro-mechanical array mentioned previously). The summation outputs are
demodulated in respective `polarization channel` and `data channel`
receivers. The `data channel` receiver is used to phase lock the `
polarization channel` receiver. Functionally, the outputs of the
respective receivers are then multiplied together in a mixer to derive an
error signal which is a function of the sine of twice the angular
polarization error. (In accordance with a preferred implementation,
multiplication is achieved digitally, after matched filtering in both
receivers.) This output of the "mixer" is coupled through a lowpass loop
filter to reduce the noise and to provide a zero steady state tracking
error. The lowpass filtered signal is used to adjust the settings of the
phase shift elements of the phased arrays, in accordance with a phased
array weight control mechanism (for steering the beam pattern of the
phased array) contained within the antenna control processor. The transmit
array's polarization angle is slaved to that of the receive array. Because
the preferred phased arrays are conformal or non-planar, it is necessary
to modify the phase shift settings produced by the antenna steering
mechanism executed by the control processor according to the degree of
departure of the conformal geometry of the array from a planar
configuration. For this purpose, a coordinate transformation look-up table
is coupled in the control feedback path from the antenna steering
mechanism and the phase shift elements of the phased array.
In addition to video communications, the present invention can accommodate
other signal formats, such as data from terminals, digital telephony, etc.
Simultaneous compressed video and data can be transmitted via TDM, FDM,
CDM or a combination thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 diagrammatically illustrates a communication system in accordance
with an embodiment of the present invention for effecting full motion and
resolution broadcast quality video communications between a
satellite-linked earth station and an aircraft;
FIGS. 2A and 2B, taken together, diagrammatically illustrate the system
architecture of an earth station for sourcing video signals to be
transmitted to an aircraft from a commercial satellite providing one or
more channels of commercial television programming and teleconference
video signals sourced from a teleconferencing site linked to the earth
station, the earth station also receiving video etc. signals from the
aircraft;
FIGS. 3A and 3B, taken together, show the system architecture of the video
transceiver on board an aircraft;
FIG. 4 diagrammatically shows fuselage-mounted conformal phased array
antenna comprising two separate pairs of transmit and receive phased
arrays, one of which is mounted on the port side of the top of the
fuselage of the aircraft and the other of which is mounted on the
starboard side of the top of the aircraft fuselage;
FIG. 5 shows a low profile, conformal configuration of a transmit, receive
phased array pair formed of a laminate structure having a top layer that
contains a two-dimensional array of antenna elements and a bottom layer
through which a transmission line interconnect is distributed; and
FIG. 6 shows the polarization tracking mechanism associated with the
antenna elements of a respective conformal phased array comprised of a
two-dimensional distribution of dual polarization antenna elements.
DETAILED DESCRIPTION
Before describing in detail the satellite-linked video communication system
in accordance with the present invention, it should be observed that the
present invention resides primarily in what is effectively a novel
combination of conventional signal processing and communication circuits
and components and not in the particular detailed configurations thereof.
Accordingly, the structure, control and arrangement of these conventional
circuits and components have been illustrated in the drawings by readily
understandable block diagrams which show only those specific details that
are pertinent to the present invention, so as not to obscure the
disclosure with structural details which will be readily apparent to those
skilled in the art having the benefit of the description herein. Thus, the
block diagram illustrations of the Figures do not necessarily represent
the mechanical structural arrangement of the exemplary system, but are
primarily intended to illustrate the major structural components of the
system in a convenient functional grouping, whereby the present invention
may be more readily understood.
FIG. 1 diagrammatically illustrates a satellite-to-aircraft communication
system in accordance with an embodiment of the present invention for
effecting full motion and resolution broadcast quality video
communications between a satellite-linked earth station 11 and an aircraft
12. Video signals to be transmitted to the aircraft may originate from a
variety of sources, such as a TV-receive only satellite receiver, CATV,
etc. For purposes of presenting an exemplary embodiment, the video signals
will be assumed to include both commercial television programming
downlinked from a commercial satellite 13, as well as private
teleconference video signals sourced from a teleconferencing site 14,
which is linked to earth station 11 by land lines, microwave or
fiber-optic links, shown generally at 15.
