Angle estimation and ambiguity resolution of radar sensors for motor vehicles with a large antenna array

Abstract

An angle-resolving radar sensor for motor vehicles, having an antenna system having a plurality of antennas set up for receiving, configured in various positions in a direction in which the radar sensor is angle-resolving, and having a control and evaluation device designed for an operating mode in which at least one antenna of the radar sensor that is set up for transmitting sends out a signal that is received by a plurality of the antennas of the radar sensor that are set up to receive, the control and evaluation device being designed, in the mentioned operating mode, for an individual estimation of an angle of a radar target to determine respective individual distances of the radar target for each of the evaluation channels, which correspond to different configurations of transmitting and receiving antennas, and to use the individual distances in the estimation of the angle of the radar target.

Claims

1. An angle-resolving radar sensor for a motor vehicle, comprising: an antenna system having a plurality of antennas set up for reception and configured in various positions in a direction in which the radar sensor is angle-resolving; and a control and evaluation device that is configured to, for an operating mode in which at least one antenna of the radar sensor that is set up to transmit sends out a signal that is received by the plurality of antennas that are set up for reception, carry out an estimation of an angle of a radar target in the operating mode; wherein in the operating mode, for an individual estimation of the angle of the radar target, the control and evaluation device is configured to determine respective individual distances of the radar target for respective evaluation channels, which correspond to different configurations of the at least one antenna that is set up to transmit and the plurality of antennas set up for reception and to use the individual distances in the individual estimation of the angle of the radar target; wherein the control and evaluation device is configured, in the operating mode, for the individual estimation of the angle of the radar target to carry out a first estimation of the angle based on amplitude and/or phase relations between signals of each of the evaluation channels and, if a plurality of possible angle values are received as an ambiguous result of the first estimation of the angle, to select one of the possible angle values as a result of the estimation of the angle, based on the individually determined distances; and wherein the control and evaluation device is configured, in the operating mode, for the selection of the one of the possible angle values received as the ambiguous result of a first estimation of the angle, to carry out a second estimation of the angle based on the equation sin θ=−Δdi/yi, where θ designates the angle to be estimated in the second estimation, i designates an evaluation channel, Δdi designates the difference between an individually determined distance for the evaluation channel i and a distance of a reference antenna position, and yi designates an antenna position for the evaluation channel i relative to the reference antenna position; and to select the one of the possible angle values based on a comparison of a result of the second estimation of the angle with the possible angle values obtained in the first estimation.

2. The radar sensor as recited in claim 1, wherein the control and evaluation device is configured to use, in the operating mode, the individual distances in the individual estimation of the angle of the radar target, taking into account positions of relevant ones of: the at least one antenna that is set up to transmit and the plurality of antennas set up for reception.

3. The radar sensor as recited in claim 1, wherein the control and evaluation device is configured, in the operating mode, for the individual estimation of the angle of the radar target, to carry out a delimitation of a region of the angle based on the individually determined distances, and to carry out, within the delimited region, an estimation of the angle based on amplitude and/or phase relations between signals of the respective evaluation channels.

4. The radar sensor as recited in claim 1, wherein the control and evaluation device is configured, in the operating mode, for the selection of the one of the possible angle values received as the ambiguous result of a first estimation of the angle, to carry out a second estimation of the angle based on the individually determined distances and to select the one of the possible angle values based on a comparison of a result of the second estimation of the angle with the possible angle values obtained in the first estimation.

5. The radar sensor as recited in claim 1, wherein in the control and evaluation device is configured, in the operating mode, for the selection of the one of the possible angle values received as the ambiguous result of a first estimation of the angle, to carry out a second estimation of the angle through trilateration or multilateration based on the individually determined distances, and to select the one of the possible angle values based on a comparison of a result of the second estimation of the angle with the possible angle values obtained in the first estimation.

6. The radar sensor as recited in claim 1, wherein the control and evaluation device is configured, in the operating mode, for the selection of the one of the possible angle values obtained as the ambiguous result of a first estimation of the angle, to check a sign of the angle value and/or to check an angle value corresponding to a linear orientation for plausibility with respect to a tendency of an allocation of the individually determined distances to positions of relevant antennas.

