Patent Publication Number: US-2023138363-A1

Title: Communication device and position estimation method

Description:
TECHNICAL FIELD 
     The present invention relates to a communication device and a position estimation method. 
     BACKGROUND ART 
     Recently, various techniques for measuring a distance between devices (hereinafter also referred to as distance measuring) have been developed. For example, Patent Literature 1 discloses a technique of measuring a distance between devices on the basis of a time required for a signal to be received after the signal has been transmitted (hereinafter also referred to as a propagation time). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: U.S. Pat. No. 9,566,945 
     SUMMARY OF INVENTION 
     Technical Problem 
     Measuring a distance between devices can be understood as estimating a position of one device relative to the other device. However, it is preferable to estimate a position of a device in more detail. 
     Therefore, the present invention was made in consideration of the aforementioned circumstances and an objective of the present invention is to provide a structure that enables a position of a device to be estimated in more detail. 
     Solution to Problem 
     To solve the above described problem, according to an aspect of the present invention, there is provided a communication device comprising: a plurality of wireless communicators configured to perform wireless communication with another communication device; and a controller configured to estimate a position at which the other communication device is located on the basis of a plurality of measured distance values which indicate distances between the plurality of wireless communicators and the other communication device and which are acquired through wireless communication performed by the plurality of wireless communicators. 
     To solve the above described problem, according to another aspect of the present invention, there is provided a position estimation method comprising: estimating a position at which another communication device is located on the basis of a plurality of measured distance values which indicate distances between a plurality of wireless communicators configured to perform wireless communication with the other communication device and the other communication device and which are acquired through wireless communication performed by the plurality of wireless communicators. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a structure that enables a position of a device to be estimated in more detail as described above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a configuration of a system according to an embodiment of the present invention. 
         FIG.  2    is a sequence diagram illustrating an example of a routine of a distance measuring process that is performed in the system according to the embodiment. 
         FIG.  3    is a diagram illustrating an example in which a first wireless communicator and a second wireless communicator according to the embodiment are arranged. 
         FIG.  4    is a diagram illustrating an example of a position estimating process according to the embodiment. 
         FIG.  5    is a diagram illustrating an example of the position estimating process according to the embodiment. 
         FIG.  6    is a sequence diagram illustrating an example of a routine of a position estimating process that is performed in the system according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     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. 
     Further, in the present specification and the drawings, different alphabets are suffixed to a same reference numeral to distinguish elements which have substantially the same functional configuration. For example, a plurality of elements which have substantially the same functional configuration are distinguished such as first wireless communicators  210 A and  210 B, as necessary. However, when there is no need in particular to distinguish elements that have substantially the same functional configuration, the same reference numeral alone is attached. For example, in the case where it is not necessary to particularly distinguish the first wireless communicators  210 A and  210 B, the first wireless communicators  210 A and  210 B are simply referred to as the first wireless communicators  210 . 
     1. Example of Configuration 
       FIG.  1    is a diagram illustrating an example of a configuration of a system  1  according to an embodiment of the present invention. As illustrated in  FIG.  1   , the system  1  according to this embodiment includes a mobile device  100  and a communication unit  200 . The communication unit  200  according to the embodiment is mounted in a vehicle  202 . The vehicle  202  is an example of an object that is used by a user. 
     A communication device of an authentication requesting party (also referred to as a first communication device) and a communication device of an authenticator party (also referred to as a second communication device) participate in the present invention. The mobile device  100  is an example of a first communication device. The communication unit  200  is an example of the second communication device. 
     In the system  1 , when a user (for example, a driver of the vehicle  202 ) with the mobile device  100  approaches the vehicle  202 , wireless communication for authentication is performed between the mobile device  100  and the communication unit  200 . When the authentication succeeds, a door of the vehicle  202  is unlocked or an engine is started such that the vehicle  202  enters a state in which it can be used by the user. The system  1  is also referred to as a smart entry system. The constituents will be sequentially described below. 
     (1) Mobile Device  100   
     The mobile device  100  is a communication device that performs wireless communication with the communication unit  200 . The mobile device  100  is configured as an arbitrary device which is carried for use by a user. Examples of the arbitrary device include an electronic key, a smartphone, and a wearable terminal. As illustrated in  FIG.  1   , the mobile device  100  includes a first wireless communicator  110 , a second wireless communicator  120 , a storage  130 , and a controller  140 . 
     The first wireless communicator  110  has a function of performing communication based on a first wireless communication standard. For example, the first wireless communicator  110  performs wireless communication with the communication unit  200 . The second wireless communicator  120  has a function of performing communication based on a second wireless communication standard. For example, the second wireless communicator  120  performs wireless communication with the communication unit  200 . 
