Signal processing device, signal processing method, and radar device

Abstract

The signal processing device (4) is configured to include a function generation unit (12) that generates an objective function relating to the sum of the differences between the absolute value of the pixel value of each pixel of interest and the absolute value of the pixel value of each adjacent pixel, with each of the multiple pixels in the radar image, whose pixel values ​​are represented by complex numbers, being a pixel of interest, and the pixels adjacent to each pixel of interest being adjacent pixels; and a pixel value update unit (13) that updates the pixel value of each pixel in the radar image based on the objective function generated by the function generation unit (12) so that the sum of the differences becomes small.

Description

[Technical Field]

[0001] This disclosure relates to a signal processing device, a signal processing method, and a radar device. [Background technology]

[0002] There are signal processing devices that reduce noise contained in images. As an example of such a signal processing device, Patent Document 1 discloses a signal processing device that reduces noise contained in an image acquired by an image sensor. The pixel value of a pixel contained in an image acquired by an image sensor represents brightness expressed as a real number. The signal processing device disclosed in Patent Document 1 provides an objective function for determining the pixel value of each pixel in an image when each of the multiple pixels in the image is designated as a "focus pixel," and the pixels adjacent to each focus pixel are designated as "adjacent pixels," such that the sum of the differences between the pixel value of each focus pixel and the pixel values ​​of each adjacent pixel is minimized. Based on this objective function, the signal processing device updates the pixel value of each pixel in the image so that the sum of the differences is minimized. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2017-98933 [Overview of the project] [Problems that the invention aims to solve]

[0004] The pixel values ​​of pixels in radar images, such as synthetic aperture radar images, represent luminance and phase as complex numbers. Therefore, when pixel values ​​are represented as complex numbers, the difference between the pixel value of the pixel of interest and the pixel values ​​of adjacent pixels is not the difference in luminance itself. Therefore, the signal processing device disclosed in Patent Document 1 has the problem that, although it can reduce noise contained in images where pixel values ​​are represented as real numbers, it cannot reduce noise contained in radar images where pixel values ​​are represented as complex numbers.

[0005] This disclosure was made to solve the above-mentioned problems and aims to provide a signal processing device that can reduce noise contained in radar images. [Means for solving the problem]

[0006] The signal processing device according to this disclosure includes a function generation unit that generates an objective function relating to the sum of the differences between the absolute value of the pixel value of each pixel of interest and the absolute value of the pixel value of each adjacent pixel, with each of the multiple pixels included in a radar image in which the pixel value is represented by a complex number being a pixel of interest and each of the pixels adjacent to each pixel of interest being an adjacent pixel; and a pixel value update unit that updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit so that the sum of the differences becomes smaller. [Effects of the Invention]

[0007] According to this disclosure, noise contained in radar images can be reduced. [Brief explanation of the drawing]

[0008] [Figure 1] This is a configuration diagram showing a radar system including a signal processing device 4 according to Embodiment 1. [Figure 2] This is a hardware configuration diagram showing the hardware of the signal processing device 4 according to Embodiment 1. [Figure 3] This is a hardware configuration diagram of a computer when the signal processing device 4 is implemented by software or firmware, etc. [Figure 4]FIG. 4A is an explanatory diagram showing a first frequency band including the frequency of the RF signal and a second frequency band including the frequency of the RF signal, and FIG. 4B is an explanatory diagram showing a first frequency band including the frequency of the RF signal, a second frequency band including the frequency of the RF signal, and a third frequency band including the frequency of the RF signal. [Figure 5] It is an explanatory diagram showing the frequency change of the electromagnetic wave repeatedly radiated into space from antenna 1. [Figure 6] It is a flowchart showing a signal processing method which is a processing procedure of the signal processing device 4. [Figure 7] It is an explanatory diagram showing the time change of the signal obtained by performing inverse FFT on the signal after pulse compression processing. [Figure 8] It is an explanatory diagram showing an example of a first vector xi and a second vector xi+1. [Figure 9] It is a flowchart showing the update processing of the pixel value update unit 13. [Figure 10] It is an explanatory diagram showing an example of the sparse signal processing by the pixel value update unit 13. [Figure 11] It is a configuration diagram showing a radar device including the signal processing device 4 according to Embodiment 2. [Figure 12] It is a hardware configuration diagram showing the hardware of the signal processing device 4 according to Embodiment 2. [Figure 13] It is an explanatory diagram showing the frequency change of two electromagnetic waves repeatedly radiated into space from antenna 1.

Embodiments of the Invention

[0009] Hereinafter, in order to explain the present disclosure in more detail, embodiments for implementing the present disclosure will be described according to the accompanying drawings.

