Angle-resolved radar sensor

Abstract

An angle-resolving radar sensor having an antenna array (10) with a number N of antenna elements (12) offset from one another in a scanning direction (y), a digital beamforming device (20) and an angle estimation device (22) configured to perform an angle estimation on the basis of signals of the beamforming device (20), characterized in that the aperture A of the antenna array (10) in units of the wavelength λ is greater than (N-1) / 2 and the center spacings between adjacent antenna elements (12) differ from one another by no more than a predetermined amount from the value A / (N-1).

Description

Technical Field

[0001] The present invention relates to an angle-resolved radar sensor having an antenna array, a digital beamforming device, and an angle estimation device. The antenna array has N antenna elements offset from each other in the scanning direction, and the angle estimation device is configured to perform angle estimation based on the signal from the beamforming device. Background Technology

[0002] Radar sensors are used in motor vehicles, for example, to measure the distance, relative speed, and azimuth angle of vehicles or other objects positioned in front of the vehicle. Individual antennas are then arranged at a distance from each other, for example, on a horizontal line, so that different azimuth angles of the positioned objects result in differences in the propagation length (the distance the radar signal must travel from the object to the corresponding antenna). These differences in propagation length result in corresponding differences in the phase of the signals received by the antenna elements and analyzed and processed in their respective analysis and processing channels. The angle of incidence of the radar signal can then be determined by comparing the (complex) amplitudes received in different channels with the corresponding amplitudes in the antenna diagram, and thus the azimuth angle of the positioned object.

[0003] In a similar manner, it is also possible to measure the elevation angle of an object. Then, the scanning direction in which the antenna elements are offset from each other is not a horizontal line but a vertical line.

[0004] To achieve high angular resolution, the antenna aperture should be as large as possible. However, if the spacing between adjacent antenna elements is too large, multivaluedness may occur in angle measurements because the received signals will have the same phase relationship for propagation length differences that are integer multiples of the wavelength λ. For example, single-valued angle measurements can be achieved using a ULA (Uniform Linear Array) structure, in which antenna elements are arranged at a spacing of λ / 2.

[0005] A process called digital beamforming can modify the main sensitivity direction of an antenna array, and thus similarly, the main receiving lobe of a radar sensor can be steered in a specific direction, where, however, side lobes with a certain sensitivity also appear on either side of the main receiving lobe. In digital beamforming, the complex amplitudes received by each individual antenna element are weighted by an angle-dependent complex phase factor, which corresponds to the difference in the propagation length of the radar radiation for a given angle. If multiple beams with different azimuth angles are formed in this way, a gain function is obtained for each beam, which describes the antenna gain for each angle, and this antenna gain is derived if the object being located is in the relevant angle. Ideally, for an object in a given angle, the signal strength corresponding to the theoretical antenna gain of that beam should be measured in each beam, and the measured amplitudes should only be correctly related to each other at exactly one angle, i.e., the angle in which the object is actually located.

[0006] However, in practice, the measured signals are more or less contaminated with noise, so the positioning angle can only be estimated by searching for an angle for which the amplitude measured in different beams best correlates with the theoretical value. This correlation can be expressed, for example, by a so-called DML function (Deterministic Maximum Likelihood function), and the angle estimation then involves searching for the maximum value of this DML function.

[0007] If the aperture of the antenna array is increased to achieve higher angular resolution, and if the measurement should still remain single-valued, the number of antenna elements must be increased. However, this also increases the number of analysis processing channels required, thus increasing the required computing power and, consequently, the hardware cost.

[0008] A radar sensor is known from WO2013 / 056880 A1, in which it operates using a sparse antenna array, in which the spacing is increased for at least some of adjacent antenna element pairs, thereby achieving a larger aperture with a given number of analysis processing channels. Then, by alternating measurements with different combinations of antenna elements, the single-valuedness of the angle measurement is restored, thereby filling the gaps in the antenna array. Summary of the Invention

[0009] The objective of this invention is to realize an angle-resolved radar sensor that enables high-resolution, single-value angle measurements while reducing the computational overhead for signal analysis and processing.

[0010] According to the present invention, this task is solved by the following method: the aperture A of the antenna array is greater than (N-1) / 2 in wavelength λ, and the center spacing between adjacent antenna elements is different from each other, but the deviation from the value A / (N-1) does not exceed a predetermined amount.