     An example of the first wireless communication standard is Bluetooth Low Energy (BLE: registered trademark). BLE is known as a wireless communication standard with low power consumption. In BLE, signals in a 2.4 GHz band are transmitted and received. 
     As an example of the second wireless communication standard, signals are transmitted and received using an ultra-wide band (UWB). In wireless communication of signals using UWB, when an impulse system is used, it is possible to measure a propagation time of radio waves with high accuracy using radio waves with a very short pulse width equal to or less than nano seconds and to measure a distance based on the propagation time with high accuracy. UWB often indicates a frequency band of about 3 GHz to about 10 GHz. 
     In wireless communication, it is known that power consumption of a receiving side becomes higher as a frequency of carrier waves becomes higher. This is because a sampling frequency of the receiving side becomes higher as the frequency of carrier waves becomes higher. Accordingly, the power consumption of the receiving side in BLE is lower than that in the wireless communication standard using UWB. 
     In the following description, it is assumed that the first wireless communication standard is BLE. That is, the first wireless communicator  110  is configured as a communication interface that can perform communication in BLE. It is also assumed that the second wireless communication standard is a wireless communication standard using UWB. That is, the second wireless communicator  120  is configured as a communication interface that can perform communication using UWB. 
     The storage  130  has a function of storing various types of information for operation of the mobile device  100 . For example, the storage  130  stores programs for operation of the mobile device  100  and an identifier (ID), a password, an authentication algorithm, and the like for authentication. The storage  130  is constituted, for example, by a storage medium such as a flash memory and a processing device that performs recording and reproduction of data in the storage medium. 
     The controller  140  has a function of performing processes in the mobile device  100 . For example, the controller  140  controls the first wireless communicator  110  and the second wireless communicator  120  such that communication with the communication unit  200  is performed. The controller  140  performs reading of information from the storage  130  and writing of information to the storage  130 . The controller  140  controls an authentication process which is performed in cooperation with the communication unit  200 . The controller  140  is constituted, for example, by an electronic circuit such as a central processing unit (CPU) and a microprocessor. 
     (2) Communication Unit  200   
     The communication unit  200  is a communication device that performs wireless communication with the mobile device  100 . The communication unit  200  is installed in correlation with the vehicle  202 . Here, it is assumed that the communication unit  200  is mounted in the vehicle  202  by installing the communication unit  200  in the cabin of the vehicle  202 , incorporating the communication unit  200  as a communication module into the vehicle  202 , or the like. 
     As illustrated in  FIG.  1   , the communication unit  200  includes a plurality of first wireless communicators  210 , a plurality of second wireless communicators  220 , a storage  230 , and a controller  240 . 
     Each first wireless communicator  210  has a function of performing communication based on the first wireless communication standard. For example, the first wireless communicator  210  performs wireless communication with the mobile device  100 . In the following description, it is assumed that each first wireless communicator  210  is configured as a communication interface that can perform communication using BLE. 
     Each second wireless communicator  220  has a function of performing communication based on the second wireless communication standard. For example, the second wireless communicator  220  performs wireless communication with the mobile device  100 . In the following description, it is assumed that the second wireless communicator  220  is configured as a communication interface that can perform communication using UWB. 
     The storage  230  has a function of storing various types of information for operation of the communication unit  200 . For example, the storage  230  stores programs for operation of the communication unit  200  and an authentication algorithm. The storage  230  is constituted, for example, by a storage medium such as a flash memory and a processing device that performs recording and reproduction of data in the storage medium. 
     The controller  240  has a function of controlling all operations of the communication unit  200  and onboard devices mounted in the vehicle  202 . For example, the controller  240  controls the first wireless communicators  210  and the second wireless communicators  220  such that communication with the mobile device  100  is performed. The controller  240  performs reading of information from the storage  230  and writing of information to the storage  230 . The controller  240  also serves as an authentication controller that controls an authentication process performed in cooperation with the mobile device  100 . The controller  240  also serves as a door lock controller that controls a door lock of the vehicle  202  and performs locking and unlocking of the door lock. The controller  240  also serves as an engine controller that controls an engine of the vehicle  202  and performs starting/stopping of the engine. A power source provided in the vehicle  202  may be a motor or the like in addition to the engine. The controller  240  is configured as, for example, an electronic control unit (ECU). 
     2. Technical Features 
     (1) Distance Measuring Process 
     The mobile device  100  and the communication unit  200  perform a distance measuring process. The distance measuring process is a process of measuring a distance between the mobile device  100  and the communication unit  200 . The distance measured through the distance measuring process is also referred to as a measured distance value in the following description. 
     In the distance measuring process, signals can be transmitted and received in a wireless manner. 
