Patent Publication Number: US-11022673-B2

Title: Terminal apparatus, location server, location measurement method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to a terminal apparatus, a location server, a location measurement method, and a program. 
     This application claims priority based on JP 2017-046786 filed on Mar. 10, 2017, the contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     In Long Terms Evolution (LTE) of 3 rd  Generation Partnership Project (3GPP), a positioning protocol for measurement and management of a position of a terminal apparatus (LPP, LTE Positioning Protocol) is defined (refer to NPL 1, NPL 2). In this positioning protocol, a location server (E-SMLC/SLP, Enhanced Serving Mobile Location. Centre, Evolved Serving Mobile Centre, Secure User Plane Location (SUPL) Location Platform) calculates a position of a terminal apparatus, based on an arrival angle of a signal from the terminal apparatus (mobile station apparatus, UE, Use Equipment) acquired by a base station apparatus (eNB, eNodeB), information on a transmission time of a signal to and/or from the terminal apparatus, and the like. 
     CITATION LIST 
     Non Patent Literature 
     NPL 3GPP, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); LTE. Positioning Protocol (LPP) (Release 13)”, TS 36.355 V13.3.0 (2016-12), 2016.12.30 
     NPL 2: 3GPP, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); LIE Positioning Protocol A (LPPa) (Release 13)”, TS 36.455 V13.1.0 (2016-03), 2016.3.19 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in measuring a position of a terminal apparatus, based on an arrival angle of a signal from the terminal apparatus acquired by a base station apparatus, information on a transmission time of a signal to and/from the terminal apparatus, and the like, there is a problem that, measurement accuracy of a position may decrease in a case that these signals are transmitted by reflection waves rather than direct waves between the base station apparatus and the terminal apparatus. 
     One aspect of the present invention has been made in view of such circumstances, and provides a terminal apparatus, a location server, a position measurement method, and a program which can suppress decrease in measurement accuracy of a position in a case that a signal between a base station apparatus and the terminal apparatus is transmitted by a reflection wave. 
     Solution to Problem 
     (1) An aspect of the present invention has been made to solve the above-described problem, and one aspect of the present invention is a terminal apparatus including: a receiver configured to detect a transmission direction of a signal used for communication with at least one base station apparatus; and a transmitter configured to transmit, to a location server for calculating a position of a terminal apparatus, base station direction information for indicating the transmission direction detected. 
     (2) Another aspect of the present invention is the above-described terminal apparatus, wherein the base station direction information indicates the transmission direction detected, based on any one of a transmission direction of a signal used for communication with one base station apparatus, a global coordinate system, and a local coordinate system. 
     (3) Another aspect of the present invention is the above-described terminal apparatus, wherein the base station direction information is based on an identical one direction with respect to transmission directions of signals used for communication with multiple base station apparatuses. 
     (4) Another aspect of the present invention is a location server including: an information acquisition unit configured to acquire terminal direction information for indicating a transmission direction of a signal, detected by each of multiple base station apparatuses, that is used for communication between each of the multiple base station apparatuses and a terminal apparatus, and base station direction information for indicating a transmission direction of a signal, detected by the terminal apparatus, that is used for communication between at least one of the multiple base station apparatuses and the terminal apparatus; and a position measurement unit configured to calculate a position of the terminal apparatus, based on the terminal direction information and the base station direction information. 
     (5) Another aspect of the present invention is the above-described location server, wherein the position measurement unit is configured to detect, based on the terminal direction information and the base station direction information, a base station apparatus, among the multiple base station apparatuses, of which a signal used for communication with the terminal apparatus, is a signal by reflection wave, and calculate the position of the terminal apparatus, by using a result of the detection and the terminal direction information or the base station direction information, 
     (6) Another aspect of the present invention is the above-described location server, wherein the information acquisition unit is configured to acquire a transmission distance of the signal between the at least one base station apparatus of the multiple base station apparatuses and the terminal apparatus, and the position measurement unit is configured to use the transmission distance in calculating the position of the terminal apparatus. 
     (7) Another aspect of the present invention is a position measurement method including: a first process of detecting a transmission direction of a signal used for communication with at least one base station apparatus; and a second process of transmitting, to a location server for calculating a position of a terminal apparatus, base station direction information for indicating the transmission direction detected. 