[0010] Embodiment 1. FIG. 1 is a configuration diagram showing a radar device including the signal processing device 4 according to Embodiment 1. FIG. 2 is a hardware configuration diagram showing the hardware of the signal processing device 4 according to Embodiment 1. The radar system shown in Figure 1 comprises an antenna 1, a transmitting / receiving unit 2, an analog-to-digital conversion unit (hereinafter referred to as "A / D conversion unit") 3, a signal processing unit 4, a tracking processing unit 5, and a display unit 6.

[0011] Antenna 1 radiates the RF (Radio Frequency) signal output from the transmitting / receiving unit 2 into space as an electromagnetic wave. The electromagnetic waves radiated into space from antenna 1 are either electromagnetic waves included in a first frequency band or electromagnetic waves included in a second frequency band separate from the first frequency band, and the electromagnetic waves included in the first frequency band and the electromagnetic waves included in the second frequency band are radiated alternately into space from antenna 1. Antenna 1 receives electromagnetic waves after they have been reflected by the target and outputs the received electromagnetic wave signal to the transmitting / receiving unit 2.

[0012] The transmitting / receiving unit 2 outputs an RF signal to the antenna 1. The transmitting / receiving unit 2 converts the frequency of the received signal output from the antenna 1 to an intermediate frequency and outputs the frequency-converted received signal to the A / D conversion unit 3.

[0013] When the A / D conversion unit 3 receives a signal from the transmitting / receiving unit 2, it converts the received signal from an analog signal to a digital signal. The digital signal after A / D conversion by the A / D conversion unit 3 is the IQ signal. The A / D conversion unit 3 outputs a digital signal to the signal processing unit 4.

[0014] The signal processing device 4 includes an image generation unit 11, a function generation unit 12, and a pixel value update unit 13. The signal processing device 4 acquires a digital signal from the A / D conversion unit 3. The signal processing device 4 generates a radar image from the digital signal, in which the pixel values ​​are represented as complex numbers. The radar image is, for example, a synthetic aperture radar image or an inverse synthetic aperture radar image. The signal processing device 4 performs noise reduction processing to reduce noise contained in the radar image. The signal processing unit 4 outputs the radar image after noise reduction processing to the tracking processing unit 5 and the display unit 6, respectively.

[0015] The tracking processing unit 5 acquires the radar image after noise reduction processing from the signal processing unit 4. The tracking processing unit 5 performs target tracking based on the radar image. The tracking processing unit 5 outputs the target tracking result to the display unit 6.

[0016] The display unit 6 acquires the radar image after noise reduction processing from the signal processing unit 4 and the target tracking result from the tracking processing unit 5. The display unit 6 displays the radar image after noise reduction processing on a display (not shown) and displays the target tracking results on the display.

[0017] The image generation unit 11 is implemented, for example, by the image generation circuit 21 shown in Figure 2. The image generation unit 11 acquires a digital signal from the A / D conversion unit 3. The image generation unit 11 generates a radar image from a digital signal, in which the pixel values ​​are represented as complex numbers. The image generation unit 11 outputs the radar image to the function generation unit 12 and the pixel value update unit 13, respectively.

[0018] The function generation unit 12 is implemented, for example, by the function generation circuit 22 shown in Figure 2. The function generation unit 12 acquires radar images from the image generation unit 11. The function generation unit 12 designates each of the multiple pixels included in the radar image as a "focus pixel," and the pixels adjacent to each focus pixel as "adjacent pixels." The function generation unit 12 generates an objective function that relates to the sum of the differences between the absolute value of the pixel value of each pixel of interest and the absolute values ​​of the pixel values ​​of each adjacent pixel. Specifically, the function generation unit 12 generates an objective function to find the pixel value of each pixel in the radar image when the sum of the differences between the pixel value of each pixel of interest and the pixel values ​​of its neighboring pixels is minimized. The function generation unit 12 outputs the objective function to the pixel value update unit 13.

[0019] The pixel value update unit 13 is implemented, for example, by the pixel value update circuit 23 shown in Figure 2. The pixel value update unit 13 obtains the objective function from the function generation unit 12. The pixel value update unit 13 updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit 12, so as to reduce the sum of the differences. Specifically, the pixel value update unit 13 differentiates the objective function generated by the function generation unit 12 and uses the derivative of the objective function to update the pixel value of each pixel included in the radar image. By updating the pixel value of each pixel through the update process of the pixel value update unit 13, the noise included in the radar image is reduced. The pixel value update unit 13 outputs the radar image after the pixel value update process to the tracking processing unit 5 and the display unit 6, respectively.

[0020] In Figure 1, the image generation unit 11, function generation unit 12, and pixel value update unit 13, which are components of the signal processing device 4, are assumed to be implemented by dedicated hardware as shown in Figure 2. That is, the signal processing device 4 is assumed to be implemented by an image generation circuit 21, a function generation circuit 22, and a pixel value update circuit 23. Each of the image generation circuit 21, function generation circuit 22, and pixel value update circuit 23 can be, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof.

