Abstract:
A method for a radar sensor for a motor vehicle, for angle estimation of radar targets based on an antenna diagram that indicates, for various configurations of radar targets, pertinent amplitudes and/or phase correlations between signals which are obtained for the relevant configuration in multiple evaluation channels of the radar sensor, wherein for a single real target, the occurrence of a number n of apparent targets, which are caused by reflection of the signal coming from the real target from elongated objects, is modeled mathematically; a correlation between the location angle of the real target and the location angles of the apparent targets is calculated; and to estimate the location angle of the real target, a multi-target estimate is performed in an n-dimensional search space and the search is limited to a sub-space that is determined by the calculated correlation.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    The present application claims priority to and the benefit of German patent application no. 10 2014 201 026.8, which was filed in Germany on Jan. 21, 2014, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for a radar sensor for motor vehicles, for angle estimation of radar targets based on an antenna diagram that indicates, for various configurations of radar targets, pertinent amplitudes and/or phase correlations between signals which are obtained for the relevant configuration in multiple evaluation channels of the radar sensor; and to a radar sensor for executing said method. 
       BACKGROUND INFORMATION 
       [0003]    Radar sensors are used in motor vehicles, for example, in order to measure the distances, relative speeds, and azimuth angles of vehicles or other objects located in the area in front of the own vehicle. Multiple antenna elements are then disposed, for example, at a distance from one another on a horizontal line, so that different azimuth angles of the located objects result in differences in the path lengths that the radar signals have traveled from the object to the respective antenna element. These path length differences result in corresponding differences in the amplitude and phase of the signals that are received by the antenna elements and evaluated in the pertinent evaluation channels. The angle of incidence of the radar signal, and thus the azimuth angle of the located object, can then be determined by comparing the (complex) amplitudes received in the various channels with corresponding amplitudes in an antenna diagram. The elevation angle of an object can also correspondingly be estimated using antenna elements disposed vertically above one another. 
         [0004]    For a single target, the comparison between the received amplitudes and the amplitudes in the antenna diagram can be made by calculating, for each angle in the antenna diagram, a correlation between the vector of the measured amplitudes (for k evaluation channels, this is a vector having k complex components) and the corresponding vector in the antenna diagram. This correlation can be expressed by a so-called deterministic maximum likelihood (DML) function which, when a specific vector of measured amplitudes is given, indicates for each angle the probability that the object is located at that angle. Angle estimation then consists in finding the maximum of this DML function. In this case the DML function is dependent on only a single variable, namely the relevant azimuth angle or elevation angle. The search for the maximum therefore occurs in a one-dimensional search space. 
         [0005]    If the radar sensor is locating multiple targets simultaneously, those targets normally differ in terms of their distance and/or relative speed, so that the targets can be separated from one another and the angle estimate can then be performed individually for each target. If the distances and relative speeds of the two targets are so similar to one that a separation is not possible given the limited resolution of the radar sensor, however, the two targets then appear as a single target, and the above-described angle estimate would yield only a single angle as a result. But because there are in fact two targets, interference occurs between the signals that are backscattered from the two targets and become superimposed at the radar sensor. The consequence of this is that the pattern of the received amplitudes no longer corresponds to the antenna diagram for a single target. 
         [0006]    It is nevertheless possible to generalize the above-described method for angle estimation to two-target or multi-target estimates. The DML function is then a function of multiple variables, namely of the angles of the various targets, and in the context of an n-target estimate the search space consequently has n dimensions. The location of the maximum of the DML function in this search space then has n components, which indicate the location angles of all n targets. 
         [0007]    Multi-target estimation has the disadvantage, however, that searching in a multi-dimensional search space is extremely calculation-intensive. The method is moreover susceptible to errors due to the unavoidable signal noise. 