     (8) Another aspect of the present invention is a position measurement method including: a first process of acquiring terminal direction information for indicating a transmission direction of a signal, detected by each of multiple base station apparatuses, that is used for communication between each of the multiple base station apparatuses and a terminal apparatus, and base station direction information for indicating a transmission direction of a signal, detected by the terminal apparatus, that is used for communication between at least one of the multiple base station apparatuses and the terminal apparatus: and a second process of calculating a position of the terminal apparatus, based on the terminal direction information and the base station direction information. 
     (9) Another aspect of the present invention is a program, for causing a computer to function as: a receiver configured to detect a transmission direction of a signal used for communication with at least one base station apparatus; and a transmitter configured to transmit, to a location server for calculating a position of a terminal apparatus, base station direction information for indicating the transmission direction detected. 
     (10).Another aspect of the present invention is a program for causing a computer to function as: an information acquisition unit configured to acquire terminal direction information for indicating a transmission direction of a signal, detected by each of multiple base station apparatuses, that is used for communication between each of the multiple base station apparatuses and a terminal apparatus, and base station direction information for indicating a transmission direction of a signal, detected by the terminal apparatus, that is used for communication between at least one of the multiple base station apparatuses and the terminal apparatus; and a position measurement unit, configured to calculate a position of the terminal apparatus, based on the terminal direction information and the base station direction information. 
     Advantageous Effects of Invention 
     According to one aspect of the present invention, it is possible to suppress a decrease in measurement accuracy of a position in a case that a signal between the base station apparatus and the terminal apparatus is transmitted by a reflection wave. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating a configuration of a mobile communication system  50  according to a first embodiment of the present invention. 
         FIG. 2  is a schematic block diagram illustrating communication between apparatuses of the mobile communication system  50  according to the present embodiment. 
         FIG. 3  is a schematic block diagram illustrating a configuration of a terminal apparatus  20  according to the present embodiment. 
         FIG. 4  is a schematic block diagram illustrating a configuration of a receiver  22  according to the present embodiment. 
         FIG. 5  is a schematic block diagram illustrating a configuration of a location server  40  according to the present embodiment. 
         FIG. 6  is a diagram illustrating base station direction information based on a global coordinate system according to the present embodiment. 
         FIG. 7  is a diagram illustrating the base station direction information based on a local coordinate system according to the present embodiment. 
         FIG. 8  is a diagram illustrating a relationship between an arrival angle detected using an array antenna  21  and a direction based on the local coordinate system according to the present embodiment. 
         FIG. 9  is a diagram illustrating rotation by Euler angles. 
         FIG. 10  is a diagram illustrating base station direction information by a reference base station according to the present embodiment. 
         FIG. 11  is a sequence diagram illustrating an example of operation of the mobile communication system  50  according to the present embodiment. 
         FIG. 12  is a diagram illustrating a method for determining, whether or not a signal is transmitted by a reflection wave according to the present embodiment. 
         FIG. 13  is a diagram illustrating another method for determining whether or not a signal is transmitted by a reflection wave according to the present embodiment. 
         FIG. 14  is a sequence diagram illustrating another example of operation of the mobile communication system  50  according to the present embodiment. 
         FIG. 15  is a diagram illustrating another method for determining whether or not a signal is transmitted by a reflection wave according to the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic block diagram illustrating a configuration of a mobile communication system  50  according to the first embodiment of the present invention. The mobile communication system  50  includes base station apparatuses  10   a,    10   b,    10   c  and  10   d,  a terminal apparatus  20 , a core network  30 , and a location server  40 . The base station apparatuses  10   a,    10   b,    10   c  and  10   d  perform communication with the terminal apparatus  20 . The base station apparatuses  10   a,    10   b,    10   c  and  10   d  are connected to the location server  40  via the core network  30 . The location server  40  calculates a two-dimensional or three-dimensional position (coordinate) of the terminal apparatus  20 . As described below, the terminal apparatus  20  and the location server  40  communicate according to a positioning protocol (e.g., LPP), which is relayed by the base station apparatuses  10   a,    10   b,    10   c  and  10   d  and the core network  30 . Furthermore, the location server  40  and the base station apparatuses  10   a,    10   b,    10   c  and  10   d  communicate in accordance with the positioning protocol A (e.g., LPPa), which is relayed by the core network  30 . 