[0021] The components of the signal processing device 4 are not limited to those implemented by dedicated hardware; the signal processing device 4 may also be implemented by software, firmware, or a combination of software and firmware. Software or firmware is stored as a program in the computer's memory. A computer refers to the hardware that executes programs, and includes, for example, a CPU (Central Processing Unit), GPU (Graphics Processing Unit), central processing unit, processing unit, arithmetic unit, microprocessor, microcomputer, processor, or DSP (Digital Signal Processor).

[0022] Figure 3 is a hardware configuration diagram of a computer when the signal processing device 4 is implemented by software or firmware, etc. If the signal processing device 4 is implemented by software or firmware, a program is stored in the memory 31 that causes the computer to execute the respective processing procedures in the image generation unit 11, the function generation unit 12, and the pixel value update unit 13. The computer's processor 32 then executes the program stored in the memory 31.

[0023] Furthermore, Figure 2 shows an example in which each component of the signal processing device 4 is implemented by dedicated hardware, and Figure 3 shows an example in which the signal processing device 4 is implemented by software or firmware, etc. However, this is only one example, and some components of the signal processing device 4 may be implemented by dedicated hardware, while the remaining components may be implemented by software or firmware, etc.

[0024] Next, we will explain the operation of the radar device shown in Figure 1. The transmitting / receiving unit 2 generates an RF signal and outputs the RF signal to the antenna 1. RF signals can include, for example, signals whose frequency increases or decreases over time. The frequency of an RF signal may vary continuously, but if there are bandwidth limitations imposed by the Radio Law or other regulations, the RF signal frequency may include gaps. Specifically, as shown in Figure 4A, an RF signal whose frequency is included in the first frequency band and an RF signal whose frequency is included in the second frequency band are generated, and a gap may exist between the first frequency band and the second frequency band. Figure 4A is an explanatory diagram showing the first frequency band containing the frequency of the RF signal and the second frequency band containing the frequency of the RF signal. In the example in Figure 4A, the frequency of the RF signal included in the first frequency band is lower than the frequency of the RF signal included in the second frequency band.

[0025] In the signal processing device 4 shown in Figure 1, the transmitting and receiving unit 2 alternately generates an RF signal whose frequency is included in a first frequency band and an RF signal whose frequency is included in a second frequency band, and alternately outputs the RF signal whose frequency is included in the first frequency band and the RF signal whose frequency is included in the second frequency band to the antenna 1. In the example shown in Figure 4A, an RF signal whose frequency is included in the first frequency band and an RF signal whose frequency is included in the second frequency band are generated, and a gap exists between the first and second frequency bands. However, this is just one example, and as shown in Figure 4B, an RF signal whose frequency is included in the first frequency band, an RF signal whose frequency is included in the second frequency band, and an RF signal whose frequency is included in the third frequency band are generated, and a gap exists between the first and second frequency bands, and a gap exists between the second and third frequency bands. In this case, the transmitting / receiving unit 2 outputs the RF signal whose frequency is included in the first frequency band, the RF signal whose frequency is included in the second frequency band, and the RF signal whose frequency is included in the third frequency band in order to the antenna 1. Figure 4B is an explanatory diagram showing the first frequency band containing the RF signal frequency, the second frequency band containing the RF signal frequency, and the third frequency band containing the RF signal frequency.

[0026] Antenna 1 radiates the RF signal into space as an electromagnetic wave, as shown in Figure 5, each time an RF signal, such as the one shown in Figure 4A, is output from the transmitting / receiving unit 2. Figure 5 is an explanatory diagram showing the frequency change of electromagnetic waves repeatedly radiated into space from antenna 1. The electromagnetic waves emitted from antenna 1 are reflected by the target. Antenna 1 receives electromagnetic waves after they have been reflected by the target and outputs the received electromagnetic wave signal to the transmitting / receiving unit 2. The transmitting / receiving unit 2 converts the frequency of the received signal output from the antenna 1 to an intermediate frequency and outputs the frequency-converted received signal to the A / D conversion unit 3.

[0027] The A / D conversion unit 3 converts the received signal from an analog signal to a digital signal each time a received signal is output from the transmitting / receiving unit 2. The A / D conversion unit 3 outputs a digital signal to the signal processing unit 4.

[0028] Figure 6 is a flowchart showing the signal processing method, which is the processing procedure of the signal processing device 4. The image generation unit 11 of the signal processing device 4 acquires a digital signal from the A / D conversion unit 3. The image generation unit 11 generates a radar image from the digital signal, in which the pixel values ​​are represented as complex numbers (step ST1 in Figure 6). The image generation unit 11 outputs the radar image to the function generation unit 12 and the pixel value update unit 13, respectively. The following describes in detail the radar image generation process performed by the image generation unit 11.

