U.S. patent application number 16/268658 was filed with the patent office on 2019-06-06 for system and method for high speed satellite-based free-space laser communications using automatic gain control.
The applicant listed for this patent is SCHAFER AEROSPACE, INC.. Invention is credited to Eric Wiswell.
Application Number | 20190173580 16/268658 |
Document ID | / |
Family ID | 60411615 |
Filed Date | 2019-06-06 |
United States Patent
Application |
20190173580 |
Kind Code |
A1 |
Wiswell; Eric |
June 6, 2019 |
SYSTEM AND METHOD FOR HIGH SPEED SATELLITE-BASED FREE-SPACE LASER
COMMUNICATIONS USING AUTOMATIC GAIN CONTROL
Abstract
A high speed satellite-based laser communications system and
method for communications between a satellite-based transmitter
system and a ground-based receiver over a free space optical link.
The satellite-based transmitter system includes an encoder to
encode data, a polarization modulator to linearly polarize the
encoded data, one or at least two transmitters to transmit the
laser beam, and a quarter-wave optical wave plate to circularly
polarize the signal to be transmitted. The ground-based receiver
includes an automatic gain control to apply AGC to the received
data before the polarizations are reversed and the data is decoded.
The system enables an increased data throughput and reduces or
eliminates the effects of signal fading.
Inventors: |
Wiswell; Eric; (Santa Fe,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHAFER AEROSPACE, INC. |
Albuquerque |
NM |
US |
|
|
Family ID: |
60411615 |
Appl. No.: |
16/268658 |
Filed: |
February 6, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15605549 |
May 25, 2017 |
10243653 |
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16268658 |
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62342454 |
May 27, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 2203/0076 20130101;
H04B 10/614 20130101; H04B 10/532 20130101; H04B 10/118 20130101;
H04B 10/2942 20130101; H04B 10/564 20130101; H04J 14/06 20130101;
H04B 10/612 20130101 |
International
Class: |
H04B 10/118 20060101
H04B010/118; H04B 10/564 20060101 H04B010/564; H04B 10/294 20060101
H04B010/294; H04B 10/532 20060101 H04B010/532; H04J 14/06 20060101
H04J014/06; H04B 10/61 20060101 H04B010/61 |
Claims
1.-20. (canceled)
21. A ground-based receiver for receiving a signal transmitted by a
satellite-based transmitter subsystem of a satellite-based laser
communications system for communication between a satellite and the
ground-based receiver using a laser beam over a free-space optical
link that uses light propagating in free space for wireless data
communications, wherein the received signal has been transmitted as
a circularly polarized signal, wherein the ground-based receiver
comprises: (a) an optical automatic gain control circuit that
processes the received signal that was transmitted by the
satellite-based transmitter subsystem using the laser beam to
account for signal fading and atmospheric conditions over the
free-space optical link, wherein the optical automatic gain control
circuit comprises: (1) an optical amplifier to amplify the received
signal to output an automatic gain controlled signal that has two
circularly polarized states; (b) a quarter-wave (.lamda./4) optical
wave plate to convert the automatic gain controlled signal from two
circularly polarized states into an optical beam having two linear
polarization states, including a first linear polarization state
and a second linear polarization state; (c) a polarizing beam
splitter to split the optical beam into a first linearly polarized
beam corresponding to the first linear polarization state and a
second linearly polarized beam corresponding to the second linear
polarization state; (d) image processing circuitry or a
computer-implemented image processing module comprising an
algorithm to generate a difference between the first linearly
polarized beam and the second linearly polarized beam to develop an
output signal that comprises the signal as encoded at the
satellite-based transmitter subsystem; (e) a decoder to decode the
output signal to obtain the transmitted data; and (f) an output
module to output the transmitted data.
22. The ground-based receiver of claim 21, wherein the decoder at
the ground-based receiver is configured to perform error correction
on the output signal when the received signal was error correction
encoded at the satellite-based transmitter subsystem.
23. The ground-based receiver of claim 21, wherein the decoder at
the ground-based receiver comprises a deinterleaver to deinterleave
the encoded output signal when the received signal was interleaved
at the satellite-based transmitter subsystem.
24. The ground-based receiver of claim 21, wherein the decoder at
the ground-based receiver comprises a demultiplexer to obtain the
multiple channels of data from the output signal when the multiple
channels of data were multiplexed at the satellite-based
transmitter subsystem.
25. The ground-based receiver of claim 21, wherein the optical
amplifier comprises one or more optical fiber amplifiers.
26. The ground-based receiver of claim 21, wherein the ground-based
receiver is configured to be used in conjunction with an on-off
keying signaling system.
27. The ground-based receiver of claim 21, wherein the ground-based
receiver is configured to be used in conjunction with a
differential phase shift keying (DPSK) system.
28. The ground-based receiver of claim 21, wherein the received
signal has been transmitted by the satellite-based transmitter
subsystem to the ground-based receiver at a data rate at least as
high as 10 Gbps.
