Method and apparatus for reducing GNSS signal interference

Abstract

A method and apparatus for reducing GNSS signal interference using an adaptive notch filter (ANF) operates based on signals received from one or more satellites of a Global Navigation Satellite System (GNSS), such as GPS, GLONASS, etc. In one embodiment, the apparatus includes a notch filter having a tunable frequency of transfer function zero, the notch filter receiving an input signal and generating an output signal. A bandpass filter coupled to an output of the notch filter receives the output signal. An adaptive block is coupled to the bandpass filter and adjusts notch filter parameters to achieve a minimization of a particular cost function.

Description

Technical Field

[0001] The present disclosure relates generally to methods for filtering, and more particularly to methods and apparatus for reducing GNSS signal interference using an adaptive notch filter. Background Art

[0002] The transmission of signals is typically subject to interference from multiple sources. The interference source may be natural or artificial and may also be intentional or unintentional. Interference and noise may prevent operations that require the reception of useful signals. For example, thermal noise or interference jamming (for example, signal blocking) may cause signal loss and prevent operations that require the reception of transmitted signals. Global navigation satellite system (GNSS) signals from satellites of these systems are typically weak in strength and are therefore susceptible to interference. A method for reducing signal interference is needed. Summary of the invention

[0003] In one embodiment, a device includes a notch filter having a tunable frequency (also referred to as a "zero frequency") of a transfer function zero, and the notch filter is configured to receive an input signal and generate an output signal. A bandpass filter is connected to the output of the notch filter and the bandpass filter is configured to receive the output signal. An adaptive block is connected to the bandpass filter, and the adaptive block is configured to adjust the notch filter parameters to minimize a specific cost function. In one embodiment, the notch filter is a 1st order digital complex filter, wherein the input of the digital complex filter is connected to the input of the device, and when the adaptation is completed, the frequency of the transfer function zero of the digital complex filter is equal to the interference frequency. In one embodiment, the bandpass filter is a 1st order digital complex filter, and the 1st order digital complex filter has a pole frequency consistent with the zero frequency of the notch filter. The digital complex filter has a bandpass filter transfer function, and the input of the digital complex filter is connected to the output of the device. In one embodiment, the adaptive block is configured to track the interference frequency and adjust the filter zero point to minimize the cost function, and the input of the adaptive block is connected to the output of the bandpass filter. In one embodiment, the device is configured to reduce multi-spectral interference using a notch filter with a specific transfer function. In one embodiment, the high-pass filter is offset using real coefficients by multiplying the real coefficients by a power function of a complex exponent. A method is also described, which has the following steps: receiving an input signal from one or more global navigation satellite system satellites. The input signal is filtered by a notch filter and the filtered signal is input to a bandpass filter, and the output signal of the bandpass filter is input to an adaptive block, which adjusts the notch filter parameters according to a minimum mean square error (LMS) algorithm. A device is also described, the device including a processor and a memory connected to the processor. The memory stores computer program instructions, and when the computer program instructions are executed, the processor performs operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 An interference reduction system for a global navigation satellite system (GNSS) receiver having a notch filter and an adaptation block according to one embodiment is shown;

[0005] Figure 2 An adaptive notch filter for a GNSS receiver according to one embodiment is shown;

[0006] Figure 3 A graph showing a reduction in spectral density of noise due to a bandpass filter according to one embodiment;

[0007] Figure 4A digital complex 1st order bandpass filter for a GNSS receiver according to one embodiment is shown.

[0008] Figure 5 Shows Figure 4 The spectrum of the bandpass filter shown in ;

[0009] Figure 6 A digital complex 3rd order filter of a GNSS receiver according to an embodiment is shown;

[0010] Figure 7 Shows a depiction Figure 6 A graph of the frequency response of the digital complex third-order filter shown in;

[0011] Figure 8 A graph showing the frequency response of a digital complex notch filter by offsetting a high pass filter with real coefficients; and

[0012] Fig. 9 A high-level block diagram of a computer for performing the operations of the components described herein is shown, according to an embodiment. DETAILED DESCRIPTION

[0013] Methods and apparatus for reducing GNSS signal interference using an adaptive notch filter (ANF) operate based on signals received from one or more satellites of a Global Navigation Satellite System (GNSS), such as GPS, GLONASS, and the like.

