MIMO radar sensor

Abstract

A MIMO radar sensor comprises: a planar antenna array having two subarrays, wherein multiple antennas in the two subarrays are respectively offset from each other in a first direction x, the subarrays being offset from each other in a second direction y perpendicular to the first direction x, the antennas of the two subarrays being more focused in the second direction y than in the first direction, and at least two of the antennas in at least one of the subarrays being also offset from each other in the second direction y; a high-frequency section for generating a transmitted signal from one subarray antenna and preprocessing a received signal from the other subarray antenna; and a control and analysis processing device configured to control the high-frequency section and determine the distance, relative velocity, azimuth angle, and elevation angle of the target object based on the preprocessed received signal; wherein at least one subarray is constructed to be symmetrical about an axis extending in the second direction y, and the antennas in the at least one subarray are also offset from each other in the second direction y.

Description

Technical Field

[0001] This invention relates to a MIMO radar sensor, which has the following features:

[0002] A planar antenna array having two subarrays, in which multiple antennas are respectively arranged offset from each other in a first direction x, wherein the subarrays are offset from each other in a second direction y perpendicular to the first direction x, the antennas of the two subarrays being more focused in the second direction y than in the first direction, and in at least one of the subarrays, at least two of the antennas are also offset from each other in the second direction y.

[0003] - A high-frequency section, which generates transmit signals for one subarray of antennas and preprocesses receive signals for another subarray of antennas.

[0004] - A control and analysis processing device configured to control a high-frequency section and determine the distance, relative velocity, azimuth, and elevation angle of the located object based on a preprocessed received signal.

[0005] This invention specifically studies a radar sensor for motor vehicles. Background Technology

[0006] When monitoring the surrounding environment in driver assistance systems for motor vehicles, in addition to the distance and relative speed of the located radar targets, the azimuth and elevation angles of these targets are also important. For example, information about the azimuth angle is needed to assign the object to a specific lane on the road. Information about the elevation angle allows for the assessment of whether the object is drivable or traversable, or whether the object represents a significant obstacle. The azimuth and elevation angles of the target can be derived from the amplitude and / or phase differences of the signals received by the antenna elements.

[0007] Based on the MIMO (Multiple Input Multiple Output) principle, the receiving antenna is combined with different transmitting antennas, for example, in a time-division multiplexing configuration, or selectively in a code-division multiplexing or frequency-division multiplexing configuration. Each combination corresponds to a virtual antenna element, the offset of which relative to another virtual antenna element is formed by adding the offsets of the participating receiving and transmitting antennas. The virtual array can have a larger aperture than the real receiving array, and thus achieve higher angular resolution.

[0008] For angle estimation, complex amplitudes obtained from different virtual antenna elements are compared with previously measured antenna patterns, and a deterministic maximum likelihood function (DML function) is calculated, which describes the probability that each angle within the positioning range is the true positioning angle of the located target. If amplitudes from at least three virtual antenna elements are available, a quality value can also be calculated, representing a measure of the quality used for angle estimation. Angle estimation becomes difficult if two targets are located in a measurement cycle, where distance and relative velocity cause the same frequency shift (in the case of FMCW radar). However, methods are known that can resolve the positioning angles of two targets even in these cases. However, angle estimation with a quality value then requires signals from at least four antenna elements.

[0009] MIMO radar of the type described at the beginning is known from DE102016203160A1, in which the antennas of the subarray are arranged at non-uniform intervals. This makes it possible to achieve a large virtual aperture using a relatively small number of receiving channels while simultaneously filling the array to a degree capable of resolving multivalued features in angle determination. The radar sensor described in this document is characterized by two transmitting antennas that are strongly focused in the azimuth, thereby achieving high resolution in the central angular range, while a weakly focused third transmitting antenna covers the outer region. Furthermore, this third transmitting antenna is also strongly offset in the vertical direction, thereby achieving accurate elevation angle estimation.

[0010] An FMCW-MIMO radar is known from WO2015 / 188987A1, in which a special FMCW analysis and processing method is used to compensate for measurement errors caused by the relative motion of the target in the case of time-division multiplexing MIMO.

[0011] US8436763 B2 describes an example of a code division multiplexing MIMO radar. Summary of the Invention

[0012] The objective of this invention is to propose a MIMO radar sensor that achieves more accurate and reliable angle estimation in both azimuth and elevation.

