# Patent

## Title
Interference signal cancellation over a broad frequency range

## Abstract

A signal interference cancellation system includes a signal filter that receives an interference signal and is in communication with a receiver that also receives the interference signal. The signal filter includes (a) a modulation system that modulates the interference signal to generate a modulated signal, (b) a first frequency range within which the modulation system amplifies quadrature components of the modulated signal, (c) a second frequency range within which the modulation system amplifies non-quadrature components of the modulated signal, and (d) a signal adder that combines the amplified quadrature and non-quadrature components of the modulated signal from (b) and (c) to generate an interference cancellation signal. The signal filter communicates the interference cancellation signal to the receiver for cancelling the interference signal.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This claims priority from U.S. provisional Application No. 62/298,496, filed Feb. 23, 2016, which is hereby incorporated by reference in its entirety.


FIELD
This relates to the field of RF devices and, more particularly, to reducing signal interference between RF devices.
BACKGROUND
Wireless signals are typically transmitted in the radio frequency (“RF”) range of the electromagnetic spectrum. Because there are so many RF signals travelling through the air at any given time, RF devices are equipped with electronics that can allow them to separate the desired signals from the RF noise and interfering signals. But distinguishing the desired signal from the noise and interfering signals can be difficult if the two signals are close or identical in frequency.
Ships and aircraft often include many different RF devices, including RADAR and radios, which serve different functions. Unfortunately, because these devices operate over the same or similar frequency ranges and because they are in close proximity to each other, they can be difficult, and sometimes impossible, to use simultaneously. When a transmit antenna is located near a receive antenna, the signal sent from the transmit antenna interferes with the signal the receive antenna is supposed to intercept, making the receive signal noisy.
Many schemes have been developed to reduce interference between nearby antennas, including using specialized electronic interference cancellers. Conventional electronic interference cancellers, however, are only able to cancel signals over a narrow frequency range with a narrow instantaneous bandwidth.
SUMMARY
In view of the foregoing, it would be advantageous to provide a signal canceller that has a broader operational frequency range and a broader instantaneous bandwidth than conventional electronic interference cancellers. Conventional modulation-based electronic interference cancellers only amplify quadrature components of the modulated signal. The system described here has a broader operational frequency range and instantaneous bandwidth because it modulates the interference signal and amplifies both quadrature and non-quadrature components of the modulated signal.
In an exemplary method of cancelling signal interference an interference signal received by a receiver is passed through a signal filter that at least partially cancels the interference signal using a modulation system. The modulation system (a) modulates the interference signal to generate a modulated signal, (b) amplifies quadrature components of the modulated signal within a first frequency range, and (c) amplifies non-quadrature components of the modulated signal within a second frequency range. An interference cancellation signal is formed from a combination of the amplified quadrature components from (c) and the amplified non-quadrature components from (d). The interference cancellation signal is provided to the receiver for cancelling the interference signal.
An exemplary signal interference cancellation system includes a signal filter that receives an interference signal and is in communication with a receiver that also receives the interference signal. The signal filter includes (a) a modulation system that modulates the interference signal to generate a modulated signal, (b) a first frequency range within which the modulation system amplifies quadrature components of the modulated signal, (c) a second frequency range within which the modulation system amplifies non-quadrature components of the modulated signal, and (d) a signal adder that combines the amplified quadrature and non-quadrature components of the modulated signal from (b) and (c) to generate an interference cancellation signal. The signal filter communicates the interference cancellation signal to the receiver for cancelling the interference signal.
An exemplary method of reducing interference in an RF communication system includes generating an interference cancellation signal by passing an interference signal received by a receiver through (i) an RF tapped delay line having a first tap and a second tap, (ii) a first modulator that receives a first input signal from the first tap, and (iii) and a second modulator that receives the second input signal from the second tap. Both of the first modulator and second modulator amplify quadrature components and non-quadrature components of the respective input signal by imparting signal weights to the quadrature components and non-quadrature components. The interference cancellation signal is provided to the receiver for cancelling the interference signal.
An exemplary RF signal filter includes an RF tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RF signal delay time. A first modulator receives a first input signal from the first tap and a second modulator that receives a second input signal from the second tap. Both of the first modulator and second modulator include (i) a first frequency range within which the respective modulator amplifies quadrature components of the input signal and (ii) a second frequency range within which the respective modulator amplifies non-quadrature components of the input signal. A signal adder combines an output signal from the first and second modulators.