       FIG. 2  is a schematic block diagram illustrating communication between apparatuses of the mobile communication system  50  according to the present embodiment. In  FIG. 1 , the mobile communication system  50  includes four base station apparatuses  10   a,    10   b,    10   c.    10   d,  but the base station apparatus  10  in  FIG. 2  is representative of the four base station apparatuses  10   a,    10   b,    10   c.    10   d.  As illustrated in.  FIG. 2 , the base station apparatus  10  transmits a downlink signal for position estimation such as a positioning reference signal (Positioning RS, PRS). For example, the terminal apparatus  20  detects (estimates) measurement information (observation information) relating to position estimation such as an arrival angle and a propagation delay time (propagation distance, transmission distance) by using the positioning reference signal. Note that the terminal apparatus  20  can detect (estimate) multiple types of measurement information from the positioning reference signal, such as the arrival angle and the propagation delay time. The terminal apparatus  20  may estimate the arrival angle by using a certain downlink signal, and estimate the propagation delay time by using another downlink signal. The terminal apparatus  20  performs communication with the location server  40  in accordance with the positioning protocol (LPP). The terminal apparatus  20  transmits (reports) information indicating the detected arrival angle (arrival direction) (base station direction information) and/or the propagation delay time (propagation distance, transmission distance), and the like, as Location Information, to the location server  40  in communication in accordance with the positioning protocol, 
     Note that the terminal apparatus  20  transmits the location information to the location server  40  in a case that the location information is requested from the location server  40 . For example, the terminal apparatus  20  is required to report the base station direction information or the propagation delay time, or the base station direction information and the propagation delay time as location information. The location server  40  can transmit assist data for location information measurement to the terminal apparatus  20 . For example, the assist data includes configuration information for the base station apparatus (cell). The configuration information for the base station apparatus (cell) includes part or all of physical cell ID, carrier frequency, number of antenna ports, and PRS information. The PRS information indicates a bandwidth or subframe at which PRS is transmitted, and a resource element on which the PRS is allocated. The resource element is defined by one subcarrier and one Orthogonal Frequency Division Multiplexing (OFDM) symbol. In a case that the reference base station apparatus (reference cell) is present, the configuration information for the base station apparatus (cell) includes configuration information for the reference base station apparatus (reference cell) and configuration information for a neighboring base station apparatus (neighboring cell). Each of the configuration information of the reference base station apparatus (reference cell) and the configuration information of the neighboring base station apparatus (neighboring cell) include some or all of physical cell ID, carrier frequency, number of antenna ports, and PRS information. 
     The location information is information for two-dimensional or three-dimensional position estimation, and the terminal apparatus  20  determines the corresponding location information, based on the indication from the location server  40 . In a case that three-dimensional location information is requested, the terminal apparatus  20  can transmit two-dimensional location information and height information (sea level, altitude, and the like) determined from an air pressure sensor and the like, as three-dimensional location information to the location server  40 . 
     The base station apparatus  10  detects (estimates) an arrival angle (arrival direction) of a signal (e.g., Sounding RS (SRS), Demodulation RS (DMRS)) from the, terminal apparatus  20 . The base station apparatus  10  performs communication with the location server  40  in accordance with the positioning protocol A (LPPa). The base station apparatus  10  transmits information indicating the detected arrival angle (terminal direction information) to the location server  40  in communication in accordance with the positioning protocol A. The location server  40  acquires the base station direction information from the terminal apparatus  20  in communication in accordance with the positioning protocol, and acquires the terminal direction information from the base station apparatus  10  in communication, in accordance with the positioning protocol A. The location server  40  determines whether or not the positioning reference signal received by the terminal apparatus  20  is transmitted by a reflection wave (reflection path) from the base station apparatus  10 , based on the acquired base station direction information and the terminal direction information. In a case that the positioning reference signal is transmitted by a reflection wave, the location server  40  measures the position of the terminal apparatus  20 , by using terminal direction information excluding the terminal direction information acquired from the base station apparatus  10  of the transmission source of the positioning reference signal. 
       FIG. 3  is a schematic block diagram illustrating a configuration of the terminal apparatus  20  according to the present embodiment. The terminal apparatus  20  includes an array antenna  21 , a receiver  22 , a controller  23 , and a transmitter  24 . The array antenna  21  is an array antenna in which multiple antenna elements are arrayed in a two-dimensional plane. 