29. A method of processing a signal received at a ground-based
receiver from a satellite-based transmitter subsystem of a
satellite-based laser communications system wherein the received
signal has been transmitted as a circularly polarized signal, the
method comprising: (a) receiving, by the ground-based receiver, the
received signal that has been transmitted using a laser beam over a
free-space optical link using light propagating in free space for
wireless data communications, wherein the signal, as transmitted,
was polarization modulated onto the laser beam by altering the
polarization state of the laser beam through adjustment of an
optical phase between two linear polarization states, including a
first linear polarization state and a second linear polarization
state, using one or more high-speed phase modulators each
comprising an electro-optical crystal aligned with its active axis
at 45.degree. to the linearly polarized input beam, and wherein the
two linear polarization states of the polarization modulated laser
beam were converted into two circularly polarized states for
transmission using a quarter-wave (.lamda./4) optical wave plate;
(b) performing, by an optical automatic gain control circuit,
automatic gain control on the received signal at an input to the
ground-based receiver to account for signal fading and atmospheric
conditions over the free-space optical link, the performance of
optical automatic gain control comprising: (1) amplifying, using
the optical amplifier, the received signal to output an automatic
gain controlled signal that has the two circularly polarized
states; (c) converting the automatic gain controlled signal from
the two circularly polarized states into an optical beam having the
two linear polarization states using a quarter-wave (.lamda./4)
optical wave plate; (d) splitting, with a polarizing beam splitter,
the optical beam into a first linearly polarized beam corresponding
to the first linear polarization state and a second linearly
polarized beam corresponding to the second linear polarization
state; (e) detecting the first linearly polarized beam and
detecting the second linearly polarized beam; (f) generating, using
image processing circuitry or a computer-implemented image
processing module, a difference between the first linearly
polarized beam and the second linearly polarized beam that have
been detected to develop an output signal that comprises the signal
as encoded at the satellite-based transmitter subsystem; (g)
decoding, using a decoder, the output signal to obtain the
transmitted data; and (h) outputting the decoded data; wherein the
method at least partially compensates for fading effects that occur
during satellite transmissions to enable improvement in data
throughput.
30. The method of claim 29, wherein the signal, as transmitted, was
error correction encoded, and wherein the method further comprises
performing error correction on the output signal.
31. The method of claim 29, wherein the signal, as transmitted, was
interleaved, and wherein the method further comprises performing
deinterleaving on the output signal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority to and the benefit
of U.S. Provisional Patent Application No. 62/342,454, filed on May
27, 2016, the entire contents of which are incorporated by
reference as if fully set forth herein.
FIELD
[0002] The present invention is generally related to an improved
system and method for transmitting optical signals from a satellite
to a ground-based receiver. In particular, the present invention
relates to high speed free-space laser communications between a
satellite-based transmitter and a ground-based receiver that uses
automatic gain control to increase data throughput and reduce or
eliminate the effects of signal fading.
BACKGROUND
[0003] Polarization shift keying (PolSK) modulation encodes data
onto an optical beam by altering the polarization state of the beam
through adjustment of optical phase between two polarization
states. This encoding allows the transmission of data through a
free-space optical ("FSO") link using existing high-speed phase
modulators. A free-space optical link is a link that uses light
propagating in free space, such as air, outer space or a vacuum, to
wirelessly transmit data for telecommunications or computer
networking. The optical transmission of a signal over a free space
optical link presents challenges as the optical signal is subject
to atmospheric turbulence, that causes the optical signal to
fade.
[0004] A recent paper by Bai, et al. in Performance Analysis of
Polarization Modulated Direct Detection Optical CDMA Systems over
Turbulent FSO Links Modeled by the Gamma-Gamma Distribution,
Photonics 2015, 2, 139-155, ISSN 2304-6732, describes optical
transmissions between a ground-based transmitter and receiver over
an FSO link when using polarization shift keying (PolSK) to
modulate the optical signal along with optical code division
multiple access (CDMA) encoding of the optical signal to be
transmitted over the FSO link. The prior art system of Bai, el al.,
shown in FIG. 1, illustrates both a transmitter 10 to process and
transmit data 11 as an optical signal over an FSO and a receiver 20
to receive and process the data. Transmitter 10 illustrated in Bai,
et al. includes a polarization controller 14 that linearly
polarizes an input laser beam 18 at a 45.degree. angle and feeds
the polarized laser beam to a modulator 16. Modulator 16 includes a
polarization beam splitter (PBS) 16a, an optical phase modulator
16b for modulating CDMA-encoded data onto the laser beam, and a
polarization beam combiner 16c that combines the phase modulated
CDMA-encoded data and the unmodulated portion of the laser beam for
transmission as an optical signal over the FSO link 19. Transmitter
10 in Bai, et al. further includes a CDMA encoder 13 to encode both
data 11 and a modified prime code (MPC) sequence for a particular
user which is then fed to modulator 16. Thus, the transmitted
signal is both CDMA-encoded and polarization modulated.
[0005] Receiver 20 in Bai, et al. reverses the polarization
modulation and the CDMA encoding. Receiver therefore includes
polarization controller 21 to receive the optical signal, a second
polarization beam splitter (PBS) 22 splits the signal based on the
polarization state of each portion of the signal and reconstructs
the data that was transmitted by applying polarizers 23, 24,
optical correlators 25, 26, and photodetectors 27, 28 to the split
polarized signal, then adding and amplifying the signal at
respective adder 28 and amplifier 29, filtering the signal at
low-pass filter (LPF) 30 and a decision processor 31 for
determining signal reconstruction of the signal output from LPF
30.