The following are optional features that may be included in the examples above.
The quadrature components of the modulated signal are 90 degrees out of phase with each other and the non-quadrature components of the modulated signal are not 90 degrees out of phase with each other.
A 90 degree RF coupler may be used. The 90 degree RF coupler separates the modulated signal into the quadrature components within the first frequency range and separates the modulated signal into the non-quadrature components within the second frequency range.
The receiver may receive the interference signal at an antenna that also generates the interference signal.
The signal filter may impart separate signal weights to the quadrature and non-quadrature components of the modulated signal.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is block diagram of an example of a signal cancellation system operating on nearby antennas.
FIG. 2 is a block diagram of an example of a signal cancelling filter that may be used in the signal cancellation system.
FIG. 3 is a block diagram of an exemplary modulator used in the filter.
FIG. 4 is a graph illustrating the modulator I and Q channel phase vs frequency for a modulator that has a quadrature operational mode from 2-4 GHz.
FIG. 5 is a set of graphs illustrating a method of cancelling signal interference. The upper panel is a graph of an interference path impulse response. The lower panel is a canceller path impulse response where a series of Nyquist samples are separated by a delay time to fit the interference path impulse response.
FIG. 6 is a block diagram of a single-antenna transmit and receive system, including the signal cancelling filter.
FIG. 7A is a graph of experimental measurements of signal cancellation between 10 MHz and 210 MHz.
FIG. 7B is a graph of experimental measurements of signal cancellation between 250 MHz and 750 MHz.
FIG. 7C is a graph of experimental measurements of signal cancellation between 1000 MHz and 1600 MHz.
FIG. 7D is a graph of experimental measurements of signal cancellation between 2000 MHz and 2600 MHz.
FIG. 7E is a graph of experimental measurements of signal cancellation between 2600 MHz and 3600 MHz.
FIG. 7F is a graph of experimental measurements of signal cancellation between 3900 MHz and 4700 MHz.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
An example of the signal cancellation system 10 is generally shown in FIG. 1. The system 10 includes a transmitter that generates a transmit signal that is transmitted by a transmit antenna 24. The system also includes a receiver 30 that is designed to receive a receive signal 34 at a receive antenna 32. The problem with interference arises when the receiver antenna 32 also receives the transmit signal, which is shown in FIG. 1 as the interference signal. Because the receiver antenna receives both the receive signal 34 and the interference signal, the total signal that reaches the receiver 30 is the combination of both.
The interference signal is cancelled by using a first directional coupler to split the transmit signal 22 into two portions: one that passes to the transmit antenna 24 and another that passes through a filter 50. A filtered transmit signal 38 exits the filter 50 and is combined with the receive signal 34 at a second direction coupler 36. The filtered transmit signal 38 is used to subtract the interference signal from the receive signal 34.
Details of the filter 50 are now described by referring to FIG. 2. The filter 50 functions like an analog tapped delay line filter. The filter 50 includes a transmit signal input port 21 receives the transmit signal 22. The transmit signal is passed through a plurality of tapped delay lines having a plurality of sequentially positioned taps 54 separated by delay times 52. The taps 54 feed a portion of the transmit signal 22 to a plurality of modulators 70. In the example shown, the letters (a, b, c . . . n) after the reference numerals 52, 54, and 70 illustrate that the filter may have any number of delay times 52, taps 54, and modulators 70. The number of taps will correspond to the number of sampling points used to cancel the interference signal. The signals exiting the modulators 70 are combined in a signal adder 56, which outputs the filtered transmit signal 38 from an output port 58.
Details of an exemplary modulator 70 are now described by referring to FIG. 3. The modulator 70 is an IQ modulator. When the modulator operates in its quadrature operational mode, an input signal is fed through a 90 degree coupler that splits the input signal into quadrature components: an in-phase component (I) and a quadrature component (Q) that are 90 degrees out of phase with each other. The respective weights of the I and Q components are adjusted, respectively, by an I-component amplifier 78 and a Q-component amplifier 74. Both of these amplifiers 74,78 are connected to a signal weight adjuster 76,80 such as a dual-polarity voltage controller. The signal weight adjuster 76,80 allows for adjustment of the weights or amplitudes of the I and Q components of the signal. The I and Q components of the signal exit the amplifiers 74, 78 and are combined at a signal combiner such as a two way 0 degree coupler 82.
In tests, the filter 50 had an operational range over which it could cancel an interference signal from 10 MHz to 6 GHz and it had an instantaneous bandwidth that ranged from 5 MHz to several hundred MHz. It was routinely capable of achieving 30 dB cancellation of the interference signal. This broad operational frequency range is possible because the modulator 70 weights signals outside of the normal quadrature operational frequency range of the 90 degree coupler.