The video signals received by earth station 11 are processed for
transmission to aircraft 12 by way of a transmit/receive antenna dish 16.
Antenna 16 uplinks an RF carrier upon which the video has been modulated
to a relay satellite 23. The video processing mechanism, to be described
more fully below with reference to FIGS. 2A and 2B, involves digitizing
the video signals to a prescribed data rate and then compressing the
digitized video to a prescribed data rate (e.g. a T1 data rate of 1.544
Mb/s). The compressed digitized video signal can be subjected to
(optional) forward error correction encoding and spread spectrum-modulated
onto a carrier for transmission to relay satellite 23. Spread
spectrum-modulation of the signal reduces its power spectral density. The
spread signal is transmitted via uplink channel 21 (e.g. Ku band) to relay
satellite 23. Relay satellite 23 then retransmits the spread signal over a
downlink transmission channel 31 (e.g. Ku-band) to aircraft 12.
The aircraft 12 receives the satellite's downlink channel retransmission
via a compact phased array antenna 35, which may be totally electronically
scanned or it may only be partially electronically scanned. As noted
above, a phased array which is only partially electronically scanned is
one which scans electronically in one dimension (e.g. elevation) and
mechanically in the other (e.g. rotational). In a preferred embodiment of
the invention, phased array antenna 35 is configured so as to be conformal
with the aircraft surface, for example, on the top of the fuselage as two
sets of transmit and receive phased arrays, one transmit, receive pair on
the port side and the other on the starboard side of the aircraft.
Alternatively, transmit and receive functions may be combined into a
single array, although generally at the expense of increased aperture size
due to additional losses and/or half duplex duty cycle. This
port/starboard separation provides approximately full hemispherical
coverage with the satellite regardless of the attitude of the aircraft. A
single antenna may be used in place of the port-starboard pair in
situations where a more restricted beam scanning volume is acceptable.
Mounting the antenna on the top of fuselage not only saves cabin space,
but, because of its relatively thin, conformal configuration, minimizes
antenna visibility and reduces drag. In addition to occupying less volume
than would a purely mechanically steered antenna, such a compact phased
array antenna 35 provides the required amount of antenna gain.
Uplink transmissions received at the aircraft from relay satellite 23 are
processed through a data recovery receiver, which despreads and
demodulates the received signal. The demodulated video is then
reconstructed for distribution to a variety of terminals and monitors on
board the aircraft. Downlink transmissions from the aircraft may include
both data and telephony transmissions, including control and overhead
signalling, such as that employed for channel selection, and also video
signalling in the case that teleconferencing capability is provided.
FIGS. 2A and 2B, taken together, diagrammatically illustrate the system
architecture of earth station 11 for the present example of sourcing video
signals to be transmitted to aircraft 12 from both commercial satellite
13, which provides commercial television programming, as well as private
teleconference video signals sourced from teleconferencing site 14 linked
to the earth station. In the present example, commercially broadcast
television signals are derived via a TV receive-only (e.g. C-band or
Ku-band) satellite receiver 201 which is coupled to receiving antenna 17,
to which satellite 13 downlinks the analog FM television programming
(eventually to be replaced with digital transmission), selected channel(s)
of which are forwarded by earth station 11 to aircraft 12. Receiver 201
outputs baseband analog video signals received by antenna 17, which are
then processed for transmission via antenna 16 to relay satellite 23.