[0011] Because the aperture is greater than (N-1) / 2, multivaluedness is inherently present in angle estimation. In the DML function used for ULA, this manifests as the function having multiple maximum values ​​with the same height at different angles. According to the invention, the antenna elements are not arranged with precisely uniform spacing, but rather with slight deviations between pairs. In this way, in the DML function, all maximum values ​​except for one are reduced in height, thus the function again has a single absolute maximum value, and therefore a single-valued angle estimation is possible. However, the deviation of the antenna spacing from the value A / (N-1) corresponding to the spacing in ULA is so limited that the same technique as in ULA can always be used for digital beamforming, and sidelobes are sufficiently attenuated. In particular, digital beamforming can be performed in a particularly efficient manner using Fast Fourier Transform (FFT).

[0012] Therefore, the radar sensor according to the present invention has quasi-regular characteristics. An array replaces the ULA, and in this quasi-regular array, the deviation from the perfect ULA is just large enough that, given the expected noise level, single-valued angle estimation can still be achieved. Here, the "predetermined amount" that allows the spacing of the antenna elements to deviate most from the ULA value A / (N-1) is chosen such that, on the one hand, sufficient robustness to signal noise is achieved, while on the other hand, the angle-dependent gain function is less distorted compared to the ULA.

[0013] Advantageous configurations and extensions of the invention are described in the dependent claims.

[0014] In one embodiment, the deviation of the center-to-center spacing between antenna elements from the value A / (N-1) is less than 25%, that is, the absolute value of the deviation is less than A / 4(N-1), and preferably, the deviation is less than 15%.

[0015] Here, the deviation in the spacing between different pairs of adjacent antenna elements can be regularly varied, for example, according to a linear function, a quadratic function, or also according to a higher-order polynomial or, for example, a sine function. The number N of antenna elements in the array is preferably a power of two, i.e., N = 8 or N = 16, thereby enabling efficient digital beamforming by means of FFT.

[0016] The following describes the embodiments in further detail with reference to the accompanying drawings. Attached Figure Description

[0017] The attached diagram shows:

[0018] Figure 1 A block diagram of a radar sensor with a quasi-regular antenna array according to the present invention is shown;

[0019] Figure 2 A schematic diagram of a regular antenna array (ULA) for a conventional radar sensor is shown.

[0020] Figure 3 The diagram shows the gain distribution functions for different beams, which have been based on... Figure 2 The antenna array is formed using digital beamforming;

[0021] Figure 4 Showing something similar to Figure 3 A simplified diagram, in which, for clarity, only the gain functions for the two beams are shown;

[0022] Figure 5 Showing the use of according to Figure 2 DML functions for antenna arrays;

[0023] Figure 6 An example of a ULA with an enlarged aperture is shown;

[0024] Figure 7 Showing something similar to Figure 4 of, but used according to Figure 6 A schematic diagram of the two gain functions of the array;

[0025] Figure 8 Showing the use of according to Figure 6 DML functions for antenna arrays;

[0026] Figure 9 Showing something similar to Figure 4 and 7 of, but used according to Figure 1 A schematic diagram of the two gain functions of the array according to the present invention;

[0027] Figure 10 Showing the use of according to Figure 1 DML functions for arrays. Detailed Implementation

[0028] exist Figure 1The image shows a radar sensor with a quasi-regular antenna array 10, illustrated graphically. This quasi-regular antenna array has eight antenna elements 12. Each antenna element 12 is formed by a column of eight serially fed antenna patches 14 extending in the vertical direction z. The columns of antenna elements 12 are arranged in a row in the scanning direction y, which corresponds to the horizontal lateral direction of the vehicle in a radar sensor used in a motor vehicle, thus providing the radar sensor with angular resolution in the azimuth angle. The spacing between the antenna elements 12 and the spacing between the individual antenna patches 14 in each column are described in units of the radar radiation wavelength λ. The spacing between pairs of adjacent antenna elements 12 is also... Figure 1 The values ​​are quantitatively described in the table, and each has a value of approximately 2, however, they deviate from the average value of 2.00 by less than 7%.

[0029] The antenna array 10 has a width of approximately 14λ in the scanning direction y, thus the array has an aperture A = 14 in the azimuth angle. Generally, when N is the number of antenna elements 12 in the array, the average spacing between the antenna elements 12 (2.00 in this example) is equal to A / (N-1).

