Patent Publication Number: US-2023132844-A1

Title: Determination device and determination method

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is based upon and claims benefit of priority from Japanese Patent Application No. 2021-176283, filed on Oct. 28, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a determination device and a determination method. 
     In recent years, technologies of estimating a direction of arrival of a radio wave have been developed. Sometimes a passive phased array antenna or the like may be used for estimating the direction of arrival of the radio wave. For example, JP 2009-081674A discloses a technology related to beam steering using the passive phased array antenna. 
     SUMMARY 
     However, in the case of using the passive phased array antenna as disclosed in JP 2009-081674A, an expensive phase shifter is necessary, and the configuration gets complicated. Therefore, the configuration disclosed in JP 2009-081674A is excessive in the case where high accuracy is not required for estimating the direction of arrival of the radio wave. 
     Accordingly, the present invention is made in view of the aforementioned issues, and an object of the present invention is to determine a direction of arrival of a radio wave by using a simpler configuration. 
     To solve the above described problem, according to an aspect of the present invention, there is provided a determination device comprising a determination section configured to determine a direction of a position of an external device on a basis of a synthetic radio wave obtained by combining a first radio wave and a second radio wave, the first radio wave being transmitted from the external device and received by a first antenna, the second radio wave being transmitted from the external device and received by a second antenna, wherein delay between the first radio wave and the second radio wave is designed to be about ¼ wavelength before generating the synthetic radio wave, and the second radio wave is further delayed by about ½ wavelength. 
     To solve the above described problem, according to another aspect of the present invention, there is provided a determination method comprising determining a direction of a position of an external device on a basis of a synthetic radio wave obtained by combining a first radio wave and a second radio wave, the first radio wave being transmitted from an external device and received by a first antenna, the second radio wave being transmitted from the external device and received by a second antenna, wherein delay between the first radio wave and the second radio wave is designed to be about ¼ wavelength before generating the synthetic radio wave, and the second radio wave is further delayed by about ½ wavelength. 
     As described above, according to the present invention, it is possible to determine a direction of arrival of a radio wave by using a simpler configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram for describing an example of determining a direction of arrival of a radio wave according to a first embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a configuration example of a determination device  10  according to the embodiment. 
         FIG.  3    is a diagram for describing a synthetic radio wave obtained in the case where an external device  60  is positioned on a first antenna  110  side (inside a room) according to the embodiment. 
         FIG.  4    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned on a second antenna  120  side (outside the room) according to the embodiment. 
         FIG.  5    is a diagram for describing an example of determining a direction of arrival of a radio wave according to a second embodiment of the present invention. 
         FIG.  6    is a diagram illustrating a configuration example of the determination device  10  according to the embodiment. 
         FIG.  7    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned in a direction along a first axis A 1  (inside or outside the room) according to the embodiment. 
         FIG.  8    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned in a direction along a second axis A 2  (upper side or lower side) according to the embodiment. 
         FIG.  9    is a diagram illustrating a case where a first antenna  110  and a second antenna  120  according to the embodiment are disposed on a ceiling of a vehicle  50 . 
         FIG.  10    is a diagram illustrating a configuration example of a determination device  10  according to a third embodiment of the present invention. 
         FIG.  11    is a diagram illustrating a configuration example of a determination device  10  according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT(S) 
     Hereinafter, referring to the appended drawings, preferred embodiments of the present invention will be described in detail. It should be noted that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation thereof is omitted. 
     1. First Embodiment 
     The above-described passive phased array antenna or the like is capable of detecting a direction of arrival of a radio wave with high accuracy. 
     On the other hand, the passive phased array antenna that needs the expensive phase shifter is excessive and unnecessary if it is sufficient to determine which one of two conflicting directions a radio wave is arrived from. 
     The technical idea of an embodiment of the present invention was conceived by focusing on the above-described points, and makes it possible to simply determine a direction of arrival of a radio wave without using an expensive component such as the phase shifter. 
       FIG.  1    is a diagram for describing an example of determining a direction of arrival of a radio wave according to a first embodiment of the present invention. 
     In this example, it is assumed that the determination device  10  according to the present embodiment determines a direction of a position of an external device  60  with reference to a partition  30 . 