7. An angle-resolving radar sensor for a motor vehicle, comprising: an antenna system having a plurality of antennas set up for reception and configured in various positions in a direction in which the radar sensor is angle-resolving; and a control and evaluation device that is configured to, for an operating mode in which at least one antenna of the radar sensor that is set up to transmit sends out a signal that is received by the plurality of antennas that are set up for reception, carry out an estimation of an angle of a radar target in the operating mode; wherein in the operating mode, for an individual estimation of the angle of the radar target, the control and evaluation device is configured to determine respective individual distances of the radar target for respective evaluation channels, which correspond to different configurations of the at least one antenna that is set up to transmit and the plurality of antennas set up for reception and to use the individual distances in the individual estimation of the angle of the radar target; wherein the operating mode is a second operating mode, and wherein the control and evaluation device is configured, in a first operating mode, to estimate the angle of the radar target based on amplitude and/or phase relations between signals of the respective evaluation channels, which correspond to different configurations of transmitting and receiving antennas, the amplitude and/or the phase relations between the signals being evaluated at, in each case, the same frequency position in the evaluation channels that are used, and, in the case of failure of one or more antennas used in the first operating mode for transmitting and/or for receiving, the control and evaluation device is configured to carry out, in the second operating mode, the estimation of an angle of a radar target using at least one remaining antenna used for transmitting and using a plurality of remaining antennas used for receiving.

8. A method for the angle estimation of radar targets for a radar sensor for a motor vehicles, the radar sensor including an antenna system having a plurality of antennas set up for receiving, configured in various positions in a direction in which the radar sensor is angle-resolving, the method comprising: determining, in a second operating mode, an individual estimation of an angle of a radar target by: determining respective individual distances of the radar target for each of a plurality of evaluation channels, which correspond to different configurations of transmitting and receiving antennas; and using the individual distances in the estimation of the angle of the radar target; and estimating, in a first operating mode, the angle of the radar target based on amplitude and/or phase relations between signals of the respective evaluation channels, which correspond to different configurations of transmitting and receiving antennas, the amplitude and/or the phase relations between the signals being evaluated at, in each case, the same frequency position in the evaluation channels that are used, and, in the case of failure of one or more antennas used in the first operating mode for transmitting and/or for receiving, carrying out, in the second operating mode, the estimation of an angle of a radar target using at least one remaining antenna used for transmitting and using a plurality of remaining antennas used for receiving.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a block diagram of a radar sensor according to the present invention.

(2) FIG. 2 shows a schematic representation of frequency bins of Fourier spectra of respective evaluation channels.

(3) FIG. 3 shows a relation between two antennas and a radar target.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(4) The radar sensor shown in FIG. 1 has a plurality of receiving antennas or antenna elements 10, 12 on a common substrate 18. The radar sensor is installed in a motor vehicle in such a way that a plurality of antennas 10, 12 are situated alongside one another at the same height, at horizontal positions yi, i=0, . . . , k, so that an angle resolution capacity of the radar sensor in the horizontal (in the azimuth) is achieved. In FIG. 1, radar beams are shown symbolically that are received by the antennas at a respective azimuth angle θi.

(5) A radio-frequency part 20 for controlling a transmitting antenna 22 includes a local oscillator 24 that produces the radar signal that is to be transmitted. The radar echoes received by antennas 10, 12 are each provided to a mixer 28, where they are mixed with a transmit signal supplied by oscillator 24. In this way, for each of the antennas 10, 12 a baseband signal or intermediate frequency signal Z0, Z1, . . . , Zi, . . . , Zk is obtained that is supplied to an electronic control and evaluation unit 30. Control and evaluation unit 30 contains a control part 32 that controls the function of oscillator 24. In the depicted example, the radar sensor is an FMCW radar unit, i.e., the frequency of the transmit signal supplied by oscillator 24 is periodically modulated in the form of a sequence of rising and/or falling frequency ramps.

(6) In addition, control and evaluation device 30 contains an evaluating part having an analog/digital converter 34 having k channels, which digitizes the intermediate frequency signals Z0-Zk received by the k antennas 10, 12, and records each of them over the duration of an individual frequency ramp. The time signals obtained in this way are then converted channel-by-channel into corresponding frequency spectra using fast Fourier transformation, in a transformation stage 36. In these frequency spectra, each radar target is shown in the form of a peak whose frequency position is a function of the signal run time from the radar sensor to the radar target and back to the radar sensor, as well as being a function of the relative speed of the radar target due to the Doppler effect. From the frequency positions of two peaks obtained for the same radar target, but on frequency ramps having different slopes, for example a climbing ramp and a falling ramp, the distance d and the relative speed v of the relevant radar target can then be calculated in a conventional manner.

