Abstract:
An angle-resolving radar sensor, e.g., for motor vehicles, includes; an antenna having multiple antenna elements which are each switchable to one of multiple evaluation channels; and an evaluation device for determining the angle of incidence of a received signal based on the amplitudes measured in the evaluation channels. The number of antenna elements is greater than the number of evaluation channels and a switching device is provided to connect the evaluation channels alternatingly to different selections of antenna elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an angle-resolving radar sensor, in particular for motor vehicles, including an antenna having multiple antenna elements which are each switchable to one of multiple evaluation channels and including an evaluation device for determining the angle of incidence of a received signal based on the amplitudes measured in the evaluation channels. 
     2. Description of the Related Art 
     Radar sensors are used in motor vehicles, for example, to measure distances, relative velocities, and azimuths of vehicles or other objects located ahead of one&#39;s own vehicle. Radar sensors including a group antenna having a planar design have the advantage in these applications that they only require little installation space. Individual antenna elements of the group antenna are situated at a distance from one another horizontally, so that different azimuths of the located objects result in different running times the radar signals need to travel from the object to the particular antenna element. These running time differences result in corresponding differences in the phase of the signals which are received by the antenna elements and evaluated in the associated evaluation channels. By comparing the (complex) amplitudes received in the different channels to the corresponding amplitudes in an antenna diagram, the angle of incidence of the radar signal and thus the azimuth of the located object may then be determined. 
     To achieve a high angular resolution, the aperture of the antenna should be as large as possible. (In the case of a planar group antenna, the aperture represents the overall extension of the group antenna in the direction of the angle measurement with regard (horizontally) to wavelength λ of the radar radiation). If, however, the distances between the adjacent antenna elements are too great, ambiguities in the angle measurement may occur, since the same phase relations between the received signals are obtained for running time differences which differ by integral multiples of wavelength λ. An unambiguous angle measurement may, for example, be obtained using a ULA (uniform linear array) structure in which the antenna elements are spaced apart at λ/2. In this case, however, the number of antenna elements and thus the number of necessary evaluation channels increases with increasing aperture, so that high hardware costs arise accordingly. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a radar sensor which enables an unambiguous angle measurement having unambiguous angular resolution using a predefined number of evaluation channels. 
     This object is achieved in that the number of antenna elements is greater than the number of evaluation channels and that a switching device is provided to connect the evaluation channels alternatingly to different selections of the antenna elements. 
     A selection of antenna elements in which each individual element is connected to one of the receive channels is to be referred to in the following as an “array.” The switching device may be used, for example, for switching between an array having a great aperture and an array having a smaller aperture. The array having a great aperture provides ambiguous angle information having a high angular resolution. The array having the smaller aperture and correspondingly smaller distances between the adjacent antenna elements may then be used to eliminate the ambiguities. Likewise, it is also possible to switch between three or more different arrays in a regular sequence. The arrays do not necessarily have to have different apertures in this case. For example, it is also possible to select different arrays having identical apertures in which the distances between the individual antenna elements are selected in such a way that the phase relations obtained for the different arrays are only consistent for one single angle of incidence, thus eliminating the ambiguity. 
     The subclaims describe advantageous embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a radar sensor according to the present invention. 
         FIG. 2  shows diagrams to illustrate the mode of operation of the radar sensor according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The radar sensor shown in  FIG. 1  has a planar group antenna  10  which is formed in the illustrated example by eleven antenna elements  12  which are situated horizontally next to one another. Each antenna element  12  includes multiple patches  14  which are situated in a vertical gap, fed in series, and together cause a beam formation in the vertical direction (elevation). 
     While in a conventional radar sensor a separate evaluation channel is assigned to each individual antenna element, the radar sensor described here only has a total of four evaluation channels  16  for evaluating the signals of a total of eleven antenna elements  12 . A switching device  18  is formed by four electronic switches  20 , each of which is associated with one of the evaluation channels  16  and has four different switching positions. Evaluation channel  16  is connected to one of antenna elements  12  in each switching position. In this way, a different selection of four antenna elements, whose signals are evaluated in the four evaluation channels  16 , is obtained for each combination of the switching positions of the four switches  20 . These different selections of antenna elements will be referred to in the following as “arrays.” 
     The radar sensor, described here as an example, is an FMCW (frequency-modulated continuous wave) radar sensor having a bistatic antenna system. Accordingly, in addition to group antenna  10  used as a receive antenna, a transmitting antenna  22  is provided which transmits a radar signal generated by a local oscillator  24 . The radar echo reflected from an object is then received by each antenna element  12  of group antenna  10 . Each evaluation channel  16  contains a mixer  26  which mixes the signal received by connected antenna element  12  with a portion of the signal generated by local oscillator  24  down to an intermediate frequency signal ZF which is then evaluated further in an evaluation device  28 . 
     The frequency of the signal generated by local oscillator  24  is modulated in a ramp-shaped manner alternating between rising and falling ramps. The frequency of intermediate frequency signal ZF corresponds to the difference between the signal transmitted by transmitting antenna  22  and the signal received by antenna element  14  and is therefore a function of the signal propagation time from transmitting antenna  22  to the object and from the object back to antenna element  14 . This propagation time is proportional to the distance from the object. If the object moves in relation to the radar sensor, the frequency of intermediate frequency signal ZF contains in addition also a Doppler component which is a function of the relative velocity of the object. By evaluating the signals obtained on multiple consecutive ramps, it is then possible in a manner known per se to assign an unambiguous distance and an unambiguous relative velocity to every located object. 