[0006] The system described in Bai, et al. suffers from problems of
signal fading over FSO links for high-speed laser communications
from a satellite-based transmitter to a ground-based receiver where
it is desired to transmit signals at very high speeds such as at 10
Gbits/sec. A signal transmitted by a satellite-based laser
communications system must necessarily pass over a long FSO link
and therefore encounters a great deal of fading. An improved
solution is therefore needed for satellite to ground laser
communications to provide for faster, more reliable, and
higher-quality laser communications that account for fading caused
by atmospheric conditions.
[0007] Satellite-based laser communications systems generally
require complex electronics and electro-optical sub-systems to
provide a robust system that can deal with atmospheric conditions.
These electronics and electro-optical sub-systems tend to
undesirably occupy a significant amount of space, which, in turn,
requires larger satellites. It is therefore desirable to develop
electronics and electro-optical sub-systems that are lighter,
cheaper and more compact.
SUMMARY
[0008] A satellite-based laser communications system in accordance
with the present invention is therefore provided with a receiver
that includes an automatic gain control to process the received
polarization modulated signal before demodulating the received
signal and thereby reduce or eliminate the effects of channel
fading.
[0009] In accordance with an embodiment of the present invention, a
ground-based receiver for receiving a signal transmitted by a
satellite-based transmitter subsystem of a satellite-based laser
communications system for communication between a satellite and the
ground-based receiver using a laser beam over a free-space optical
link that uses light propagating in free space for wireless data
communications, includes (a) an optical automatic gain control
circuit that processes the received signal that was transmitted by
the satellite-based transmitter subsystem using the laser beam to
account for signal fading and atmospheric conditions over the
free-space optical link. The optical automatic gain control circuit
has a channel state estimator that receives a fraction of the
received signal, estimates a state of the communication channel
parameters that may have degraded the received signal, and outputs
a control signal comprising the estimated communication channel
parameters, and an optical amplifier to receive the control signal
that is output by the channel state estimator and to adjust and
amplify the received signal based, at least in part, on the control
signal to output an automatic gain controlled signal that has two
circularly polarized states.
[0010] The ground-based receiver in this embodiment of the present
invention further includes (b) a quarter-wave (.lamda./4) optical
wave plate to convert the automatic gain controlled signal from two
circularly polarized states into an optical beam having two linear
polarization states, including a first linear polarization state
and a second linear polarization state; (c) a polarizing beam
splitter to split the optical beam into a first linearly polarized
beam corresponding to the first linear polarization state and a
second linearly polarized beam corresponding to the second linear
polarization state; (d) two detectors, including a first detector
to detect the first linearly polarized beam and a second detector
to detect the second linearly polarized beam; (e) image processing
circuitry or a computer-implemented image processing module
comprising an algorithm to generate a difference in the output of
the two detectors to develop an output signal that comprises the
signal as encoded at the satellite-based transmitter; (f) a decoder
to decode the output signal to obtain the transmitted data; and (g)
an output module to output the transmitted data, wherein the
optical automatic gain control circuit at least partially
compensates for fading effects that occur during satellite
transmissions to enable improvement in data throughput. A
ground-based receiver of the present invention enables the received
signal to be transmitted by the satellite-based transmitter
subsystem to the ground-based receiver at a data rate at least as
high as 10 Gbps.
[0011] In embodiments, the decoder at the ground-based receiver is
configured to perform error correction on the output signal when
the received signal was error correction encoded at the
satellite-based transmitter subsystem. Also, in embodiments, the
decoder at the ground-based receiver includes a deinterleaver to
deinterleave the encoded output signal when the received signal was
interleaved at the satellite-based transmitter subsystem. Further,
in embodiments, the decoder at the ground-based receiver includes a
demultiplexer to obtain multiple channels of data from the output
signal when the multiple channels of data were multiplexed at the
satellite-based transmitter subsystem.
[0012] In embodiments, the optical amplifier comprises one or more
optical fiber amplifiers. Further, in embodiments, the first
detector is configured to detect a first area on an imaging sensor
focal plane and the second detector is configured to detect a
second area on the imaging sensor focal plane. Also, in
embodiments, the ground-based receiver may be configured to be used
in conjunction with an on-off keying signaling system or a
differential phase shift keying (DISK) system.
