Abstract:
A radar system for detecting positions of targets using a high resolution algorithm is presented. The number of incorrect target detections due to incorrect estimation of the number of radar reflections is reduced. A two-dimensional (azimuth and distance) peak having maximum power is detected, and a virtual beam formed centering on that peak. If that two-dimensional peak is buried in the virtual beam it is assumed to be a noise peak and removed. This is repeated for all two-dimensional peaks, thereby removing noise peaks and reducing the number of unnecessary target detections. The system can also correctly detect peaks that are close together at a certain resolution but have a power larger than the virtual beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2011-272475 filed Dec. 13, 2011, the description of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a radar system that detects the position of a target using a high-resolution algorithm. 
     2. Related Art 
     Some types of radar systems are well known. One type of well-known radar system uses array antennas composed of a plurality of antenna elements to detect the distance to a target that has reflected radio waves or the direction of arrival of radio waves (i.e. a direction in which the target is present). 
     As mentioned in JP-A-2006-047282, so-called high-resolution algorithms, such as MUSIC (Multiple Signal Classification), ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) and the like, are well known as methods of estimating the direction of arrival of radio waves. In a method using such a high-resolution algorithm, an angle spectrum is prepared according to a correlation matrix that shows correlation between signals received by antenna elements (also referred to as “channels”). Then, in the method, the angle spectrum is scanned to estimate the direction of arrival of radio waves. 
     In a high-resolution algorithm, an angle spectrum is prepared in a process of calculating the direction of arrival of radio waves. Specifically, in preparing an angle spectrum in the process, the number of reflections is estimated so that the estimated number of peaks is formed. Accordingly, if the estimated number of incoming waves is incorrect, unnecessary peaks (noise peaks) may be formed to unnecessarily detect a target that is not actually present (hereinafter this detection is referred to as “unnecessary detection”). An unnecessary detection may cause unnecessary control such as in an ACC (Adaptive Cruise Control) system of a vehicle. Therefore, the unnecessary detections have to be reduced as much as possible. 
     When a high-resolution algorithm is used, peaks are detected in the calculation with unnecessary fine resolution. Therefore, use of a high-resolution algorithm raises a problem of exposing noise peaks that would have been buried in peaks based on the waves reflected from a target in so-called Beam-former method. 
     On the other hand, as described in JP-A-2006-047282 as well, several attempts are made to enhance the accuracy of estimating the number of reflections. However, currently, sufficiently satisfactory results are not yet available. 
     SUMMARY 
     Accordingly, regarding a radar system for detecting the position of a target using a high-resolution algorithm, such a radar system that is able to reduce the occurring of unnecessary detections of targets due to an erroneous estimation of the number of incoming waves is desired. 
     Hence present application presents a following radar system as exemplary embodiment, of which includes: i) an electromagnetic wave transmitting means for transmitting electromagnetic waves; ii) an electromagnetic wave receiving means for receiving electromagnetic waves reflected from targets; iii) a position estimating means for estimating a position of each target that has reflected the electromagnetic waves, using at least a high resolution algorithm, according to reception signals acquired from the electromagnetic wave receiving means; iv) a virtual beam creating means for creating virtual beams having a predetermined beam width centering on respective targets&#39; positions estimated by the position estimating means; and v) a removing means for removing the targets that are located within a range of the respective virtual beams and a received power with respect to the respective position is smaller than the power of the respective virtual beams (The first aspect of the radar system). 
     Accordingly, by means of the virtual beam, the first aspect of the radar system can remove the small power peak which appears in the vicinity of the peak regarding an angle spectrum, that is, can decrease unnecessary detection of nonexistent targets. Hence the present radar system can prevent causing unnecessary control of the vehicle. 
     It is preferable that a width of the virtual beam is just set to a value being able to gain a desired resolution (The second aspect of the radar system). According to the second aspect, the radar system can prevent over performance and offers a preferred detecting result to the downstream component for vehicle control. 
     It is preferable that the removing means processes the estimated targets in descending order of their power (The third aspect of the radar system). According to the third aspect, the processing load of the removing means can be reduced. 
     Though the position estimated by the estimating means may be a one-dimensional position represented by azimuth, a two-dimensional position represented by both distance and azimuth is more preferable (The fourth aspect of the radar system). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram illustrating a configuration of a radar sensor applied to an ACC system, according to the exemplary embodiment; 
         FIG. 2  is a flow diagram illustrating a processing performed by a signal processor in the radar sensor; 
         FIG. 3  is a flow diagram specifically illustrating a two-dimensional peak extraction performed in the processing; 
         FIG. 4  is an explanatory diagram illustrating an outline of a virtual beam; 
         FIG. 5  is an explanatory diagram exemplifying positions of temporary peaks and gain distribution of virtual beams on a two-dimensional map expressed by distance and azimuth; and 
         FIG. 6  is a table showing measurements of the effects of the radar sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, hereinafter is described an embodiment of the present invention.  FIG. 1  is a block diagram illustrating a configuration of a radar sensor (corresponding to the “radar system” recited in the claims)  1  applied to an ACC (Adaptive Cruise Control) system, according to the exemplary embodiment. 
