
The invention relates to a radar method for determining the location and / or a speed and / or an angle of an object according to the preamble of claim 1, in which a transmission signal is emitted by means of a radar transmitter, which is received by the object reflected transmission signal by means of at least one radar antenna element and a baseband signal is mixed from a current transmission signal and the received transmission signal.

Furthermore, the invention relates to a radar system according to the preamble of claim 10.

Radar methods or radar systems are generally known from the prior art. By means of a radar system or radar electromagnetic waves are bundled emitted as a transmission signal (primary signal), wherein the object reflected by the transmission signals (secondary signals), which are also referred to as "echoes" are received and evaluated. From the received waves reflected from the object, the angle or the direction to the scanned object, the distance to the object and the speed of the object with respect to the radar device (transmitter) can then be determined.

Such a radar method is z. B. from the
DE 10 2006 028 465 A1 known. Thereafter, in order to determine speeds and distances of objects relative to a radar system of a motor vehicle, wherein a detection range of the radar system is divided into at least two subregions, the detection range is examined for reflecting objects in successive measuring cycles, radar signals received in a measuring cycle separately processed into subregions and processed signals are combined into a total result differentiated according to spatial directions. In this case, hypotheses for the distance and speed of reflecting objects are formed from signals received in a first measuring cycle, and the hypotheses are validated in dependence on signals received in at least one further measuring cycle.

In general, the radar method or systems described at the outset involves the problem that slowly moving pedestrians or weakly scattering objects can hardly be resolved simultaneously in terms of speed, distance and angle with the aid of radar sensors in front of stationary obstacles.

The present invention is therefore based on the problem to provide a radar method and a radar system, which is improved in terms of the aforementioned disadvantages.

This problem is solved by a radar method having the features of claim 1.

Thereafter, it is provided that the baseband signal and / or a signal obtained with the aid of the baseband signal is continued by means of an estimation method, in particular in the form of a spectral estimation method.

As a result, in particular a synthetic increase in the number of sampling points, which are applied in the distance, speed and angle range, is effected, so that the resolution of the method or system is significantly improved.

In summary, the bandwidth and the received signal duration can be synthetically increased by the measure according to the invention in determining the distance; in the velocity determination, the number of sampling points (increase in the number of ramps) and thus the speed resolution can be improved, and in the angle determination, the number of radar antenna elements (receiving elements) is synthetically increased, so that a correspondingly improved angular resolution can be achieved.

The estimation method is particularly preferably the linear prediction known from mathematics (linear prediction). As linear prediction, autoregressive methods are possible, whereby the linear prediction itself can be classified under the parametric spectral estimation methods. Possibly. In the present case, nonparametric spectral estimation methods may also be used. In particular, the MUSIC method known from the prior art or also the ESPRIT estimation method are suitable for use in connection with this invention.

In a variant of the invention, the baseband signal or a signal obtained with the aid of the baseband signal is continued by the estimation method along the abscissa to the "back" and / or "forward", i. h., to larger Abzissenwerte (time values) and smaller Abzissenwerten (time values), in particular in each case over at least a period of time or a multiple of the time duration of the baseband signal. As a result, the duration of the baseband signal is synthetically prolonged. In particular, in the case of simple harmonic and periodic signals, the baseband signal can be continued identically (adding additional periods of the signal). In the manner set out above, it is also possible to proceed with signals derived from the baseband signal.

The baseband signal is mixed from the instantaneous transmit signal and the signal reflected and received on the object, so that for a sampled object in the baseband signal a harmonic oscillation results with a frequency which is the difference of the instantaneous transmit signal frequency and the frequency of the reflected, received transmit signal ( before mixing).

Due to the signal propagation time of the transmission signal (to the object and back), which depends on the distance R between the radar system and the reflecting object, the distance R in the said frequency f (socalled beat frequency) is formed. Due to the Doppler effect, the radial velocity, that is to say the relative velocity oriented in the direction of a connecting line between the radar system and the reflecting object, also forms in the said frequency f. This frequency f can be obtained by means of a Fourier transformation of the baseband signal and be assigned directly to a location or a distance R (f α R).

The radar method or system according to the invention is preferably an FMCW method ("frequencymodulated continuous wave"), specifically in the form of a fast ramp method. In this case, the transmission signal has a plurality of successive frequency ramps (socalled "chirps"), which are repeated periodically. The frequency deviation corresponds to the bandwidth of the transmission signal.