     Here, the external device  60  is a device configured to transmit radio waves compliant with a designated communication standard. 
     Examples of the designated communication standard include Bluetooth Low Energy (BLE) (registered trademark) and the like. 
     For example, the external device  60  may be a smartphone, a tablet, or the like. 
     On the other hand, the partition  30  may be a structure such as a door or a wall. 
     In this case, the determination device  10  according to the present embodiment determines the direction of the position of the external device  60  on the basis of radio waves received by the first antenna  110  and the second antenna  120  that are disposed across the partition  30 . 
     More specifically, a determination section  180  of the determination device  10  according to the present embodiment determines the direction of the position of the external device  60  on the basis of a synthetic radio wave obtained by combining a first radio wave and a second radio wave, the first radio wave being transmitted from the external device  60  and received by the first antenna  110 , the second radio wave being transmitted from the external device  60  and received by the second antenna  120 . 
     For example, on the basis of the synthetic radio wave, the determination section  180  of the determination device  10  according to the present embodiment may determine whether the external device  60  is positioned on the first antenna side or on the second antenna side along a first axis A 1  connecting the first antenna  110  to the second antenna  120 . 
     In the example illustrated in  FIG.  1   , the first antenna  110  is disposed inside a room with reference to the partition  30  disposed in a house or the like, and the second antenna  120  is disposed outside the room with reference to the partition  30  disposed in the house or the like. 
     In this case, the determination section  180  according to the present embodiment may determine whether the external device  60  is positioned inside or outside the room on the basis of radio waves received by the first antenna  110  and the second antenna  120 . 
     To make a determination as described above, both the first antenna  110  and the second antenna  120  have to receive the radio waves transmitted from the external device  60 . 
     Accordingly, the partition  30  may be formed of material that is transparent to the radio waves that are transmitted from the external device  60  in conformity with the designated communication standard. 
     However, the material used for the partition  30  is not limited thereto in the case where the partition  30  is capable of diffracting the radio waves transmitted from the external device  60 . 
     In addition, the first antenna  110  and the second antenna  120  do not have to be disposed across the partition  30 . For example, the second antenna  120  may be disposed near a door inside the room in the case of determining whether or not the external device  60  is positioned inside or outside the room. In addition, the results of the determination according to the present embodiment are not limited to the result indicating an inside of the room and the result indicating an outside of the room. 
     In addition, one of features of the determination device  10  according to the present embodiment is to design delay between the first radio wave and the second radio wave to be about ¼ of wavelength λ before generating the synthetic radio wave. 
     For example, the above-described feature is achieved when the first antenna  110  and the second antenna  120  are disposed in such a manner that a physical (or spatial) distance between the first antenna  110  and the second antenna becomes λ/4. 
     However, the physical distance between the first antenna  110  and the second antenna does not have to become λ/4. It is sufficient to achieve the above-described feature by adding a delay line. 
     For example, in the case where the physical distance between the first antenna  110  and the second antenna  120  is λ/8, it is sufficient to add a delay line in such a manner that a difference between a transmission line of the first antenna  110  and a transmission line of the second antenna  120  becomes λ/8. In this case, a sum of the physical distance and the delay line is λ/4. 
     Alternatively, for example, it is also assumed that the physical distance between the first antenna  110  and the second antenna is λ/2. In this case, a delay line may be added in such a manner that a difference between the transmission line of the first antenna  110  and the transmission line of the second antenna  120  becomes 3λ/4. In this case, a sum of the physical distance and the delay line is λ/4+nλ (note that, n represents 0 or a natural number. In this example, n=1), and the above-described feature is achieved. 
     Next, details of the configuration example for making the above-described determination according to the present embodiment will be described with reference to  FIG.  2   . 
       FIG.  2    is a diagram illustrating the configuration example of the determination device  10  according to the present embodiment. 
     In the example illustrated in  FIG.  2   , the determination device  10  includes the first antenna  110 , the second antenna  120 , amplification circuits  130  and  135 , a delay line  140 , a first wave height adjustment circuit  150 , a second wave height adjustment circuit  155 , a synthesis circuit  160 , a detection circuit  170 , and the determination section  180 . 