     The radar signals which are reflected from the same point of an object and are then received by different antenna elements  12  travel at different running times (at least in the case of a 0° different azimuth of the object) and therefore differ in their phases. The signal lines from antenna elements  12  to mixers  26  are coordinated in their lengths in such a way that the phase differences of the signals are maintained. Since all signals are mixed with the same oscillator signal, the corresponding phase differences are also obtained in intermediate frequency signals ZF. Based on these phase differences, the angle of incidence of the received radar radiation and thus the azimuth of the associated object may be determined in evaluation device  28 . 
       FIG. 1  shows above group antenna  10  a longitudinal scale which indicates the positions of individual antenna elements  14  in units of wavelength λ of the radar radiation. The position of leftmost antenna element  12  is defined as position  0 . Rightmost antenna element  12  is then in position  11 . The overall extension of group antenna  10  is thus 11λ, i.e., its maximum aperture has the value 11. 
     The four leftmost antenna elements  12  are in positions 0.0, 0.5, 1.0, and 1.5 and thus together form a ULA structure having four elements. When all switches  20  of switching device  18  are in switching position “a,” the four elements of this ULA structure are switched to the four evaluation channels  16 . In this switching position, an unambiguous angle measurement is possible, but only having a low angular resolution due to the small aperture of this array. 
     When all four switches  20  are in switching position “b,” antenna elements  12 , which have positions 0.0, 1.5, 5.5, and 11.0, [are] switched to the four evaluation channels  16 , as indicated in  FIG. 1  by dashed lines with short interruptions. The use of this array allows a measurement having maximum angular resolution, but at the expense of unambiguity. 
     If all four switches  20  are in switching position “c,” antenna elements  12  of the selected array have positions 0.0, 5.5, 7.7, and 9.0 (dashed lines having somewhat shorter dash lengths). In switching position “d,” antenna elements  12  of the selected array have positions 0.0, 3.3, 4.4, and 6.6 (finely dashed lines). 
     For each of these four selectable arrays, an antenna diagram may be prepared which indicates the amplitude and/or the phase relations of the signals received in the four evaluation channels  16  as a function of assumed angle of incidence θ of the radar echo. In general, the azimuth of the located object corresponds, as actual angle of incidence α, to assumed angle of incidence θ for which the best match between the actual amplitude and/or phase relations measured in evaluation channels  16  and the corresponding values result in the antenna diagram. For the evaluation, a DML (deterministic maximum likelihood) function may be computed which indicates the correlation between the actually measured values and the values in the antenna diagram as a function of angle of incidence θ. The function value of the DML function varies between 0 (small correlation) and 1 (perfect match). The amplitudes and/or phases (complex amplitudes) measured in the four evaluation channels  16  may be understood as a vector having four components. Accordingly, the values in the antenna diagram also form a vector having four components for each angle of incidence θ. The DML function may then be computed by normalizing each of these two vectors to 1 and then forming a scale product. 
       FIG. 2  shows examples of such DML functions for the four arrays which correspond to switching positions “a” through “d” in  FIG. 1 , in each case assuming that the incident radar radiation is frontal (actual angle of incidence α=0°) and the received signals do not contain noise. In the case of unambiguity, the actual azimuth of the located object should then be at angle θ for which the DML function reaches value 1 (i.e., in this example at 0°). For other values of actual angle of incidence α, other (asymmetric) DML functions would be obtained in which the maxima are located at other points. Each function would then have at least one maximum at point θ=α. 
     Upper diagram (a) in  FIG. 2  shows the DML function for the array (ULA) which corresponds to switching position “a” in  FIG. 1 . As expected, this function has an unambiguous maximum at θ=0. Diagrams (b) through (d) in  FIG. 2  show the corresponding DML functions for switching positions “b” through “d” in  FIG. 1 . It is apparent that considerably stronger maxima occur here, thus corresponding to a greater angular resolution; however, multiple maxima are present in each case which reach at least approximately value 1. Since the signals will contain more or less noise in practice, unambiguous determination of the angle of incidence is not possible using these arrays. 
     Unambiguous angle determination having a high resolution is however possible, when the signals obtained by all four arrays are combined, is e.g., by forming the sum of the four DML functions. This sum is illustrated in  FIG. 2  in diagram (Σ). It is apparent that in this sum, only one strongly pronounced maximum is present at θ=0, while the remaining maxima are suppressed to the extent that they do not reach value 1 even if the noise is taken into account. 
     Switching device  18  ( FIG. 1 ) may now be controlled in such a way, for example, that after every frequency ramp of local oscillator  24 , it is switched over to another switching position, so that a measurement has been carried out after four ramps by each of the four possible arrays. The results contained in evaluation channels  16  are then stored (for each individual object) so that after four ramps the azimuth of each object may be determined with high resolution and without ambiguities based on the sum of the DML functions. 
     Instead of the sum of the DML functions, it is optionally also possible to use a weighted sum, the weighting for every array, for example, being a function of the aperture of the array and/or of the particular inclination of the frequency ramp on which the measurement takes place using this array. 
     The leftmost switch  20  is plotted in  FIG. 1  only systematically. In practice, this switch has no functions, since the signal of the leftmost antenna element  14  is evaluated in this channel. This switch may therefore be replaced in practice by a fixed connection. 
     The configuration of antenna elements  14  shown here is only to be understood as an example. Other positions may also be selected for the antenna elements. Likewise, the number of the antenna arrays and/or the number of the evaluation channels could be varied. The positions of antenna elements  14  in the different arrays may be optimized, for example, with the aid of a computer simulation. Particularly advantageous is a configuration of the antenna elements and a selection of arrays in which all arrays have a relatively large aperture and the DML functions fulfill the condition that for each actual angle of incidence α there is only one single value θ for which all DML functions have a maximum, which approaches value 1. At this value θ, the sum function has an absolute maximum.