     An example of the signals transmitted and received in the distance measuring process is a distance-measuring signal. The distance-measuring signal is a signal that is transmitted and received to measure an inter-device distance. The distance-measuring signal is also a signal that is to be measured. For example, a time required for transmission and reception of the distance-measuring signal is measured. Typically, the distance-measuring signal is formed in a frame format not including a payload part in which data is stored. The distance-measuring signal may be formed in a frame format including a payload part in which data is stored. 
     In the distance measuring process, a plurality of distance-measuring signals can be transmitted and received between devices. A distance-measuring signal transmitted from one device to the other device out of the plurality of distance-measuring signals is also referred to as a first distance-measuring signal. A distance-measuring signal transmitted from the device having received the first distance-measuring signal to the device having transmitted the first distance-measuring signal is also referred to as a second distance-measuring signal. 
     Another example of the signals transmitted and received in the distance measuring process is a data signal. The data signal is a signal for storing and carrying data. The data signal is formed in a frame format including a payload part in which data is stored. 
     Transmitting and receiving of signals in the distance measuring process is also referred to as distance-measuring communication in the following description. In this embodiment, it is assumed that the second wireless communicator  120  and the second wireless communicator  220  perform distance-measuring communication. In the distance measuring process, a distance between the second wireless communicator  120  and the second wireless communicator  220  having performed distance-measuring communication is measured as the distance between the mobile device  100  and the communication unit  200 . 
     An example of the distance measuring process will be described with reference to  FIG.  2   . 
       FIG.  2    is a sequence diagram illustrating an example of a routine of the distance measuring process that is performed in the system  1  according to this embodiment. The mobile device  100  and the communication unit  200  participate in this sequence. 
     As illustrated in  FIG.  2   , first, the second wireless communicator  120  of the mobile device  100  transmits a first distance-measuring signal (Step S 12 ). For example, the first distance-measuring signal may be transmitted as a signal using UWB. 
     Subsequently, when the first distance-measuring signal is received from the mobile device  100 , the second wireless communicator  220  of the communication unit  200  transmits a second distance-measuring signal as a response to the first distance-measuring signal (Step S 14 ). For example, the second distance-measuring signal may be transmitted as a signal using UWB. 
     At this time, the controller  240  of the communication unit  200  measures a time period ΔT 2  from a time point at which the communication unit  200  has received the first distance-measuring signal to a time point at which the communication unit  200  has transmitted the second distance-measuring signal. On the other hand, when the second distance-measuring signal is received from the communication unit  200 , the controller  140  of the mobile device  100  measures a time period ΔT 1  from a time point at which the mobile device  100  has transmitted the first distance-measuring signal to a time point at which the mobile device  100  has received the second distance-measuring signal. 
     Then, the second wireless communicator  220  of the mobile device  100  transmits a data signal including information indicating the time period ΔT 1  (Step S 16 ). For example, the data signal may be transmitted as a signal using UWB. 
     Then, when the second wireless communicator  220  receives a data signal from the mobile device  100 , the controller  240  of the communication unit  200  acquires a measured distance value indicating the distance between the mobile device  100  and the second wireless communicator  220  on the basis of the time period ΔT 1  indicated by the received data signal and the measured time period ΔT 2  (Step S 18 ). For example, first, the controller  240  calculates a propagation time by dividing ΔT 1 −ΔT 2  by 2. Here, the propagation time is a time period in which a signal is transmitted or received one way between the mobile device  100  and the second wireless communicator  220 . Then, the controller  240  calculates the measured distance value indicating the distance between the mobile device  100  and the second wireless communicator  220  by multiplying the propagation time by a speed of the signal. 
     Here, it is preferable that a signal using UWB be transmitted or received in the distance-measuring communication. It is preferable that at least the distance-measuring communication be transmitted or received as a signal using UWB. With this configuration, it is possible to measure a distance with high accuracy as described above in association with UWB. 
     (2) Arrangement of Wireless Communicators 
     Relative positions of the first wireless communicator  210  and the second wireless communicator  220  with respect to the vehicle  202  are fixed. For example, the first wireless communicator  210  and the second wireless communicator  220  are disposed at predetermined positions of the vehicle  202 . 
       FIG.  3    is a diagram illustrating an example in which the first wireless communicator  210  and the second wireless communicator  220  are arranged according to this embodiment. In the example illustrated in  FIG.  3   , the first wireless communicator  210 A, the second wireless communicator  210 B, and the second wireless communicators  220 A to  220 D are provided in the vehicle  202 . As illustrated in  FIG.  3   , a traveling direction of the vehicle  202  is also referred to as a forward direction. The opposite direction of the traveling direction of the vehicle  202  is also referred to as a rearward direction. Directions perpendicular to the traveling direction of the vehicle  202  are also referred to as a rightward direction and a leftward direction. 