     As described above, the first antenna  110  and the second antenna  120  according to the present embodiment receive radio waves transmitted from the external device  60  in conformity with the designated communication standard. To simplify the explanation, the present example assumes that the first antenna  110  and the second antenna  120  are disposed in such a manner that the physical distance between the first antenna  110  and the second antenna is λ/4. 
     The amplification circuit  130  according to the present embodiment amplifies a first radio wave received by the first antenna  110 , and the amplification circuit  135  according to the present embodiment amplifies a second radio wave received by the second antenna  120 . 
     The delay line  140  according to the present embodiment further delays the second radio wave received by the second antenna  120 . In the present example, delay between the first radio wave and the second radio wave are designed to be about ¼ of the wavelength λ, and the delay line  140  further delays the second radio wave by about (¼+½) of the wavelength λ. 
     The first wave height adjustment circuit  150  according to the present embodiment has a function of adjusting the wave height of the first radio wave to a designated wave height, and the second wave height adjustment circuit  155  according to the present embodiment has a function of adjusting the wave height of the second radio wave to a designated wave height. 
     The adjustment functions of the first wave height adjustment circuit  150  and the second wave height adjustment circuit  155  may be achieved through automatic gain control (AGC), for example. Sometimes the AGC is also referred to as automatic level control (ALC). 
     The synthesis circuit  160  according to the present embodiment combines the first radio wave and the second radio wave, and generates a synthetic radio wave. 
     The detection circuit  170  according to the present embodiment extracts a direct current (DC) component of the synthetic radio wave. 
     The determination section  180  according to the present embodiment determines a direction of a position of the external device  60  on the basis of the synthetic radio wave. 
     As an example, the determination section  180  according to the present embodiment may make the above-described determination by comparing the DC component to a designated value. The DC component is extracted from the synthetic radio wave by the detection circuit  170 . In this case, the determination section  180  may be implemented as a comparator. 
     The configuration example of the display device  10  according to the present embodiment has been described above. Next, details of an example of the determination according to the present embodiment will be described with reference to  FIG.  3    and  FIG.  4   . 
       FIG.  3    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned on the first antenna  110  side (inside the room) under the conditions illustrated in  FIG.  1    and  FIG.  2   . 
     In addition,  FIG.  4    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned on the second antenna  120  side (outside the room) under the conditions illustrated in  FIG.  1    and  FIG.  2   . 
     Note that, to simplify the explanation,  FIG.  3    and  FIG.  4    assume that the radio waves have a same wave height. As described above, there is no divergence from the assumption because the first wave height adjustment circuit  150  and the second wave height adjustment circuit  155  adjust the wave height of the first radio wave and the wave height of the second radio wave to certain wave heights before synthesis. 
     First, description will be given with reference to  FIG.  3   . In the case where the external device  60  is positioned inside the room, the first antenna  110  closer to the external device  60  first receive a radio wave, and then the second antenna  120  receives a radio wave. 
     Therefore, as illustrated in  FIG.  3   , the second radio wave is delayed behind the first radio wave by λ/4. 
     In addition, in this example, the delay line  140  further delays the second radio wave by λ/2. 
     Therefore, the second radio wave obtained at a point P 2  illustrated in  FIG.  2    has substantially the same phase as the first radio wave obtained at a point P 1 . The first radio wave obtained at the point P 1  has no delay. 
     Accordingly, the wave height of a synthetic radio wave generated by the synthesis circuit  160  is about twice as high as the wave height of the first radio wave obtained at the point P 1  and the wave height of the second radio wave obtained at the point P 2  as illustrated in  FIG.  3   . 
     On the other hand, in the case where the external device  60  is positioned outside the room, the second antenna  120  closer to the external device  60  first receive a radio wave, and then the first antenna  110  receives a radio wave. 
     Therefore, as illustrated in  FIG.  4   , the first radio wave is delayed behind the second radio wave by λ/4. 
     Note that, in this example, the delay line  140  delays the second radio wave by λ/4+λ/2. 
     Therefore, the second radio wave obtained at the point P 2  illustrated in  FIG.  2    has a phase that is almost reverse of the first radio wave obtained at the point P 1 . 