[0014] Figure 1 An interference reduction system 100 of a GNSS receiver is shown. The interference reduction system 100 includes a notch filter 102 and an adaptation block 104. In one embodiment, the interference reduction system 100 is used to reduce intentionally caused interference. For example, the reduction system 100 can be used to perform anti-jamming on a jammed signal (or anti-jamming on a signal that has been blocked using interference). The signal y(t) at the input of the GNSS receiver may include both thermal noise and interference. Using a sampling frequency f S =(T S ) -1 After filtering (e.g., by an RF bandpass filter), down-conversion, and analog-to-digital conversion (ADC), the resulting GNSS digital signal y[n] at the input of the notch filter 102 of the interference reduction system 100 for the GNSS receiver may be expressed as:

[0015] y[n]=x[n]+i[n]+η[n] (1),

[0016] Where x[n] = ∑ k x k[n] is the combination of GNSS signals from different satellites, i[n]- is the interference signal, and η[n]- is the thermal noise with spectral density N0.

[0017] GNSS signal interference reduction in the time domain may involve removing the interfering signal i[n] from the signal y[n]:

[0018]

[0019] Narrowband quasi-harmonic interference and linear frequency modulation signals can be represented in the following single-component form:

[0020]

[0021] In this case, the instantaneous value of the interference signal frequency (Equation 3) has the following form:

[0022]

[0023] In the case of a single component signal (3), the following formula can be used to predict the form of the interference signal i[n] based on the previous sample i[n-1]:

[0024] i[n]=a[n]i[n-1] (5).

[0025] Considering that for adjacent samples, if the noise amplitude is assumed to be constant, the linear prediction coefficient of the error signal is determined by estimating only the instantaneous value of the interference frequency f, so the expression for the coefficient a[n] can be obtained from equations (3) and (5):

[0026]

[0027] In one embodiment, an interference reduction system for a GNSS receiver uses a digital notch filter with a complex transfer function of 1st order (also referred to as a 1st order digital complex filter) having a transfer function:

[0028]

[0029] where z0[n] is the complex zero of the transfer function (Eq. (7)), and k α <1 are real coefficients at the poles of the filter transfer function.

[0030] The numerator of the transfer function of equation (7) is called the moving average (MA) part. In one embodiment, Figure 1The MA component 106 shown in FIG. 1 is used to implement the function numerator of equation (7). In one embodiment, the MA part is used for interference signal reduction. The MA part H having a filter transfer function MA (z) = 1 - z0[n]z -1 The output signal can then be expressed as:

[0031] y MA [n]=y[n]-z0[n]y[n-1]=x[n]-z0[n]x[n-1]+(i[n]-z0[n]i[n-1])+n[n]-z0[n]n[n-1] (8).

[0032] From equation (8), it can be seen that when the interference signal estimate (equation (6)) coincides with the zero value of the notch filter tuned to the interference with the filter transfer function (equation (7)), the single component interference signal can be reduced. The filter transfer function can be expressed as:

[0033]

[0034] Autoregressive (AR) portion of the filter transfer function The effect of MA on the distortion of the desired (i.e., useful) signal can be reduced by reducing the suppression bandwidth of the filter. Figure 1 The AR component 108 shown in FIG. 1 is used to implement the AR part of the filter transfer function. The smaller the frequency distortion of the desired signal, the smaller the coefficient k α The closer the value is to 1.

[0035] According to the minimum mean square error (LMS) algorithm, the search for the optimal value of the zero frequency of the notch filter (also called the target value) is iteratively performed at each sample. The direction of the search and the value of the correction addition itself are related to the value of the gradient of the cost function, which is equal to the power of the output signal:

[0036]

[0037] where x f [n] is the output signal of the MA block in the notch filter. By successful adaptation, i.e. reaching the minimum of the cost function, the zero frequency of the notch filter coincides with the interference frequency with a given accuracy.

[0038] The limit to increasing the depth of interference reduction is the ratio of the interference signal power J to the noise power N = N0B at the notch filter output: where N0 is the thermal noise spectral density and B is the bandwidth of the useful GNSS signal.

