Title: Cosite interference rejection system using an optical approach
Abstract: A cosite interference rejection system allows cancellation of large interfering signals with an optical cancellation subsystem. The rejection system includes an interference subsystem coupled to a transmit system, where the interference subsystem weights a sampled transmit signal based on a feedback signal such that the weighted signal is out of phase with the sampled transmit signal. The optical cancellation subsystem is coupled to the interference subsystem and a receive antenna. The optical cancellation subsystem converts an optical signal into a desired receive signal based on an interfering coupled signal and the weighted signal. The weighted signal is therefore used to drive the optical cancellation subsystem. The rejection system further includes a feedback loop for providing the feedback signal to the interference subsystem based on the desired receive signal.
Patent Number: 6,934,476 Issued on 08/23/2005 to LaGasse
| Inventors:
|
LaGasse; Michael J. (Lexington, MA)
|
| Assignee:
|
The Boeing Company (Chicago, IL)
|
| Appl. No.:
|
944974 |
| Filed:
|
August 31, 2001 |
| Current U.S. Class: |
398/135; 398/137; 398/139; 398/115; 398/140; 398/182; 398/183; 398/188; 398/192; 398/194; 398/195; 398/202; 398/208; 398/209; 455/63; 455/673; 455/82; 455/83; 455/296; 455/303 |
| Intern'l Class: |
H04B 010/00 |
| Field of Search: |
398/182,183,188,194,195,202,135,139,137,140,115,192,208,209
455/63,673,82,83,296,303
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Phan; Hanh
Attorney, Agent or Firm: Harness Dickey & Pierce P.L.C.
Claims
1. A cosite interference rejection system comprising:
an interference subsystem coupled to a transmit system, the interference subsystem
weighting a sampled transmit signal based on a feedback signal such that the weighted
signal is out of phase with the sampled transmit signal;
an optical cancellation subsystem coupled to the interference subsystem and a
receive antenna, the optical cancellation subsystem converting an optical signal
into a desired receive signal based on an interfering coupled signal and the weighted
signal; and
a feedback loop for providing the feedback signal to the interference subsystem
based on the desired receive signal;
wherein the optical cancellation subsystem includes:
an optical source for generating the optical signal;
a modulation network coupled to the optical source, the receive antenna, and
the interference subsystem, the modulation network phase modulating the optical
signal based on the interfering coupled signal and the weighted signal; and
a demodulation system coupled to the modulation network and the feedback loop,
the demodulation system demodulating the phase modulated optical signal;
wherein the modulation network includes:
a first modulator coupled to the optical source and the receive antenna, the
first modulator phase modulating the optical signal based on the interfering coupled
signal;
a second modulator coupled to the interference subsystem, the second modulator
phase modulating the optical signal based on the weighted signal; and
a fiber optic subsystem for transferring the optical signal from the first modulator
to the second modulator.
2. The rejection system of claim 1 wherein the first modulator is remotely located
from the second modulator.
3. The rejection system of claim 2 wherein the rejection system is fixed to an
aircraft, the first modulator and the second modulator being positioned to obtain
a desired weight distribution within the aircraft.
4. The rejection system of claim 1 wherein the optical source comprises a laser.
5. The rejection system of claim 1 wherein power transmitted by the transmit
system is coupled to the receive antenna.
6. The rejection system of claim 1 wherein the interference subsystem includes:
an amplifier for amplifying the sampled transmit signal; and
an amplitude and phase module for weighting the sampled transmit signal.
7. The rejection system of claim 1 wherein the feedback loop includes a coupler
for sampling the desired receive signal.
8. The rejection system of claim 1 further including a low noise amplifier for
amplifying the desired receive signal.
9. An optical cancellation subsystem for a cosite interference rejection system,
the optical cancellation subsystem comprising:
an optical source for generating an optical signal;
a modulation network coupled to the optical source, a receive antenna, and an
interference subsystem, the modulation network phase modulating the optical signal
based on an interfering coupled signal from the receive antenna and a weighted
signal from the interference subsystem; and
a demodulation system coupled to the modulation network, the demodulation system
demodulating the phase modulated optical signal to generate a desired receive signal;
wherein the modulation network includes:
a first modulator coupled to the optical source and the receive antenna, the
first modulator phase modulating the optical signal based on the interfering coupled
signal;
a second modulator coupled to the interference subsystem, the second modulator
phase modulating the optical signal based on the weighted signal; and
a fiber optic subsystem for transferring the optical signal from the first modulator
to the second modulator.