[0013] In accordance with another embodiment of the present
invention, a satellite-based laser communications system for
communication between a satellite and a ground-based receiver
comprises a satellite-based transmitter subsystem to transmit a
signal to a ground-based receiver over a free-space optical link,
wherein the free space optical link uses light propagating in free
space, such as air, outer space or a vacuum, to wirelessly transmit
data for telecommunications or computer networking, and the
ground-based receiver. The satellite-based transmitter subsystem
includes (a) an input module for receiving data to be transmitted
to the ground-based receiver, (b) an encoder to encode the data to
be transmitted, (c) a processor configured to generate a
transmission signal comprising the encoded data, (d) a laser light
source to generate a linearly polarized laser beam, (e) at least
one polarization modulator that further encodes the encoded data in
the transmission signal onto the laser beam by polarization
modulation of the laser beam through adjustment of an optical phase
between two linear polarization states using one or more high-speed
phase modulators each comprising an electro-optical crystal aligned
with its active axis at 45.degree. to the linearly polarized input
beam, (f) at least one transmitter for transmitting the
polarization modulated laser beam, wherein, the amount of energy in
each of the two linear polarization states is dependent on the
applied voltage, and (g) a quarter-wave (.lamda./4) optical wave
plate to convert the two linear polarization states of the
polarization modulated laser beam into circularly polarized states
in which to transmit the laser beam via the free-space optical link
so that the ground-based receiver need not be aligned in rotation
with respect to the transmitter.
[0014] In embodiments, the ground-based receiver of the
satellite-based laser communications system includes (a) an optical
automatic gain control circuit that processes the received signal
to account for signal fading and atmospheric conditions over the
free-space optical link. The optical automatic gain control circuit
has a channel state estimator that receives a fraction of the
received signal and estimates a state of communication channel
parameters that may have degraded the received signal, and outputs
a control signal comprising estimated communication channel
parameters to the optical amplifier, and has an optical amplifier
to receive the control signal that is output from the channel state
estimator and to adjust and amplify the received signal based, at
least in part, on the control signal to output an automatic gain
controlled signal that has two circularly polarized states. The
ground-based receiver further includes (b) a quarter-wave
(.lamda./4) optical wave plate to convert the automatic gain
controlled signal from the two circularly polarized states into an
optical beam having two linear polarization states, including a
first linear polarization state and a second linear polarization
state, (c) a polarizing beam splitter to split the optical beam
into a first linearly polarized beam corresponding to the first
linear polarization state and a second linearly polarized beam
corresponding to the second linear polarization state, (d) two
detectors, including a first detector to detect the first linearly
polarized beam and a second detector to detect the second linearly
polarized beam, (e) image processing circuitry or a
computer-implemented image processing module comprising an
algorithm to generate the difference in the output of the two
detectors to develop an output signal that comprises the signal as
encoded at the satellite-based transmitter subsystem, (f) a decoder
to decode the output signal to obtain the transmitted data; and (g)
an output module to output the transmitted data. The optical
automatic gain control circuit at least partially compensates for
fading effects that occur during satellite transmissions to enable
improvement in data throughput. Thus, the signal is not overcome by
interference like atmospheric scintillation when transmitted at
high speeds from the satellite-based transmitter subsystem to the
ground-based receiver.
[0015] In accordance with an alternative exemplary embodiment of
the present invention, a satellite-based laser communications
system for communication between a satellite and a ground-based
receiver comprises a satellite-based transmitter subsystem to
transmit a signal to a ground-based receiver over a free-space
optical link, wherein the free space optical link uses light
propagating in free space, such as air, outer space, or a vacuum,
to wirelessly transmit data for telecommunications or computer
networking, and the ground-based receiver. The satellite-based
transmitter subsystem includes (a) an input module for receiving
data to be transmitted to the ground-based receiver, (b) an encoder
to encode the data to be transmitted, (c) a processor configured to
generate a transmission signal comprising the encoded data, (d) a
laser light source to generate a linearly polarized laser beam, (e)
a polarization modulator that further encodes the encoded data in
the transmission signal onto the laser beam by altering the
polarization state of the laser beam through adjustment of an
optical phase between two linear polarization states using one or
more high-speed phase modulators each comprising an electro-optical
crystal aligned with its active axis at 45.degree. to the linearly
polarized input beam; wherein the amount of energy in each of the
two linear polarization states is dependent on the applied voltage,
(f) at least two transmitters, wherein each of the two transmitters
transmits a portion of the polarization modulated laser beam that
corresponds to a respective one of the two linear polarization
states, and (g) a quarter-wave (.lamda./4) optical wave plate to
convert the two linear polarization states into circularly
polarized states in which to transmit the polarization modulated
laser beam via the free-space optical link so that the ground-based
receiver need not be aligned in rotation with respect to the
transmitter.
[0016] In accordance with this alternative exemplary embodiment,
the ground-based receiver comprises (h) an optical automatic gain
control circuit that processes the received signal to account for
signal fading and atmospheric conditions over the free-space
optical link, the optical automatic gain control circuit comprising
a channel state estimator that receives a fraction of the received
signal and estimates a state of communication channel parameters
that may have degraded the received signal, and outputs a control
signal comprising estimated communication channel parameters to the
optical amplifier, and an optical amplifier to receive the control
signal that is output from the channel state estimator and to
adjust and amplify the received signal based, at least in part, on
the control signal to output an automatic gain controlled signal
that has two circularly polarized states. The ground-based receiver
further comprises (i) a quarter-wave (.lamda./4) optical wave plate
to convert the automatic gain controlled signal with the circularly
polarized states transmitted with the received laser beam back into
an optical beam having two linear polarization states, (j) a
polarizing beam splitter to split the optical beam into two beams,
one for each of the two linear polarization states, (k) two
detectors, one for each of the two beams, to capture the linear
polarization state for the beam to be detected, (l) circuitry or an
image processing module comprising an algorithm to generate the
difference in the output of the two detectors or focal plane areas
to develop an output signal that comprises the signal as encoded at
the satellite-based transmitter subsystem, (m) a decoder to decode
the output signal to obtain the transmitted data, and (n) an output
module to output the transmitted data.