[0029] The image generation unit 11 performs an FFT (Fast Fourier Transform) on the digital signal in the range direction each time a digital signal is output from the A / D conversion unit 3. The image generation unit 11 generates a radar image by performing pulse compression processing on the signal after the FFT. Since the radar image generation process using pulse compression is a well-known technique, a detailed explanation is omitted. If a gap exists between the first and second frequency bands, the radar image may contain noise related to grating lobes. Figure 7 is an explanatory diagram showing the time evolution of the signal obtained by inverse FFT of the signal after pulse compression processing. As shown in Figure 7, the signal after the inverse FFT includes a grating lobe in addition to the main lobe related to the target. In Figure 7, the black dots indicate the sampling timing of the digital signal.

[0030] The function generation unit 12 acquires a radar image from the image generation unit 11. The radar image contains I pixels i (i=1,...,I), where I is an integer greater than or equal to 2. The function generation unit 12 uses each of the I pixels i (i=1,···,I) as the pixel of interest, and the pixels adjacent to each pixel of interest as neighboring pixels, to generate an objective function h(x) as shown in equation (0) below (step ST2 in Figure 6). The neighboring pixels adjacent to the pixel of interest are, for example, the pixel to the right of the pixel of interest, the pixel above the pixel of interest, or the pixel below the pixel of interest. The search for the minimum value of the objective function h(x) is a function for finding the optimal pixel values ​​of I pixels i(i=1,···,I), as shown in equation (1). Specifically, the objective function h(x) is a function for finding the pixel values ​​of I pixels i(i=1,···,I) when the sum of the differences Δx is minimized while satisfying the condition y=Φx. The difference Δx is the first vector x representing the pixel value of the pixel of interest, as shown in equation (2) below. i The absolute value of and a second vector x that indicates the pixel values ​​of adjacent pixels. i+1 This is the difference from the absolute value of [the given value].

[0031] In equations (0) to (2) of TIFF0007829815000001.tif56166, y is the spectrum of the digital signal, x is the range profile, and Φ is the inverse discrete Fourier transform matrix, all of which are known. λ is a hyperparameter. Figure 8 shows the first vector x i and the second vector xi+1 This is an explanatory diagram illustrating an example. The difference Δx becomes small when the first vector x i The absolute value of |x i | and the second vector x i+1 The absolute value of |x i+1 This occurs when | and are close together.

[0032] In the signal processing device 4 shown in Figure 1, the function generation unit 12 generates an objective function h(x) for finding the pixel values ​​of I pixels i (i=1,...,I) when the sum of the differences Δx is minimized. However, the objective function h(x) generated by the function generation unit 12 is not limited to a function for finding the pixel values ​​of I pixels i when the sum of the differences Δx is minimized. Therefore, within a range that does not pose practical problems, the function generation unit 12 may also generate an objective function h(x) for finding the pixel values ​​of I pixels i (i=1,...,I) when the sum of the differences Δx is the second smallest, or when the sum of the differences Δx is the third smallest.

[0033] In order for the pixel value update unit 13, described later, to calculate the optimal solution for the pixel values ​​of I pixels i (i=1,···,I), it is necessary to find the derivative of the objective function h(x). However, the objective function h(x) shown in equation (1) cannot be differentiated in its current form. The function generation unit 12 performs an approximation of the objective function h(x) using positive real scalar values ​​δ1, δ2, and δ3, as shown in equation (3) below, in order to make the objective function h(x) differentiable (step ST3 in Figure 6). Equation (3) represents the difference Δx contained in the objective function h(x) after the approximation. As real scalar values ​​δ1, δ2, and δ3, for example, values ​​smaller than the sampling interval of the digital signal used to generate the radar image are used. The function generation unit 12 outputs the objective function h(x) after differentiable approximation processing to the pixel value update unit 13.

[0034] TIFF0007829815000002.tif19166

[0035] The pixel value update unit 13 acquires a radar image from the image generation unit 11 and obtains the approximated objective function h(x) from the function generation unit 12. First, the pixel value update unit 13 identifies the pixel in the radar image that has grating lobe noise superimposed on it, in order to suppress the noise related to grating lobes included in the radar image.

[0036] Specifically, the pixel value update unit 13 performs an inverse FFT on the signal after pulse compression processing, and recognizes that signals with a signal intensity of 1 or greater than or equal to the first threshold Th1 in the signal after the inverse FFT (see Figure 7) are the main lobes. The pixel value update unit 13 then recognizes, among the signals after the inverse FFT (see Figure 7), that the signal intensity is greater than or equal to the second threshold Th2, less than the first threshold Th1, and temporally close to the main lobe, as the grating lobe. The first threshold Th1 is greater than the second threshold Th2. The first threshold Th1 and the second threshold Th2 may be stored in the internal memory of the pixel value update unit 13, or they may be provided from outside the signal processing device 4 shown in Figure 1. Grating lobes often occur temporally consecutively with the main lobe. Therefore, if the time difference between the occurrence time of a peak (hereinafter referred to as a "grating lobe candidate") where the signal intensity is above the second threshold Th2 but below the first threshold Th1, and the occurrence time of the main lobe peak is smaller than a certain time difference, the grating lobe candidate is recognized as a grating lobe.

