Abstract:
A wireless communication device includes: a plurality of antennas configured to receive a radio wave; a processor configured to execute a program; and a memory configured to store the program, wherein the processor preforms operations to: measure reception strength of a radio wave of an estimation target at each of the plurality of antennas; executes first calculation processing of calculating first correlation coefficients of antenna pairs among the plurality of antennas and calculating a second correlation coefficient of the plurality of antennas by using the first correlation coefficients for each of a plurality of direction-of-arrival candidates of the radio wave; and estimates a first direction of arrival of the radio wave of the estimation target based on the second correlation coefficient for each of the direction-of-arrival candidates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-183649, filed on Sep. 9, 2014, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a wireless communication device and an estimation method. 
       BACKGROUND 
       [0003]    The direction of the arrival of radio waves is estimated. 
         [0004]    A related art is disclosed in Japanese Laid-open Patent Publication No. 2004-257820, Japanese Laid-open Patent Publication No. 2006-234767, and non-patent document 1, Eddy Taillefer et al., “Direction-of-Arrival Estimation Using Radiation Power Pattern With an ESPAR Antenna,” IEEE TRANSACTION ON ANTENNAS AND PROPAGATION, February 2005, VOL. 53, NO. 2. 
       SUMMARY 
       [0005]    According to an aspect of the embodiment, a wireless communication device includes: a plurality of antennas configured to receive a radio wave; a processor configured to execute a program; and a memory configured to store the program, wherein the processor preforms operations to: measure reception strength of a radio wave of an estimation target at each of the plurality of antennas; executes first calculation processing of calculating first correlation coefficients of antenna pairs among the plurality of antennas and calculating a second correlation coefficient of the plurality of antennas by using the first correlation coefficients for each of a plurality of direction-of-arrival candidates of the radio wave; and estimates a first direction of arrival of the radio wave of the estimation target based on the second correlation coefficient for each of the direction-of-arrival candidates. 
         [0006]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  illustrates one example of a wireless communication device; 
           [0009]      FIG. 2  illustrates one example of a control unit; 
           [0010]      FIG. 3  illustrates one example of processing of a wireless communication device; 
           [0011]      FIG. 4  illustrates one example of an average estimation error; 
           [0012]      FIG. 5  illustrates one example of a wireless communication device; 
           [0013]      FIG. 6  illustrates one example of a control unit; 
           [0014]      FIG. 7  illustrates one example of processing of a wireless communication device; 
           [0015]      FIG. 8  illustrates one example of an estimation error; 
           [0016]      FIG. 9  illustrates one example of an estimation error; and 
           [0017]      FIG. 10  illustrates one example of a hardware configuration of a wireless communication device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    The direction of the arrival of radio waves is estimated by using an array antenna. The method for estimating the direction of the arrival of radio waves by using an array antenna may be e.g. multiple signal classification (MUSIC), estimation of signal parameters via rotational invariance techniques (ESPRIT), or the like. In the MUSIC or ESPRIT, processing with a large amount of calculation, such as eigenvalue expansion, is executed. In addition, many kinds of processing are executed in order to acquire phase information of received radio waves used for calculation at the time of reception of radio waves. 
         [0019]    To reduce the amount of processing taken for the direction-of-arrival estimation, the direction-of-arrival estimation is carried out by utilizing the reception strength (received signal strength indicator (RSSI)) of a signal received by a directional antenna. 
         [0020]    A wireless communication device used in the direction-of-arrival estimation method with use of the reception strength includes a “directional antenna group” in which directional antennas are so disposed as to be different from each other in the direction of high receiving sensitivity. The wireless communication device stores a “reference table” in which plural “direction-of-arrival candidates” of radio waves are associated with reception strength expectations of the respective directional antennas according to each direction-of-arrival candidate. For example, the placement direction of a transmission source of radio waves is sequentially moved and the reception strength of each directional antenna is measured in advance regarding the respective placement directions. The measured reception strength is stored in the “reference table” as the reception strength expectations of each direction-of-arrival candidate. The wireless communication device calculates a “correlation coefficient of the unit of the directional antenna group” regarding each direction-of-arrival candidate by using the following expression (1). For example, for each direction-of-arrival candidate, the “correlation coefficient of the unit of the directional antenna group” is calculated by using expression (1). In expression (1), a calculation expression of the “correlation coefficient of the unit of the directional antenna group” when the directional antenna group is four directional antennas is represented. 
         [0000]    
       