     The receiver  22  receives signals from the base station apparatus  10  by using the array antenna  21  and decodes the signals to generate received data. The receiver  22  detects the arrival angle of the positioning reference signal from the base station apparatus  10  by using the array antenna  21 . 
     The controller  23  controls the entire terminal apparatus  20 . For example, the controller  23  generates transmission data, based on the received data acquired from the receiver  22 , input to the terminal apparatus  20  by the user, performance of an application, and the like, and inputs the generated data to the transmitter  24 . The controller  23  according to the present embodiment inputs, to the transmitter  24 , base station direction information indicating an arrival angle of the positioning reference signal from at least one base station apparatus  10 , which is the arrival angle detected by, the receiver  22 , as transmission data directed to the location server  40  in accordance with the positioning protocol. The transmitter  24  encodes and modulates the input transmission data to generate a transmit signal. The transmitter  24  transmits the generated transmit signal to the base station apparatus  10  via the array antenna  21 . 
       FIG. 4  is a schematic block diagram illustrating a configuration of the receiver  22  according to the present embodiment. The receiver  22  includes a combiner unit  221 , a radio unit  222 , and a reception processing unit  223 . The combiner unit  221  extracts a signal of a particular arrival angle by providing a phase rotation to signals received by the antenna elements of the array antenna  21  and then synthesizing the signals. The radio unit  222  converts the signal extracted by the combiner unit  221  by down-conversion and analog-to-digital conversion to generate a digital baseband signal. The reception processing unit  223  decodes the digital baseband signal to generate received data. The reception processing unit  223  detects (estimates) an arrival angle of the positioning reference signal from each base station apparatus  10 , by changing the arrival angle of the signal extracted by the combiner unit  221 , and determining which positioning reference signal of any base station apparatus  10  is included in the digital baseband signal at which arrival angle. 
       FIG. 5  is a schematic block diagram illustrating a configuration of the location server  40  according to the present embodiment. The location server  40  includes an information acquisition unit  41 , a position measurement unit  42 , and a location information storage unit  43 . The information acquisition unit  41  communicates with the terminal apparatus  20  in accordance with the positioning protocol, and acquires base station direction information indicating an arrival angle of the positioning reference signal from at least one base station apparatus  10 . The information acquisition unit  41  communicates with each base station apparatus  10  in accordance with the positioning protocol A to acquire terminal direction information. 
     The position measurement unit  42  determines, based on the base station direction information and the terminal direction information acquired by the information acquisition unit  41 , the base station apparatus(es), of which the received positioning reference signal is transmitted by a reflection wave, among the base station apparatuses  10 . The position measurement unit  42  measures the position of the terminal apparatus  20 , by using the terminal direction information detected by the base station apparatuses  10  excluding base station apparatus(es)  10  in which the positioning reference signal is determined to be transmitted by a reflection wave. A known method such as a triangulation method or a least square method, can be used for measuring the position. The position measurement unit  42  stores location information indicating the measured position in the location information storage unit  43 . 
     The base station direction information will be described in detail below. The base station direction information transmitted by the terminal apparatus  20  to the location server  40  may be based on any of A) a global coordinate system, B) a local coordinate system, and C) an arrival direction of a signal used for communication with one base station apparatus  10  (reference base station apparatus). Hereinafter, base station direction information for A) is referred to as base station direction information based on a global coordinate system, base station direction information for B) is referred to as base station direction information based on a local coordinate system, and base station direction information for C) is referred to as base station direction information based on the reference base station. 
       FIG. 6  is a diagram illustration the base station direction information based on the global coordinate system according to the present embodiment. The base station direction information based on a global coordinate system includes, for example, information indicating the azimuth angle ϕ globe  (horizontal direction) and information indicating the zenith angle θ globe  (vertical direction), based on the global coordinates where the zenith direction is a z-axis direction and the magnetic north direction is a y-axis direction. For example, the terminal apparatus  20  includes a gravity (acceleration) sensor and a geomagnetic sensor, with the direction opposite to the gravity direction detected by the gravity (acceleration) sensor being the z-axis direction, and the magnetic north direction detected by the geomagnetic sensor being the y-axis direction. At this time, since a direction vector (x y z) is represented by Equation (1), the base station direction information based on the global coordinate system may include information indicating these x, y, and z. 
     