[0030] In the example shown, the spacing between pairs of antenna elements 12 increases linearly from 1.87 to 2.13.

[0031] Eight antenna elements 12 are connected to an analysis and processing circuit 18 via corresponding signal lines 16, in which the received signals are analyzed and processed in independent receiving channels. For example, the radar sensor shown here could be an FMCW (Frequency Modulated Continuous Wave) radar. Each analysis and processing channel then includes a mixer in which the signal received by the antenna elements is mixed with a portion of the transmitted radar signal to obtain an intermediate frequency (IF) signal. The frequency of this IF signal depends on both the travel time of the radar signal from the radar sensor to the object and back, and the relative velocity of the object. The IF signal is digitized in the analysis and processing circuit 18 and recorded separately within a certain sampling period, during which the frequency of the transmitted signal is modulated in a ramp manner. Then, based on the frequencies of the IF signals obtained at multiple modulation ramps with different ramp slopes, the distance and relative velocity of the located objects can be determined in a known manner.

[0032] The signal lines 16 connecting the antenna elements and the analysis and processing circuit 18 are configured such that all of these signal lines have the same length, so that the phase relationship between the signals is not distorted on the path to the analysis and processing circuit 18. Then, by comparing the amplitude and phase (i.e., complex amplitude) of the signals received in the eight receiving channels, an angle (azimuth angle) indicating the direction from the radar sensor to the object can be determined for each object positioned at a given spacing and relative velocity. For this purpose, the signals received in the eight receiving channels are digitally beamformed in the beamformer 20, for example, using a Fast Fourier Transform (FFT). The beamforming result is passed to the angle estimation device 22, where the azimuth angle φ of the positioned object is determined using maximum likelihood estimation.

[0033] To illustrate how the invention works, one should first consider a fully filled, regular antenna array 24 (ULA), as used in conventional radar sensors and as in... Figure 2 As shown in the graphic representation. In this array, the spacing between adjacent antenna elements 12 is λ / 2, thus satisfying the single-valuedness condition. However, this array (in the case of eight receiving channels) only has an aperture of A = 3.5, thus significantly limiting the angular resolution.

[0034] Figure 3 This is a schematic diagram, in which, according to Figure 2 The antenna array 24 illustrates the antenna gain of the beamforming device 20 as a function of the azimuth angle φ. Specifically, the schematic diagram shows graphs of normalized gain functions 28 to 40 for ten received beams, with maximum sensitivity values ​​located outside the ranges of 0°, + / -15°, + / -30°, + / -45°, and + / -60°. Each beam has a main lobe with maximum gain and multiple side lobes attenuating by approximately 13 dB. This schematic diagram is based on beamforming using an FFT with a rectangular window.

[0035] For clarity, in Figure 4 The curves for gain functions 30 and 36 are shown separately again. The curve for gain function 30 is indicated by a thick line, while the curve for gain function 36 is indicated by a dashed line. It can be seen that gain function 30, which has a maximum value at 0°, has symmetrical sidelobes, while the gain function of the beam that has a maximum sensitivity at -45° is asymmetrical.

[0036] In digital beamforming, a weighted sum is formed from the complex amplitudes of the signals received in the eight antenna elements 12 using complex weighting factors (which reflect the differences in propagation length between antenna elements). Since these propagation length differences depend on the azimuth angle φ, a different set of weighting factors is obtained for each beam (which has the maximum sensitivity at a given azimuth angle). If the object is positioned at a given azimuth angle φ, a signal with the maximum sensitivity at 0° is obtained, the strength of which is proportional to the gain function 28, while for the same object, a signal with the maximum sensitivity at -45° is obtained, the strength of which is proportional to the gain function 36. Accordingly, for each of the remaining beams, a value given by its respective gain function is also obtained.

[0037] By analyzing the different amplitude values ​​obtained after beamforming different beams, the actual azimuth angle of the located object can be inferred. Therefore, the search is conducted for the angle at which the measured value best correlates with the value given by the gain function.

[0038] As an example, Figure 5 The diagram illustrates the DML function 42 (deterministic maximum likelihood function), also known as the angle spectrum, for a target at an azimuth angle φ = 0°. The DML function 42 describes the correlation between the measured value and the gain function used for different beams for each azimuth angle φ. The function is normalized such that its maximum value has a value of 1. It can be seen that the DML function 42 has a unique and distinct maximum value only at the angle φ = 0° where the located object is situated. Alternatively, if a signal is received from an object at an angle φ = 20°, the DML function will be shifted so that its maximum value is located at 20°.