         
           
             
               
                 
                   ρ 
                   = 
                   
                     
                       
                         
                           
                             
                               ( 
                               
                                 
                                   ref 
                                   1 
                                 
                                 × 
                                 
                                   RSSI 
                                   1 
                                 
                               
                               ) 
                             
                             + 
                             
                               ( 
                               
                                 
                                   ref 
                                   2 
                                 
                                 × 
                                 
                                   RSSI 
                                   2 
                                 
                               
                               ) 
                             
                             + 
                           
                         
                       
                       
                         
                           
                             
                               ( 
                               
                                 
                                   ref 
                                   3 
                                 
                                 × 
                                 
                                   RSSI 
                                   3 
                                 
                               
                               ) 
                             
                             + 
                             
                               ( 
                               
                                 
                                   ref 
                                   4 
                                 
                                 × 
                                 
                                   RSSI 
                                   4 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               
                                 
                                    
                                   
                                     ref 
                                     1 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     ref 
                                     2 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     ref 
                                     3 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                   
                                      
                                     
                                       ref 
                                       4 
                                     
                                      
                                   
                                   2 
                                 
                                 × 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 
                                    
                                   
                                     RSSI 
                                     1 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     RSSI 
                                     2 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     RSSI 
                                     3 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     RSSI 
                                     4 
                                   
                                    
                                 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0021]    The symbol ref denotes the reception strength expectation stored in the reference table and ref 1  denotes the reception strength expectation of antenna  1 . RSSI denotes the measured present reception strength and RSSI 1  denotes the present reception strength of antenna  1 . 
         [0022]    For example, in the direction-of-arrival estimation method with use of the reception strength, a bias in the degree of influence on the “correlation coefficient of the unit of the directional antenna group” might be caused among the directional antennas and the calculation accuracy of the “correlation coefficient of the unit of the directional antenna group” might decrease. 
         [0023]    For example, by dividing the denominator and the numerator by “ref 1 ×RSSI 1 ” in expression (1), expression (1) is modified to the following expression (2). 
         [0000]    
       
         
           
             
               
                 
                   ρ 
                   = 
                   
                     
                       1 
                       + 
                       
                         ( 
                         
                           
                             
                               ref 
                               2 
                             
                             
                               ref 
                               1 
                             
                           
                           × 
                           
                             
                               RSSI 
                               2 
                             
                             
                               RSSI 
                               1 
                             
                           
                         
                         ) 
                       
                       + 
                       
                         ( 
                         
                           
                             
                               ref 
                               3 
                             
                             
                               ref 
                               1 
                             
                           
                           × 
                           
                             
                               RSSI 
                               3 
                             
                             
                               RSSI 
                               1 
                             
                           
                         
                         ) 
                       
                       + 
                       
                         ( 
                         
                           
                             
                               ref 
                               4 
                             
                             
                               ref 
                               1 
                             
                           
                           × 
                           
                             
                               RSSI 
                               4 
                             
                             
                               RSSI 
                               1 
                             
                           
                         
                         ) 
                       
                     
                     
                       
                         
                           
                             
                               
                                 1 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         ref 
                                         2 
                                       
                                       
                                         ref 
                                         1 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         ref 
                                         3 
                                       
                                       
                                         ref 
                                         1 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         ref 
                                         4 
                                       
                                       
                                         ref 
                                         1 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                               
                             
                             × 
                           
                         
                       
                       
                         
                           
                             
                               1 
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       RSSI 
                                       2 
                                     
                                     
                                       RSSI 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       RSSI 
                                       3 
                                     
                                     
                                       RSSI 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       RSSI 
                                       4 
                                     