     As shown in  FIG. 1 , the ACC system includes the radar sensor  1 , an electronic control unit for inter-vehicle distance control (hereinafter referred to as an “inter-vehicle distance control ECU”)  30 : herein “inter-vehicle distance” means distance between the vehicles, an electronic control unit for engine (hereinafter referred to as an “engine ECU”)  32  and an electronic control unit for brake (hereinafter referred to as a “brake ECU”)  34 . The radar sensor  1  detects conditions ahead of the vehicle. The inter-vehicle distance control ECU  30  automatically controls inter-vehicle distance according to the conditions ahead of the vehicle, which are detected by the radar sensor  1 , and the conditions of the vehicle detected by various sensors, not shown. The engine ECU  32  controls the conditions of the engine according to a manipulated variable of the accelerator pedal or an instruction from the inter-vehicle distance control ECU  30 . The brake ECU  34  controls the conditions of the braking device according to the state of manipulation of the brake pedal or an instruction from the inter-vehicle distance control ECU  30 . The ECUs  30 ,  32  and  34  are each mainly configured by a well-known microcomputer and connected to each other via an on-vehicle LAN (Local Area Network). 
     The radar sensor  1  is configured as a so-called “millimeter-wave radar” based on an FMCW (Frequency-Modulated Continuous Wave) system. The radar sensor  1  transmits and receives frequency-modulated electromagnetic waves in a millimeter-wave band as radar waves to and from capture targets, such as preceding vehicles or roadside objects (hereinafter referred to as “captured targets”). The radar sensor  1  generates information regarding the captured targets (hereinafter referred to as “target information”) and transmits the information to the inter-vehicle distance control ECU  30 . 
     The target information at least includes positions (distance and azimuth) of the captured targets and relative velocity of the captured targets with respect to the vehicle equipped with the radar system (hereinafter also referred to as a “system-equipped vehicle”). 
     [Configuration of the Radar Sensor] 
     The radar sensor  1  is specifically described below. The radar sensor  1  includes an oscillator  10 , amplifier  12 , distributor  14 , transmission antenna  16  and reception antenna array  20 . The oscillator  10  generates high-frequency signals of a millimeter-wave band. The high-frequency signals are modulated so as to have a section in which the frequency linearly increases with time (this section is hereinafter referred to as a “frequency-increase section”) and a section in which the frequency linearly decreases with time (this section is hereinafter referred to as a “frequency-decrease section”). The amplifier  12  amplifies the high-frequency signals generated by the oscillator  10 . The distributor  14  distributes power of the output from the amplifier  12  into transmission signals (Ss) and local signals (L). The transmission antenna  16  radiates radar waves according to the transmission signals (Ss). The reception antenna array  20  is composed of an n number of reception antenna elements (CH 1  to CHn) that receive the radar waves. 
     The radar sensor  1  also includes a reception switch  21 , amplifier  22 , mixer  23 , filter  24 , A/D converter  25  and signal processor  26 . The reception switch  21  sequentially selects any one of the antenna elements composing the reception antenna array  20  to supply reception signals (Sr) from the selected antenna element to a downstream component. The amplifier  22  amplifies the reception signals (Sr) supplied from the reception switch  21 . The mixer  23  mixes the reception signals (Sr) amplified by the amplifier  22  with the local signals (L) to generate beat signals (BT). The filter  24  removes unnecessary signal components from the beat signals (BT) generated by the mixer  23 . The A/D converter  25  samples the output from the filter  24  and converts the sampled output into digital data. The signal processor  26  activates or deactivates the oscillator  10 , controls sampling of the beat signals (BT) performed via the A/D converter  25 , processes the sampled data so as to generate target information or the like. 
     Hereinafter, the reception systems using the antenna elements CH 1  to CHn composing the reception antenna array  20  are referred to as channels CH 1  to CNn, respectively. The signal processor  26 , which is mainly composed of a well-known microcomputer, includes a processing unit (e.g., DSP (Digital Signal Processor)). The processing unit has a function, for example, of performing Fast Fourier Transform (FFT) processing with respect to data acquired via the A/D converter  25 . 
     [Operation of the Radar Sensor] 
     In the radar sensor  1  configured in this way, when the oscillator  10  is activated according to the instruction from the signal processor  26 , the high-frequency signals generated in the oscillator  10  is transmitted to the amplifier  12  for amplification. Then, the output power of the amplified high-frequency signals is distributed by the distributor  14  into the transmission signals (Ss) and the local signals (L). Of these signals, the transmission signals (Ss) are transmitted as radar waves via the transmission antenna  16 . 