[0037] The pixel value update unit 13 performs signal processing to reduce the signal intensity of the grating lobe included in the signal after the inverse FFT. One example of signal processing to reduce the signal intensity of the grating lobe is to reduce the signal intensity of the grating lobe to approximately 0. As a result, if the pixel i included in the radar image is a pixel with noise related to the grating lobe superimposed thereon, the pixel value of the pixel i is updated, and the grating lobe is reduced.

[0038] After reducing the grating lobe, the pixel value update unit 13 updates the pixel values of the I pixels i (i = 1, ···, I) so that the sum of the differences Δx between the absolute value |x i | of the first vector x i and the absolute value |x i+1 | of the second vector x i+1 becomes small, based on the objective function h(x) after the approximation process (step ST4 in FIG. 6). When updating the pixel value of the pixel i, the pixel value update unit 13 uses, as the initial value of the pixel value, for example, the pixel value of the pixel i after the signal processing for reducing the signal intensity of the grating lobe. Hereinafter, the update process of the pixel value of the pixel i by the pixel value update unit 13 will be specifically described. FIG. 9 is a flowchart showing the update process of the pixel value update unit 13.

[0039] The pixel value update unit 13 differentiates the objective function h(x) after the approximation process, as shown in the following formula (4) (step ST11 in FIG. 9).

[0040] TIFF0007829815000003.tif26166

[0041] The pixel value update unit 13 updates the pixel value of each pixel i included in the radar image by using the differential value ∇h(x) of the objective function h(x), as shown in the following formula (5) (step ST12 in FIG. 9).

[0042] TIFF0007829815000004.tif12166 In formula (5), x[k] is the pixel value of the pixel i indicated by the first vector x i before the update process, and x[k + 1] is the pixel value of the pixel i indicated by the first vector x i after the update process.

[0043] If the pixel value update unit 13 has not satisfied the termination condition for the update process that updates the pixel value of each pixel i included in the radar image (step ST13 in Figure 9: NO), it will again perform the process of differentiating the objective function h(x) (step ST11 in Figure 9) and the process of updating the pixel value (step ST12 in Figure 9). The termination condition for the update process is, for example, that the number of update processes that update the pixel value has reached the upper limit, or that the change in the pixel value due to the update is less than the threshold σ. The change in pixel value due to the update being less than the threshold σ means that x[k+1]-x[k]<σ. The upper limit number of operations and the threshold σ may be stored in the internal memory of the pixel value update unit 13, or they may be provided from outside the signal processing device 4 shown in Figure 1. The pixel value update unit 13 terminates the update process for updating pixel values ​​if the termination condition for the update process is met (in step ST13: YES in Figure 9).

[0044] The update of the value shown in equation (5) is a sparse signal processing method based on the gradient method. Sparse signal processing, as shown in Figure 10, is a process that reduces the difference in signal intensity between adjacent pixels. By performing sparse signal processing, noise contained in radar images is reduced. Figure 10 is an explanatory diagram showing an example of sparse signal processing by the pixel value update unit 13. In Figure 10, the dashed line shows the signal after sparse signal processing. The pixel value update unit 13 outputs the radar image after the pixel value update process to the tracking processing unit 5 and the display unit 6, respectively.

[0045] In the signal processing device 4 shown in Figure 1, the pixel value update unit 13 determines the pixel value of each pixel included in the radar image when the sum of the differences Δx is minimized. However, the pixel value determined by the pixel value update unit 13 is not limited to the pixel value when the sum of the differences Δx is minimized. Therefore, within a range that does not pose a practical problem, the pixel value update unit 13 may, for example, determine the pixel value of each pixel included in the radar image when the sum of the differences Δx is second smallest, or when the sum of the differences Δx is third smallest.

[0046] The tracking processing unit 5 acquires the radar image after the pixel value update process, which is the radar image after noise reduction processing, from the pixel value update unit 13. The tracking unit 5 performs target tracking based on the radar image. Since the target tracking process itself is a well-known technique, a detailed explanation is omitted. The tracking processing unit 5 outputs the target tracking result to the display unit 6.

[0047] The display unit 6 acquires the radar image after the pixel value update process from the pixel value update unit 13 and acquires the target tracking result from the tracking processing unit 5. The display unit 6 displays the radar image after the pixel value update process on a display (not shown), and displays the target tracking results on the display.