[0039] For a Nyquist frequency f S / 2 digital signal two-sided spectrum, bandwidth frequency is B = 2·(f S / 2)=f S In the case of a narrowband interference signal, due to the bandwidth Δω of the interference signal i Much smaller than the bandwidth of the noise: It is usually not possible to reduce the spectral density of the interfering signal to the level of the spectral density of the thermal noise.

[0040] Figure 2 An adaptive notch filter 200 of a GNSS receiver according to an embodiment is shown. A bandpass filter 206 is arranged at the input of an adaptive block 204. Using a bandpass filter at the input of an adaptive block allows the depth of reduction of harmonic interference (e.g., continuous wave (CW) interference) and linear frequency modulation interference to be increased by 20 dB compared to other ANF-based interference reduction systems that can be used for anti-jamming. The bandpass filter 206 has a center frequency that coincides with the zero frequency of the notch filter 202. In one embodiment, the notch filter 202 has a tunable frequency of the transfer function zero. In one embodiment, the bandpass filter 206 is a digital complex filter. After the signal is processed by the bandpass filter 206, the contribution of the power of the thermal noise signal present in the output signal of the notch filter 202 will be reduced in the adaptive block 204 when the adaptive block 204 calculates the cost function gradient.

[0041] It should be noted that the bandpass filter 206 at the input of the adaptation block 204 does not alter the signal in the direct transmission channel, since the bandpass filter is used in the calculation circuitry of the adaptive notch filter 200 of the GNSS receiver.

[0042] The rejection band / stop band width of the notch filter 202 having a transfer function of the form shown in equation (7) is B -3dB =(1-k α )f S π / 10. In one embodiment, it is assumed that the stop band of the notch filter 202 coincides with the frequency band of the interference signal (B -3dB =Δω i ). When the bandwidth of the notch filter 202 is related to the bandwidth of the interfering signal, the noise level at the input of the adaptation block 204 is reduced and the gain in the ratio J / N is

[0043] Figure 3 Graph 300 shows the reduction in noise spectral density caused by using bandpass filter 202. Figure 3As shown, the input noise signal 302 has a uniform power spectral density (PSD) distribution over a frequency range from -80 MHz to 80 MHz. The bandpass output signal 304 has a resonant response with a center frequency of approximately 23 MHz.

[0044] The bandpass filter at the input of the adaptive block should be tunable, because during the adaptation of the notch filter, its pole frequency changes and is also tuned to the frequency of the interfering signal. However, the adjustment of the digital filter requires the recalculation of the transfer function coefficients in real time. For example, in one embodiment, a biquad filter is used as a filter with a resonant frequency ω i =2πf i And Q factor Q bandpass filter. After the biquadruple bilinear frequency conversion After that, and this is an analog implementation, the transfer function of the digital biquad in the z-domain has the following form:

[0045]

[0046] in as well as

[0047] In order to adjust the resonant frequency of the digital filter using the transfer function (Equation (11)), the coefficients a1 and a2 are changed. In one embodiment, when the adaptation is completed, the zero frequency of the transfer function of the digital complex filter is equal to the interference frequency.

[0048] In addition, after the frequency conversion, the frequency characteristics are distorted, and the degree of distortion increases as the resonant frequency approaches the Nyquist frequency. There is an offset between the value of the resonant frequency and the calculated value. In order to accurately implement the digital filter at the frequency ω, according to Predistortion is introduced (see equation (8)). In this case, the coefficient β in expression (11) should be in the following form:

[0049] Therefore, when tuning the zero frequency of the notch filter in the adaptive process, the coefficients of the transfer function of the digital biquad filter with the sampling frequency need to be calculated according to the following formula:

[0050]

[0051] When using a field programmable gate array (FPGA) to implement interference reduction, equation (12) requires relatively large computing resources.

[0052] In general, a transfer function with real coefficients (see equation (11)) has complex conjugate zeros and poles. For a double-sided spectrum, the frequency response of a filter with real coefficients has a symmetrical response in both the positive frequency region and the conjugate negative part. However, the interference signal is located in one of the two conjugate parts of the spectrum according to the rule. Then, a bandpass filter with a real transfer function has an excess bandwidth in the conjugate part of the spectrum, where there is no interference signal, which reduces the value of the ratio J / N by at least 3 dB.