     Accordingly, the wave height of a synthetic radio wave generated by the synthesis circuit  160  is about zero as illustrated in  FIG.  4   . 
     Therefore, it is possible for the determination section  180  according to the present embodiment to determine that the external device  60  is positioned on the second antenna  120  side (outside the room in this example) along the first axis A 1  in the case where the synthetic radio wave has a wave height that falls below a designated wave height. 
     On the contrary, it is also possible for the determination section  180  according to the present embodiment to determine that the external device  60  is positioned on the first antenna  110  side (inside the room in this example) along the first axis A 1  in the case where the synthetic radio wave has a wave height that is a designated wave height or higher. 
     As an example, the determination section  180  may compare a DC component to a designated value. The DC component is extracted from the synthetic radio wave by the detection circuit  170 . 
     Such a configuration makes it possible to simply and inexpensively determine which one of two conflicting directions a radio wave is arrived from. 
     Note that, although not illustrated, in the case where the delay added by the delay line  140  is switched between the first radio wave and the second radio wave, it is also possible to determine that the radio wave has arrived from the direction of the wave height of 0 in view of the switching condition. 
     Most of the detection circuits have a function of outputting the wave height in logarithmic format. Accordingly, a wave height obtained by doubling an original wave form results in increase by 3 dB in the case where the two radio waves to be combined have almost the same phases. On the other hand, a wave height obtained by multiplying an original wave form by 0.01 results in decrease by 20 dB in the case where the two radio waves to be combined have almost reverse phases. Therefore, higher accuracy is obtained when a radio wave having a wave height close to zero is detected. 
     Accordingly, for example, it is sufficient to adopt a highly accurate detection method in which the external device  60  tends to be detected as a device positioned inside the room if it is desirable to prohibit a third person from unlocking a door because the external device  60  is erroneously determined as a device positioned outside the room and an authentication process is completed although the external device  60  is positioned inside the room actually. 
     2. Second Embodiment 
     Next, an example of determination according to a second embodiment of the present invention will be described with reference to  FIG.  5    to  FIG.  9   . 
     In the first embodiment, it is determined which one of the two directions along the first axis A 1  the external device  60  is positioned. 
     On the contrary, according to the second embodiment illustrated in  FIG.  5   , it is determined whether the external device  60  is positioned in a direction along the first axis A 1  (inside or outside the room) or in a direction along a second axis A 2  (upper side or lower side) perpendicular to the first axis A 1 . 
       FIG.  6    is a diagram illustrating a configuration example of a determination device  10  according to the second embodiment of the present invention. The configuration of the determination device  10  according to the second embodiment is almost similar to the configuration of the determination device  10  according to the first embodiment illustrated in  FIG.  2   . 
     However, the second embodiment is different from the first embodiment in that a delay line  140  according to the second embodiment delays the second radio wave by λ/2. 
     In other words, one of features of the determination device  10  according to the second embodiment of the present invention is to design delay between the first radio wave and the second radio wave to be about ¼ of the wavelength λ before generating the synthetic radio wave, to further delay the second radio wave by about ½ of the wavelength λ. 
       FIG.  7    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned in a direction along the first axis A 1  (inside or outside the room) under the conditions illustrated in  FIG.  5    and  FIG.  6   . 
     In the case where the external device  60  is positioned inside or outside the room, the second radio wave received by the second antenna  120  is delayed behind the first radio wave received by the first antenna  110  by about  214  as illustrated in  FIG.  7   . 
     Here, a second radio wave obtained at the point P 2  has a phase that is almost reverse of a second radio wave received by the second antenna  120  because the delay line  140  has delayed the second radio wave by λ/2. 
     Accordingly, the wave height of a synthetic radio wave generated by the synthesis circuit  160  is higher than the wave heights of the first radio wave and the second radio wave as illustrated in  FIG.  7   . 
     On the contrary,  FIG.  8    is a diagram for describing a synthetic radio wave obtained in the case where the external device  60  is positioned in a direction along the first axis A 2  (upper side or lower side) under the conditions illustrated in  FIG.  5    and  FIG.  6   . 