(7) As is shown schematically in FIG. 1 on the basis of the radar beams, the various positions of antennas 10, 12 have the result that the radar beams emitted by one and the same antenna are reflected by the radar target and are then received by the various antennas, traveling different run lengths and thus having phase differences that are a function of the azimuth angle θ of the radar target. The associated intermediate frequency signals Z0-Zk have corresponding phase differences. The amplitudes (magnitudes) of the received signals are different from antenna to antenna, and are also a function of the azimuth angle θ.

(8) For each located object, i.e., each radar target (each peak in the frequency spectrum), an angle estimator 38 compares the complex amplitudes received in the k receive channels with the antenna diagram, in order in this way to estimate the azimuth angle θ of the radar target. As a result, for example an ambiguous result can be obtained having a plurality of possible angle values θe1, θe2, θe3 for azimuth angle θ.

(9) However, given a high bandwidth, corresponding to a large frequency sweep of the FMCW modulation, and a large extension of the antenna system, the complex amplitudes in the individual receive channels are contained at different frequency positions fa(i) in the frequency spectrum of the received signal, according to the azimuth angle θ of the radar target and its distance d. This is illustrated schematically in FIG. 2, which shows successive frequency bins of the Fourier spectrum in the direction of increasing frequency f. In the Fourier transformation, a peak in the received signal at a frequency position fa is mapped to more than one frequency bin, according to the evaluation channel and angle, as is shown by hatching in FIG. 2. The frequency fref designates a middle frequency position that corresponds to the bin evaluated by the angle estimation.

(10) As FIG. 2 illustrates schematically, via a peak over the respective frequency spectrum, the more accurate position (frequency position) of a peak is then determined by a distance estimator 40, for example through interpolation of the spectrum at support points that have finer resolution than the distances of the frequency bins, and seeking the peak maximum, or through local adaptation (fitting) of a peak function to the spectrum.

(11) As is shown in FIG. 1, for each evaluation channel i a decision unit 42 receives the estimated individual value of the distance di from the distance estimator, and receives the possibly ambiguous result of the angle estimation, i.e., the possible angle values θe1, θe2, θe3 of the azimuth angle, from angle estimator 38. Based on this, and taking into account the positions yi of each of the antennas, decision unit 42 chooses the angle value that best fits the individual distances di. This is explained in more detail in the following.

(12) In a top view, for two antennas designated by indices 0 and i at the coordinates (0,y0) and (0,yi), FIG. 3 illustrates the relation to a point target as radar target at the coordinates (x,y). The distances of the point target from the individual antennas are designated d0, di, and the angle of incidence (azimuth angle) of the received radar signal is designated GO or θi. To simplify the representation, it is assumed that the origin (0, 0) is the midpoint of the antenna array and corresponds to a center position of receiving antennas 10, 12, and a monostatic system is described.

(13) For each antenna having the index i, the following holds:
di=(x.sup.2+y−yi.sup.2).sup.1/2
and θi=a tan((y−yi)/x)
for the individual positions and angles of the radar target.

(14) As estimated variables of the radar sensor, the coordinates of the radar target relative to the origin are to be ascertained, i.e.
d=(x.sup.2+y.sup.2).sup.1/2
and θ=a tan(y/x).

(15) For each antenna, the difference from the average variables is:
Δdi=di−d=(x.sup.2+(y−yi).sup.2).sup.1/2−(x.sup.2+y.sup.2).sup.1/2
and Δθ=θi−θ=a tan((y−yi)/x)−a tan(y/x),
where Δdi is the distance difference and Δθi is the azimuth angle difference.

(16) For the distance difference, the following holds:

(17) $\begin{array}{cc}\Delta d_i=& d_i-d=\sqrt{x^2+{\left(y-y_i\right)}^2}-\sqrt{x^2+y^2}=\\ & \sqrt{x^2+y^2-2{\mathrm{yy}}_i+y_i^2}-\sqrt{x^2+y^2}\\ =& \sqrt{x^2+y^2}\sqrt{1-\frac{2{\mathrm{yy}}_i+y_i^2}{x^2+y^2}}-\sqrt{x^2+y^2}=\\ & \left(\sqrt{1-\frac{y_i\left(2y+y_i\right)}{d^2}}-1\right)d\\ =& \left(\sqrt{1+z}-1\right)d=\\ & \left(\frac{1}{2}z-\frac{1}{8}{z}^2+\frac{1}{16}{z}^3-\frac{5}{128}{z}^4+\Lambda \right)d\end{array}$
with the Taylor series expansion in the last line for

(18) $z=\frac{y_i\left(2y+y_i\right)}{d^2}.$

(19) The following results as an approximation for |y|>>|y.sub.i| and thus small z:

(20) $z\approx -\frac{2{\mathrm{yy}}_i}{d^2}=-\frac{2y_i}{d}\sin \theta .$
There then results: Δd.sub.i≈½zd≈−y.sub.i sin θ.