[0053] Figure 4 A filter section 400 of a GNSS receiver with a digital complex 1st order bandpass filter according to an embodiment is shown. The filter section 400 has a bandpass filter 406 at the input of an adaptation block 404. The bandpass filter 406 has a complex transfer function which provides selection in only one of the two conjugate parts of a double-sided spectrum.

[0054] In one embodiment, the bandpass filter 406 operates as a transfer function, specifically:

[0055]

[0056] In one embodiment, the bandpass filter 406 has the following characteristics:

[0057] The transfer function shown in equation (13) corresponds to a complex pole A digital complex 1st order bandpass filter, where is the zero frequency of the transfer function of the notch filter 402 (see equation (7)), that is, the value of the resonant frequency of the proposed bandpass filter is consistent with the zero frequency of the transfer function of the notch filter 402.

[0058] Real coefficient k b <1 determines the bandwidth of the bandpass filter, and its value can also be expressed by formula B -3dB =(1-k b )f S π / 10 is used to estimate k b The closer the value of the coefficient is to 1, the smaller the bandwidth of a given bandpass filter.

[0059] The proposed filter has a resonant frequency The transfer coefficient under is equal to:

[0060] because

[0061] Finally, the bandpass complex filter operating according to the transfer function shown in equation (13) ensures that only one (one of the two conjugated parts) of the double-sided spectrum is selected.

[0062] In one embodiment, the bandpass filter comprises a digital complex first-order filter having a pole frequency that coincides with the zero frequency of the notch filter, the digital complex filter having The transfer function of .

[0063] Figure 5 A spectrum 500 is shown for a bandpass filter 406 operating according to the transfer function shown in equation (13) and having the characteristics described above.

[0064] Methods used to reduce interference are usually limited to reducing one type of interference. For example, the PB (pulse blanker) method works well for wideband pulse interference signals, but cannot be designed to address continuous interference. Multispectral methods using discrete Fourier transform (DFT), discrete wavelet transform (DWT) or Karhunen-Loève transform (KLT) provide great flexibility in reducing interference, but require very large computing resources.

[0065] The adaptive notch filtering (ANF) method described herein provides favorable results in the case of quasi-harmonic interference and chirp signals, but may be considered less effective for reducing multi-spectral or pulsed interference signals. However, the effectiveness of the adaptive notch filtering method can be improved as follows.

[0066] In one embodiment, a notch-type digital complex first-order bandpass filter operates according to the transfer function of equation (7). The digital complex first-order bandpass filter described herein does not require additional computing resources to recalculate the transfer function and provides tracking and reduction of linear frequency modulation interference with a frequency of up to 10 MHz / μs. In one embodiment, the adaptive block is configured to track the interference frequency and adjust the filter zero point to minimize the cost function. In the operation of the digital complex first-order filter, the real coefficient k α At the same time, it affects the signal reduction depth and suppression bandwidth. In order to reduce broadband (or for example multi-spectrum OFDM signal) interference, the suppression band of the notch filter is increased, that is, k α However, in this case, the signal reduction in the stop band is reduced. To resolve this contradiction, the filter order is increased and the stop band of the notch filter is widened. One of two possible solutions can be implemented to resolve the contradiction.

[0067] In the first approach, a digital complex 3rd order filter consisting of three cascaded notch filters is used, where the resulting transfer function is of the form:

[0068]

[0069] in

[0070] Figure 6 The filter section 600 of a GNSS receiver is shown comprising a digital complex 3rd order filter as described above with three cascaded notch filters 602A, 602B and 603C. The GNSS receiver 600 also has an adaptation block 604 with a bandpass filter 606 at its input.

[0071] Figure 7 Shows a depiction Figure 6 Graph 700 of the frequency response of a digital complex 3rd order filter is shown in FIG.