       
         
           
             
               
                 
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       FIG. 7  is a diagram illustrating the base station direction information based on the local coordinate system according to the present embodiment. The base station direction information based on the local coordinate system includes, for example, information indicating the horizontal direction and information indicating the vertical direction θ local , based on local coordinates with the lateral (horizontal) direction of the array antenna  21  being a x′″ axis direction, the longitudinal (vertical) direction being a z′″ axis direction, and the direction perpendicular to a plane of the array antenna  21  being a y′″ axis direction. At this time, since the direction vector (x′″ y′″ z′″) is represented by Equation (2), the base station direction information based on the local coordinate system may include information indicating these x′″, y′″, z′″, 
     
       
         
           
             
               
                 
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       FIG. 8  is a diagram illustrating a relationship between an arrival angle detected using the array antenna  21  and a direction based on the local coordinate system according to the present embodiment. In detecting the arrival angle by using the array antenna  21 , the angle of the antenna elements in the array direction is detected. In  FIG. 8 , the antenna elements of the array antenna  21  are arrayed in the lateral direction and the longitudinal direction, with the lateral direction being the x-axis direction and the longitudinal direction being the z-axis direction. At this time, the lateral direction ϕ of the arrival angle detected using the array antenna  21  coincides with the horizontal direction ϕ local  of  FIG. 7 . However, the longitudinal direction ψ of the arrival angle detected using the array antenna  21  is different from the vertical direction θ local  of  FIG. 7 , but can be converted by Equation (3). 
     
       
         
           
             
               
                 
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     The controller  23  of the terminal apparatus  20  uses Equation (3) to determine the horizontal direction, and the vertical direction θ local  from the lateral direction of the arrival angle and the longitudinal direction ψ of the arrival angle detected using the array antenna  21 , and uses information indicating these as the base station direction information. Note that the base station direction information based on the local coordinate system may include information indicating the lateral direction ϕ of the arrival angle and information indicating the longitudinal direction ψ of the arrival angle. The base station direction information based on the local coordinate system may be information indicating a configuration for the terminal apparatus  20  for generating a beam in the lateral direction ϕ and the longitudinal direction ψ (e.g. PMI; Precoding. Matrix Indicator, CRI; CSI-RS Resource Indicator, SRI; SRS Resource Indicator). Note that the CRI is information indicating one or more of multiple channel state information reference signal (CSI-RS) resources, and the SRI is information indicating one or more of multiple SRS resources. In a case that different beamforming (precoding) is applied to multiple CSI-RS resources or multiple SRS resources, the index of the resource indicates the arrival angle (arrival direction). 
     Furthermore, in a case that the base station direction information is the base station direction information based on the local coordinate system, the base station direction information may include information (coordinate conversion information) indicating a relationship between the global coordinate system and the local coordinate system. The coordinate conversion information is, for example, information indicating a rotation for converting the local coordinate system to the global coordinate system. This rotation may be expressed by Euler angles (e.g., yaw angle α, roll angle β, pitch angle γ) or may be expressed by a quaternion. FIG — 9  is a diagram illustrating rotation by Euler angles. In  FIG. 9 , rotating the local coordinate system x′″, y′″, z′″ by a about the z′″ axis achieves the coordinate system x″, y″, z″ Next, rotating the coordinate system x″, y″, z″ by β around the y″ axis achieves the coordinate system x′, y′, and z′, Next, rotating the coordinate system x′, y′, z′ by γ about the x′ axis achieves the global coordinate system x, y, z. 
     Rotation by quaternion is represented by Equation (4) using a rotation vector q. In Equation (4). v is a vector in the local coordinate system, and u is a vector in the global coordinate system. Furthermore, the rotation vector q is expressed by Equation (5) with the rotation axis n and the rotation amount ω. 
     
       
         
           
             
               
                 