Preferably, the frequency of a frequency ramp increases linearly to drop abruptly at a certain frequency to the initial value (sawtooth pattern). Due to the linear change of the frequency and the continuous transmission, it is possible to determine not only the relative speed between the transmitter and the object but also their absolute distance from one another at the same time.

Preferably, the baseband signal is in each case mixed from a current frequency ramp of the transmission signal and a frequency ramp reflected at the object and received by the at least one radar antenna element and then subjected to a Fourier transformation for further evaluation. Ie. the Fourier transform of the baseband signal is preferably carried out for each individual frequency ramp.

To determine the distance of the object to the radar system or to the at least one radar antenna element, the continued baseband signal is subjected to a discrete Fourier transformation, in particular an FFT (Fast Fourier Transform), for each frequency ramp in order to determine the respectively assigned frequency spectrum Maximum at a frequency f, which can be assigned directly to the distance of the object (see above).

The continuation of the baseband signal by a corresponding signal processing leads to an artificial widening of the bandwidth of the baseband signal and thus to an increased resolution of the radar method according to the invention with respect to the distance determination. As a result, objects with different Radarquerschnitt can be distinguished from each other, in particular pedestrians who z. B. in front of a vehicle, can be detected.

In order to determine the speed of the object with respect to the at least one radar antenna element, the base band signal is preferably subjected to frequency analysis for each frequency ramp as described above. From frequency ramp to frequency ramp, the distance of a relative to at least one radar antenna element moving object changes only slightly, d. that is, the frequency peak of the spectrum can be assigned to the same object distance, but has a shifted phase. The frequency of the phase rotation of the frequency peaks corresponds in a known manner to the Doppler frequency and is thus proportional to the relative velocity V of the object with respect to the at least one radar antenna element of the radar system (Δφ α V).

Preferably, the rotation of the phase of the maxima (frequency peaks) of said spectra as a signal derived from the baseband is continued by means of the estimation method (eg linear prediction). The Fourier transformation of this signal provides the Doppler frequency f _{D} and consequently the sought velocity V.

The continuation of the signal described above corresponds to an artificial increase in the number of emitted frequency ramps and thus an artificial increase in the entire measurement duration and thus also an artificial increase in the number of sample points for the velocity determination. In this way, despite a limited number of frequency ramps ("chirps"), the speed resolution can be increased. In particular, the slow movements of pedestrians can thus be better detected.

In order to determine the angle that the object has with respect to the radar system or of a motor vehicle in which the radar system is arranged, the radar method is furthermore preferably carried out by means of a plurality of radar antenna elements which can each receive the emitted frequency ramps after reflection at the object to be detected.

With an appropriate orientation of the radar system, that angle can be defined as the angle of the object, which includes a position vector pointing from the radar system to the object with the vehicle longitudinal axis of the motor vehicle. In the case of a plurality of radar antenna elements z. B. are arranged equidistantly along a straight line in a plane, the angle of the object can be defined as the angle which includes the said position vector with a normal of the line and plane. Of course, the angle can be expressed in azimuth and elevation angle in a known manner.

For the concrete determination of the angle of the object, the transmission signal or baseband signal reflected at the object and received by the radar antenna elements is spatially continued by the estimation method, whereby the in particular continued baseband signals, which are received at equal times by the radar antenna elements, are subjected to a Fourier transformation In each case, a phase difference of a frequency (corresponding to a finite frequency interval or a socalled frequency bin in the case of discrete numerical methods) of the respective spectrum of the same frequency ramp between adjacent radar antenna elements is determined and compared.

This spatial continuation of the received signal corresponds to an artificial increase in the number of radar antenna elements and causes an increased angular resolution of the radar method according to the invention, which makes it possible to separate objects with different radar cross section from each other. In this case, the signal continuation according to the invention can be carried out simultaneously both with regard to the azimuth and with respect to the elevation angle or for both components. In particular, with a continuation of a signal concerning the elevation angle, the height resolution can be improved, so that there is the possibility to separate objects of different heights from each other, for. For example, a person in front of a wall or street lamp.

Furthermore, the problem according to the invention is solved by a radar system having the features of claim 10, wherein that radar system is intended in particular for use in the radar method according to the invention.