                                     
                                       RSSI 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0024]    If ref 1  and ref 2  are sufficiently larger than ref 3  and ref 4  and RSSI 1  and RSSI 2  are sufficiently larger than RSSI 3  and RSSI 4 , the third term and the fourth term in the numerator become almost zero. Furthermore, the third term and the fourth term in the former root in the denominator also become almost zero and the third term and the fourth term in the latter root also become almost zero. Therefore, directional antenna  3  and directional antenna  4  might have little influence on the “correlation coefficient of the unit of the directional antenna group.” 
         [0025]    In the embodiments, a configuration having substantially the same or similar function might be given the same numeral and overlapping description might be omitted or reduced. 
         [0026]      FIG. 1  illustrates one example of a wireless communication device. A wireless communication device  10  illustrated in  FIG. 1  includes directional antennas  11 - 1  to  11 - 4 , measuring units  12 - 1  to  12 - 4 , a storing unit  13 , and a control unit  14 .  FIG. 2  illustrates one example of a control unit. The control unit illustrated in  FIG. 2  may be the control unit  14  illustrated in  FIG. 1 . The control unit  14  illustrated in  FIG. 2  includes a calculating block  21  and an estimating block  22 . Hereinafter, if the directional antennas  11 - 1  to  11 - 4  are not discriminated, the directional antennas  11 - 1  to  11 - 4  might be collectively referred to as the directional antenna  11 . The whole of the directional antennas  11 - 1  to  11 - 4  might be collectively referred to as the directional antenna group. If the measuring units  12 - 1  to  12 - 4  are not discriminated, the measuring units  12 - 1  to  12 - 4  might be collectively referred to as the measuring unit  12 . The number of pairs of the directional antenna  11  and the measuring unit  12  may be four or may be another arbitrary number. 
         [0027]    The directional antennas  11 - 1  to  11 - 4  are so disposed as to be different from each other in the direction of high receiving sensitivity. Each directional antenna  11  receives radio waves of a direction-of-arrival estimation target. 
         [0028]    The measuring units  12 - 1  to  12 - 4  may correspond to the directional antennas  11 - 1  to  11 - 4 , respectively. The measuring unit  12  measures the reception strength of radio waves received by the corresponding directional antenna  11  and outputs the value of the measured reception strength to the calculating block  21 . 
         [0029]    The storing unit  13  stores the reference table. The reference table holds plural direction-of-arrival candidates of radio waves and the reception strength expectations of the respective directional antennas  11  according to each direction-of-arrival candidate in association with each other. The reception strength expectations of the respective directional antennas  11  may be e.g. measured values obtained by measuring the reception strength of each directional antenna  11  in advance regarding the respective placement directions resulting from sequential movement of the placement direction of a transmission source of radio waves from a reference direction of the wireless communication device  10 . 
         [0030]    Regarding each direction-of-arrival candidate, the calculating block  21  carries out a first-stage calculation to calculate correlation coefficients of the unit of an antenna pair and a second-stage calculation to calculate a correlation coefficient of the unit of the directional antenna group. For example, the calculating block  21  may repeatedly carry out the first-stage calculation and the second-stage calculation while sequentially changing a target direction-of-arrival candidate among plural direction-of-arrival candidates. 
         [0031]    In the first-stage calculation, the calculating block  21  calculates correlation coefficients of the unit of an antenna pair in the directional antenna group by using the reception strength expectations stored in the storing unit  13  and the reception strength measured by the respective measuring units  12 . For example, the correlation coefficient may be calculated regarding each of an antenna pair of the directional antennas  11 - 1  and  11 - 2 , an antenna pair of the directional antennas  11 - 1  and  11 - 3 , an antenna pair of the directional antennas  11 - 1  and  11 - 4 , an antenna pair of the directional antennas  11 - 2  and  11 - 3 , an antenna pair of the directional antennas  11 - 2  and  11 - 4 , and an antenna pair of the directional antennas  11 - 3  and  11 - 4 . 
         [0032]    Each correlation coefficient of the unit of an antenna pair regarding the target direction-of-arrival candidate is calculated by using the following expression (3). 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                      
                     
                       ( 
                       
                         x 
                         , 
                         y 
                         , 
                         θ 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             
                               ref 
                               x 
                             
                              
                             
                               ( 
                               θ 
                               ) 
                             
                           
                           × 
                           
                             RSSI 
                             x 
                           
                         
                         ) 
                       
                       + 
                       
                         ( 
                         
                           
                             
                               ref 
                               y 
                             
                              
                             
                               ( 
                               θ 
                               ) 
                             
                           
                           × 
                           
                             RSSI 
                             y 
                           
                         
                         ) 
                       
                     
                     
                       
                         
                           
                             
                                
                               
                                 ref 
                                 x 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 ref 
                                 y 
                               
                                
                             
                             2 
                           
                         
                       
                       × 
                       
                         
                           
                             
                                