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     Note that each terminal apparatus  20  determines the global coordinate system using a sensor, and thus in a case that the terminal apparatuses  20  use different sensors from each other, variations in the global coordinate system between the terminal apparatuses may be problematic. Moreover, the estimation in the global coordinate system may be difficult due to influences of surrounding buildings or the like. Thus, the base station direction information based on the global coordinate system and the coordinate conversion information may include information indicating the estimation performance of the global coordinate system, such as a variance value of errors of the global coordinate, reliability information, and the like. 
       FIG. 10  is a diagram illustrating base station direction information by a reference base station (reference cell) according to the present embodiment. In  FIG. 10 , the x-axis, y-axis, and z-axis are in the global coordinate system, a base station apparatus  10   a  is a reference base station, and a base station apparatus  10   b  is a neighboring base station (neighboring cell). At this time, the base station direction information of the base station apparatus  10   b  based on the reference base station includes information indicating a relative zenith angle Δθ globe  obtained by subtracting the zenith angle of the base station apparatus  10   a  from the zenith angle of the base station apparatus  10   b,  and information indicating a relative azimuth angle Δϕ globe  obtained by subtracting the azimuth angle of the base station apparatus  10   a  from the azimuth angle of the base station apparatus  10   b.  The controller  23  of the terminal apparatus  20  may select the reference base station to include information indicating the reference base station in the base station direction information based on the reference base station. 
     Note that the x-axis, y-axis, and z-axis are assumed to be in the global coordinate system, but it is sufficient that at least one of the three axes corresponds to the global coordinate system. 
     In a case that the base station direction information based on the reference base station is used, the controller  2  of the terminal apparatus  20  may select the transmission source of the signal having the greatest receive power as the reference base station, since it is important that the positioning reference signal from the reference base station is transmitted by a direct wave (direct pass). Alternatively, any of the base station apparatuses  10   a,    10   b,    10   c.    10   d  may select the base station apparatus  10 , which has the greatest receive power of the signal from the terminal apparatus  20 , as the reference base station, and notify the terminal apparatus  20  of the reference base station, 
     The base station direction information based on the reference base station may be information indicating an angle formed by an arrival direction of a signal from the base station apparatus  10   b  and an arrival direction of a signal from the base station apparatus  10   a.  Note that in a case that the reference base station and the terminal apparatus  20  are transmitted by a direct wave, the terminal apparatus  20  may not report the base station direction information of the reference base station to the location server  40 . 
       FIG. 11  is a sequence diagram illustrating an example of operation of the mobile communication system  50  according to the present embodiment. The example operation illustrated in  FIG. 11  is an example in which the terminal apparatus  20  transmits the base station direction information based on the global coordinate system to the location server  40 . First, each of the base station apparatuses  10  detects an arrival direction of a signal from the terminal apparatus  20  in the global coordinate system (Sa 1 ). Next, each of the base station apparatuses  10  transmits terminal direction information indicating the detected arrival direction to the location server  40  (Sa 2 ). 
     In parallel with these sequences Sa 1  and Sa 2 , the terminal apparatus  20  performs the following sequences Sa 3  to Sa 5 . First, the controller  23  of the terminal apparatus  20  calculates the conversion parameter (coordinate conversion information) from the local coordinate system to the global coordinate system (Sa 3 ). Next, the receiver  22  of the terminal apparatus  20  detects an arrival direction (an arrival angle) of the positioning reference signal of each of the base station apparatuses  10  (Sa 4 ). The controller  23  of the terminal apparatus  20  converts the detected arrival direction into an arrival direction based on the global coordinate system to generate base station direction information (Sa 5 ). The transmitter  24  transmits the base station direction information to the location server  40  (Sa 6 ). The location server  40  calculates the position of the terminal apparatus  20 , by using the terminal direction information acquired from each of the base station apparatuses  10  and the base station direction information related to each of the base station apparatuses  10  acquired from the terminal apparatus  20  (Sa 7 ). 
     Note that in a case that the terminal apparatus  20  transmits the base station direction information based on the local coordinate system to the location server  40 , the following two sequences Sa 5 ′ and Sa 6 ′ are included instead of the sequences Sa 5  and Sa 6  in  FIG. 11 . In the sequence Sa 5 ′, the transmitter  24  of the terminal apparatus  20  transmits, to the location server  40 , base station direction information based on the local coordinate system and coordinate conversion information. In the sequence Sa 6 ′, the location server  40  converts the base station direction information based on the local coordinate system received from the terminal apparatus  20  to base station direction information based on the global coordinate system, by using the coordinate conversion information. 