10. The cancellation subsystem of claim 9 wherein the first modulator is remotely
located from the second modulator.
11. The cancellation subsystem of claim 10 wherein the cancellation subsystem
is fixed to an aircraft, the first modulator and the second modulator being positioned
to obtain a desired weight distribution within the aircraft.
12. The cancellation subsystem of claim 8 wherein the optical source comprises
a laser.
13. A method for rejecting cosite interference, the method comprising the steps of:
weighting a sampled transmit signal based on a feedback signal such that the
weighted signal is out of phase with the sampled transmit signal;
converting an optical signal into a desired receive signal based on an interfering
coupled signal and the weighted signal;
generating the feedback signal based on the desired receive signal;
generating the optical signal;
phase modulating the optical signal based on the interfering coupled signal and
the weighted signal;
demodulating the phase modulated optical signal;
phase modulating the optical signal with a first phase modulator based on the
interfering coupled signal;
transferring the optical signal to a second phase modulator with a fiber optic
subsystem; and
phase modulating the optical signal with the second phase modulator based on
the weighted signal.
14. The method of claim 13 further including the step of generating the optical
signal with a single wavelength laser.
Description
TECHNICAL FIELD
The present invention relates generally to cosite interference rejection systems.
More particularly, the invention relates to a cosite interference rejection system
having an optical cancellation subsystem.
BACKGROUND OF THE INVENTION
Modern commercial and military aviation applications often require communication
systems to transmit high power RF signals in the presence of relatively small RF
receive signals. In fact, there is a growing demand in the commercial aircraft
industry to increase the number of radios present on a given platform. Similarly,
the defense industry is constantly increasing the required number of signals to
be simultaneously transmitted and received. Given the limited amount of space available
on most platforms, it is therefore easy to understand that high power transmit
antennas may interfere with nearby receive antennas. In fact, a typical transmit
antenna will radiate hundreds or thousands of watts of power, whereas the power
of the desired receive signal will be a fraction of that. If the receive antenna
is located in relatively close proximity to the transmit antenna, residual transmitted
power will be coupled to the nearby receive antenna. The result is saturation of
the low noise amplifier (LNA) associated with the receive antenna. While the common
sense approach to this problem is to physically separate the receive antenna from
the transmit antenna, on platforms such as aircraft, helicopters, spacecraft, ships,
and building tops, such a solution may not be possible due to limited space. Another
solution is to use a cosite interference rejection system to cancel the coupled
power from the interfering coupled signal obtained by the receive antenna.
A modern day interference rejection system is shown in FIG. 1 at 20. Generally,
it can be seen that a transmit system 24 amplifies an input signal with
a power amplifier 28 for transmission with a transmit antenna 21.
The transmit signal is commonly sampled by a 10 dB coupler 23 for use by
an interference subsystem 22. The interference subsystem 22 amplitude
and phase weights the sampled transmit signal based on a feedback signal such that
the weighted signal is effectively out of phase with the sampled transmit signal.
A cancellation coupler 29 couples the weighted signal to an interfering
coupled signal obtained from a nearby receive antenna 25. It is important
to note that cancellation occurs in the electrical domain. Thus, the cancellation
coupler 29 functions as an electrical cancellation subsystem. A feedback
loop 26 provides the feedback signal to the interference subsystem 22
based on the desired receive signal produced by the cancellation coupler 29.
The feedback loop 26 typically uses a feedback coupler 27 to effectively
sample the desired receive signal. The desired receive signal is then passed on
to an LNA 15 for amplification.
While the above described conventional interference rejection system 20
partially addresses the issue of cosite interference, there is still room for considerable
improvement. For example, the conventional interference rejection system 20
is limited in the amount of coupled power that can be cancelled. In fact, when
the coupled power exceeds the threshold of the rejection system 20, the
system 20 can no longer transmit and receive simultaneously. The result
can be a loss of information. This problem is generally due to the non-linearity
of the electrical components used in the system 20. Specifically, the exact
reduction in amplitude of the interfering signal depends on how accurately the
phase and amplitude of the weighted signal matches the interfering signal. The
combination of a high level interfering signal and loss in the couplers 23,
27, 29 makes it difficult for the interference subsystem 22
to maintain linearity. When the linearity degrades, the cancellation performance
may be reduced. Eventually, as the interfering levels increase, large signals will
reach the input to the LNA 15 causing saturation and additional non-linearities.