                               
                                 
                                   RSSI 
                                   
                                     x 
                                      
                                     
                                         
                                     
                                   
                                 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 
                                   RSSI 
                                   y 
                                 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                                
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0033]    θ denotes an angle formed between the target direction-of-arrival candidate and the reference direction of the wireless communication device  10 . The symbols ref x (θ) and ref y (θ) each correspond to a respective one of the reception strength expectations of two directional antennas  11  configuring a target antenna pair about the target direction-of-arrival candidate. RSSI x  and RSSI y  each correspond to a respective one of the reception strength measured by the measuring units  12  regarding the two directional antennas  11  configuring the target antenna pair. 
         [0034]    In the second-stage calculation, the calculating block  21  calculates the correlation coefficient of the unit of the directional antenna group regarding the target direction-of-arrival candidate based on the correlation coefficients of the unit of an antenna pair calculated in the first-stage calculation. 
         [0035]    The correlation coefficient of the unit of the directional antenna group regarding the target direction-of-arrival candidate may be calculated by using the following expression (4). In expression (4), a calculation expression when the directional antenna group is composed of four directional antennas is represented. 
         [0000]      ρ(θ)= f (1,2,θ)· f (1,3,θ)· f (1,4,θ)· f (2,3,θ)· f (2,4,θ)· f (3,4,θ)  (4)
 