[0017] In the alternative embodiment of the satellite-based laser
communications system, the satellite-based transmitter subsystem
may comprise at least two transmitters that use a time division
diversity scheme to transmit the laser beam to the ground-based
receiver to account for possible different arrival times at the
ground-based receiver for different channels.
[0018] In embodiments of the satellite-based laser communications
system, the polarization modulation boosts signal strength and is
not overcome by interference like atmospheric scintillation and the
automatic gain control increases data throughput and eliminates the
effects of fading that occur during satellite transmissions.
[0019] In embodiments, the encoder may be a code division multiple
access (CDMA) encoder that is configured to (1) encode a
synchronization channel using asynchronous CDMA encoding with
pseudo-random modulation, and (2) separately encode the data to be
transmitted as data symbols using CDMA encoding with modified Walsh
matrix modulation to distribute the encoded data symbols across
multiple channels so as to maximize a data transfer rate; wherein,
the modified Walsh matrix modulation values are derived from the
pseudo-random modulation vector and Walsh vector within each data
symbol by successively projecting each Walsh vector out of the
sub-space spanned by the pseudo-random modulation vector and any
previous modified Walsh vectors, and then normalizing. Further, in
embodiments, the decoder at the ground-based receiver comprises a
CDMA decoder to decode the received multiple channels of the
CDMA-encoded signal using the synchronization signal to obtain the
transmitted data. Use of CDMA encoding may further increase data
throughput and reduce fading effects on the CDMA-encoded signal
that results from the satellite transmissions over the free space
optical link.
[0020] In embodiments, the satellite-based transmitter subsystem
includes an error correction encoder to encode the CDMA-encoded
data with error correction codes, and the ground-based receiver
includes an error correction decoder to decode the error correction
coding for the CDMA-encoded data. Also, in embodiments, the
satellite-based transmitter subsystem further includes an
interleaver to interleave the CDMA-encoded data, and the
ground-based receiver further includes a deinterleaver to restore
the CDMA-encoded data.
[0021] A relative intensity between the two linear polarization
states may be used by the encoder to encode the data to be
transmitted into an analog or digital format. Moreover, in
embodiments, a ratio of energy between the two linear polarization
states is not affected by atmospheric scintillation, and can thus
be used to send more than one bit of digital information, or analog
information, independent of atmospheric scintillation or link
transmission properties.
[0022] In embodiments, the optical amplifier of the ground-based
receiver may comprise one or more optical fiber amplifiers. Also,
in embodiments, a first detector of the two detectors is configured
to detect a first area on an imaging sensor focal plane and a
second detector of the two detectors is configured to detect a
second area on the imaging sensor focal plane.
[0023] Moreover, in embodiments, the ground-based receiver may also
be used in conjunction with OOK signaling systems (on-off keying or
OOK) or in conjunction with Differential phase shift keying (DPSK)
systems.
[0024] The present invention further includes a method of
performing automatic gain control on a polarization modulated
signal at the input to a ground-based receiver in accordance with
any of the embodiments of the present invention.
[0025] In accordance with embodiments of the present invention, a
method of processing a signal received at a ground-based receiver
from a satellite-based transmitter subsystem of a satellite-based
laser communications system, includes (a) receiving, by the
ground-based receiver, the signal that has been transmitted using a
laser beam over a free-space optical link using light propagating
in free space for wireless data communications, wherein the
transmitted signal was polarization modulated onto the laser beam
by altering the polarization state of the laser beam through
adjustment of an optical phase between two linear polarization
states, including a first linear polarization state and a second
linear polarization state, using one or more high-speed phase
modulators each comprising an electro-optical crystal aligned with
its active axis at 45.degree. to the linearly polarized input beam,
and wherein the two linear polarization states of the polarization
modulated laser beam were converted into two circularly polarized
states for transmission using a quarter-wave (.lamda./4) optical
wave plate. The method further includes: (b) performing, by an
optical automatic gain control circuit, automatic gain control on
the received signal at an input to the ground-based receiver to
account for signal fading and atmospheric conditions over the
free-space optical link, the performance of optical automatic gain
control comprising (1) estimating, using a channel state estimator,
a state of communication channel parameters that may have degraded
the received signal and outputting a control signal comprising
estimated communication channel parameters to an optical amplifier;
and (2) adjusting and amplifying, using the optical amplifier, the
received signal based, at least in part, on the control signal
output to output an automatic gain controlled signal that has the
two circularly polarized states. The method further includes (c)
converting the automatic gain controlled signal from the two
circularly polarized states into an optical beam having two linear
polarization states using a quarter-wave (.lamda./4) optical wave
plate, (d) splitting, with a polarizing beam splitter, the optical
beam into a first linearly polarized beam corresponding to the
first linear polarization state and a second linearly polarized
beam corresponding to the second linear polarization state, (e)
detecting the first linearly polarized beam using a first detector
and detecting the second linearly polarized beam using a second
detector, (f) generating, using image processing circuitry or a
computer-implemented image processing module, a difference in the
output of the two detectors to develop an output signal that
comprises the signal as encoded at the satellite-based transmitter,
(g) decoding, using a decoder, the output signal to obtain the
transmitted data, and (h) outputting the decoded data. The method
at least partially compensates for fading effects that occur during
satellite transmissions to enable improvement in data
throughput.