     In the case where the external device  60  is positioned on an upper side or a lower side, the second radio wave received by the second antenna  120  has substantially the same phase as the first radio wave received by the first antenna  110  as illustrated in  FIG.  8   . 
     Here, a second radio wave obtained at the point P 2  has a phase that is almost reverse of a first radio wave obtained at the point P 1  because the delay line  140  has delayed the second radio wave by λ/2. 
     Accordingly, the wave height of a synthetic radio wave generated by the synthesis circuit  160  is higher than the wave heights of the first radio wave and the second radio wave as illustrated in  FIG.  7   . 
     Accordingly, the wave height of a synthetic radio wave generated by the synthesis circuit  160  is about zero as illustrated in  FIG.  7   . 
     Therefore, it is possible for the determination section  180  according to the present embodiment to determine that the external device  60  is positioned in the direction along the second axis A 2  (upper side or lower side in this example) in the case where the synthetic radio wave has a wave height that falls below a designated wave height. 
     On the contrary, it is also possible for the determination section  180  according to the present embodiment to determine that the external device  60  is positioned in the direction along the first axis A 1  (inside or outside the room in this example) in the case where the synthetic radio wave has a wave height that is a designated wave height or higher. 
     Note that,  FIG.  5    illustrates the example in which the first antenna  110  and the second antenna  120  are disposed across the partition  30 . However, the arrangement of the first antenna  110  and the second antenna  120  is not limited thereto. 
       FIG.  9    is a diagram illustrating a case where the first antenna  110  and the second antenna  120  according to the present embodiment are disposed on a ceiling of a vehicle  50 . 
     In the example illustrated in  FIG.  9   , a first axis A 1  represents a left-right direction of the vehicle  50 , and a second axis A 2  represents an up-down direction of the vehicle  50 . 
     In this case, a region below the first antenna  110  and the second antenna  120  covers almost the whole vehicle cabin of the vehicle  50 . 
     This allows the determination section 180  according to the present embodiment to determine whether or not the external device  60  is positioned inside or outside the vehicle cabin of the vehicle  50  on the basis of the synthetic radio wave. 
     Note that, in principle, a phase difference between the first radio wave and the second radio wave is zero in a front-rear direction of the vehicle  50 . However, it is also possible to differentiate the front-rear direction from the up-down direction (inside the vehicle cabin) by installing two more antennas in the front-rear direction of the vehicle  50 . 
     As described above, when using a system  1  according to the present embodiment, it is also possible to detect the external device  60  positioned below the first antenna  110  and the second antenna  120 . 
     Note that,  FIG.  9    illustrates the example in which the system determines whether the external device  60  is positioned inside or outside the vehicle cabin of the vehicle  50 . However, the system  1  configured to determine the position of the external device  60  according to the present embodiment is not limited thereto. 
     For example, it is also possible for the system  1  according to the present embodiment to detect whether or not the external device  60  is positioned in a specific line in a situation where a plurality of lines is formed such as ticket gates in a station. 
     In other words, the system  1  according to the present embodiment makes it possible to determine whether a user carrying the external device  60  has passed through a specific line among a plurality of lines (such as ticket gates in a station) that are arrayed side by side. 
     The details of the determination made by the determination device  10  according to the second embodiment of the present invention has been described above. 
     Note that, in the first embodiment and the second embodiment, the determination conditions may further include a condition that received signal strength indicators (RSSI) of the first radio wave and the second radio wave is designated values or more. 
     By adding such a condition, it is possible to determine the position of the external device with higher accuracy. 
     3. Third Embodiment 
     Next, a third embodiment of the present invention will be described. 
     In the first embodiment and the second embodiment, it is possible to determine the position of the external device  60  with high accuracy in the case where an environment includes no other devices that emit radio waves than the external device  60 . 
     However, in recent years, devices that emit radio waves such as smartphones and tablets have been widespread, and it is expected that many environments are filled with a plurality of radio waves emitted from various devices. 
     In the environment filled with a plurality of radio waves as described above, there is a possibility that the first antenna  110  and the second antenna  120  receive radio waves emitted from devices other than the target external device  60 , and the determination device erroneously determines the position of the external device  60  on the basis of such radio waves. 