[0048] In the above embodiment 1, the signal processing device 4 is configured to include a function generation unit 12 that generates an objective function relating to the sum of the differences between the absolute value of the pixel value of each of the multiple pixels included in a radar image in which the pixel values ​​are represented by complex numbers, with each of the pixels included in the radar image being a pixel of interest and each pixel adjacent to each pixel of interest being an adjacent pixel, and a pixel value update unit 13 that updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit 12 so that the sum of the differences becomes smaller.Therefore, the signal processing device 4 can reduce the noise included in the radar image.

[0049] In Embodiment 1, the signal processing device 4 is configured such that the pixel value update unit 13 repeatedly updates the pixel value of each pixel included in the radar image until the number of update processes reaches the upper limit, or until the change in pixel value due to the update falls below a threshold. Therefore, the signal processing device 4 can enhance the noise reduction effect while suppressing the processing load of the noise reduction process.

[0050] In Embodiment 1, the signal processing device 4 is configured such that the pixel value update unit 13 updates the pixel value of each pixel included in the radar image in order to suppress noise related to grating lobes included in the radar image, and then updates the pixel value of each pixel included in the radar image in order to reduce the sum of the differences based on the objective function generated by the function generation unit 12. Therefore, by providing a gap between the first frequency band and the second frequency band, the signal processing device 4 can suppress noise related to grating lobes even if noise related to grating lobes is generated in the radar image.

[0051] Embodiment 2. Embodiment 2 describes a signal processing device 4 in which the pixel value update unit 14 differentiates the objective function after approximation processing by the function generation unit 12 twice, and updates the pixel value of each pixel included in the radar image using the second derivative of the objective function.

[0052] Figure 11 is a configuration diagram showing a radar system including a signal processing device 4 according to Embodiment 2. In Figure 11, the same reference numerals as in Figure 1 indicate the same or corresponding parts, so a detailed explanation is omitted. Figure 12 is a hardware configuration diagram showing the hardware of the signal processing device 4 according to Embodiment 2. In Figure 12, the same reference numerals as in Figure 2 indicate the same or corresponding parts, so a detailed explanation is omitted. The signal processing device 4 includes an image generation unit 11, a function generation unit 12, and a pixel value update unit 14.

[0053] The pixel value update unit 14 is implemented, for example, by the pixel value update circuit 24 shown in Figure 12. The pixel value update unit 14 obtains the objective function from the function generation unit 12. The pixel value update unit 14 updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit 12, so as to reduce the sum of the differences. Specifically, the pixel value update unit 14 performs an update process that updates the pixel value of each pixel in the radar image by differentiating the objective function generated by the function generation unit 12 twice and using the second derivative of the objective function. By updating the pixel value of each pixel through the update process of the pixel value update unit 14, the noise contained in the radar image is reduced. The pixel value update unit 14 outputs the radar image after the pixel value update process to the tracking processing unit 5 and the display unit 6, respectively.

[0054] In Figure 11, the image generation unit 11, function generation unit 12, and pixel value update unit 14, which are components of the signal processing device 4, are assumed to be implemented by dedicated hardware as shown in Figure 12. That is, the signal processing device 4 is assumed to be implemented by an image generation circuit 21, a function generation circuit 22, and a pixel value update circuit 24. Each of the image generation circuit 21, function generation circuit 22, and pixel value update circuit 24 can be, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC, an FPGA, or a combination thereof.

[0055] The components of the signal processing device 4 are not limited to those implemented by dedicated hardware; the signal processing device 4 may also be implemented by software, firmware, or a combination of software and firmware. If the signal processing device 4 is implemented by software or firmware, a program that causes the computer to execute the respective processing procedures in the image generation unit 11, the function generation unit 12, and the pixel value update unit 14 is stored in the memory 31 shown in Figure 3. Then, the processor 32 shown in Figure 3 executes the program stored in the memory 31.

[0056] Furthermore, Figure 12 shows an example in which each component of the signal processing device 4 is implemented by dedicated hardware, and Figure 3 shows an example in which the signal processing device 4 is implemented by software or firmware, etc. However, this is only one example, and some components of the signal processing device 4 may be implemented by dedicated hardware, while the remaining components may be implemented by software or firmware, etc.

[0057] Next, the operation of the radar system shown in Figure 11 will be explained. However, since it is the same as the radar system shown in Figure 1 except for the pixel value update unit 14, only the operation of the pixel value update unit 14 will be explained here. The pixel value update unit 14, similar to the pixel value update unit 13 shown in Figure 1, reduces grating lobes and then differentiates the approximated objective function h(x) twice, as shown in equation (6) below.