(21) As an approximation, via this easily implemented equation, the azimuth angle θ is estimated for each evaluation channel i from the differences Δdi of the distance and the relevant antenna positions yi. Through comparison with the possible, more accurately estimated angle values θe1, θe2, θe3, estimator 42 determines angle value ee, recognized as correct, as estimated azimuth value θ.

(22) The individual distances di of the evaluation channels, “seen” by the radar sensor based on the run length differences, are a function of the antenna configuration. Thus, in a bistatic system, or an MIMO system, the effects (distance, or run time) for the path from the transmit antenna to the target, and from the target to the receive antenna, are added and averaged. The estimated distance di is determined for example via the overall run time of the signal, divided into the path out and the return path, and thus as the average distance over the average run time of the signal. The center position of the relevant transmit and receive antennas is determined as antenna position yi.

(23) In an MIMO radar sensor, k evaluation channels correspond to different configurations of transmitting and receiving antennas.

(24) In the example, angle estimator 38 forms a first stage of an angle estimator 44 that includes angle estimator 38, distance estimator 40, and decision unit 42.

(25) In another example, decision unit 42 is set up to approximately estimate the angle θ from the individual distances di, taking into account the antenna positions yi through trilateration or multilateration, and to determine the angle θe as the estimated azimuth angle θ through comparison with the possible more accurately estimated angle values θe1, θe2, θe3.

(26) Decision unit 42 is for example set up to check the possible angle values θe1, θe2, θe3 for plausibility with respect to a tendency of an allocation of the individually determined distances di to the positions yi of relevant antennas. For example, if the angles −30°, 0°, or 30°, as the ambiguous result of an angle estimation, are to be checked for plausibility, then the following cases can be distinguished: 1) for antenna positions to the left and to the right of a center antenna position, substantially identical individual distances are determined; the angle 0° is determined as plausible; 2) for a left antenna position, a greater individual distance is determined than for a right antenna position; the angle −30° (to the right of center) is determined as plausible; and: 3) for a left antenna position, a smaller individual distance is determined than for a right antenna position; the angle +30° is determined as plausible.

(27) In another exemplary embodiment, the angle estimation takes place in a corresponding manner, but first a delimited angle region is inferred on the basis of a tendency of an allocation of the individually determined distances di to the positions yi of relevant antennas, for example “at the left side,” “in the center with a tolerance width,” “at the right side,” and the angle estimation is then carried out in an unambiguous manner within the delimited angle region. The angle regions may overlap.

(28) In another exemplary embodiment, an angle estimation takes place immediately on the basis of the individual distances determined by distance estimator 40, e.g., through trilateration or multilateration, without angle estimator 38 being present or being used.

(29) In an exemplary embodiment, the operating mode, described above on the basis of examples, of control and evaluation device 30 is an operating mode provided for emergency operation. In normal operation, angle estimator 44 carries out a conventional angle estimation using angle estimator 38, and uses a conventional method to resolve ambiguities of the angle estimation. When there is a failure of one or more antennas, control and evaluation device 30 switches over to an emergency operating mode using the remaining antennas, in which the operating mode described above for angle estimation, or for resolving ambiguities, is used.

(30) Antennas 10, 12, and 22 can be group antennas, each including an array of patches that are controlled with the same phase, or can be combined while receiving the phases to form a receive signal.

(31) The exemplary embodiments described here are based on a bistatic antenna design. Optionally, however, a monostatic antenna design could also be used in which the same (group) antennas are used for transmission and for reception.

(32) The described operation of the control and evaluation device can advantageously be used in particular in FMCW radar sensors that operate with so-called rapid chirp sequences. Here, a multiplicity of frequency ramps (chirps) having a large slope and a relatively short duration are gone through quickly in sequence.