     The first wireless communicator  210 A and the first wireless communicator  210 B are disposed at different positions. In the example illustrated in  FIG.  3   , the first wireless communicator  210 A and the first wireless communicator  210 B are disposed at positions which are laterally separated from each other in the vicinity of the center in the longitudinal direction of the vehicle  202 . Specifically, the first wireless communicator  210 A is disposed on the right side of the center of the vehicle  202 . The first wireless communicator  210 B is disposed on the left side of the center of the vehicle  202 . For example, the first wireless communicator  210 A and the first wireless communicator  210 B may be disposed in the cabin. The cabin is a space which is provided in the vehicle  202  for a user to get in the vehicle  202 . 
     The second wireless communicators  220 A to  220 D are disposed at different positions. As illustrated in  FIG.  3   , the second wireless communicators  220 A to  220 D are disposed outside of the cabin of the vehicle  202 . Specifically, the second wireless communicators  220 A to  220 D are disposed at ends in the longitudinal direction of the vehicle  202  and at ends in the lateral direction of the vehicle  202 . For example, the second wireless communicator  220 A is disposed at the front-right end of the vehicle  202 . The second wireless communicator  220 B is disposed at the rear-right end of the vehicle  202 . The second wireless communicator  220 C is disposed at the rear-left end of the vehicle  202 . The second wireless communicator  220 D is disposed at the front-left end of the vehicle  202 . For example, the second wireless communicators  220 A to  220 D may be disposed in bumpers of the vehicle  202 . The bumper is a shock absorber that reduces shocks and vibration when the vehicle  202  collides with another object. 
     (3) Position Estimating Process 
     The controller  240  performs a position estimating process. The position estimating process is a process of estimating a position at which the mobile device  100  is located. 
     The position of the mobile device  100  estimated through the position estimating process is used for authentication which is performed between the mobile device  100  and the communication unit  200 . For example, the authentication succeeds when the estimated position of the mobile device  100  is included in a prescribed range, and the authentication fails when the estimated position of the mobile device  100  is not included in the prescribed range. 
     Position Estimating Process Based on a Plurality of Measured Distance Values 
     The controller  240  estimates the position at which the mobile device  100  is located on the basis of a plurality of measured distance values which indicate distances between the plurality of second wireless communicators  220  and the mobile device  100  and which have been acquired through wireless communication performed by the plurality of second wireless communicators  220 . Here, wireless communication is the aforementioned distance-measuring communication. The controller  240  acquires the plurality of measured distance values indicating the distances between the plurality of second wireless communicators  220  and the mobile device  100  by causing the plurality of second wireless communicators  220  to perform the distance-measuring communication. Then, the controller  240  estimates the position at which the mobile device  100  is located on the basis of the plurality of acquired measured distance values. Particularly, the controller  240  according to this embodiment estimates the position at which the mobile device  100  is located on the basis of two measured distance values acquired by two different second wireless communicators  220 . 
     The position at which the mobile device  100  is located and which is estimated in this embodiment is a relative position of the mobile device  100  with respect to the communication unit  200 . Specifically, the position at which the mobile device  100  is located is coordinates of the mobile device  100  in a first coordinate system. The first coordinate system is a coordinate system in which an arbitrary position of which a relative position with respect to the communication unit  200  (more accurately each of the plurality of second wireless communicators  220 ) is fixed is set as the origin. An example of the first coordinate system is a coordinate system in which the position of one of the plurality of second wireless communicators  220  is set as the origin. Another example of the first coordinate system is a coordinate system in which an arbitrary position of the vehicle  202  is set as the origin. An example of the arbitrary position of the vehicle  202  is a center point of the vehicle  202 . 
     The storage  230  stores information indicating the positions of the plurality of second wireless communicators  220 . The information indicating the positions may be coordinates in the first coordinate system. The controller  240  estimates the position at which the mobile device  100  is located additionally on the basis of the positions of the plurality of second wireless communicators  220 . 
     An example of the position estimating process based on a plurality of measured distance values will be described with reference to  FIG.  4   .  FIG.  4    is a diagram illustrating an example of the position estimating process according to this embodiment. In  FIG.  4   , an example in which the position at which the mobile device  100  is located is estimated on the basis of a plurality of measured distance values acquired through distance-measuring communication performed by the second wireless communicator  220 A and the second wireless communicator  220 B is illustrated. A measured distance value L A  is a measured distance value acquired through distance-measuring communication performed between the second wireless communicator  120  of the mobile device  100  and the second wireless communicator  220 A of the communication unit  200 . A measured distance value L B  is a measured distance value acquired through distance-measuring communication performed between the second wireless communicator  120  of the mobile device  100  and the second wireless communicator  220 B of the communication unit  200 . 