The processing mechanism employed in accordance with the present invention
initially involves digitizing (via an A-D converter, included in video
compression unit 203) the television channel(s) supplied by receiver 201
to a prescribed data rate and compressing the digitized television
signal(s) by way of video compression unit 203. For this purpose, video
compression unit 203 may comprise a Rembrandt II/VP
compressor/decompressor unit the CTX Plus.TM. algorithm, manufactured by
CLI (Compression Labs Inc.). Compressing the video reduces (e.g. by nearly
two orders of magnitude) its data rate which, for example, may be on the
order of 100 Mb/s, while maintaining both full resolution and motion
associated with broadcast quality television signals. Since the video
compression operation effectively narrows the information bandwidth, it
inherently contributes to a reduction in the receive aperture size
required to maintain a given bit error rate. The compressed information
bandwidth also facilitates spread spectrum processing to be subsequently
performed.
The compressed digitized television signal produced by video compression
unit 203 is supplied to transmit stage 204-1. Additional transmitter
stages 204 may be included which are controllably tunable to respective
ones of a plurality of video channels that are available for transmission
to the aircraft and are operative to place compressed video (or auxiliary
data and telephony) signals onto a carrier for transmission to the relay
satellite 23.
For this purpose each modulation stage 204-i has a multiplexer 205, to a
first input 202 of which a compressed video channel (or other data) of
interest is coupled. As set forth above, the video channel may be derived
either from the downlinked channels output by TVRO receiver 201, from one
or more teleconferencing sites 14 served by earth station 11, or any other
desired data source. The output of multiplexer 205 is coupled to (an
optional) forward error correction unit 207. Error correction unit 207 may
comprise an STEL-2020 Convolutional Encoder Viterbi Decoder manufactured
by Standford Telecommunications, Inc.
Using error correction coding in conjunction with efficient (e.g. coherent
PSK-type or MSK) data modulation further reduces the aperture size for a
given bit error rate. A second input 206 of multiplexer 205 is derived
from an earth station control processor 210 through which the operation of
earth station 11 is controlled. Control processor 210 may comprise a
processor-based transceiver controller, such as an LCP III Local Control
Processor manufactured by TelMac., the supervisory functionality is
effected by means of a resident communication control program, such as a
System 90, manufactured by CCS (Corporate Computer Systems). The second
input provides data/overhead signalling capability customarily employed
for communication system control functions. The output of forward error
correction unit 207 is coupled to a PSK/Spread Spectrum modulator 216,
such as an STEL-2173 NCO in conjunction with an STEL-1032 PRN Coder,
manufactured by Standford Telecommunications, Inc. (or a CD7000 cellular
telephone by Qualcomm), which performs initial carrier modulation of the
compressed video along with PN spreading onto an IF carrier.
Spread spectrum modulation reduces the spectral density of the modulated
signal. The spread spectrum processing performed by unit 213 can take the
form of direct PN sequence modulation and/or frequency hopping, for
example. As described previously, spread spectrum processing reduces power
spectral density for FCC compliance, prevents unauthorized users from
demodulating the video signal and enables the receiver on the aircraft to
reject interfering transmissions from other satellites.
The spread IF signal is coupled to an up-converter 217, which up-converts
the spread IF by mixing it with the output of frequency synthesizer 214,
such as that used in the CV-121 Ku-band SATCOM transmitter manufactured by
Comstream Corp., which is referenced to a stable frequency source 215. The
synthesizer frequency is selected by control processor 210. The level of
the resulting spread RF signal may be (optionally) adjusted by a variable
attenuator 219 for application to one port of a summing unit 222, the
output of which is supplied to high power amplifier 221. The output of
high power amplifier 221 is coupled to input port 223 of a diplexer 225.
Summing unit 222 is coupled to receive the outputs of the respective signal
transmitter stages 204-1 . . . 204-N. The T1 data rate information
channels that are coupled to modulation stages 204, may include
teleconference and data channels, as discussed above. The resulting
multi-channel summation signal from summing unit 222 is coupled from
diplexer 225 to antenna 16, which outputs the multi-channel signal over
uplink channel 21 to relay satellite 23. Relay satellite 23 then
retransmits the combined channels over a downlink channel 31 to aircraft
12.
The receiver section of earth terminal 11, shown in FIG. 2B, includes a low
noise amplifier 231 which is coupled to an output port 232 of diplexer
225. The o | | |