[0039] To improve angular resolution, now, based on Figure 2 In ULA 24, the spacing between antenna elements 12 is increased to 2λ, however, the number of antenna elements is not increased. Then, as in Figure 6 As shown, ULA44 with an aperture of A=14 was obtained.

[0040] Figure 7 This illustrates how these changes affect the gain function from 28 to 40. Similar to... Figure 4 As in, in Figure 7 Only the graphs of gain functions 28 and 36 are shown. It can be seen that these gain functions each have multiple maximum values ​​with approximately the same height. For example, gain function 28 has a maximum value at 0° (as shown in...). Figure 4(as in [the previous text]), but with other maximum values ​​of the same height at + / -30°. Between these, there are larger numbers of sidelobes with stronger attenuation. The same applies accordingly to gain function 36 and any others (in [the previous text]). Figure 7 (Gain function not shown in the diagram)

[0041] Used according to Figure 6 ULA's DML functions in Figure 8 As shown in the diagram. The function now also has multiple principal maxima of the same height, thus no longer enabling single-value angle estimation.

[0042] Conversely, if using according to Figure 1 The quasi-regular antenna array 10 can then achieve single-value angle estimation. The gain functions 28 and 36 used for this array... Figure 9 As shown in [the image]. Similar to [the image shown in the image]. Figure 7 As in the example, each gain function has three prominent principal maxima in the angular range of -50° to +50°. However, here, the sidelobes are more "angeschultert", meaning that the sidelobes are raised and closer to the flanks of the principal maxima.

[0043] Figure 10 The corresponding DML function 42 is shown. Due to the beamforming in the beamforming device 20, it is aligned with the beamforming in accordance with... Figure 2 and 6 The same approach is used in the ULA, with the same weighting factors, but the spacing between adjacent antenna elements 12 is slightly uneven. Therefore, (when the target is at 0°) only the main maximum value 46 at φ = 0° retains the full height, while the flank maximum value 48 at + / -30° is strongly suppressed. This means that the maximum value of a single value is found in the angle estimation, and thus a single-value angle estimation can be achieved.

[0044] In most cases, when the signal is so heavily contaminated by noise that the difference between the maximum values ​​46 and 48 becomes blurred, and one of the maximum values ​​48 is the highest and is incorrectly chosen to determine the azimuth angle, erroneous angle estimations occur. The more non-uniform the spacing between adjacent antenna elements 12, the more the maximum value 48 is suppressed, and the more robust the angle estimation is to signal noise. However, on the other hand, with increasing array non-uniformity, sidelobes become increasingly prominent. However, by appropriately selecting the non-uniformity of the spacing between antenna elements 12, it is possible to achieve single-valued angle estimation even with normally present signal noise, and to achieve higher angular resolution due to the increased aperture, without requiring additional analysis processing channels.

Claims

1. An angle-resolved radar sensor, the radar sensor having an antenna array (10), a digital beamforming device (20), and an angle estimation device (22), the antenna array having a number N>2 antenna elements (12) offset from each other in the scanning direction (y), in wavelength λ, the aperture A of the antenna array (10) being greater than (N-1) / 2, the angle estimation device being configured to perform angle estimation based on the signal from the beamforming device (20), characterized in that, The center spacing between adjacent antenna elements (12) is different from each other, but the deviation from the value A / (N-1) does not exceed a predetermined amount, wherein the spacing between the adjacent antenna elements deviates from the value A / (N-1) by no more than 25%, wherein the value A / (N-1) corresponds to the spacing in a uniform linear array.

2. The radar sensor according to claim 1, wherein the spacing between adjacent antenna elements deviates from the value A / (N-1) by no more than 15%.

3. The radar sensor according to claim 1 or 2, wherein the spacing between the antenna elements (12) changes according to a regular pattern.

4. The radar sensor according to claim 3, wherein the spacing between the antenna elements (12) is changed according to a polynomial function.

5. The radar sensor according to claim 4, wherein the spacing between the antenna elements (12) changes according to a linear function.

6. The radar sensor according to claim 1 or 2, wherein N is a power of two.

7. The radar sensor according to claim 1 or 2, wherein A ≥ 1.

8. The radar sensor according to claim 7, wherein A ≥ 2.

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