[0026] In embodiments, the detection of the first linear
polarization state and the second polarization state of the two
optical beams comprises detecting a first area on an imaging sensor
focal plane using the first detector and detecting a second area on
the imaging sensor focal plane using the second detector.
[0027] In embodiments, the method further comprises performing
error correction on the output signal where the transmitted signal
was error correction encoded. Also, in embodiments, the method
comprises performing deinterleaving on the output signal when the
transmitted signal was interleaved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Exemplary embodiments of the present invention will be
described with references to the accompanying figures, wherein:
[0029] FIG. 1 is a diagram illustrating a prior art free-space
laser communications system for transmission of an optical signal
that has been polarization modulated and CDMA-encoded over an FSO
link;
[0030] FIG. 2 is a diagram illustrating a high-speed free-space
laser communications system for satellite-to-ground communications
in accordance with an embodiment of the present invention;
[0031] FIG. 3 is a diagram illustrating a high-speed free-space
laser communications system for satellite-to-ground communications
in accordance with a second embodiment of the present
invention;
[0032] FIG. 4 is a block diagram of an automatic gain control
circuit in accordance with an embodiment of the present invention
for use in a high-speed free-space laser communications;
[0033] FIG. 5 is a diagram illustrating a variation of the first
embodiment of the high-speed free-space laser communication system
shown in FIG. 2;
[0034] FIG. 6 is a diagram illustrating a variation of the second
embodiment of the high-speed free-space laser communication system
shown in FIG. 3; and
[0035] FIG. 7 is a flow chart illustrating a method of processing a
signal received at a ground-based receiver from a satellite-based
transmitter subsystem of a satellite-based laser communications
system in accordance with an exemplary embodiment of the present
invention.
DETAILED DESCRIPTION
[0036] The present invention is generally related to an improved
system and method for transmitting optical signals from a satellite
to a ground-based receiver. In particular, the present invention
relates to high speed free-space laser communications between a
satellite-based transmitter and a ground-based receiver that uses
automatic gain control to increase data throughput and reduce or
eliminate the effects of signal fading.
[0037] A system and method in accordance with the present invention
provides a high-speed free-space laser communications that at the
transmitter polarization modulates an optical signal to be
transmitted over an FSO link into two orthogonal quarter-wave
circularly polarized states and at the receiver applies an
automatic gain control to the optical signal that has been received
at the receiver while the received optical signal is circularly
polarized and before the signal is polarization demodulated.
[0038] FIG. 2 is a diagram of the satellite-based laser
communications system 1 of the present invention that comprises a
satellite-based PolSK transmitter 40 in a transmitter subsystem
(input device 42 to wave plate 54) to transmit data over an FSO
link and a ground-based receiver 41 that is configured to receive
and process the transmitted data. In communications system 1, data
is input via an input device/input module 42 at transmitter 40. The
input device/input module 42 collects input data from data sources
and formats the collected data, as necessary, for input to the
transmitter subsystem. The input data is then encoded at encoder 44
by an encoding scheme, such as, for example, an optical CDMA
encoding scheme. Encoder 44 may have optional circuitry to perform
one or more operations on the data to format the data and improve
its robustness. For example, encoder 44 may include an optional ECC
encoder to add error correction codes to the encoded data to allow
for error correction to be performed at receiver 41, and may be
optionally interleaved at an interleaver to enhance the ability to
perform error correction, especially when there are burst errors.
Encoder 44 may also reformat the data, multiplex multiple channels
of data, perform time division diversity, and allow for multiple
accesses. A processor 46 coupled to encoder 44 is configured to
generate a transmission signal comprising the encoded data.
[0039] The encoded data output from encoder 44 is then modulated
onto a linearly polarized laser beam that is generated by a laser
light source 50. A polarization modulator 52, which may be included
in transmitter 40, alters the polarization state of a portion of
the laser beam through adjustment of an optical phase between two
orthogonal linear polarization states, where the amount of energy
in each of the two linear polarization states is dependent on the
applied voltage. The voltage that is applied for each of the two
linear polarization states may be varied such that the relative
intensity between the two linear polarization states can also be
used to encode additional information into the signal using another
analog or digital format. The polarization of the laser beam into
the two linear polarization states is performed by an
electro-optical crystal that is aligned with its active axis at
45.degree. to output the linearly polarized input beam.
[0040] After the laser beam is linearly polarized into two states,
the laser beam is transmitted by transmitter 40 through a
quarter-wave (.lamda./4) optical wave plate 54 to convert the two
linear polarization states into circularly polarized states and the
laser beam is then transmitted via the free-space optical link 19
to the ground-based receiver 41. By transforming the linearly
polarized states into circularly polarized states, the receiver 41
need not be aligned in rotation with respect to the transmitter 40
to receive the transmitted optical signal.