[0072] It should be noted that the zero frequency f0 of the notch filter, i.e. the value Δf1 that determines the suppression band, does not need to be changed when changing (i.e. tuning) during adaptation. The values ​​of the two additional first-order filter complex poles are No complicated calculations are required; it is determined by the sum of the arguments of two complex numbers.

[0073] Generally speaking, for multi-spectral interference with a large bandwidth, the order of the notch filter needs to be increased. In addition, the position of the zero point in the approximate function must be symmetrical about the center frequency. The transfer function of this filter can be expressed as:

[0075]

[0076] The number and location of zero points (Δf1<Δf2<…<Δf k ) is determined by the required interference reduction depth in the stopband of the 2(k+1)th order notch filter. The frequency value f0 must correspond to the center frequency of the interference spectrum and the value 2·Δf k Must correspond to the signal width of this interference.

[0077] The second method implements a notch-type high-order complex filter for interference reduction with real coefficients by multiplying the real coefficients by a power function of the complex exponent. A finite impulse response (FIR) filter with real coefficients or a transfer function An infinite impulse response (IIR) filter can be used as a high-pass filter. A high-pass filter with real coefficients is a band [-ω c ,+ω c ] provides signal attenuation within the double-sided spectrum.

[0078] Figure 8 Graph 800 shows the frequency response of a digital complex notch filter by offsetting a high pass filter with real coefficients.

[0079] The coefficients of the complex filter are further formed by multiplying the coefficients of the original high-pass filter by a power function of the complex exponent: in the case of the FIR filter, this is and in the case of an IIR filter, it is and (For all m=0..(M 1(2) -1))).

[0080] Therefore, the transfer function of the complex IIR filter is obtained Or the impulse response characteristics of the complex FIR filter

[0081] When the notch filter drifts during the adaptation process, is used as a complex exponent, where ω0 is the center frequency of the complex filter. The notch filter synthesized by shifting the frequency characteristics to the center frequency ω0 ensures that the approximate function of the original high-pass filter is within the frequency band [ω c -ω c ,ω0+ω c The choice of high-pass filter determines the required selectivity of the frequency response, the minimum value of the width of the transition zone of the frequency response, thereby reducing the distortion of the useful signal.

[0082] In one embodiment, the method may be used to detect and reduce multi-spectral interference (a type of Orthogonal Frequency Division Multiplexing (OFDM) signal) when using high-order notch filters in an ANF structure.

[0083] The methods and techniques described in this paper provide detection and mitigation of not only single component interference (CW and chirp) but also multi-spectral interference, which has qualitative advantages over other works devoted to ANF-based interference reduction systems.

[0084] The techniques described in this article can be used not only to detect and mitigate single-component jammers (such as CW and chirp), but also to counter multi-spectrum broadband jammers.

[0085] In one embodiment, a computer is used to perform the operations described herein and, for example, Figure 1 , Figure 2 , Figure 4 and Figure 6 The components and operations of the formula are shown in FIG. These components can be, for example, notch filters, bandpass filters, and adaptive blocks. Fig. 9A high-level block diagram of such a computer is shown in . The computer 902 includes a processor 904 that controls the overall operation of the computer 902 by executing computer program instructions that define such operations. The computer program instructions may be stored in a storage device 912 or other computer-readable medium (e.g., a disk, CD ROM, etc.), and the computer program instructions are loaded into the memory 910 when the computer program instructions need to be executed. Therefore, the components and formulas described herein may be defined by the computer program instructions stored in the memory 910 and / or the storage device 912 and the components and formulas described herein may be controlled by the processor 904 executing the computer program instructions. For example, the computer program instructions may be implemented as a computer executable code written by a person skilled in the art to perform the algorithms defined by the components and formulas described herein. Therefore, by executing the computer program instructions, the processor 904 performs the algorithms defined by the components and formulas described herein, such as Figure 1 , Figure 2 , Figure 4 and Figure 6 The computer 902 also includes one or more network interfaces 906 for communicating with other devices via a network. The computer 902 also includes input / output devices 708 (e.g., a display, keyboard, mouse, speakers, buttons, etc.) that enable a user to interact with the computer 902. Those skilled in the art will recognize that the actual computer implementation may also include other components, and Fig. 9 is a high-level representation of some of the components of such a computer for purposes of illustration.