90 degree couplers are only designed to separate quadrature signals into I and Q components over an octave bandwidth, such as 1-2 GHz for example. When the input RF signal is within the quadrature operational frequency range of the 90 degree coupler, the 90 degree coupler will divide the signal into true I and Q vector components that are 90 degrees out of phase, where the I-component is the real vector component and the Q component is the imaginary vector component. In this scenario, the modulator 70 functions as a true vector modulator. The I and Q component amplifiers 74,78 may be adjusted to apply a desired weight to each component by supplying different voltages with the signal weight adjuster 76,80.
Conventional wisdom would suggest that the operational bandwidth of modulator employing a narrow band 90 degree coupler would be no more than the operational bandwidth of the 90 degree coupler. In other words, if the 90 degree coupler can only separate a modulated signal into true 90 degrees out of phase IQ components between 1-2 GHz, one would not expect the modulator to work very well outside the 1-2 GHz range.
A particularly advantageous feature of the modulator 70 is that its operational bandwidth is much larger than the 90 degree coupler's 72 operational bandwidth. This is because the modulator 70 can also operate outside the frequency range in which the 90 degree coupler can separate quadrature signals into IQ components.
Outside the quadrature operational frequency range, it does not separate the input signal into true real and imaginary vector components; instead, it simply splits the input signal into two non-quadrature components that are not 90 degrees out of phase with each other.
Adjusting the weights of the two non-quadrature component signals via the I and Q component amplifiers 74,78 may still allow for interference signal cancellation both above and below the quadrature operational frequency range of the 90 degree coupler. By taking advantage of this functionality, the operational frequency range of the modulator 70 is expanded beyond what would be expected when using a conventional narrow-band 90 degree coupler.
FIG. 4 is an illustrative I and Q channel phase vs. frequency plot for a hypothetical modulator having a quadrature operational frequency range of 2-4 GHz. Between 2-4 GHz, the I and Q channels receive the 0 degree and 90 degree components of the modulated input signal. Above and below this range, however, the modulator operates in its non-quadrature operational mode because the modulated signals that reach the I and Q channels are not 90 degrees out of phase.
As illustrated in FIG. 5, the interference signal may be characterized by an impulse response that varies over time. The filter 50 generates a simulated time-dependent cancellation impulse response by fitting the true impulse response with a series of Nyquist samples that are separated by the delay time 52 in the tapped delay line. The simulated impulse response may be subtracted from the true impulse path response to substantially cancel the interference signal. The Nyquist samples are fitted by adjusting the I and Q component weights, whether they be from a quadrature component signal or a non-quadrature component signal.
The filter 50 may also be used in conjunction with a single antenna system 90, such as simultaneous transmit and receive (“STAR”) antenna as illustrated in FIG. 6. The single antenna system 90 includes a transmit/receive antenna 92 that transmits the transmit signal 22 and also receives the receive signal 34. This results in substantial RF interference between the transmit signal 22 and the receive signal.
The interference may be cancelled by feeding a portion of the transmit signal 22 to the filter 50 to create a filtered signal 38 as discussed above. The receiver 30 then receives a combination of the receive signal 34 and filtered signal.
EXAMPLE
Interference Signal Cancellation Measurements
This example is provided to show that the filter can be used to cancel an interference signal over a broad operational bandwidth and with a wide instantaneous bandwidth. This example is provided by way of illustration and does not limit the scope of possible embodiments.
FIGS. 7A-F are a series of graphs that illustrate the functionality of the signal filter from 10 MHz to 4700 MHz. The bold line is a combination of the receive signal and interference signal. The narrower line is the received signal with the interference signal cancelled over an instantaneous bandwidth. The quadrature operational frequency of the 90 degree coupler was 1.7-2.7 GHz.
In FIGS. 7A and B, the filter operated in non-quadrature mode because the frequency was below the quadrature operational frequency of the 90 degree coupler. In FIG. 7A, the filter provided about 30 dB cancellation of the interference signal over about 50 MHz. In FIG. 7B, the filter provided about dB cancellation of the interference signal over about 100 MHz.
In FIGS. 7C and D the filter operated in quadrature mode because the frequency was within the quadrature operational frequency of the 90 degree coupler. In FIG. 7C, The filter provided about 30 dB cancellation of the interference signal over about 200 MHz.
In FIG. 7D, the filter provided about 30 dB cancellation of the interference signal over about 200 MHz.
In FIGS. 7E and F, the filter operated in non-quadrature mode. In FIG. 7E, the filter provided about 45 dB cancellation of the interference signal over about 200 MHz. In FIG. 7F, the filter provided about 35 dB cancellation of the interference signal over about 200 MHz.
This example shows that over the range of 10 MHz to 4700 MHz, the filter was able to cancel the interference signal very well, regardless of whether it operates on a quadrature signal or non-quadrature signal.
This disclosure describes certain example embodiments, but not all possible embodiments of the RF signal cancelling system, filter, and associated methods. Where a particular feature is disclosed in the context of a particular embodiment, that feature can also be used, to the extent possible, in combination with and/or in the context of other embodiments. The signal cancelling system, filter, and associated methods may be embodied in many different forms and should not be construed as limited to only the embodiments described here.