The radar system, which is set up and provided for determining a distance and / or a speed and / or an angle of an object, in particular a pedestrian, with respect to a reference object, in particular in the form of a motor vehicle, then has at least one radar transmitter for transmitting a transmission signal. at least one radar antenna element for receiving the transmitted signal reflected by the object, and an evaluation means for generating a baseband signal from the current transmission signal and the reflected transmission signal, wherein according to the invention the evaluation means is adapted to the baseband signal and / or a signal obtained by means of the baseband signal an estimation procedure, in particular in the form of a spectral estimation procedure. The estimation method is preferably the linear prediction (see above).

The radar method and the radar system according to the invention can advantageously detect the position and the movement of weak targets (eg a person) at an early stage, so that future driver assistance systems effectively avoid accidents in urban traffic as well and promptly alert the driver to an impending one To raise the danger.

Showing:

1 a schematic overview of the radar method and radar system according to the invention;

2 a schematic representation of a fast ramping process;

3 a schematic representation of the artificial extension of the duration of a baseband signal by means of the linear prediction;

4 a schematic representation of the improvement of the spatial or distance resolution by the linear prediction of baseband signals;

5 a schematic representation of the improvement in the velocity resolution by the linear prediction of the phase rotation of the frequency peaks of the Fouriertransformed baseband signals;

6 a schematic representation of the improvement of the angular resolution by the spatial continuation of the received signals by means of linear prediction in several radar antenna elements;

7 a schematic representation of the reduction of the duration of the transmitted transmission signal or the increase of the bandwidth by linear prediction; and

8th a schematic representation of the increase of the bandwidth for the same time duration of the transmission signal or the increase of the frequency ramp number;

1 shows a schematic representation of an overview of the radar method according to the invention and the functions of the radar system according to the invention 1 ,

In the case of a radar antenna element 3 For example, the fast ramping method is used to determine the distance R and the velocity V (along the distance R).

The distance R and the velocity V can be considered to be independent of each other, since the Doppler frequency shift with respect to the distance frequency shift is small.

Be M frequency ramps 200 (Comp. 2 ) or "chirp", the velocity V thus mainly affects the phase information of the baseband signal 22 a ramp 200 out. Through the linear P prediction P, the baseband signal 22 be extended synthetically (continued baseband signal 22 ' ). This corresponds to an increase in the number of points in the time domain t.

With constant steepness of the frequency ramps 200 Does such an increase in the number of points affect the bandwidth B _{w used} (cf. 2 ) out. This means that in addition results in a larger synthetic bandwidth. By applying the Fourier transform F (in particular discrete Fourier transform F or fast Fourier transform, abbreviated FFT) to this extended time signal 22 ' the frequency resolution is increased, thus improving the location resolution. For an object at a certain distance R, the result is the Fourier transformation of the continued baseband signal 22 ' a frequency peak at f _{R} , which can be directly assigned to a distance R. Due to the influence of the Doppler shift, this frequency peak additionally receives a phase difference Δφ from chirp to chirp, which is constant at constant speed. If one applies the Fourier transform F to one frequency interval of the Fouriertransformed baseband signal 22 ' on, the speed V can be determined directly. If very slow movements are recorded with this method, only very few samples are available. This means that this leads to a poor speed resolution. Now apply the linear prediction P to this sampled Doppler signal (cf. 5 ), the speed resolution can be increased.

In the case of several, in particular N radar antenna elements, the angle of the object W, W ', W "can additionally be determined according to 6 be determined. For the processing of the angle in each case the previously described extended and Fouriertransformed baseband signals 22 ' at the same time on different radar antenna elements 3 were scanned, processed together. In this case, the phase difference of a frequency interval (frequency bin) between the adjacent antenna elements 3 and the same chirps (frequency ramps) evaluated and compared. This will be the received signal 21 spatially scanned. This signal can also be artificially continued with the linear prediction P. This corresponds to an artificial increase in Radarantennenlementanzahl on z. B. N _{p} (see. 1 ). The angular resolution is thus improved from the Fourier transformation F of all antenna elements N _{p} at the same frequency bin per chirp.

2 shows in detail in connection with the 2 to 4 the location or distance determination in the fast ramp method according to the invention.