Under these conditions, it is not possible to receive low-level signals near the
system noise floor, and information will be lost. It is therefore desirable to
provide a cosite interference rejection system that does not fall subject to the
non-linearities associated with high level interfering signals.
Another concern relates to applications where weight distribution is important.
For example, it is well known that conventional interference rejection systems
can significantly effect the distribution of weight on modern day aircraft. In
fact, it is quite difficult to arrange the components of the rejection system to
redistribute weight towards the center of gravity in order to improve performance
of the aircraft. This is largely due to the electrical nature of the components
and connections associated with conventional interference rejection systems. It
is therefore desirable to provide a cosite interference rejection system that allows
for more efficient weight distribution.
SUMMARY OF THE INVENTION
The above and other objectives are provided by a cosite interference rejection
system in accordance with the present invention having an optical cancellation
subsystem. Specifically, the rejection system includes an interference subsystem
coupled to a transmit system, where the interference subsystem weights a sampled
transmit signal based on a feedback signal such that the weighted signal is out
of phase with the sampled transmit signal. The optical cancellation subsystem is
coupled to the interference subsystem and a receive antenna. The optical cancellation
subsystem converts an optical signal into a desired receive signal based on an
interfering coupled signal and the weighted signal. The rejection system further
includes a feedback loop for providing the feedback signal to the interference
subsystem based on the desired receive signal. Using the optical cancellation subsystem
to convert an optical signal into the desired receive signal allows the above problems
associated with non-linearity to be eliminated.
Further, in accordance with the present invention, an optical cancellation
subsystem is provided. The preferred optical cancellation subsystem has an optical
source for generating an optical signal, a modulation network coupled to the optical
source, a receive antenna and an interference subsystem. The modulation network
phase modulates the optical signal based on an interfering coupled signal from
the receive antenna and a weighted signal from the interference subsystem. A demodulation
system is coupled to the modulation network, where the demodulation system demodulates
the phase modulated optical signal to generate a desired receive signal.
The present invention also provides a method for rejecting cosite interference.
The method includes the step of weighting a sampled transmit signal based on a
feedback signal such that the weighted signal is out of phase with the sampled
transmit signal. An optical signal is converted into a desired receive signal based
on an interfering coupled signal and the weighted signal. The method further provides
for generating the feedback signal based on the desired receive signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The various advantages of the present invention will become apparent to one skilled
in the art by reading the following specification and subjoined claims and by referencing
the following drawings, in which:
FIG. 1 is a block diagram of a conventional cosite interference rejection system
useful in understanding the present invention;
FIG. 2 is a block diagram of a cosite interference rejection system in accordance
with a preferred embodiment of the present invention;
FIG. 3 is a flowchart of a method for rejecting cosite interference in accordance
with the principles of the present invention; and
FIG. 4 is a flowchart of a process for converting an optical signal into a desired
receive signal in accordance with the principles of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to FIG. 2, the preferred cosite interference rejection system
is shown at
10. Generally, the rejection system
10 has an interference
subsystem
30, an adaptive optical cancellation subsystem
50, and
a feedback loop
70. It can be seen that the interference subsystem
30
is coupled to a transmit system
24. The interference subsystem
30
amplitude and phase weights a sampled transmit signal based on a feedback signal
such that the weighted signal is out of phase with the sampled transmit signal.
The weighted signal is therefore also out of phase with the actual transmitted
signal. It can also be seen that the interference subsystem
30 has an amplifier
32 for amplifying the sampled transmit signal, in addition to an amplitude
and phase module
34 for weighting the sampled transmit signal.
The optical cancellation subsystem
50 is coupled to the interference subsystem
30 and a receive antenna
25. The receive antenna
25 is located
in close enough proximity to the transmit antenna
21 for power transmitted
by the transmit system
24 to be coupled to the receive antenna
25.
The result is an interfering coupled signal made up of both the transmitted signal
and the desired receive signal. It can be seen that the optical cancellation subsystem
50 converts an optical signal into the desired receive signal based on the
interfering coupled signal and the weighted signal.
The feedback loop
70 provides the feedback signal to the interference
subsystem
30 based on the desired receive signal. The rejection system
10
may further include an LNA
15 if amplification is necessary. In fact, the
optical cancellation subsystem
50 is effectively able to set the system
noise figure, and therefore behaves like a very high-dynamic range, low-noise amplifier.
It is important to note that under the conventional electronic approach, the LNA
is required and the large interfering signal must be removed before the LNA. If
this is not done, non-linearities will degrade the system noise figure.