     The controller  240  estimates coordinates in the first coordinate system satisfying conditions that a distance from the coordinates of the second wireless communicator  220 A is the measured distance value L A  and the distance from the coordinates of the second wireless communicator  220 B is the measured distance value L B  as the position at which the mobile device  100  is located. For example, the controller  240  estimates an intersection point V 1  and an intersection point V 2  between a circle  10 A in which the radius centered on the coordinates of the second wireless communicator  220 A is the measured distance value L A  and a circle  10 B in which the radius centered on the coordinates of the second wireless communicator  220 B is the measured distance value L B  as the position at which the mobile device  100  is located. 
     With the aforementioned position estimating process based on a plurality of measured distance values, it is possible to estimate that the mobile device  100  is located at one of the intersection point V 1  and the intersection point V 2 . Accordingly, according to this embodiment, it is possible to specifically estimate the position of the mobile device  100  from simple measured distance values. 
     Position Estimating Process Based on a Plurality of Measured Distance Values and Space Information 
     As the position estimating process, the controller  240  may estimate the position at which the mobile device  100  is located on the basis of space information that is information indicating a space in which the mobile device  100  is assumed to be located in addition to the plurality of measured distance values. That is, the controller  240  may estimate the position at which the mobile device  100  is located on the basis of the space information in addition to the measured distance values in the position estimating process. With this configuration, it is possible to improve position estimation accuracy. 
     As the position estimating process, the controller  240  may estimate a position belonging to the space which is indicated by the space information and in which the mobile device  100  is assumed to be located out of a plurality of positions at which the mobile device  100  can be located and which have been estimated on the basis of a plurality of measured distance values as the position of the mobile device  100 . As described above with reference to  FIG.  4   , the controller  240  can estimate a plurality of positions as the position at which the mobile device  100  is located through the position estimating process based on a plurality of measured distance values. Therefore, the controller  240  decreases the number of positions estimated through the position estimating process based on a plurality of measured distance values to a smaller number (for example, one position) on the basis of the space information. With this configuration, it is possible to more specifically estimate the position at which the mobile device  100  is located. 
     The space information may be information indicating a direction in which the mobile device  100  is located as the space in which the mobile device  100  is assumed to be located. For example, the space information may indicate that the mobile device  100  is assumed to be located on the right side of the vehicle  202 . For example, the space information may indicate that the mobile device  100  is assumed to be located on the left side of the vehicle  202 . With this configuration, a position in a direction indicated by the space information out of a plurality of positions estimated through the position estimating process based on a plurality of measured distance values can be estimated as the position at which the mobile device  100  is located. 
     The space information may be acquired through wireless communication which has been performed using a wireless communication standard different from the wireless communication standard used for distance-measuring communication performed by the plurality of second wireless communicators  220  to acquire measured distance values. For example, the space information may be acquired through wireless communication performed using BLE by the first wireless communicator  210 . For example, the space information may indicate a direction in which the mobile device  100  is located and which is indicated by a magnitude relationship of radio field intensities in a plurality of first wireless communicators  210  when the plurality of first wireless communicators  210  have received signals from the first wireless communicator  110  of the mobile device  100 . An example of the radio field intensity is a received signal strength indicator (RSSI). The first wireless communicator  210  with a greater RSSI has a smaller distance from the mobile device  100 . Accordingly, it is assumed that the mobile device  100  is located in a direction which is close to the first wireless communicator  210  with the greatest RSSI out of the plurality of first wireless communicators  210 . With this configuration, it is possible to acquire space information using BLE in which power consumption on a receiving side is lower than that of UWB. That is, it is possible to decrease power consumption of the communication unit  200 . 
     The storage  230  stores information indicating the positions of the plurality of first wireless communicators  210 . The information indicating the positions may be coordinates in the first coordinate system. The controller  240  estimates the position at which the mobile device  100  is located additionally on the basis of the positions of the first wireless communicators  210 . 
     An example of the position estimating process based on a plurality of measured distance values and space information will be described with reference to  FIG.  5   .  FIG.  5    is a diagram illustrating an example of the position estimating process according to this embodiment. As described above with reference to  FIG.  4   , the intersection point V 1  and the intersection point V 2  are estimated as the position at which the mobile device  100  is located through the position estimating process based on a plurality of measured distance values. In the example illustrated in  FIG.  5   , it is assumed that the magnitude relationship between RSSI A  and RSSI B  is acquired as the space information. RSSI A  is an RSSI acquired by the first wireless communicator  210 A. RSSI B  is an RSSI acquired by the first wireless communicator  210 B. 