     To prevent the above-described erroneous position determination, it is necessary to determine whether or not radio waves received by the first antenna  110  and the second antenna  120  are desired radio waves emitted from the target external device  60 . 
     However, for example, in the case where BLE or the like is adopted as the designated communication standard, there is a possibility that the devices other than the target external device  60  may emit radio waves compliant with the same communication standard. This makes it difficult to distinguish the radio waves emitted from the external device  60  from the radio waves emitted from the devices other than the target external device  60 . 
     Here, it is also assumed that a radio wave having an extremely strong RSSI is treated as the desired radio wave to determine whether or not a received radio wave is the desired radio wave. 
     In the case of adopting such a method, a receiver configured to receive radio waves compliant with a designated communication standard may further be disposed in addition to the first antenna  110  and the second antenna  120 , for example. In addition, a position determination result obtained in the case where the receiver has received a radio wave having an RSSI of a designated value or more is used. 
     In addition, examples of another method of determining whether or not the received radio wave is the desired radio wave may include a method of catching only a radio wave of a BLE advertising channel (such as 2402 MHz), analyzing the radio wave, and determining whether or not the analyzed radio wave is the desired radio wave. 
     The third embodiment of the present invention adopts the above-described determination method based on the radio wave analysis. 
     Alternatively, according to the first embodiment and the second embodiment, there is a possibility that a synthetic radio wave may have a strength indicating an in-phase situation even if the first radio wave has a phase that is almost reverse of the second radio wave before synthesis in the case where the radio wave  60  is extremely close to the first antenna  110  and the second antenna  120 . 
     A configuration to be described below makes it possible to determine the position of the external device  60  with high accuracy even in the above-described situation. 
       FIG.  10    is a diagram illustrating a configuration example of a determination device  10  according to a third embodiment of the present invention. 
     As illustrated in  FIG.  10   , the determination device  10  according to the third embodiment of the present invention includes a first distributor  210  configured to distribute a portion of the first radio wave. 
     The portion of the first radio wave distributed by the first distributor is processed by a mixer  221  connected to an oscillator  230  in such a manner that the portion of the first radio wave is in an intermediate frequency (IF) band, and then the portion of the first radio wave is input to a band-pass filter (BPF)  241 . 
     The BPF  241  only passes center frequencies ±1 MHz in a designated frequency band of the input radio wave (for example, center frequency of 100 MHz obtained by converting 2402 MHz to the IF (100 MHz may be replaced with any value)). 
     The radio wave that has passed through the BPF  241  is detected by the detection circuit  171 , and the first wave height adjustment circuit  150  generates gain for an amplifier on the basis of a detection result (wave height value) obtained by the detection circuit  171 . 
     In addition, the determination device  10  according to the present embodiment includes a second distributor  215  configured to distribute a portion of the second radio wave. 
     The portion of the second radio wave distributed by the second distributor  215  is processed by the mixer  222 , the BPF  242 , the detection circuit  172 , and the second wave height adjustment circuit  155  in a way similar to the above-described processes performed on the portion of the first radio wave. 
     The above-described configuration makes it possible to ignore most of radio waves emitted from devices other than the target external device  60 , even in the case where there are the devices configured to emit radio waves compliant with the designated communication standard in addition to the target external device  60 . 
     If radio waves having a same frequency are received by coincidence, it is difficult to determine which of the radio waves is the desired radio wave. However, in this case, a determination result obtained by the determination section  180  may be treated as a valid result only when an additional receiver (not illustrated) has received a radio wave including a designated ID. 
     In the example illustrated in  FIG.  10   , the first wave height adjustment circuit  150  and the second wave height adjustment circuit  155  of the determination device  10  uniform wave heights of radio waves in such a manner that the radio waves to be input to the synthesis circuit  160  have a certain wave height. 
     Therefore, it is possible to accurately determine the position of the external device  60  by using a single threshold even in the case where the radio wave  60  is extremely close to the first antenna  110  and the second antenna  120  and the first radio wave has a phase that is almost reverse of the second radio wave before synthesis. 
     In addition, as illustrated in  FIG.  10   , a synthetic radio wave generated by the synthesis circuit  160  is processed by a mixer  223  and a BPF  243 , and is input to a detection circuit  173 . 