         [0008]    In practical utilization of a radar sensor in motor vehicles, even when only a single radar target is present it often happen that as a result of reflections of the backscattered signal from the road surface or from a guardrail, additional apparent targets, which are in reality merely mirror images of the located object, are simulated. In this case the distances and relative speeds are almost identical. Although the signal reflected from the road surface or guardrail takes a certain detour, it is hardly measurable given the almost raking reflection. It is thus not possible to differentiate between the real target and the apparent target, so that strictly speaking a multi-target estimate would need to be carried out for estimates of the elevation angle when reflections from the road surface are to be expected, and a multi-target estimate would need to be carried out for estimates of the azimuth angle when reflections from a guardrail or a similarly elongated object are to be expected. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the invention is to describe a method that enables simpler and more accurate angle estimation in situations in which reflections of the radar signal from elongated objects are to be expected. 
         [0010]    This object may be achieved according to the present invention in that for a single real target, the occurrence of a number n of apparent targets, which are caused by reflection of the signal coming from the real target from elongated objects, is modeled mathematically; that a correlation between the location angle of the real target and the location angles of the apparent targets is calculated; and that in order to estimate the location angle of the real target, a multi-target estimate is performed in an n-dimensional search space and the search is limited to a sub-space that is determined by the calculated correlation. 
         [0011]    In the case of a two-target estimate, the search space has two dimensions. The laws of reflection yield, for the location angle of the real target and the location angle of the apparent target, a correlation that defines a one-dimensional sub-space in the two-dimensional search space. 
         [0012]    The aforesaid correlation is in general non-linear, so that the sub-space is not necessarily a vector space. If the two-dimensional search space is depicted as a portion of a plane, the one-dimensional sub-space is then represented by a (generally curved) line in that plane. For small angles, however, the correlation between the location angles is approximately linear, and what is obtained as the sub-space is a vector space that is represented by a straight line in the plane. The search for the maximum of the DML function can in any case be limited to locations that lie on the line or curve or, in consideration of unavoidable errors in the determination of the aforesaid correlation, in the close vicinity of said line, i.e. in a strip in the plane that contains the line or curve and whose width is determined by the permitted tolerances. For purposes of this application, the term “sub-space” refers to this entire strip and not only to the one-dimensional line within said strip. 
         [0013]    Limiting the search space results in a considerable reduction in calculation outlay. It is moreover apparent that the accuracy of the angle estimate is also improved by this method. 
         [0014]    Advantageous embodiments and refinements are indicated in the dependent claims. 
         [0015]    Exemplifying embodiments are explained in further detail below with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram of a radar sensor for motor vehicles with which the method according to the present invention is executable. 
           [0017]      FIG. 2  schematically depicts a situation in which a signal scattered from a radar target is reflected from a guardrail. 
           [0018]      FIG. 3A  shows sketch of possible antenna configuration of a radar sensor. 
           [0019]      FIG. 3B  shows sketch of possible antenna configuration of a radar sensor. 
           [0020]      FIG. 4  graphically depicts a DML function and a sub-space in a two-dimensional search space for angle estimation. 
           [0021]      FIGS. 5 and 6  show results of simulation calculations for comparing different angle estimation methods. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The radar sensor shown in  FIG. 1  has four receiving antenna elements  10 ,  12 ,  14 ,  16  that together form a planar group antenna  18 . The radar sensor is incorporated into a motor vehicle in such a way that antenna elements  10  to  16  are located next to one another at the same height, so an angular resolution capability of the radar sensor in the horizontal (in azimuth) is achieved.  FIG. 1  symbolically depicts radar beams that are received by the antenna elements at an azimuth angle θ. 
         [0023]    A high-frequency section  20  for applying control to a transmitting antenna element  22  is constituted, for example, by a monolithic microwave integrated circuit (MMIC), and encompasses a local oscillator  24  that generates the radar signal to be transmitted. The radar echoes received by antenna elements  10  to  16  are respectively delivered to a mixer  28 , where they are mixed with the transmitted signal supplied by oscillator  24 . This yields, for each of the antenna elements, an intermediate-frequency signal Z 1 , Z 2 , Z 3 , Z 4  that is delivered to an electronic control and evaluation unit  30 . 
         [0024]    Control and evaluation unit  30  contains a control section  32  that controls the functions of oscillator  24 . In the example shown, the radar sensor is a frequency modulated continuous wave (FMCW) radar, i.e. the frequency of the transmitted signal supplied from oscillator  24  is periodically modulated in the form of a sequence of rising and/or falling frequency ramps. 