## Claims

1. A method of cancelling signal interference, the method comprising:
passing an interference signal received by a receiver through a signal filter that at least partially cancels the interference signal using a modulation system by modulating the interference signal, including:
(a) amplifying quadrature components of an input signal associated with the interference signal within a first frequency range,
(b) amplifying non-quadrature components of the input signal within a second frequency range,
(c) forming an interference cancellation signal from a combination of the amplified quadrature components from (a) and the amplified non-quadrature components from (b), and
(d) providing the interference cancellation signal to the receiver.

2. The method of cancelling signal interference of claim 1, wherein:
the quadrature components of the input signal are 90 degrees out of phase with each other; and
the non-quadrature components of the input signal are not 90 degrees out of phase with each other.

3. The method of cancelling signal interference of claim 1, wherein the signal filter further comprises:
a tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RF signal delay time; and
the modulation system includes a first modulator that receives the input signal from the first tap and a second modulator that receives the input signal from the second tap.

4. The method of cancelling signal interference of claim 1, wherein the modulation system comprises a 90 degree RF coupler that separates the input signal into the quadrature components within the first frequency range and separates the input signal into the non-quadrature components within the second frequency range.

5. The method of cancelling signal interference of claim 1, wherein the receiver receives the interference signal at an antenna that also generates the interference signal.

6. The method of cancelling signal interference of claim 1, further comprising imparting separate signal weights to the quadrature and non-quadrature components of the input signal.

7. The method of cancelling signal interference of claim 1, wherein:
the signal filter further comprises a tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RF signal delay time;
the modulation system comprises:
(a) a first modulator that receives an input signal from the first tap and a second modulator that receives an input signal from the second tap and
(b) each of the first modulator and the second modulator comprises a 90 degree RF coupler that separates the respective input signal into the quadrature components within the first frequency range and separates the respective input signal into the non-quadrature components within the second frequency range; and
the quadrature components of the input signal are 90 degrees out of phase with each other and the non-quadrature components of the input signal are not 90 degrees out of phase with each other.

8. A signal interference cancellation system comprising:
a signal filter that receives an interference signal and is in communication with a receiver that also receives the interference signal, the signal filter comprising:
(a) a modulation system that modulates the interference signal, the modulation system having a first frequency range within which the modulation system amplifies quadrature components of an input signal associated with the interference signal and a second frequency range within which the modulation system amplifies non-quadrature components of the input signal, and
(b) a signal adder that combines the amplified quadrature and non-quadrature components of the input signal from (a) to generate an interference cancellation signal and that communicates the interference cancellation signal to the receiver.