[0013] According to the present invention, this task is solved by a MIMO radar sensor having: a planar antenna array having two subarrays in which multiple antennas are respectively arranged offset from each other in a first direction x, wherein the subarrays are offset from each other in a second direction y perpendicular to the first direction x, the antennas of the two subarrays being more focused in the second direction y than in the first direction, and at least two of the antennas in at least one of the subarrays are also offset from each other in the second direction y; a high-frequency section for generating a transmit signal for the antennas of one subarray and for preprocessing a receive signal for the antennas of the other subarray; and a control and analysis processing device (14) configured to control the high-frequency section (12) and determine the distance, relative velocity, azimuth angle, and elevation angle of the located object based on the preprocessed receive signal; wherein at least one subarray is constructed to be symmetrical about an axis extending in the second direction y: in at least one subarray, the antennas are also offset from each other in the second direction y.

[0014] For example, if direction x is horizontal and direction y is vertical, azimuth angle estimation is achieved by offsetting the antennas in the subarray in direction x, and elevation angle estimation is achieved by offsetting them in direction y. However, if the elevation angle of the target is not 0°, the offset in direction y results in a phase difference that can adversely affect azimuth angle estimation. If the antennas are arranged at uneven distances in the horizontal direction, the phase difference that occurs at large elevation angles usually results in a smaller difference between the primary and secondary maximum values ​​when estimating the azimuth angle. In extreme cases, this can lead to the secondary maximum angle being incorrectly identified as the target's location angle, especially with noisy signals. This invention is based on the understanding that this undesirable effect can be suppressed by symmetrical arrangement of the antennas.

[0015] Advantageous configurations and extensions of the invention are described below.

[0016] In one implementation, the antennas of the two subarrays have the same aperture in the x-direction, preferably an aperture small enough to uniformly cover the entire positioning angle range.

[0017] In subarrays where the antennas do not need to be arranged symmetrically, at least one antenna element can also be offset in the y-direction, preferably by a greater amount than the offset in a symmetrical subarray. This achieves higher angular resolution in the elevation angle. However, these offsets are not considered when estimating the azimuth angle to avoid systematic errors that might otherwise be caused by a strong offset of the antenna in the vertical direction. Attached Figure Description

[0018] In the following, an embodiment is described in more detail with reference to the accompanying drawings.

[0019] The attached diagram shows:

[0020] Figure 1 A block diagram of a radar sensor according to the present invention is shown; and

[0021] Figures 2 to 5 The diagram shows the DML functions of the antenna array and comparator array of the radar sensor according to the present invention for targets at different azimuth and elevation angles. Detailed Implementation

[0022] exist Figure 1 The radar sensor shown has a circuit board 10 on which a planar antenna array with two subarrays TX and RX is formed. The subarray TX includes three transmitting antennas TX1, TX2, and TX3, which are offset from each other in a first direction x (horizontal direction). The subarray RX includes four receiving antennas RX1, RX2, RX3, and RX4, which are also offset from each other in the x-direction. The subarray RX is offset relative to the subarray TX in a second direction y (vertical direction) such that the subarrays do not overlap each other in the vertical direction. In the gaps between the subarrays, a high-frequency section 12 is arranged on the circuit board. This high-frequency section is connected to the transmitting and receiving antennas via microwave lines (not shown) and is used to feed the transmitted signal to the transmitting antennas and to intercept and preprocess the received signal from the receiving antennas. The high-frequency section 12 is formed, for example, by an MMIC chip (Mikrowave Monolithic Integrated Circuit) and connected to a digital control and analysis processing direction 14. This digital control and analysis processing direction controls the high-frequency section 12 and digitally analyzes and processes the pre-processed received signal to determine the distance, relative velocity, azimuth, and elevation angle of the located object. Signal digitization can be achieved, for example, by an analog-to-digital converter integrated into the MMIC chip, or selectively in the input stage of the control and analysis processing device.

[0023] To illustrate the geometry of the transmitting and receiving antennas, circuit board 10 is shown here as having a square grid pattern, where each grid cell has a side length of 1 / 4 of the wavelength of a microwave. In the following description, all distances between objects on the circuit board are given in units of this wavelength. All transmitting and receiving antennas have the same shape and are constructed as a group of antennas with two columns of ten antenna patches 16 each. These columns extend in the vertical direction y. The distance between each antenna patch 16 is 1 / 2, as is the distance between two columns of an antenna. Guide lines associated with reference numerals RX1, RX2, etc., lead to the phase center points (black squares) of the antennas. All descriptions of the positional relationships between the antennas below refer to the locations of these phase center points.