[0058] TIFF0007829815000005.tif28166

[0059] The pixel value update unit 14 calculates the second derivative of the objective function h(x) as shown in equation (7) below. 2 The pixel value of each pixel i included in the radar image is updated using h(x).

[0060] TIFF0007829815000006.tif10166

[0061] The pixel value update unit 14 performs the process of differentiating the objective function h(x) twice and updating the pixel value again if the number of update processes for updating the pixel value has not reached the upper limit and the change in the pixel value due to the update is greater than or equal to the threshold σ. The pixel value update unit 14 terminates the pixel value update process if the number of update processes for updating the pixel value reaches the upper limit, or if the change in the pixel value due to the update is less than the threshold σ.

[0062] The update of the value shown in equation (7) is sparse signal processing based on Newton's method. Sparse signal processing based on Newton's method has slightly lower update accuracy than sparse signal processing based on the gradient method as shown in equation (5), but it takes less processing time than sparse signal processing based on the gradient method. The pixel value update unit 14 outputs the radar image after the pixel value update process to the tracking processing unit 5 and the display unit 6, respectively.

[0063] In the above embodiment 2, the signal processing device 4 is configured such that the pixel value update unit 14 takes the second derivative of the objective function after approximation processing by the function generation unit 12, and updates the pixel value of each pixel included in the radar image using the second derivative of the objective function. Therefore, the signal processing device 4 can reduce noise included in the radar image.

[0064] Embodiment 3. Embodiment 3 describes a radar device in which the transmitting / receiving unit 2 simultaneously radiates a first electromagnetic wave included in a first frequency band and a second electromagnetic wave included in a second frequency band separate from the first frequency band into space from the antenna 1.

[0065] The configuration of the radar device according to Embodiment 3 is the same as the configuration of the radar device according to Embodiment 1 or the configuration of the radar device according to Embodiment 2. Therefore, the configuration diagram showing the radar device according to Embodiment 3 is Figure 1 or Figure 11.

[0066] The transmitting / receiving unit 2 simultaneously generates an RF signal whose frequency is included in the first frequency band and an RF signal whose frequency is included in the second frequency band. The transmitting / receiving unit 2 simultaneously outputs to the antenna 1 an RF signal whose frequency is included in the first frequency band and an RF signal whose frequency is included in the second frequency band.

[0067] Antenna 1 simultaneously radiates two RF signals as electromagnetic waves into space, as shown in Figure 13, each time two RF signals are output from the transmitting / receiving unit 2. Figure 13 is an explanatory diagram showing the frequency changes of two electromagnetic waves repeatedly radiated into space from antenna 1. The two electromagnetic waves emitted from antenna 1 are reflected by the target. Antenna 1 receives the two electromagnetic waves after reflection from the target and outputs the received signals of the two electromagnetic waves to the transmitting / receiving unit 2.

[0068] The transmitting / receiving unit 2 has, for example, two or more filters, including a high-pass filter, a band-pass filter, or a low-pass filter. When the transmitting / receiving unit 2 receives two received signals from the antenna 1, it filters the two received signals to separate them into a received signal whose frequency is included in the first frequency band and a received signal whose frequency is included in the second frequency band. The transmitting / receiving unit 2 converts the frequency of each separated received signal to an intermediate frequency and outputs each of the frequency-converted received signals to the A / D conversion unit 3.

[0069] Here, the transmitting / receiving unit 2 separates the two received signals output from antenna 1 into two signals: one whose frequency is included in the first frequency band and another whose frequency is included in the second frequency band, by passing them through a filter. However, the signal separation process performed by the transmitting / receiving unit 2 is not limited to passing the two received signals through a filter.

[0070] The A / D conversion unit 3 converts each of the two received signals from the transmitting / receiving unit 2 from an analog signal to a digital signal each time two received signals are output. The A / D conversion unit 3 outputs each digital signal to the signal processing unit 4. The processing content of the signal processing device 4 is the same as that shown in Figure 1 or Figure 11, so a detailed explanation is omitted. As a result, the radar device according to Embodiment 3 can reduce noise contained in the radar image, just like the radar devices according to Embodiments 1 and 2.

[0071] Furthermore, this disclosure allows for free combination of each embodiment, modification of any component in each embodiment, or omission of any component in each embodiment. [Industrial applicability]

[0072] The signal processing device, signal processing method, and radar device according to this invention include a function generation unit that generates an objective function relating to the sum of the differences between the absolute value of the pixel value of each of the multiple pixels included in a radar image in which the pixel values ​​are represented by complex numbers, with each of the multiple pixels included in the radar image being a pixel of interest and each pixel adjacent to each pixel of interest being an adjacent pixel, and a pixel value update unit that updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit so as to reduce the sum of the differences, thereby reducing noise included in the radar image, and is suitable for signal processing devices, signal processing methods, and radar devices. [Explanation of symbols]

[0073] 1 Antenna, 2 Transceiver unit, 3 A / D converter unit, 4 Signal processing unit, 5 Tracking processing unit, 6 Display unit, 11 Image generation unit, 12 Function generation unit, 13 Pixel value update unit, 14 Pixel value update unit, 21 Image generation circuit, 22 Function generation circuit, 23 Pixel value update circuit, 24 Pixel value update circuit, 31 Memory, 32 Processor.