9. The signal interference cancellation system of claim 8, wherein:
the quadrature components of the input signal are 90 degrees out of phase with each other; and
the non-quadrature components of the input signal are not 90 degrees out of phase with each other.

10. The signal interference cancellation system of claim 8, wherein the signal filter further comprises:
a tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RE signal delay time; and
the modulation system includes a first modulator that receives the input signal from the first tap and a second modulator that receives the input signal from the second tap.

11. The signal interference cancellation system of claim 8, wherein the modulation system comprises a 90 degree RF coupler that separates the input signal into the quadrature components within the first frequency range and separates the input signal into the non-quadrature components within the second frequency range.

12. The signal interference cancellation system of claim 8, wherein the receiver receives the interference signal at an antenna that also generates the interference signal.

13. The signal interference cancellation system of claim 8, wherein the signal filter further includes a signal weight adjuster connected to modulation system, the signal weight adjuster being adapted to impart separate signal weights to the quadrature and non-quadrature components of the input signal.

14. The signal interference cancellation system of claim 8, wherein:
the signal filter further comprises a tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RF signal delay time;
the modulation system comprises:
(a) a first modulator that receives an input signal from the first tap and a second modulator that receives an input signal from the second tap and
(b) each of the first modulator and the second modulator comprise a 90 degree RF coupler that separates the respective input signal into the quadrature components within the first frequency range and separates the respective input signal into the non-quadrature components within the second frequency range; and
the quadrature components of the input signal are 90 degrees out of phase with each other and the non-quadrature components of the input signal are not 90 degrees out of phase with each other.

15. A method of reducing interference in an RF communication system, the method comprising:
generating an interference cancellation signal by passing an interference signal received by a receiver through (i) an RF tapped delay line having a first tap and a second tap, (ii) a first modulator that receives a first input signal from the first tap, and (iii) and a second modulator that receives a second input signal from the second tap, both of the first modulator and second modulator amplifying quadrature components and non-quadrature components of the respective input signal by imparting signal weights to the quadrature components and non-quadrature components; and
providing the interference cancellation signal to the receiver.

16. The method of reducing interference in an RF communication system of claim 15, wherein:
the quadrature components are 90 degrees out of phase with each other; and
the non-quadrature components are not 90 degrees out of phase with each other.

17. The method of reducing interference in an RF communication system of claim 15, wherein the receiver receives the interference signal at an antenna that also generates the interference signal.

18. An RF signal filter comprising:
an RF tapped delay line having a first tap and a second tap separated by a delay line having a characteristic RF signal delay time;
a first modulator that receives a first input signal from the first tap and a second modulator that receives a second input signal from the second tap;
both of the first modulator and second modulator operationally having (i) a first frequency range within which the first and second modulator amplify quadrature components of the respective first or second input signal and (ii) a second frequency range within which the first and second modulator amplify non-quadrature components of the respective first or second input signal; and
a signal adder that combines an output signal from the first and second modulators.

19. The RF signal filter of claim 18, further comprising:
an input port positioned to receive a transmit signal from a transmitter and direct the transmit signal to the RF tapped delay line; and
output port positioned to receive a filtered signal from the signal adder and direct the filtered signal to a receiver.

20. The RF signal filter of claim 18, wherein:
the quadrature components of the first input signal are 90 degrees out of phase with each other; and
the non-quadrature components of the first input signal are not 90 degrees out of phase with each other.

21. The RF signal filter of claim 18, wherein both of the first modulator and second modulator include a 90 degree RF coupler that separates the respective first and second input signal into the quadrature components within the first frequency range and separates the respective first and second input signal into the non-quadrature components within the second frequency range.

22. The RF signal filter of claim 18, further comprising a receiver that receives an Interference signal at an antenna that also generates the interference signal and the first input signal includes at least a portion of the interference signal.

23. The RF signal filter of claim 18, wherein the signal filter further includes a signal weight adjuster connected to the first modulator, the signal weight adjuster being adapted to impart separate signal weights to the quadrature and non-quadrature components of the first input signal.