[0024] Transmitting antennas RX1, RX2, and RX3 are arranged at the same height in the vertical y-direction. The horizontal distance between receiving antennas RX1 and RX2 is 1, and the distance between receiving antennas RX2 and RX3 is 2.

[0025] Receiving antenna RX4 is offset downwards by 3.5 units in the vertical y-direction relative to the other three receiving antennas. Its horizontal distance from receiving antenna RX3 is 1.75 units.

[0026] The horizontal distances (x) between the three transmitting antennas TX1, TX2, and TX3 are 1.75. The transmitting antenna TX2, located at the center between TX1 and TX3, is offset upwards by a value of 1 in the vertical direction (y). Therefore, the subarray TX is symmetrical about an axis A extending in the direction y.

[0027] In the vertical direction, the lower end of the receiving antenna RX4 is directly connected to the upper end of the transmitting antenna TX2, thereby minimizing the vertical dimension of the circuit board 10 to a value that prevents subarray overlap. In the horizontal direction, the size of the circuit board is minimized by aligning antennas RX1 and TX1 with each other on the left edge of the circuit board.

[0028] Three transmitting antennas, TX1, TX2, and TX3, are used sequentially in a time-division multiplexing configuration to transmit radar signals. Overall, the radar sensor is configured as a so-called Joint-Compression-FMCW radar and operates according to the principles described in WO2015 / 188987A1. This compensates for range variations in the located radar targets, which occur due to the targets' own motion within time intervals that separate the activity periods of the different transmitting antennas.

[0029] If the radar target is located at an azimuth angle other than 0°, the signal paths from the transmitting antenna to the target and back from the target to the receiving antenna have different lengths for each combination of the transmitting and receiving antennas. Therefore, each combination of the transmitting and receiving antennas corresponds to a virtual antenna element, and the position of these virtual antenna elements in the x-direction is given by the x-component of the signal path. Consequently, the virtual aperture of this virtual antenna array is significantly larger than the actual aperture of the receiving antenna. Due to this enlarged aperture, the azimuth angle can be measured with greater separation accuracy.

[0030] Since one of the transmitting or receiving antennas in the subarray TX and the subarray RX is offset relative to the other antennas in the vertical y direction, a larger virtual aperture is obtained when measuring the elevation angle, and thus a greater separation capability is achieved.

[0031] However, in the case of a target with an elevation angle significantly deviating from 0°, the vertical offset of the antenna can introduce systematic errors when measuring the azimuth angle, because the phase shift between different virtual antenna elements depends not only on the azimuth angle but also on the elevation angle. For this reason, in the example shown here, the vertical offset of the transmitting antenna TX2 relative to the other transmitting antennas is significantly smaller than the offset of the receiving antenna RX4 relative to the other receiving antennas. The control and analysis processing unit 14 is configured such that it considers only the virtual antenna elements generated with the participation of the receiving antennas RX1 to RX3 when measuring the azimuth angle; that is, the signal of RX4 is ignored when measuring the azimuth angle, so that the systematic error caused by the strong offset of the receiving antenna RX4 is not included in the measurement result. Due to the relatively small offset of the transmitting antenna TX2, the remaining systematic error can then be tolerated.

[0032] Not only in measuring azimuth, but also in measuring elevation, the measurement principle is based on maximum likelihood estimation, which is based on the correlation between the phase, or complex amplitude, measured using different virtual antenna elements and the antenna pattern used for the given antenna array. This correlation is described by a deterministic maximum likelihood function (DML function), which has a maximum value corresponding to the true azimuth of the target. However, this DML function also has a minor maximum value at angles significantly deviating from the true azimuth of the radar target.

[0033] If the transmitting and receiving antennas in an antenna array are offset not only horizontally but also vertically, this typically results in the size ratio of the primary and secondary maximum values ​​when measuring the azimuth angle also depending on the target's elevation angle. This effect tends to cause the height of the (or at least some) secondary maximum values ​​to increase relative to the height of the primary maximum value as the elevation angle increases, thus enabling erroneous measurements because the measurement signal is always more or less contaminated with noise in practice, incorrectly treating one of the secondary maximum values ​​as the primary maximum value. However, in the antenna array described herein, this effect is largely suppressed by the fact that the vertically offset transmitting antenna TX2 is positioned precisely between the two other transmitting antennas TX1 and TX3. This symmetry results in the secondary maximum value increasing less rapidly relative to the primary maximum value as the elevation angle increases compared to an asymmetrical transmitting subarray. This effect will be illustrated below with reference to some example figures.