         [0025]    Control and evaluation device  30  furthermore contains an evaluation section having a four-channel analog/digital converter  34  that digitizes the intermediate-frequency signals Z 1  to Z 4  obtained from the four antenna elements and plots them respectively against the duration of an individual frequency ramp. The time signals thereby obtained are then converted channel by channel by fast Fourier transformation, in a transformation stage  36 , into corresponding frequency spectra. In these frequency spectra, each located object emerges in the form of a peak whose frequency location is dependent on the signal travel time from the radar sensor to the object and back to the radar sensor and—because of the Doppler effect—on the relative speed of the object. The distance d and the relative speed v of the relevant object can be calculated in known fashion from the frequency locations of two peaks that have been obtained for the same object but on frequency ramps having different slopes, for example a rising ramp and a falling ramp. 
         [0026]    As schematically depicted in  FIG. 1  based on the radar beams, the different positions of antenna elements  10  to  16  cause the radar beams that have been emitted from the same antenna element, have been reflected from the object, and are then received by the various antenna elements, to travel over different path lengths and thus to exhibit phase differences that are dependent on the azimuth angle θ of the object. The pertinent intermediate-frequency signals Z 1  to Z 4  also exhibit corresponding phase differences. The amplitudes (absolute values) of the received signals are also different from one antenna element to another, again as a function of the azimuth angle θ. The dependence of the complex amplitudes, i.e. absolute values and phases, of the received signals on the azimuth angle θ can be stored for each antenna element in the form of a diagram in control and evaluation unit  30 . The diagrams for the individual antenna elements can be combined into an antenna diagram that indicates, for each antenna element, the amplitude of the received signal as a function of the azimuth angle. For each located object (each peak in the frequency spectrum), an angle estimator  38  compares the complex amplitudes obtained in the four reception channels with the antenna diagram, so as thereby to estimate the azimuth angle θ of the object. The value at which the measured amplitudes best correlate with the values read off from the antenna diagram is taken as the most probable value for the azimuth angle. 
         [0027]      FIG. 2  is a plan view illustrating a traffic situation in which a radar sensor according to  FIG. 1 , which is incorporated into a motor vehicle  40 , is locating a target  42 , in this example a preceding vehicle. Target  42  is assumed in idealized fashion to be a point. The signal emitted from the radar sensor is scattered from target  42  and travels along the same path back to the radar sensor. Based on this signal, the radar sensor would measure an object distance d. 
         [0028]    The radar radiation striking target  42  is also scattered or reflected in other directions, however, so that a portion of said radiation, for example, strikes a guardrail  44  and is reflected from it back into the radar sensor. This reflection appears to the radar sensor as a further target, an apparent target  46  that is the mirror image of the real target  42 . 
         [0029]    In practice, in contrast to the not-to-scale depiction in  FIG. 2 , the distance between motor vehicle  40  and target  42  is substantially greater than the distance from the two vehicles to guardrail  44 . The difference between the object distance d and the apparent distance of apparent target  46  is therefore so small that it is normally below the resolution limit of the radar sensor. The relative speeds of target  42  and of apparent target  46  are also practically identical, so that the radar sensor cannot separate the two targets. Superimpositions and interference nevertheless occur between the signal backscattered on the direct path to the radar sensor and the signal reflected at guardrail  44 , and distort the result of the angle estimate. In order to take this effect into consideration, it would be necessary to carry out a two-target estimate that supplies two azimuth angles, namely an azimuth angle θd for target  42  and another azimuth angle θr for apparent target  46 . 
         [0030]    The two azimuth angles θd and θr are not independent of one another, however. If y 1  is the lateral distance between the radar sensor in motor vehicle  40  and guardrail  44 , y 2  is the lateral distance between target  42  and the guardrail, and x is the distance, measured in the travel direction, between target  42  and motor vehicle  40 , then: 
         [0000]      tan θ d =( y 1− y 2)/ x ,tan θ r =( y 1+ y 2)/ x   (1).