In this case, a transmission signal 20 that consists of a sequence of frequency ramps 200 exists on an object in the form of a motor vehicle 10 reflected and as a reflected transmission signal 21 (Received signal) after a running time .DELTA.t _{1} from a radar antenna element 3 receive. This signal is in the radar system 1 with the current transmission signal 21 to a baseband signal 22 mixed. For the object 10 then results in a harmonic signal with a frequency f _{target1} , which _{corresponds} to the difference of the current transmission signal frequency and the received signal frequency before mixing. This frequency can be determined by Fourier transformation F (cf. 4 ) and assigned directly to a distance R.

To improve the spatial resolution, the baseband signal 22 according to 3 continued by linear prediction P to a continued baseband signal 22 ' to obtain.

This has an artificially extended period of time (signal duration). As a result, according to 4 the bandwidth B of the frequency ramps 200 artificially on z. For example, twice the bandwidth 26 be raised when z. B. the duration of the baseband signal from T to T 'is extended.

The Fourier transform F of the thuscontinued baseband signals 22 ' (corresponding to the transmitted signals reflected by the moving motor vehicle 20 with delays Δt _{1} and Δt _{2} ) then yields in the frequency spectrum  A  (lower right in the 4 ) has a narrower peak (dotted line and dotdash line) compared to the discontinued baseband signals 22 (solid line and dashed line in the spectrum  A ). Thus, the frequencies f _{1} and f _{2} , the respective location of the motor vehicle 10 correspond, be specified, from which the improvement of the spatial resolution is apparent.

5 shows in detail the speed determination of the object 10 , For this purpose, the frequency peaks of the spectra of the individual baseband signals belonging to the same distances become 22 respectively. 22 ' evaluated. Due to the Doppler effect, these frequency peaks have a phase difference Δφ of chirp 200 to Chirp 200 , whose chronology in the 5 with A, B, C, D, E, ... is marked.

The rotation of the phase A (t) takes place at a frequency which corresponds to the Doppler frequency f _{D} and the speed V of the motor vehicle relative to the radar system 1 is proportional. The Doppler frequency f _{D} can be obtained in a known manner by Fourier transformation of the phase rotation A (t), as shown on the right side of FIG 5 is indicated.

If now the phase rotation A (t) or 23 continued by linear prediction P, resulting in an artificial increase in the number of chirps 200 or the times A, B, C, ... corresponds, the speed resolution can be increased accordingly.

6 shows the angle determination using three radar antenna elements 3 in detail. Here, the equidistant to the (equidistance d, corresponds to half the wavelength λ of the received signal 21 ) and linearly arranged radar antenna elements 3 incident wavefront of the received signal 21 considered, where the zero crossings of the deflection of the wave under consideration at the positions 50 lie.

At an angle at W = 0 incident wave results in a signal 24 with vanishing spatial frequency (no phase difference between the three antenna elements 3 ), where the Fourier transform F of this signal 24 provides a spectrum which has a peak in a known manner at the corresponding angle W = 0. By continuing the signal 24 the resolution of the angle W is increased by means of the linear prediction P (solid line in the spectrum of the signal 24 ). For the angles W 'and W ", an improvement in the angular resolution by linear prediction P. is obtained in the same way.

The 7 and 8th Clearly show the possibilities of linear prediction in the context of the radar method according to the invention.

7 shows in the upper part of the transmitted (original) transmission signal from frequency ramps 200 the signal duration (duration) D. According to a first case ( 7 , bottom left), the bandwidth B can be kept artificially constant by the physical bandwidth B 'is artificially increased to B at reduced signal duration by linear prediction (dotted lines). As a result, the process times of the radar system can be shortened with the same bandwidth with advantage.

According to a second case ( 7 Alternatively, the signal duration D can be left unchanged, but the physical bandwidth B 'can be increased to a synthetic bandwidth B by linear prediction (dotted lines), which improves the spatial resolution (see above).

Further shows 8th in the upper part again the transmitted (original) transmission signal from frequency ramps 200 the signal duration (duration) D. According to another third case ( 8th , bottom right) can with equal duration and bandwidth B (each increased by linear prediction), the number of frequency ramps 200 be increased to increase the maximum speed (see above). The representation according to 8th , bottom left, corresponds to the second case described above.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

 DE 102006028465 A1 [0004]