[0086] The foregoing detailed description should be understood to be illustrative and exemplary in all respects, rather than restrictive, and the scope of the inventive concept disclosed herein should be interpreted in accordance with the full scope permitted by patent law. It should be understood that the embodiments shown and described herein are only illustrative of the principles of the inventive concept, and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the inventive concept. Various other feature combinations may be implemented by those skilled in the art without departing from the scope and spirit of the inventive concept.

Claims

1. A device, comprising: a notch filter having a tunable zero frequency of a transfer function and configured to receive an input signal and generate an output signal; a bandpass filter coupled to an output of the notch filter and configured to receive the output signal; and An adaptation block is coupled to the bandpass filter and is configured to adjust parameters of the notch filter to minimize a specific cost function.

2. The device according to claim 1, wherein: The notch filter is a 1st order digital complex filter, wherein the input of the digital complex filter is coupled to the input of the device, and when adaptation is completed, the frequency of the transfer function zero of the digital complex filter is equal to the interference frequency.

3. The device according to claim 1, wherein: The bandpass filter is a first-order digital complex filter having a pole frequency that coincides with the zero frequency of the notch filter and having A transfer function wherein the input of the digital complex bandpass filter is coupled to the output of the device.

4. The device according to claim 1, wherein: The adaptation block is configured to track interference frequencies and adjust the filter zeros to minimize a cost function, an input of the adaptation block being coupled to an output of the bandpass filter.

5. The apparatus of claim 1, wherein the apparatus is configured to reduce multi-spectral interference, the notch filter having the following transfer function:

6. The apparatus of claim 1, wherein the apparatus is configured to reduce multi-spectral interference, wherein: The notch filter is implemented by multiplying the real coefficients of the transfer function of the high-pass filter by a power function of a complex exponent and then shifting the high-pass filter.

7. A method comprising: receiving an input signal from one or more global navigation satellite system satellites; filtering the input signal using a first transfer function of a notch filter to generate a filtered signal; filtering the filtered signal using a second transfer function of the bandpass filter to generate a filtered bandpass signal; as well as An interference frequency of the filtered bandpass signal is tracked.

8. The method according to claim 7, wherein: The notch filter is a 1st order digital complex filter, wherein an input of the digital complex filter is coupled to an input of a device for receiving the input signal, and when adaptation is completed, the frequency of the transfer function zero of the digital complex filter is equal to the interference frequency.

9. The method according to claim 7, wherein: The bandpass filter is a first-order digital complex filter having a pole frequency that is consistent with the zero frequency of the notch filter. The transfer function of .

10. The method according to claim 7, further comprising: The interference frequency is tracked and the filter zeros are adjusted to minimize the cost function.

11. The method according to claim 7, wherein: The notch filter has the following transfer function:

12. The method according to claim 7, wherein: The notch filter is a high pass filter using real coefficients shifted by multiplying the real coefficients by a power function of a complex exponent.

13. A device, comprising: processor; as well as A memory connected to the processor, the memory storing computer program instructions, which, when executed, causes the processor to perform the following operations, the operations comprising: receiving an input signal from one or more global navigation satellite system satellites; filtering the input signal using a first transfer function of a notch filter to generate a filtered signal; filtering the filtered signal using a second transfer function of the bandpass filter to generate a filtered bandpass signal; and An interference frequency of the filtered bandpass signal is tracked.

14. The device according to claim 13, wherein: The notch filter is a first-order digital complex filter, and when adaptation is completed, the zero frequency of the transfer function of the digital complex filter is equal to the interference frequency.

15. The device according to claim 13, wherein: The bandpass filter is a first-order digital complex filter having a pole frequency that coincides with the zero frequency of the notch filter and having The transfer function of .

16. The apparatus of claim 13, wherein the operations further comprise: The interference frequency is tracked and the zero frequency of the notch filter is adjusted to minimize the cost function.

17. The device according to claim 13, wherein: The notch filter has the following transfer function:

18. The device according to claim 13, wherein: The notch filter is shifted toward the interfering frequency by multiplying the real coefficients of the bandpass filter transfer function by a power function of the complex exponential.

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