 
         [0031]    It follows from the first equation that 
         [0000]        y 2= y 1− x *tan θ d   (2)
 
         [0000]    and consequently 
         [0000]      θ r =tan −1 (( y 1+ y 2)/ x )=tan −1 (2 y 1/ x )−tan θ d )  (3).
 
         [0032]    For x=d cos θd, then: 
         [0000]      θ r =tan −1 (2 y 1/( d  cos θ d )−tan θ d   (4).
 
         [0033]    For small angles θr, θd, it is approximately true that 
         [0000]      θ r =(2 y 1/ d )−θ d   (5).
 
         [0034]    This correlation between θd and θr can be used to carry out a two-target estimate with reduced calculation outlay, and to improve the accuracy of the angle estimate. The lateral distance y 1  to guardrail  44  must, however, be known. 
         [0035]    One possibility is to estimate the distance y 1  based on the typical roadway width. For roads having two or more lanes in each travel direction, by locating vehicles in adjacent lanes it is also possible to establish the lane in which the own vehicle is traveling, and accordingly to establish what the distance to the guardrail would need to be. 
         [0036]    The radar sensor will often also receive radar echoes from posts on which guardrail  44  is mounted. These signals can then be used to measure the distance y 1  directly. 
         [0037]    As will be further explained later on, another possibility is also to correct any errors in the estimate of y 1  after the fact, based on the result of the angle estimate. If a sufficiently accurate value for y 1  has been found for a single target  42 , the same value can also be used for an improved angle estimate of other radar targets. 
         [0038]    In motor vehicles it is often also desirable to estimate the elevation angle of a located object, for example in order to decide whether an object can be driven over (e.g. a manhole cover) or driven under (e.g. a bridge). In this case as well, the angle estimate can be distorted by reflections, in particular by reflections from the road surface, which would then play the part of guardrail  44  in  FIG. 2 . The value that corresponds to the distance y 1  is then provided directly by the installation height of the radar sensor on motor vehicle  40 . 
         [0039]      FIG. 3A  schematically shows an example of a planar single input/multiple output (SIMO) antenna assemblage for a radar sensor that is angularly resolving in both azimuth and elevation. In this antenna assemblage multiple receiving antenna elements  50  (circles) are disposed in both horizontal rows and vertical rows, so that the azimuth angle of an object can be estimated based on path length differences in the horizontal rows, and the elevation angle based on path length differences in the vertical rows. Here as well, as in  FIG. 1 , what is implemented is a bistatic antenna concept in which antenna elements  50  serve exclusively to receive the radar signals while a single additional antenna element  52  (square) is provided in order to transmit the signal. 
         [0040]      FIG. 3B , in contrast, shows an example of a multiple input/multiple output (MIMO) antenna assemblage in which multiple transmitting antenna elements  52  are also disposed horizontally and vertically at a distance from one another. Antenna elements  52  can be activated sequentially in time or alternatively also simultaneously. In the latter case (MIMO mode), consideration must be given in creating the antenna diagrams to the fact that signals which have been transmitted from each of the transmitting antenna elements  52  become superimposed at each receiving antenna element  50 . Alternatively, the transmitting antenna elements  52  can be switchable, so that different configurations of active transmitting antenna elements can be worked with depending on the situation. The signals received by the receiving antenna elements  50  can likewise, depending on the situation, be evaluated selectively for specific groups of said antenna elements. 
         [0041]    A two-target estimate in accordance with the method according to the present invention is depicted in  FIG. 4  using the example of an estimate for the elevation angle φ of an object. 
         [0042]      FIG. 4  shows a square portion of a two-dimensional search space  54 . Elevation angles φ 1  from −15° to +15° for a real target are plotted on the horizontal axis, and elevation angles φ 2  from −15° to +15° for an associated apparent target, which is produced by reflection from the road surface, are plotted on the vertical axis. Because of the symmetry between the real target and apparent target, consideration can be limited to the triangular region located in  FIG. 4  below the diagonal. For each value pair (φ 1 , φ 2 ), the antenna diagram of antenna configuration  48  shown in  FIG. 3  supplies a set of amplitudes that would need to be measured by antenna elements  50  if the targets were located at the elevation angles φ 1  and φ 2 . 