       FIG. 12  is a diagram illustrating a method for determining whether or not a signal is transmitted by a reflection wave according to the present embodiment. The method illustrated in  FIG. 12  is an example of a case where the base station direction information based on the global coordinate system and the terminal direction information based on the global coordinate system are used. As illustrated in  FIG. 12 , in a case that the signal is transmitted by a direct wave, an absolute value of a difference between an azimuth angle ϕ globe  of base station direction information of any of the base station apparatuses  10  based on the global coordinate system and an azimuth angle ρ p  of terminal direction information in the base station apparatus  10  based on the global coordinate system is π. 
     Therefore, the position measurement unit  42  of the location server  40  calculates the difference between an absolute value of a value obtained by subtracting ρ p  from ϕ globe  and π, and determines that the signal is transmitted by a reflection wave in a case that an absolute value of the calculation result exceeds a prescribed threshold. Furthermore, since the same applies to the zenith angle θ globe  of base station direction information of any of the base station apparatuses  10  based on the global coordinate system and the zenith angle μ p  of terminal direction information in the base station apparatus  10  based on the global coordinate system, the position measurement unit  42  calculates a difference between an absolute value of a value obtained by subtracting μ p  from θ globe  and π, and determines that the signal is transmitted by a reflection wave in a case that an absolute value of the calculation result exceeds a prescribed threshold. In any of these two determinations, the position measurement unit  42  determines that the signal is transmitted by a direct wave in a case that the threshold is not exceeded. Note that the thresholds for these two determinations may not be the same. 
       FIG. 13  is a diagram illustrating another method for determining whether or not a signal is transmitted by a reflection wave according to the present embodiment. The method illustrated in  FIG. 13  is an example of a case where the base station direction information based on the local coordinate system and the terminal direction information based on the global coordinate system are used. As illustrated in  FIG. 13 , in a case that the signal is transmitted by a direct wave, an absolute value of a difference between a value obtained by subtracting ϕ d  from an azimuth angle ϕ local  of base station direction information of any of the base station apparatuses  10  based on the local coordinate system and an azimuth angle ρ p  of terminal direction information in the base station apparatus  10  based on the global coordinate system is π. Here, ϕ d  is the angle of rotation about the z-axis for converting the local coordinate system to the global coordinate system. 
     The position measurement unit  42  calculates a difference between the absolute value of the difference between the value obtained by subtracting ϕ d  and ϕ local  and ρ p , an π, and determines that the signal is transmitted by a reflection wave in a case that an absolute value of the calculation result exceeds a prescribed threshold value. A similar determination is made for the zenith angle. At this time, instead of ϕ d , an angle of rotation about an axis perpendicular to the zenith direction and perpendicular to the base station direction, for converting the local coordinate system to the global coordinate, system is used. In any of these two determinations, the position measurement unit  42  determines that the signal is transmitted by a direct wave in a case that the threshold is not exceeded. Note that the thresholds for these two determinations may not be the same. 
       FIG. 14  is a sequence diagram illustrating another example of operation of the mobile communication system  50  according to the present embodiment. The example operation illustrated in  FIG. 14  is an example in which the terminal apparatus  20  transmits base station direction information based on a reference base station to the location server  40 . In  FIG. 14  as well as in  FIG. 11 , each base station apparatus  10  performs the sequences Sa 1  and Sa 2 . In parallel with these sequences Sa 1  and Sa 2 , the terminal apparatus  20  performs the following sequences Sb 3  to Sb 5 . 
     First, the controller  23  of the terminal apparatus  20  selects the reference base station among the base station apparatuses  10  (Sb 3 ). Next, the receiver  22  detects an arrival angle (arrival direction) of the positioning reference signal from each of the base station apparatuses  10  (Sb 4 ). The controller  23  calculates a difference between each of the arrival directions among the base station apparatuses  10  other than the reference base station and the arrival direction of the reference base station (Sb 5 ), and generates base station direction information indicating the calculated value. The transmitter  24  transmits the, base station direction information to the location server  40  (Sb 6 ). The location server  40  calculates the position of the terminal apparatus  20 , by using the terminal direction information acquired from each of the base station apparatuses  10  and the base station direction information for each of the base station apparatuses  10  excluding the reference base station acquired from the terminal apparatus  20  (Sb 7 ). 
       FIG. 15  is a diagram illustrating another method for determining whether or not a signal is transmitted by a reflection wave according to the present embodiment. The method illustrated in  FIG. 15  is an example of a case where base station direction information Δϕ globe  based on the reference base station and terminal direction information ρ r  and  92   p  based on the global coordinate system are used. Note that in  FIG. 15 , the base station apparatus  10   a  is the reference base station. Thus, the terminal direction information p r  is terminal direction information based on the reference base station, and the base station direction information Δϕ globe  is base station direction information of the base station apparatus  10   b.  The position measurement unit  42  of the location server  40  assumes a triangle having points, projected on a horizontal plane, of the base station apparatus  10   a,  the base station apparatus  10   b,  and the terminal apparatus  20 , and determines that, in a case that a difference between the sum of the internal angles and π exceeds a prescribed threshold, the positioning reference signal of the base station apparatus  10   b  is transmitted by a reflection wave. The position measurement unit  42  makes the similar determination on the zenith angle. 