         [0036]    In the expression, f(1, 2, θ) represents the correlation coefficient of the unit of an antenna pair regarding the antenna pair of the directional antennas  11 - 1  and  11 - 2  about the target direction-of-arrival candidate whose angle formed with the reference direction of the wireless communication device  10  is θ. 
         [0037]    For example, the calculating block  21  calculates the correlation coefficient of the unit of the directional antenna group regarding each direction-of-arrival candidate by calculating the product of the correlation coefficients of the unit of an antenna pair calculated regarding the respective direction-of-arrival candidates. 
         [0038]    The calculating block  21  outputs, to the estimating block  22 , the calculated values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates. 
         [0039]    The estimating block  22  estimates the direction of the arrival of radio waves of the estimation target based on the values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates, received from the calculating block  21 . For example, the estimating block  22  may estimate, as the direction of the arrival of radio waves of the estimation target, the direction-of-arrival candidate corresponding to the largest value among the values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates, received from the calculating block  21 . 
         [0040]      FIG. 3  illustrates one example of processing of a wireless communication device. The wireless communication device  10  illustrated in  FIG. 1  may execute the processing illustrated in  FIG. 3 . 
         [0041]    The calculating block  21  sets θ to an initial value (operation S 101 ). For example, if the number of direction-of-arrival candidates θ k  is N (N is a natural number equal to or larger than two), the calculating block  21  sets the target direction-of-arrival candidate to a direction-of-arrival candidate θ 1 . 
         [0042]    The calculating block  21  sets the antenna pair to an initial value (operation S 102 ). For example, if the number of directional antennas  11  is four, the calculating block  21  sets the target antenna pair to the antenna pair of the directional antennas  11 - 1  and  11 - 2  for example. 
         [0043]    The calculating block  21  calculates the correlation coefficient of the unit of an antenna pair for the set target antenna pair (operation S 103 ). For example, the calculating block  21  calculates the correlation coefficient of the unit of an antenna pair for the set target antenna pair by using the above expression (3). 
         [0044]    The calculating block  21  determines whether or not all of the antenna pairs have been set (operation S 104 ). 
         [0045]    If the antenna pair that has not yet been set exists (No of the operation S 104 ), the calculating block  21  sets the target antenna pair to the next antenna pair, for example, the antenna pair of the directional antennas  11 - 1  and  11 - 3  (operation S 105 ). The operations S 103  to S 105  may be repeated until the calculation of the correlation coefficient of the unit of an antenna pair is completed for all of the antenna pairs. 
         [0046]    If all of the antenna pairs have been set (Yes of the operation S 104 ), the calculating block  21  calculates the correlation coefficient of the unit of the directional antenna group by using the correlation coefficients of the unit of an antenna pair repeatedly calculated in the operation S 103  about the set target direction-of-arrival candidate (operation S 106 ). For example, the calculating block  21  calculates the “correlation coefficient of the unit of the directional antenna group” by using expression (4). 
         [0047]    The calculating block  21  determines whether or not all of θ have been set (operation S 107 ). For example, the calculating block  21  determines whether or not the setting of all of θ from the direction-of-arrival candidate θ 1  to the direction-of-arrival candidate θ N  has been completed. 
         [0048]    If the direction-of-arrival candidate θ k  that has not yet been set exists (No of the operation S 107 ), the calculating block  21  sets the direction-of-arrival candidate θ k  to the next θ (operation S 108 ). The operations S 102  to S 108  may be repeated until the calculation of the “correlation coefficient of the unit of the directional antenna group” is completed for all of the direction-of-arrival candidates θ k . 
         [0049]    If all of θ have been set (Yes of the operation S 107 ), the estimating block  22  identifies θ corresponding to the maximum correlation coefficient among the correlation coefficients of the unit of the directional antenna group calculated by the calculating block  21  for all of θ (operation S 109 ). For example, the estimating block  22  estimates, as the direction of the arrival of radio waves of the estimation target, the direction-of-arrival candidate θ k  corresponding to the maximum value among the values of the “correlation coefficient of the unit of the directional antenna group” regarding the respective direction-of-arrival candidates θ k , received from the calculating block  21 . 
         [0050]    First calculation processing in which the correlation coefficient of the unit of the directional antenna group is calculated by the first-stage calculation and the second-stage calculation will be compared with second calculation processing in which the correlation coefficient of the unit of the directional antenna group is directly calculated by expression (1).  FIG. 4  illustrates one example of an average estimation error. In  FIG. 4 , the abscissa axis indicates the number of directional antennas included in the wireless communication device and the ordinate axis indicates the average estimation error. In  FIG. 4 , values plotted by squares indicate the average estimation errors resulting from the second calculation processing and values plotted by diamonds indicate the average estimation errors resulting from the first calculation processing. 
         [0051]    As illustrated in  FIG. 4 , whichever number the number of directional antennas included in the wireless communication device is, the average estimation error resulting from the first calculation processing is smaller than the average estimation error resulting from the second calculation processing. The estimation error might be improved by the first calculation processing. 
         [0052]    In the wireless communication device  10 , the calculating block  21  executes the first calculation processing including the first-stage calculation and the second-stage calculation. For example, in the first-stage calculation, the calculating block  21  calculates the correlation coefficients of the unit of an antenna pair in the directional antenna group by using the reception strength expectations stored in the storing unit  13  and the reception strength measured by the respective measuring units  12 . In the second-stage calculation, the calculating block  21  calculates the correlation coefficient of the unit of the directional antenna group regarding the target direction-of-arrival candidate based on the correlation coefficients of the unit of an antenna pair calculated in the first-stage calculation. The estimating block  22  estimates the direction of the arrival of radio waves of the estimation target based on the values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates, calculated by the calculating block  21 . 
         [0053]    In the wireless communication device  10 , the degree of influence on the correlation coefficient of the unit of the directional antenna group is leveled among the directional antennas  11 . Therefore, the calculation accuracy of the correlation coefficient of the unit of the directional antenna group might be improved. The estimation accuracy of the direction of arrival might be enhanced. 
         [0054]    In the second-stage calculation, the calculating block  21  calculates the correlation coefficient of the unit of the directional antenna group regarding each direction-of-arrival candidate by calculating the product of the correlation coefficients of the unit of an antenna pair calculated regarding the respective direction-of-arrival candidates. 
         [0055]    The correlation coefficient of the unit of the directional antenna group regarding each direction-of-arrival candidate may be calculated by calculating the product of the correlation coefficients of the unit of an antenna pair. For example, through calculation of the sum instead of the product, the correlation coefficient of the unit of the directional antenna group regarding each direction-of-arrival candidate may be calculated. For example, the “correlation coefficient of the unit of the directional antenna group” may be calculated by using the following expression (5). 
         [0000]      ρ(θ)= f (1,2,θ)+ f (1,3,θ)+ f (1,4,θ)+ f (2,3,θ)+ f (2,4,θ)+ f (3,4,θ)  (5)
 