Claims

1. A function generation unit generates an objective function that relates to the sum of the differences between the absolute value of the pixel value of each pixel of interest and the absolute value of the pixel value of each adjacent pixel, where each of the multiple pixels in the radar image, whose pixel values ​​are represented as complex numbers, is designated as the "pixel of interest," and the pixels adjacent to each pixel of interest are designated as the "adjacent pixels." A pixel value update unit updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit, so as to reduce the sum of the differences. A signal processing device equipped with [a specific component].

2. The function generation unit, As the aforementioned objective function, we generate an objective function for determining the pixel value of each pixel in the radar image when the sum of the differences between the pixel value of each pixel of interest and the pixel values ​​of its neighbors is minimized. The signal processing apparatus according to claim 1, characterized in that it is a signal processing apparatus.

3. The function generation unit, The objective function is approximated using a positive real scalar value. The signal processing apparatus according to claim 1 or 2, characterized in that it is a signal processing apparatus.

4. The function generation unit, The real scalar value used is smaller than the sampling interval of the signal used to generate the radar image. The signal processing apparatus according to claim 3, characterized in that it is a signal processing apparatus.

5. The aforementioned pixel value updating unit, The objective function after approximation processing by the function generation unit is differentiated, and the pixel value of each pixel included in the radar image is updated using the derivative of the objective function. The signal processing apparatus according to claim 3, characterized in that it is a signal processing apparatus.

6. The aforementioned pixel value updating unit, The objective function, after approximation processing by the function generation unit, is differentiated twice, and the pixel value of each pixel included in the radar image is updated using the second derivative of the objective function. The signal processing apparatus according to claim 3, characterized in that it is a signal processing apparatus.

7. The aforementioned pixel value updating unit, The pixel value of each pixel included in the radar image is repeatedly updated until the number of update processes that update the pixel value of each pixel reaches the upper limit, or until the change in the pixel value due to the update falls below a threshold. The signal processing apparatus according to claim 1 or 2, characterized in that it is a signal processing apparatus.

8. The aforementioned pixel value updating unit, In order to suppress the noise related to grating lobes included in the radar image, the pixel values ​​of each pixel included in the radar image are updated, and then, based on the objective function generated by the function generation unit, the pixel values ​​of each pixel included in the radar image are updated so that the sum of the differences is reduced. The signal processing apparatus according to claim 1 or 2, characterized in that it is a signal processing apparatus.

9. The function generation unit takes multiple pixels in a radar image, where pixel values ​​are represented as complex numbers, as "focus pixels," and the pixels adjacent to each focus pixel as "adjacent pixels." It then generates an objective function related to the sum of the differences between the absolute values ​​of the pixel values ​​of each focus pixel and the absolute values ​​of the pixel values ​​of each adjacent pixel. The pixel value update unit updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit, so as to reduce the sum of the differences. Signal processing method.

10. A transmitting and receiving unit that radiates electromagnetic waves into space, receives the electromagnetic waves after they have been reflected by a target, and outputs a signal indicating that the electromagnetic waves have been received, An image generation unit generates a radar image in which pixel values ​​are represented by complex numbers from the received signal output from the aforementioned transmitting and receiving unit, A function generation unit generates an objective function relating to the sum of the differences between the absolute value of the pixel value of each pixel of interest and the absolute value of the pixel value of each adjacent pixel, with each of the multiple pixels included in the radar image generated by the image generation unit being designated as a pixel of interest, and the pixels adjacent to each pixel of interest being designated as adjacent pixels. A pixel value update unit updates the pixel value of each pixel included in the radar image based on the objective function generated by the function generation unit, so as to reduce the sum of the differences. A radar device equipped with [a specific feature / feature].

11. The aforementioned transmitting and receiving unit is The system alternately radiates electromagnetic waves into space, including those contained in a first frequency band and those contained in a second frequency band that is separate from the first frequency band. The radar device according to claim 10, characterized in that it is a radar device.

12. The aforementioned transmitting and receiving unit is After simultaneously radiating a first electromagnetic wave contained in a first frequency band and a second electromagnetic wave contained in a second frequency band separate from the first frequency band into space, the first electromagnetic wave and the second electromagnetic wave after reflection by the target are received, and the received signals of the first electromagnetic wave and the second electromagnetic wave are output to the image generation unit. The radar device according to claim 10, characterized in that it is a radar device.

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