     This is because a wave height of a radio wave having a desired frequency is treated as a determination target. 
     For example, in the case where it is determined that a radio wave of an advertising channel emitted from the external device  60  is a desired radio wave, it is also determined that the radio wave include a valid ID, but a device other than the external device  60  has emitted a radio wave in a different frequency band by coincidence, such a radio wave is also combined by the synthesis circuit  160 . 
     However, by installing the mixer  223  and the BPF  243 , it is possible to abandon the determination result based on the synthetic radio wave generated in the above-described situation. 
     4. Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described. 
       FIG.  11    is a diagram illustrating a configuration example of a determination device  10  according to the fourth embodiment of the present invention. 
     The determination device  10  according to the fourth embodiment is different from the third embodiment in that the determination device  10  according to the fourth embodiment does not include the first wave height adjustment circuit  150  or the second wave height adjustment circuit  115 , and in that the determination section  180  receives a radio wave output from the detection circuit  171 , a radio wave output from the detection circuit  172  and a radio wave output from the detection circuit  173 . 
     The first radio wave received by the first antenna  110  and the second radio wave received by the second antenna  120  according to the present embodiment are a same radio wave emitted from the external device  60  (but the radio waves are designed intentionally in such a manner that radio waves have different phases). 
     However, the antennas have directivity, and the ability to receive radio waves varies depending on the directions of arrival of the radio waves. This does not guarantee that the first antenna  110  and the second antenna  120  receive radio waves having a same wave height. Accordingly, in general, a function of adjusting wave heights to a certain value is necessary (for the first wave height adjustment circuit  150  and the wave height adjustment circuit  155 ). 
     However, the determination device  10  does not have to include the first wave height adjustment circuit  150  or the second wave height adjustment circuit  155  in the case where it is not assumed that a difference between the wave height of the first radio wave and the wave height of the second radio wave is about 100 to 1000 times but it is assumed that the difference between the wave height of the first radio wave and the wave height of the second radio wave is about 2 to 20 times (about 3 to 13 dB). 
     As an example, a case where a first radio wave has wave height of 10 and a second radio wave has wave height of 2 will be described. 
     In this example, a synthetic radio wave has wave height of 12 if the phase of the first radio wave is the same as the phase of the second radio wave before synthesis. 
     On the other hand, a synthetic radio wave has wave height of 8 if the first radio wave has a phase that is reverse of the second radio wave before synthesis. 
     The determination section  180  according to the present embodiment selects a radio wave having higher wave height from among the first radio wave input from the detection circuit  171  and the second radio wave input from the detection circuit  172 , and compares the wave height of the selected radio wave with the wave height of a synthetic radio wave input from the detection circuit  173 . 
     In this example, the radio wave having the wave height of 10 is selected and compared with the wave height of the synthetic radio wave (12 is obtained in the case of the same phase, and 8 is obtained in the case of different phases). 
     On the basis of such comparison, the synthetic radio wave has a higher wave height (12&gt;10) in the case where the first radio wave has the same phase as the second radio wave, but the synthetic radio wave has a lower wave height (8&lt;10) in the case where the first radio wave has a different phase from the second radio wave. 
     As another example, a case where a first radio wave has wave height of 300 and a second radio wave has wave height of 15 will be described. 
     In this example, a synthetic radio wave has wave height of 315 if the first radio wave has a same phase as the second radio wave before synthesis. On the other hand, a synthetic radio wave has wave height of 285 if the radio waves are opposite to one another. 
     In addition, the synthetic radio wave has a higher wave height (315&gt;300) if the first radio wave has the same phase as the second radio wave, but the synthetic radio wave has a lower wave height (285&lt;300) if the radio waves are opposite to one another. 
     As described above, it is possible for the determination device  10  according to the present embodiment to determine the position of the external device  60  without using the first wave height adjustment circuit  150  or the second wave height adjustment circuit  155  while reducing the cost. 
     5. Supplement 
     Heretofore, preferred embodiments of the present invention have been described in detail with reference to the appended drawings, but the present invention is not limited thereto. It should be understood by those skilled in the art that various changes and alterations may be made without departing from the spirit and scope of the appended claims.