In one preferred embodiment, the optical cancellation subsystem
50 includes
an optical source
52, a modulation network
60, and a demodulation
system
54. It will be appreciated that the optical source
52 can
be a laser or any other device capable of generating a modulatable optical signal.
The modulation network
60 is coupled to the optical source
52, the
receive antenna
25, the interference subsystem
30, and the demodulation
system
54. The modulation network
60 phase modulates the optical
signal based on the interfering coupled signal and the weighted signal. The demodulation
system
54 is coupled to the modulation network
60 and the feedback
loop
70, where the demodulation system
54 demodulates the phase modulated
optical signal to obtain the desired receive signal.
It is highly preferred that the modulation network
60 includes a first
modulator
62 coupled to the optical source
52 and the receive antenna
25. The first modulator
62 phase modulates the optical signal based
on the interfering coupled signal. The modulation network
60 further includes
a second modulator coupled to the interference subsystem
30, where the second
modulator
64 phase modulates the optical signal based on the weighted signal.
The weighted signal therefore functions as a drive signal to the second modulator
64. It is important to note that this approach is quite different to that
of conventional approaches wherein the weighted signal is merely coupled to the
interfering coupled signal in the electrical domain. A fiber optic subsystem
66
transfers the optical signal from the first modulator
62 to the second modulator
64.
It is important to note that the use of low-loss fiber optics allows the different
subsystems to be more efficiently located. For example, the first modulator
62
can be located at the receive antenna
25, where it can immediately set the
system noise figure. The second modulator
64 and the components associated
with the feedback loop
70 can be located in a central, conveniently accessible
electronics bay. Hence, in an aircraft application, redistributing weight toward
the center of gravity can improve the performance of the aircraft. This concept
can be extended using techniques well known in the art to allow several systems
to share cancellation hardware through a fiber switched network. This type of architecture
reduces weight and cost, and increases performance.
Thus, FIG. 3 shows a method
100 for rejecting cosite interference in
accordance with the present invention for programming purposes. It will be appreciated
that method
100 can be readily implemented with a combination of hardware
and software using techniques well known in the art. It therefore can be seen that
at step
110 a sampled transmit signal is weighted based on a feedback signal
such that the weighted signal is out of phase with the sampled transmit signal.
At step
120, an optical signal is converted into a desired receive signal
based on an interfering coupled signal and the weighted signal. The method further
provides for generating the feedback signal at step
140 based on the desired
receive signal.
The preferred approach to converting the optical signal at step
120 is
shown in FIG.
4. Specifically, it can be seen that at step
122 the
optical signal is generated, and at step
124 the optical signal is phase
modulated based on the interfering coupled signal and the weighted signal. As already
discussed, phase modulation preferably occurs via a first phase modulator based
on the interfering coupled signal, and a second phase modulator based on the weighted
signal. The optical signal can be transferred to the second phase modulator with
a fiber optic subsystem as already discussed. The process at step
120 further
includes the step
126 of demodulating the phase modulated optical signal.
Returning now to FIG. 2, it will be appreciated that multiplexing the large
interfering signal with the second modulator
64 eliminates the RF loss associated
with the cancellation coupler used in the conventional approach. This is significant
because the amplifier driving the second modulator
64 has lower power and
is more linear than all-electric cancellation subsystems. This increase in linearity
gives a better cancellation ratio over a higher range of coupled power. Furthermore,
high dynamic range optical links having a laser, optical phase modulator, and optical
phase demodulator are commercially available. In fact, mathematical models are
able to predict the performance of the link with high accuracy. The result is negligible
non-linearities in the phase modulation process. Publications by the Navy Research
Laboratory, and MIT Lincoln Laboratory have demonstrated noise figures below 3
dB, and modulators that have sufficient sensitivity to operate at frequencies below
1 GHz. Other evidence can be found in the laser gyroscope field, which has measured
data showing residual amplitude modulation that is over 60 dB less than conventional
phase modulation approaches. The present invention is therefore able to address
the need for an increased number of radios as well as the need for an increased
number of signals with respect to modern day aviation platforms.
Those skilled in the art can now appreciate from the foregoing description
that the broad teachings of the present invention can be implemented in a variety
of forms. Therefore, while this invention has been described in connection with
particular examples thereof, the true scope of the invention should not be so limited
since other modifications will become apparent to the skilled practitioner upon
a study of the drawings, specification and following claims.
*