         [0056]    For example, when the above-described second calculation processing is executed before the above-described first calculation processing and a given condition is satisfied, the first calculation processing may be executed. 
         [0057]      FIG. 5  illustrates one example of a wireless communication device. A wireless communication device  110  illustrated in  FIG. 5  includes a control unit  114 .  FIG. 6  illustrates one example of a control unit. The control unit illustrated in  FIG. 6  may be the control unit  114  illustrated in  FIG. 5 . The control unit  114  illustrated in  FIG. 6  includes a calculating block  131 , an estimating block  132 , and an estimation controller  133 . 
         [0058]    Regarding each direction-of-arrival candidate, the calculating block  131  calculates the correlation coefficients of the unit of the directional antenna group regarding the respective direction-of-arrival candidates by using the reception strength expectations stored in the storing unit  13  and the reception strength measured by the measuring units  12 . For example, the calculating block  131  calculates the correlation coefficients of the unit of the directional antenna group regarding the respective direction-of-arrival candidates by using expression (1) in the above-described second calculation processing. 
         [0059]    The estimating block  132  estimates the direction of the arrival of radio waves of the estimation target based on the values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates, calculated by the calculating block  131 . For example, the estimating block  132  may estimate, as the direction of the arrival of radio waves of the estimation target, the direction-of-arrival candidate corresponding to the largest value among the values of the correlation coefficient of the unit of the directional antenna group regarding the respective direction-of-arrival candidates, received from the calculating block  131 . 
         [0060]    If the given condition is satisfied, the estimation controller  133  causes the calculating block  21  to execute the first calculation processing and causes the estimating block  22  to execute the estimation processing. The estimation controller  133  employs the direction-of-arrival candidate estimated by the estimating block  22  as the direction of the arrival of radio waves of the estimation target. If the given condition is not satisfied, the estimation controller  133  employs the direction-of-arrival candidate estimated by the estimating block  132  as the direction of the arrival of radio waves of the estimation target. 
         [0061]    For example, the given condition may include that the direction of arrival estimated by the estimating block  132  does not fall within a given angle range whichever antenna in the directional antenna group is selected as the antenna whose high receiving sensitivity direction serves as a reference, or that the direction of arrival estimated by the estimating block  132  falls within the given angle range based on the high receiving sensitivity direction of one antenna in the directional antenna group and the difference between the reception strength of the one antenna and the reception strength of an adjacent antenna of the one antenna is equal to or smaller than a threshold. 
         [0062]      FIG. 7  illustrates one example of processing of a wireless communication device. 
         [0063]    The estimation controller  133  acquires, from the estimating block  132 , information relating to the direction of arrival estimated based on the correlation coefficients of the unit of the directional antenna group calculated in the second calculation processing (operation S 201 ). For example, the estimation controller  133  acquires information relating to the direction of arrival estimated by the estimating block  132 . 
         [0064]    The estimation controller  133  determines whether or not the given condition is satisfied (operation S 202 ). For example, the given condition may include that the direction of arrival estimated by the estimating block  132  does not fall within a given angle range whichever antenna in the directional antenna group is selected as the antenna whose high receiving sensitivity direction serves as the basis, or that the direction of arrival estimated by the estimating block  132  falls within the given angle range based on the high receiving sensitivity direction of one antenna in the directional antenna group and the difference between the reception strength of the one antenna and the reception strength of an adjacent antenna of the one antenna is equal to or smaller than a threshold. 
         [0065]    If the given condition is satisfied (Yes of the operation S 202 ), the estimation controller  133  causes the calculating block  21  to execute the first calculation processing and causes the estimating block  22  to execute the estimation processing (operation S 203 ). The processing illustrated in  FIG. 7  ends. If the given condition is not satisfied (No of the operation S 202 ), the processing illustrated in  FIG. 7  ends. 
         [0066]      FIGS. 8 and 9  illustrate one example of an estimation error.  FIG. 8  is a diagram illustrating estimation errors according to the direction of arrival estimated based on the respective kinds of calculation processing. As illustrated in  FIG. 8 , as the result of a simulation, if the direction of arrival falls within a given angle range based on the high receiving sensitivity direction of any antenna in the directional antenna group, the accuracy of the direction of arrival estimated based on the result of the second calculation processing is higher than the accuracy of the direction of arrival estimated based on the result of the first calculation processing. 