         [0043]    Given a set of actually measured amplitudes, it is then possible to calculate for each value pair (φ 1 , φ 2 ), based on the correlation between the measured amplitudes and the amplitudes expected for that pair, a DML function which indicates the probability that for the (a priori) given measurement result, the targets are in fact located at the relevant angles φ 1  and φ 2 . In  FIG. 4  a DML function  56  of this kind is depicted in the form of altitude contour lines of a “mountain” above the φ 1 -φ 2  plane. The different altitude regions are symbolized by different cross-hatchings: the finer the cross-hatching, the higher the function value of the DML function. In the example shown, DML function  56  has a “valley”  58  and three “peaks”  60 ,  62 ,  64  separated from one another. The best estimate for the elevation angles φ 1  and φ 2  is the maximum of this function, i.e. the location of the highest peak. 
         [0044]    But because the correlation applicable to elevation angles φ 1  and φ 2  is analogously the same as for the azimuth angles in equation (4) or (5), it is sufficient to limit the search for the maximum to those locations which conform to this correlation. In  FIG. 4  these are those locations which lie on a line  66 . All that is necessary in order to find the maximum of the function is therefore to follow line  66  and look for the point on that line at which DML function  56  has the greatest value. 
         [0045]    But because the exact location of line  66  is dependent on the variable y 1  (which here represents the installation height of the radar sensor above the road surface), and because this variable is affected by some uncertainties, it appears useful to expand the search to those locations which are located within a certain tolerance zone away from line  66 . In the example shown, the search is thus limited to a sub-space  68  that is in the shape of a strip which contains line  66 . 
         [0046]    In the example shown, the maximum of DML function  56 , and thus the desired elevation angles φ 1  and φ 2 , are thus found at the highest point of peak  64  within sub-space  68 . If it becomes apparent in this context that the maximum is located not exactly on line  66  but rather slightly away from it, the variable y 1  and thus line  66  can be adapted so that they pass through the maximum of the function. The result is to produce, for the next angle estimate, a more accurate value y 1  that characterizes the correlation between elevation angles φ 1  and φ 2 . 
         [0047]    In the example shown here, peak  64  constitutes the absolute maximum of the function, but the other two peaks  60 ,  62  are only slightly lower. As a result of unavoidable signal noise, it can therefore happen that for one or more of the periodically repeated angle estimates, the absolute maximum is located in the region of peak  60  or  62 , so that if a conventional two-target estimate were performed in the entire search space  54 , the result of the angle estimate would abruptly change as a result of noise. Limiting the search to sub-space  68  eliminates such noise-related outliers, so that the accuracy of the angle estimate is also improved. 
         [0048]    The decision as to which of the two elevation angles φ 1  and φ 2  then represents the real target is made based on the plausible consideration that the real target is located above the road surface and not below it. The same applies to the situation shown in  FIG. 2 : the real target  42  is located on the actual roadway and not beyond guardrail  44 . 
         [0049]    In order to illustrate the improvement in measurement accuracy,  FIG. 5  indicates, for various estimating methods, the root mean square error (RMSE) of the estimated angles as a function of the elevation angle of a real target. Curve  70  shows the results for the above-described method in accordance with the invention. For comparison, curve  72  shows the results for a conventional two-target estimate in which the entire search space  54  is searched, and curve  74  shows the results for a one-target estimate based on the assumption that the measurement results represent only a single target. 
         [0050]      FIG. 6  shows the results of a simulation calculation in which the elevation angle is continuously measured while the vehicle equipped with the radar sensor approaches the real target and the object distance d correspondingly decreases (from right to left in  FIG. 6 ) from 30 m to 4 m. 
         [0051]    Curve  76  in the upper graph in  FIG. 6  indicates the results for the method according to the present invention, curve  78  in the center graph the results for a conventional two-target estimate, and curve  80  in the lower graph the results for a one-target estimate. The smoother, thicker line  82  in all three graphs indicates the actual elevation angle of the target. Because the target, in this example, is located at a lower height above the roadway than the radar sensor, as the distance to the target decreases (to the left), the elevation angle becomes negative and greater in absolute value. 
         [0052]    The results clearly show the superiority of the method according to the present invention.