     When RSSI A  is greater than RSSI B , it is assumed that the mobile device  100  be located in the direction which is closer to the first wireless communicator  210 A with respect to an axis  11  passing through the center between the coordinates of the first wireless communicator  210 A and the coordinates of the first wireless communicator  210 B in the first coordinate system. More briefly, when RSSI A  is greater than RSSI B , it is assumed that the mobile device  100  be located on the right side of the vehicle  202 . Accordingly, when RSSI A  is greater than RSSI B , the controller  240  estimates that the mobile device  100  is located on the intersection point V 1  which is on the right side of the vehicle  202  out of the intersection point V 1  and the intersection point V 2 . 
     On the other hand, when RSSI B  is greater than RSSI A , it is assumed that the mobile device  100  be located in the direction which is closer to the first wireless communicator  210 B with respect to the axis  11  passing through the center between the coordinates of the first wireless communicator  210 A and the coordinates of the first wireless communicator  210 B in the first coordinate system. More briefly, when RSSI B  is greater than RSSI A , it is assumed that the mobile device  100  be located on the left side of the vehicle  202 . Accordingly, when RSSI B  is greater than RSSI A , the controller  240  estimates that the mobile device  100  is located on the intersection point V 2  which is on the left side of the vehicle  202  out of the intersection point V 1  and the intersection point V 2 . 
     With the aforementioned position estimating process based on a plurality of measured distance values and the space information, it is possible to determine at which of the intersection point V 1  and the intersection point V 2  estimated through the position estimating process based on the plurality of measured distance values the mobile device  100  is located. Accordingly, according to this embodiment, it is possible to specifically estimate the position of the mobile device  100  on the basis of the simple measured distance values and the position estimating process based on the plurality of measured distance values. 
     (4) Routine of Processes 
       FIG.  6    is a sequence diagram illustrating an example of a routine of the position estimating processes that are performed in the system  1  according to this embodiment. The mobile device  100  and the communication unit  200  participate in this sequence. In this sequence, it is assumed that the second wireless communicator  220 A and the second wireless communicator  220 B perform distance-measuring communication. 
     As illustrated in  FIG.  6   , first, the first wireless communicator  110  of the mobile device  100  transmits a radio signal (Step S 102 ). The radio signal is transmitted, for example, using BLE. When the radio signal is received by the first wireless communicator  210 A, the controller  240  of the communication unit  200  acquires RSSI A  which is the radio field intensity when the radio signal has been received by the first wireless communicator  210 A (Step S 104 ). 
     Similarly, the first wireless communicator  110  of the mobile device  100  transmits a radio signal (Step S 106 ). The radio signal is transmitted, for example, using BLE. When the radio signal is received by the first wireless communicator  210 B, the controller  240  of the communication unit  200  acquires RSSI B  which is the radio field intensity when the radio signal has been received by the first wireless communicator  210 B (Step S 108 ). 
     Subsequently, the second wireless communicator  120  of the mobile device  100  and the second wireless communicator  220 A of the communication unit  200  perform distance-measuring communication (Step S 110 ). In the distance-measuring communication, for example, a signal using UWB is transmitted. The controller  240  of the communication unit  200  acquires a measured distance value L A  indicating the distance between the mobile device  100  and the second wireless communicator  220 A on the basis of the information acquired through distance-measuring communication of the second wireless communicator  220 A (Step S 112 ). 
     Similarly, the second wireless communicator  120  of the mobile device  100  and the second wireless communicator  220 B of the communication unit  200  perform distance-measuring communication (Step S 114 ). In the distance-measuring communication, for example, a signal using UWB is transmitted. The controller  240  of the communication unit  200  acquires a measured distance value L B  indicating the distance between the mobile device  100  and the second wireless communicator  220 B on the basis of the information acquired through distance-measuring communication of the second wireless communicator  220 B (Step S 116 ). 
     Then, the controller  240  of the communication unit  200  estimates the position at which the mobile device  100  is located on the basis of the measured distance value L A , the measured distance value L B , and the magnitude relationship between RSSI A  and RSSI B  which is space information (Step S 118 ). For example, first, the controller  240  identifies coordinates of an intersection point V 1  and an intersection point V 2  between a circle  10 A in which the radius centered on the coordinates of the second wireless communicator  220 A is the distance L A  and a circle  10 B in which the radius centered on the coordinates of the second wireless communicator  220 B is the distance L B . Then, when RSSI A  is greater than RSSI B , the controller  240  estimates that the mobile device  100  is located at the intersection point V 1  which is on the right side of the vehicle  202  out of the intersection point V 1  and the intersection point V 2 . On the other hand, when RSSI B  is greater than RSSI A , the controller  240  estimates that the mobile device  100  is located at the intersection point V 2  which is on the left side of the vehicle  202  out of the intersection point V 1  and the intersection point V 2 . 