         [0067]      FIG. 9  illustrates the superiority and inferiority of the estimation error based on the respective kinds of calculation processing according to the difference in the received power between the directional antenna having the highest reception strength and the directional antenna having the second highest reception strength. In  FIG. 9 , the abscissa axis indicates the difference in the received power between the highest reception strength and the second highest reception strength among the antennas. The ordinate axis indicates the rate at which the direction estimation error resulting from the first calculation processing is smaller than the direction estimation error resulting from the second calculation processing. As illustrated in  FIG. 9 , as the result of a simulation, when the difference in the received power between the directional antenna having the highest reception strength and the directional antenna having the second highest reception strength is larger than a threshold, the accuracy of the direction of arrival estimated based on the result of the second calculation processing is higher than the accuracy of the direction of arrival estimated based on the result of the first calculation processing. 
         [0068]    Therefore, in the control by the estimation controller  133 , either one of the first calculation processing and the second calculation processing may be selected as the calculation processing for enhancement in the estimation accuracy depending on the situation. 
         [0069]    In the wireless communication device  110 , the calculating block  21  executes the first calculation processing if the given condition is satisfied. The given condition may include that the direction of arrival estimated by the estimating block  132  does not fall within a given angle range whichever antenna in the directional antenna group is selected as the antenna whose high receiving sensitivity direction serves as the reference, or that the direction of arrival estimated by the estimating block  132  falls within the given angle range based on the high receiving sensitivity direction of one antenna in the directional antenna group and the difference between the reception strength of the one antenna and the reception strength of an adjacent antenna of the one antenna is equal to or smaller than a threshold. 
         [0070]    In the wireless communication device  110 , the first calculation processing is executed if the accuracy of the direction of arrival estimated based on the result of the first calculation processing is higher than the accuracy of the direction of arrival estimated based on the result of the second calculation processing. Therefore, the estimation accuracy might be improved and the amount of processing might be reduced. 
         [0071]    The first calculation processing may be executed if the second calculation processing is executed before the first calculation processing and the given condition is satisfied. For example, the following estimation control may be carried out as a variation of the estimation control. 
         [0072]    For example, the second calculation processing may be executed if the first calculation processing is executed and a second condition is satisfied. The second condition may include that the direction of arrival estimated by the estimating block  22  falls within a given angle range based on the high receiving sensitivity direction of any antenna in the directional antenna group, or that the difference in the received power between the directional antenna having the highest reception strength and the directional antenna having the second highest reception strength is larger than a threshold. 
         [0073]    All or part of the above-described respective constituent elements may be distributed or integrated functionally or physically in an arbitrary unit according to various kinds of loads, the use condition, and so forth. 
         [0074]    All or an arbitrary part of various kinds of processing functions carried out by the respective devices may be carried out on a central processing unit (CPU) or a microcomputer such as a micro processing unit (MPU) or a micro controller unit (MCU). All or an arbitrary part of the various kinds of processing functions may be carried out on a program analyzed and executed on a CPU or a microcomputer such as an MPU or MCU or on hardware based on wired logic. 
         [0075]    The wireless communication device illustrated in  FIG. 1  or  FIG. 5  may be implemented by a hardware configuration for example. 
         [0076]      FIG. 10  illustrates one example of a hardware configuration of a wireless communication device. As illustrated in  FIG. 10 , a wireless communication device  200  includes radio frequency (RF) circuits  201 - 1  to  201 - 4 , a processor  202 , and a memory  203 . As the processor  202 , e.g. a CPU, a digital signal processor (DSP), a field programmable gate array (FPGA), or the like may be used. As the memory  203 , a random access memory (RAM) such as a synchronous dynamic random access memory (SDRAM), a read only memory (ROM), a flash memory, or the like may be used. The wireless communication devices  10  and  110  illustrated in  FIGS. 1 and 5  may have the configuration illustrated in  FIG. 10 . 
         [0077]    The various kinds of processing functions carried out by the above-described wireless communication device may be implemented through execution of a program stored in various kinds of memories such as a non-volatile storage medium by a processor. For example, programs corresponding to the respective kinds of processing executed by the measuring units  12 - 1  to  12 - 4  and the control unit  14  or  114  may be recorded in the memory  203  and the respective programs may be executed by the processor  202 . The directional antennas  11 - 1  to  11 - 4  may be implemented by the RF circuits  201 - 1  to  201 - 4 . The storing unit  13  may be implemented by the memory  203 . 
         [0078]    The various kinds of processing functions carried out by the above-described wireless communication device may be carried out by the processor  202  or may be carried out by plural processors. 
         [0079]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.