     The radar waves emitted from the transmission antenna  16  and reflected from a target are received by the reception antenna elements composing the reception antenna array  20 . In this case, only the reception signals (Sr) of a reception channel Chi (i=1 to n) selected by the reception switch  21  are supplied to the amplifier  22  for amplification. The reception signals (Sr) amplified by the amplifier  22  are supplied to the mixer  23 . Then, the mixer  23  mixes the reception signals (Sr) with the local signals (L) from the distributor  14  to generate the beat signals (BT), i.e. frequency components equivalent to the difference between the signals (Ss) and the signals (Sr). The beat signals (BT) are subjected to filtering in the filter  24  to remove unnecessary signal components. Then, the filtered signals are sampled by the A/D converter and the sampled signals as digital data are supplied to the signal processor  26 . 
     The reception switch  21  is ensured to switch the channels CH 1  to CHn such that all of the channels CH 1  to CHn are selected by a predetermined number of times during one modulation cycle of the radar waves. Further, the A/D converter  25  is ensured to perform sampling in synchronization with the switching timing. Specifically, during one modulation cycle of the radar waves, sampled data are stored for each of the channels CH 1  to CHn and for each of the frequency-increase and frequency-decrease sections of the radar waves. 
     [Main Processing] 
     The main processing performed by the signal processor  26  is described along the flow diagram illustrated in  FIG. 2 . The main processing is repeatedly started on the basis of one modulation cycle of the radar waves as one measurement cycle. 
     When the main processing is started, frequency analysis (FFT processing here) is performed first, at step S 110 , for the sampled data corresponding to one modulation cycle stored during one measurement cycle. During the frequency analysis, the signal processor  26  calculates a power spectrum of the beat signals (BT) for each of the channels CH 1  to CHn and for each of the frequency-increase and frequency-decrease sections of the radar waves. 
     Then, at step S 120 , the signal processor  26  searches through the power spectrum calculated at step S 110  to extract frequency components as peaks (hereinafter referred to as “distance-direction peaks”). At step S 130 , the signal processor  26  performs azimuth calculation to obtain directions of arrival of the reflected waves that have caused the distance-direction peaks extracted at step S 120 , for each of the frequencies and for each of the modulation sections of the distance-direction peaks. Specifically, the signal processor  26  performs a high resolution algorithm, such as MUSIC, using the same distance-direction peaks simultaneously detected in the channels CH 1  to CHn to obtain an angle spectrum. 
     At step S 140 , the signal processor  26  performs two-dimensional peak extraction. In the two-dimensional peak extraction, the signal processor  26  removes unnecessary noise peaks from the peaks (temporary peaks) whose positions are two-dimensionally expressed by the distance and azimuth obtained through steps S 110  to S 130 , to extract peaks based on the waves reflected from the target (hereinafter referred to as “two-dimensional peaks”). 
     At step S 150 , the signal processor  26  performs pair matching. In the pair matching, the signal processor  26  combines, among the two-dimensional peaks extracted at step S 140  for every frequency-increase and frequency-decrease section, the peaks based on the waves reflected from the same target. Specifically, the signal processors  26  combines the two-dimensional peaks which coincide with each other in the signal levels of the distance-direction peaks as extracted at step S 120  and in the directions of arrival as calculated at step S 130 . 
     At step S 160 , the signal processor  26  calculates a distance and a relative velocity for each of the combinations set at step S 150 , using a well-known process in an FMCW radar. Then, the signal processor  26  generates target information according to the calculated distance and relative velocity as well as the azimuth calculated at step S 130  and then ends the main processing. The target information generated in the main processing is transmitted to the inter-vehicle distance control ECU  30  for use in ACC or the like. 
     [Two-Dimensional Peak Extraction] 
     The two-dimensional peak extraction is specifically described along the flow diagram shown in  FIG. 3 . 
     With the start of the two-dimensional peak extraction, the results obtained at step S 130  (angle spectrum obtained through MUSIC) is subjected to peak scanning, at step S 210 , to thereby extract temporary peaks and prepare a list indicating two-dimensional positions and powers of the temporary peaks. The number of temporary peaks equals to the number of incoming waves, which has been estimated during the process of MUSIC. 
     Then, at step S 220 , the temporary peaks extracted at step S 210  are sorted in descending order of their power. At step S 230 , among the temporary peaks, the one having a maximum power is registered as a two-dimensional peak and, at the same time, is removed from the list of the temporary peaks. 