     3. 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. 
     For example, in the aforementioned embodiment, information indicating the direction in which the mobile device  100  is located and which is indicated by RSSI in each of a plurality of first wireless communicators  210  is used as an example of the space information, but the present invention is not limited to this example. 
     For example, the space information may be information indicating a space including the position at which the mobile device  100  is located and which was estimated in the past at the position at which the communication unit  200  is located as the space in which the mobile device  100  is assumed to be located. Specifically, the communication unit  200  stores position information indicating the position of the communication unit  200  and position information indicating the position of the mobile device  100  estimated through the position estimating process in correlation. Then, when the position estimating process is performed again at the position at which the position estimating process was performed in the past, the communication unit  200  estimates that the mobile device  100  is located in the same space which includes the position of the mobile device  100  and which was estimated through the position estimating process performed in the past. For example, when a house of a user is located on the right side of a garage, the user often approaches the vehicle  202  parking in the garage from the right side of the vehicle  202 . Therefore, when the vehicle  202  is in the garage, the communication unit  200  estimates that the mobile device  100  is located at a position on the right side of the vehicle  202  out of the positions estimated through the position estimating process based on a plurality of measured distance values. With this configuration, the communication unit  200  can perform the position estimating process based on the space information without causing the first wireless communicator  210  to perform wireless communication. 
     For example, the space information may indicate the inside or the outside of the cabin of the vehicle  202  as the space in which the mobile device  100  is assumed to be located. For example, the communication unit  200  may further include an imaging unit configured to capture an image of the inside of the cabin. The space information may be acquired on the basis of the image captured by the imaging unit. 
     Specifically, when a user appears in the inside of the cabin, the communication unit  200  acquires the space information indicating the inside of the cabin as the space in which the mobile device  100  is assumed to be located. In this case, the communication unit  200  estimates that the mobile device  100  is located at a position in the inside of the cabin out of the positions estimated through the position estimating process based on a plurality of measured distance values. That is, the communication unit  200  may estimate the position at which the mobile device  100  is located in the inside of the cabin. 
     On the other hand, when a user does not appear in the inside of the cabin, the communication unit  200  acquires the space information indicating the outside of the cabin as the space in which the mobile device  100  is assumed to be located. In this case, the communication unit  200  estimates that the mobile device  100  is located at a position in the outside of the cabin out of the positions estimated through the position estimating process based on a plurality of measured distance values. That is, the communication unit  200  may estimate the position at which the mobile device  100  is located in the outside of the cabin. 
     For example, an example in which the position at which the mobile device  100  is located is estimated on the basis of the plurality of measured distance values and space information has been described in the aforementioned embodiment, but the present invention is not limited to this example. For example, the position at which the mobile device  100  is located may be estimated on the basis of one measured distance value and space information. In this case, the controller  240  estimates that the mobile device  100  is located at one position on a circle in which the distance from the second wireless communicator  220  is the measured distance value in the space in which the mobile device  100  is assumed to be located and which is indicated by the space information. 
     For example, an example in which the controller  240  calculates the measured distance values has been described above in the aforementioned embodiment, but the present invention is not limited to this example. For example, the second wireless communicator  220  may calculate the measured distance values. In this case, the second wireless communicator  220  reports the calculated measured distance values to the controller  240 . 
     For example, in the aforementioned embodiment, information indicating the time period ΔT 1  from the time point at which the mobile device  100  has transmitted the first distance-measuring signal to the time point at which the mobile device  100  has received the second distance-measuring signal is included in a data signal, but the present invention is not limited to this example. The data signal has only to include information on the time point at which the first distance-measuring signal has been transmitted and the time point at which the second distance-measuring signal has been received. Another example of the information included in the data signal will be described below. 
     Another example of the information included in the data signal is information indicating the time point at which the mobile device  100  has transmitted the first distance-measuring signal and the time point at which the mobile device  100  has received the second distance-measuring signal. That is, the mobile device  100  may not calculate the time period ΔT 1  but transmit time stamps of a start point and an end point of the time period ΔT 1 . 
     Another example of the information included in the data signal is a measured distance value indicating the distance between the mobile device  100  and the communication unit  200  which has been calculated on the basis of a time period from the time point at which the first distance-measuring signal has been transmitted and the time point at which the second distance-measuring signal has been received. That is, the mobile device  100  may calculate the measured distance value indicating the distance between the mobile device  100  and the communication unit  200  and transmit the calculated measured distance value to the communication unit  200 . For example, when ΔT 2  is a fixed value, the mobile device  100  can calculate the measured distance value by measuring the time period ΔT 1 . 