[0034] exist Figure 2 In this context, for radar targets with azimuth angles ranging from -60° to +60° and with elevation angles of 0° (dashed line) or 15° (solid line), the DML function values ​​are plotted. Figure 1 The antenna array shown in the figure was calculated. At larger elevation angles (solid lines), the maximum value at 0° is significantly weaker than at 0° elevation, however, the next higher minor maximum value (at ±38°) also decreases, while the first major maximum value (at ±19°) strengthens at larger elevation angles. However, overall, the distance between the major maximum value and the next higher minor maximum value is so large that there is no need to worry about erroneous measurements due to noise.

[0035] Figure 3 The same DML diagram is shown for the antenna array, in which... Figure 1 In contrast, the horizontal position of the transmitting antenna TX2 is shifted 0.25 (in wavelengths) to the right, thus breaking the symmetry of the subarray TX. The dashed curve at 0° elevation remains symmetrical, while the curve (solid line) at 15° elevation now exhibits asymmetry. The first major maximum at -19° is greater than the first major maximum at +19°, while the opposite is true for the second major maximum at ±38°. The distance between the major maximum and the largest minor maximum at +38° is here greater than... Figure 1 It's smaller, but still sufficient.

[0036] Figure 4 and Figure 5The same DML diagram is shown for the radar target, which is located at an azimuth angle of -60°. In the antenna array according to the invention ( Figure 4 Even at an elevation angle of 15° (solid line), there is still a significant distance between the primary maximum and the highest secondary maximum. Figure 4 (Horizontal dashed lines and arrows in the image). Conversely, in asymmetric antenna arrays (…). Figure 5 This distance has been reduced dramatically, making it impossible to rule out erroneous measurements in the case of heavily noise-contaminated signals.

[0037] As in Figures 2 to 5 As illustrated by the graph in the figure, in Figure 3 and 5 The relatively large height of the secondary maximum is, among other things, a result of the asymmetry of these curves. Therefore, at large elevation angles, a sufficient distance between the primary maximum and the highest secondary maximum can generally be achieved by symmetrically arranging the transmitting antennas. However, in other embodiments, the number of transmitting antennas may be greater than three, and an odd number of antennas can be conceived if the number of vertically offset antennas is also even. Similarly, an embodiment in which receiving antennas, rather than transmitting antennas, are symmetrically arranged can also be conceived.

Claims

1. A MIMO radar sensor, comprising: - A planar antenna array having two subarrays, one of which is a transmitting subarray (TX) and the other is a receiving subarray (RX), wherein multiple antennas in the two subarrays are arranged offset from each other in a first direction x, wherein... The two subarrays are offset relative to each other in a second direction y perpendicular to the first direction x. The antennas of the two subarrays are more focused in the second direction y than in the first direction. Furthermore, in at least one of the two subarrays, at least two of the antennas are also offset relative to each other in the second direction y. - High-frequency section (12), which is used to generate the transmit signal for the antenna of the transmit subarray (TX) and to preprocess the receive signal for the antenna of the receive subarray (RX), and - A control and analysis processing device (14), configured to control the high-frequency section (12) and determine the distance, relative velocity, azimuth, and elevation angle of the located object based on the preprocessed received signal. The feature is that at least one subarray is constructed symmetrically about an axis (A) extending in the second direction y: in said at least one subarray, the antennas are also offset relative to each other in the second direction y. Wherein, one of the two subarrays is offset relative to the other of the two subarrays in the second direction y such that the two subarrays do not overlap each other in the second direction y.

2. The radar sensor according to claim 1, wherein the first direction x is a horizontal direction and the second direction y is a vertical direction.

3. The radar sensor according to claim 1 or 2, wherein a subarray is formed by transmitting antennas (TX1, TX2, TX3): in the subarray, the antennas are also offset relative to each other in the second direction y.

4. The radar sensor of claim 3, wherein in another subarray formed by receiving antennas, at least one antenna is also offset relative to the other antennas of this subarray in the second direction y.

5. The radar sensor according to claim 4, wherein the offset in the second direction y in the subarray of the receiving antenna is greater than the offset in the subarray of the transmitting antenna, and wherein the control and analysis processing device (14) is configured to ignore the signal of the receiving antenna offset in the second direction y when performing angle estimation in the first direction x.

6. The radar sensor according to claim 1 or 2, wherein the symmetrically constructed subarray has an odd number of antennas.

7. The radar sensor according to claim 6, wherein only the central antenna of the symmetrical subarray is offset in the second direction y.

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