[0041] The polarization modulation receiver can be used in
conjunction with OOK (on-off keying) signaling systems to convey
additional information or in conjunction with DPSK (differential
phase shift keying) systems to modulate additional information onto
the transmitted optical signal.
[0042] Receiver 41 receives the optical signal in the form of a
laser beam transmitted over FSO link 56 and subjects the signal to
an optical automatic gain control (AGC) circuit 58. FIG. 4 shows an
implementation of the AGC circuit 58. In FIG. 4, the optical
automatic gain control circuit 58 processes the received signal to
account for signal fading and atmospheric conditions over the
free-space optical link 56. The optical automatic gain control
circuit 58 includes an optical amplifier 80, such as one or more
optical fiber amplifiers, to adjust and amplify the received signal
in the laser beam to output an automatic gain controlled signal,
and a channel state estimator 81. Channel state estimator 81
captures a fraction of the incoming channel and uses it to estimate
the communication channel parameters, particularly the fade state
of the channel, that degraded the received signal as it passed
through the free-space optical link 56. Control signals 82,
comprising the estimated communication channel parameters, are sent
to the optical amplifier 80 where they are used to adjust and
amplify the signal output from automatic gain control 58.
[0043] Significantly, the optical automatic gain control circuit 58
limits the bandwidth range of the received laser beam signal to be
closer to the bandwidth of the signal that was actually transmitted
so that detectors 64, 66 at receiver 41 can process a signal having
a more limited bandwidth. For the AGC circuit 58 to work
effectively, the fade rate must be slower than the data rate, as is
the case in high-speed satellite communications where fast data
rates, such as data rates at least as high as 10 Gb/sec (Gbps), are
desired.
[0044] After performing automatic gain control, the signal is
transmitted through a quarter-wave (.lamda./4) optical wave plate
60 to transform the signal from a circularly polarized state back
to a linearly polarized signal. Next, a polarizing beam splitter 62
splits the beam into two beams, one for each of the two linear
polarization states. Each of the beams is input to a respective
detector 64, 66, to recapture the entire signal and the outputs are
fed to an image processing circuit that includes circuitry, such as
comparator 68, or a computer-implemented image processing module
(not shown) using an image processing algorithm, to generate the
difference in the output of the two detectors 64, 66, that detect
separate areas on an imaging focal plane, to develop an output
signal that comprises the encoded signal as encoded at the
satellite-based transmitter subsystem.
[0045] The output of the comparator 68 may then be optically
amplified, such as optical fiber amplifier 70, and may then be
decoded at decoder 76. Decoder 76 performs operations that are the
inverse to the operations that were performed at encoder 44. For
example, if CDMA encoding was used at the encoder 44, then signal
must be CDMA decoded at decoder 76. If interleaving was performed
at encoder 44, decoder 76 must perform deinterleaving and an ECC
encoded signal must be ECC decoded. The transmitted data is output
via an output module 78 where the data may be reformatted as
necessary for transmission to users via a network, such as a
telecommunications network or the Internet.
[0046] As noted above, in embodiments, encoder 44 may perform
optical CDMA encoding. The optical CDMA coding may be performed for
example, by encoding a synchronization channel using asynchronous
CDMA encoding with pseudo-random modulation, and separately
encoding the data to be transmitted as data symbols using CDMA
encoding with modified Walsh matrix modulation to distribute the
encoded data symbols across multiple channels so as to maximize a
data transfer rate. The modified Walsh matrix modulation values are
derived from the pseudo-random modulation vector and Walsh vector
within each data symbol by successively projecting each Walsh
vector out of the sub-space spanned by the pseudo-random modulation
vector and any previous modified Walsh vectors, and then
normalizing the result. The multiple channels and the separate
synchronization channel may be multiplexed into the transmission
signal by direct addition of each channel's signal value, and the
transmission signal may be normalized by scaling the multiplexed
signal values to the full modulation range within each
pseudo-random period.
[0047] For CDMA decoding, the received multiple channels of the
CDMA-encoded signal uses the synchronization signal to obtain the
transmitted data. Using CDMA encoding and decoding as well as the
error correction processing and interleaving, further reduces
interference and fading in the satellite transmission and increases
data throughput. However, the CDMA transmitted signal nevertheless
may still be subject to fading during transmission that is not
compensated for by the CDMA encoding or other encoding steps. Thus,
the automatic gain control circuit 58 at the input to the receiver
41 beneficially compensates for the fading effects of satellite
transmissions, increases the data throughput via the FSO link 56
and boosts throughput. As the ratio of energy between the two
polarization states in a satellite-based laser communications
system of the present invention is not affected by atmospheric
scintillation, the ratio of energy can also be used to send more
than one bit of digital information, or analog information,
independent of atmospheric scintillation or link transmission
properties.
[0048] FIG. 3 shows an alternative embodiment of a satellite-based
laser communications system 100 that includes two transmitters 40'
and 40'', rather than just one transmitter 40, with polarization
modulator 52 preceding transmitters 40' and 40'' and a multiplexer
53 between laser 50 and optical wave plate 54. The other elements
of system 100 are similar to the elements of system 1 shown in FIG.