     For example, an example in which the mobile device  100  transmits the first distance-measuring signal has been described above in the aforementioned embodiment, but the present invention is not limited to this example. For example, the communication unit  200  may transmit the first distance-measuring signal. In this case, when the first distance-measuring signal has been received, the mobile device  100  transmits the second distance-measuring signal as a response thereto. Then, the mobile device  100  transmits a data signal including information indicating the time period ΔT 2  from the time point at which the first distance-measuring signal has been received to the time point at which the second distance-measuring signal has been received. On the other hand, the communication unit  200  calculates the measured distance value on the basis of the time period ΔT 1  from the time point at which the first distance-measuring signal has been transmitted and the time point at which the second distance-measuring signal has been received and the time period ΔT 2  included in the data signal. 
     For example, an example in which the measured distance value is calculated on the basis of the propagation time has been described above in the aforementioned embodiment, but the present invention is not limited to this example. For example, the measured distance value may be calculated on the basis of radio field intensities. 
     For example, an example in which the first wireless communication standard is BLE and the second wireless communication standard is a wireless communication standard using UWB has been described above in the aforementioned embodiment, but the present invention is not limited to this example. For example, a wireless communication standard using a signal in a ultra-high frequency (UHF) band and a low frequency (LF) band may be used as the first wireless communication standard or the second wireless communication standard. For example, Wi-Fi (registered trademark), near-field communication (NFC), or a wireless communication standard using infrared, may be used as the first wireless communication standard or the second wireless communication standard. The first wireless communication standard and the second wireless communication standard may be the same or different from each other. 
     For example, although not particularly described in the aforementioned embodiment, various connection modes between the second wireless communicator  220  and the controller  240  can be considered. For example, the plurality of second wireless communicators  220  may be directly connected to the controller  240 . That is, the plurality of second wireless communicators  220  may be connected in parallel to the controller  240 . For example, the plurality of second wireless communicators  220  may be classified into a second wireless communicator  220  serving as a master and a second wireless communicator  220  serving as a slave. The slave may be indirectly connected to the controller  240  via the master, and the master may be directly connected to the controller  240 . That is, one or more slaves may be connected in parallel to the master, and the master may be connected to the controller  240 . The same is true of the connection mode between the first wireless communicator  210  and the controller  240 . 
     For example, an example in which the communication unit  200  estimates the relative position of the mobile device  100  with respect to the communication unit  200  has been described above in the aforementioned embodiment, but the present invention is not limited to this example. For example, the mobile device  100  may estimate the relative position of the mobile device  100  with respect to the communication unit  200 . In this case, the mobile device  100  acquires information acquired through wireless communication performed by the first wireless communicator  210  and the second wireless communicator  220  of the communication unit  200  from the communication unit  200 . Then, the controller  140  of the mobile device  100  performs the same process as in the controller  240  of the communication unit  200  described above in the aforementioned embodiment. An application of this configuration is a car finder. The car finder is a function of ascertaining the parking position of the vehicle  202 . For example, the relative position of the mobile device  100  with respect to the communication unit  200  may be estimated by a device other than the mobile device  100  and the communication unit  200 . 
     For example, an example in which the communication unit  200  is a communication device mounted in a vehicle has been described above in the embodiment, but the present invention is not limited to the example. The communication unit  200  may be mounted in an arbitrary mobile object such as an aircraft and a watercraft. Here, a mobile object is a moving device. 
     For example, an example in which the present invention is applied to a smart entry system has been described above in the embodiment, but the present invention is not limited to the example. The present invention is applicable to an arbitrary system that performs wireless communication. For example, the present invention is applicable to a pair including two of a mobile device, a vehicle, a smartphone, a drone, a house, and a home appliance. This pair may include two devices of the same type or include two devices of two different types. 
     Note that, a series of processes performed by the devices described in this specification may be achieved by any of software, hardware, and a combination of software and hardware. A program that configures software is stored in advance in, for example, a recording medium (non-transitory medium) installed inside or outside the devices. In addition, for example, when a computer executes the programs, the programs are read into random access memory (RAM), and executed by a processor such as a CPU. The recording medium may be a magnetic disk, an optical disc, a magneto-optical disc, flash memory, or the like. Alternatively, the above-described computer program may be distributed via a network without using the recording medium, for example. 
     Further, in the present specification, the processes described using the flowcharts and the sequence diagrams are not necessarily executed in the order illustrated in the drawings. Some processing steps may be executed in parallel. In addition, additional processing steps may be employed and some processing steps may be omitted. 
     REFERENCE SIGNS LIST 
       1  system
 
 100  mobile device
 
 110  first wireless communicator
 
 120  second wireless communicator
 
 130  storage
 
 140  controller
 
 200  communication unit
 
 202  vehicle
 
 210  first wireless communicator
 
 220  second wireless communicator
 
 230  storage
 
 240  controller