     At step S 240 , it is determined whether or not any temporary peaks remain unremoved. If no temporary peaks remain unremoved (NO at step S 240 ), the two-dimensional peak extraction is ended. On the other hand, if any temporary peak remains (YES at step S 240 ), control proceeds to step S 250 . At step S 250 , virtual beams are created for the registered two-dimensional peaks. Then, at step S 260 , it is determined whether or not there are any temporary peaks buried in the virtual beams. 
       FIG. 4  is an explanatory diagram illustrating an outline of a virtual beam. As indicated by the broken line in  FIG. 4 , a virtual beam is created so as to have a beam shape (beam width) in which gain is lowered by 6 dB at a position distanced from the two-dimensional peak to be processed, by a half of a target azimuth resolution (or distance resolution). An expression “buried in a virtual beam” refers to the received power of the temporary peak not being more than the power of the virtual beam at the position. The diagram of  FIG. 4  is based on only a relationship with azimuth. In the case of using a relationship with distance, the range of lowering by 6 dB is specified not by azimuth resolution but by range resolution. Otherwise, a relationship with distance is treated similar to a relationship with azimuth. 
       FIG. 5  is an explanatory diagram exemplifying positions of temporary peaks and gain distribution of virtual beams on a two-dimensional map expressed by distance and azimuth. Specifically, in  FIG. 5 , the positions of temporary peaks (shown by circled numerals 1 to 5 in a descending order of the power of the peaks) are shown on the two-dimensional map expressed by distance and azimuth. Also, for the temporary peaks having the three highest powers, the gain distribution of virtual beams is shown in the map by grayscale. The gain distribution of each virtual beam appears, for example, as shown in  FIG. 5 . 
     If no temporary peaks are buried in the virtual beams (NO at step S 260 ), control returns to step S 230  to repeat steps S 230  to S 260 . If any peaks are buried in the virtual beams (YES at step S 260 ), control proceeds to step S 270 . At step S 270 , the temporary peaks buried in the virtual beams are determined to be the peaks originated from noise and removed. 
     At step S 280 , it is determined whether or not there are temporary peaks that remain without being removed. If any temporary peaks remain unremoved (YES at step S 280 ), control returns to step S 230  to repeat steps S 230  to S 280 . If no temporary peaks remain unremoved (NO at step S 280 ), i.e. if all of the temporary peaks have been processed, the two-dimensional peak extraction is ended. 
     Advantages 
     As described above, in the radar sensor  1 , the temporary peak having the maximum power is registered as a two-dimensional peak. Then, a virtual beam is formed centering on the position of the two-dimensional peak. Then, the temporary peak, if it is buried in the virtual beam, is removed as a noise peak. The same process is repeatedly performed for the temporary peaks remaining without being removed, thereby ensuring extraction of two-dimensional peaks. 
     Thus, the radar sensor  1  is able to remove noise peaks in the event that the noise peaks are caused in the vicinity of the two-dimensional peaks, and thereby avoid erroneously estimating the number of reflections to be larger than the actual number. Accordingly, the radar sensor  1  is able to prevent unnecessary detections. 
     Further the radar sensor  1  is able to individually and separately detect a peak having a larger power than that of the virtual beam, in the event that the peaks are located close to each other with respect to a required resolution. 
       FIG. 6  is a table showing measurements of the effects of the radar sensor  1 . Specifically, the table of  FIG. 6  shows measurements of the frequency of unnecessary detections performed by a radar (using high resolution algorithm (Music)) of conventional art and by the radar sensor  1  of the present embodiment (using combination of high-resolution algorithm and virtual beams). The measurements are based on a scenario that a certain vehicle running on the adjacent lane passes the system-equipped vehicle. As shown in the table, the radar sensor  1  of the present embodiment has been confirmed to reduce the number of unnecessary detections to ⅓ to ¼ of those of the radar of conventional art. 
     In the present embodiment, the components other than the signal processor  26  of the radar sensor  1  correspond to the electromagnetic transmitting/receiving means; the signal processor  26  that performs steps S 120  to S 130  corresponds to the position estimating means; the signal processor  26  that performs steps S 250  corresponds to the virtual beam creating means; and the signal processor  26  that performs steps S 260  to S 270  corresponds to the removing means. 
     [Modifications] 
     An embodiment of the present invention has been described so far. However, the present invention is not limited to the above embodiment but may be implemented in various modifications in a range not departing from the spirit of the present invention. 
     The embodiment described above exemplifies that the radar sensor  1  is applied to an ACC system. However, this should not impose a limitation. Further, as an alternative to MUSIC as a high-resolution algorithm used in azimuth detection in the above embodiment, ESPRIT or other algorithms may be used. 
     In the embodiment described above, virtual beams are applied to the temporary peaks whose positions are two-dimensionally expressed by distance and azimuth. Alternative to this, virtual beams may be applied to the peaks whose positions are one-dimensionally expressed (i.e., only by either azimuth or distance) as a result of using a high-resolution algorithm.