2 and are identified by similar reference numerals. In the
embodiment of FIG. 3, polarization modulator 52 may have two phase
modulators, which polarizes the laser beam into one of the two
orthogonal linear polarization states using an electro-optical
crystal that is aligned with its active axis at 45.degree. to the
input linearly polarized input beam. The output of each phase
modulator is input to one of two transmitters 40' and 40''. The
signals from the two phase modulators are then multiplexed at
multiplexer 52 and input to quarter-wave (.lamda./4) optical wave
plate 54 to convert the two linear polarization states into
circularly polarized states. The circularly polarized laser beam is
then transmitted via the free-space optical link 19 from
transmitters 40' and 40'' to the ground-based receiver 41. The use
of two transmitters increases the transmission speed and throughput
of the optical signal through satellite-based laser communications
system 100 to ground-based receiver 41.
[0049] FIG. 5 is a diagram illustrating a variation of the first
embodiment of the high-speed free-space laser communication shown
in FIG. 2. In FIG. 5, polarization modulator 52 precedes
transmitter 40, but the elements shown in FIG. 5 are otherwise
identical to FIG. 2.
[0050] FIG. 6 is a diagram illustrating a variation of the first
embodiment of the high-speed free-space laser communication shown
in FIG. 3. In FIG. 6, separate polarization modulators 52', 52''
are used to feed respective transmitters 40', 40'', but the figure
is otherwise identical to FIG. 3.
[0051] The use of an AGC circuit at the input to a receiver to
address the fading of signals transmitted by satellite obviates the
need for alternative circuitry/processing at the satellite-based
transmitter for more intensive encoding and error correction. As a
result, the transmitter design may be simplified and made more
compact, allowing the size of the satellite can be reduced so that
it is lighter, cheaper and more compact. Moreover, space may be
freed up in the satellite for other equipment that provides
additional functionality.
[0052] FIG. 7 illustrates a method of processing a signal received
at a ground-based receiver from a satellite-based transmitter
subsystem of a satellite-based laser communications system in
accordance with an exemplary embodiment of the present invention,
such as the embodiments of the ground-based receiver and the
satellite-based laser communications systems described above. The
ground-based receiver receives, at step 90, the signal that has
been transmitted using a laser beam over a free-space optical link
using light propagating in free space for wireless data
communications, wherein the transmitted signal was polarization
modulated onto the laser beam by altering the polarization state of
the laser beam through adjustment of an optical phase between two
linear polarization states, including a first linear polarization
state and a second linear polarization state, using one or more
high-speed phase modulators each including an electro-optical
crystal aligned with its active axis at 45.degree. to the linearly
polarized input beam, and wherein the two linear polarization
states of the polarization modulated laser beam were converted into
two circularly polarized states for transmission using a
quarter-wave (.lamda./4) optical wave plate. At step 91, an optical
automatic gain control circuit performs automatic gain control on
the received signal at an input to the ground-based receiver to
account for signal fading and atmospheric conditions over the
free-space optical link, the performance of optical automatic gain
control including (1) estimating, using a channel state estimator,
a state of communication channel parameters that may have degraded
the received signal and outputting a control signal including
estimated communication channel parameters to an optical amplifier;
and (2) adjusting and amplifying, using the optical amplifier, the
received signal based, at least in part, on the control signal
output to output an automatic gain controlled signal that has the
two circularly polarized states. At step 92, the automatic gain
controlled signal is converted from the two circularly polarized
states into an optical beam having two linear polarization states
using a quarter-wave (.lamda./4) optical wave plate. At step 93, a
polarizing beam splitter splits the optical beam into a first
linearly polarized beam corresponding to the first linear
polarization state and a second linearly polarized beam
corresponding to the second linear polarization state. At step 94,
a first detector detects the first linearly polarized beam using a
first detector and a second detector detects the second linearly
polarized beam using a second detector. In embodiments, the
detection of the first linear polarization state and the second
polarization state of the two optical beams includes detecting a
first area on an imaging sensor focal plane using the first
detector and detecting a second area on the imaging sensor focal
plane using the second detector.
[0053] At step 95, image processing circuitry or a
computer-implemented image processing module is used to generate a
difference in the output of the two detectors to develop an output
signal that includes the signal as encoded at the satellite-based
transmitter. At step 96, a decoder decodes the output signal to
obtain the transmitted data. At step 97, the decoded data is
output. The method at least partially compensates for fading
effects that occur during satellite transmissions to enable
improvement in data throughput.
[0054] In embodiments, the method further includes performing error
correction on the output signal where the transmitted signal was
error correction encoded. Also, in embodiments, the method includes
performing deinterleaving on the output signal when the transmitted
signal was interleaved.
[0055] While particular embodiments of the present invention have
been shown and described in detail, it would be obvious to those
skilled in the art that various modifications and improvements
thereon may be made without departing from the spirit and scope of
the invention. It is therefore intended to cover in the appended
claims all such modifications and improvements that are within the
scope of this invention.
* * * * *