     In any of these two determinations, the position measurement unit  42  determines that the signal is transmitted by a direct wave in a case that the threshold is not exceeded. Note that the thresholds for these two determinations may not be the same. 
     Note that the terminal apparatus  20  may use an arrival angle detected using a signal other than the positioning reference signal as the base station direction information. As the base station direction information, information indicating a transmission direction (direction of a transmission beam) of a transmit signal to the base station apparatus  10  may be used instead of an arrival angle. In other words, the base station direction information may be information indicating a transmission direction of a signal used for communication between the terminal apparatus  20  and the base station apparatus  10 , and a transmission direction detected by the terminal apparatus  20 . 
     The terminal direction information may be information indicating a transmission direction of a signal used for communication between the terminal apparatus  20  and the base station apparatus  10 , and a transmission direction detected by the base station apparatus  10 . 
     Instead of the terminal direction information, the position measurement unit  42  may acquire, from the base station apparatus  10  or the terminal apparatus  20 , a distance between the base station apparatus  10  and the terminal apparatus  20 , which is calculated based on a transmission time of a signal between the base station apparatus  10  and the terminal apparatus  20 , and measure the position of the terminal apparatus  20 . 
     In this manner, the position measurement unit  42  determines the base station apparatus in which the signal is transmitted by a reflection wave, by using base station direction information, and measures the position of the terminal apparatus  20 , based on the determination result. As a result, a decrease in the measurement accuracy of the position can be suppressed in a case that a signal between any of the base station apparatuses  10  and the terminal apparatus  20  is transmitted by a reflection wave. 
     Second Embodiment 
     A second embodiment of the present invention will be described below. In the first embodiment, the location server  40  measures the position of the terminal apparatus  20  except for terminal direction information related to a signal transmitted by a reflection wave. In the second embodiment, the location server  40  estimates a reflected position of the signal, and measures the position of the terminal apparatus  20 , by using the terminal direction information relating to the signal transmitted by a reflection wave. 
     In addition to the base station direction information and the terminal direction information, the information acquisition unit  41  according to the present embodiment also acquires transmission (propagation) distance information. The transmission distance information is information indicating a transmission (propagation) distance of a signal between each of the base station apparatuses  10  and the terminal apparatus  20 . This transmission distance information is calculated based on round trip time of the signal between the apparatuses, by the base station apparatus  10  or the terminal apparatus  20 . 
     Alternatively, the transmission distance between the base station apparatus  10  as reference (reference base station) and the terminal apparatus  20  is calculated based on the round trip time, and the transmission distance between each of other base station apparatuses  10  (neighboring base stations) and the terminal apparatus  20  is calculated based on a difference between the arrival time of the signal from the reference base station and the arrival time of the signal from the corresponding base station apparatuses  10 . 
     The position measurement unit  42  according to the present embodiment first estimates a temporary position of the terminal apparatus  20  in the similar manner to the manner in the first embodiment. Next, the position measurement unit  42  again estimates the position of the terminal apparatus  20 , by using the transmission distance information for the base station apparatus  10 , in which it is determined that the signal is transmitted (once) by a reflection wave, and the estimated temporary position The position measurement unit  42  repeats, using the position of the terminal apparatus  20  estimated again as a temporary position, the estimation of the position of the terminal apparatus  20 , by using the transmission distance information and the temporary position, until the predetermined end condition is satisfied, and makes the final estimated temporary position be the position of the terminal apparatus  20 . The end condition is, for example, the number of repetitions. 
     The position measurement unit  42  estimates the position of the terminal apparatus  20  by using the distance information and the temporary position, for example, as follows. First, based on the terminal direction information, the position measurement unit  42  determines a reflection position in which the transmission distance to the temporary position on a straight line extended from the base station apparatus  10  is the distance indicated by the transmission distance information as the reflection position for the current calculation. The position measurement unit  42  again estimates the position of the terminal apparatus  20 , by using the reflection position, base station direction information of the corresponding base station apparatus  10 , and terminal direction information of each of the base station apparatuses  10  in which the signal is determined to be transmitted by a direct wave. Specifically, the position measurement unit  42  estimates the position of the terminal apparatus  20 , with the opposite direction of the base station direction information of the corresponding base station apparatus  10  as the direction of the terminal apparatus  20 , for the reflection position, and with the direction of the terminal direction information as the direction of the terminal apparatus  20 , for each of the base station apparatuses  10  in which the signal is determined to be transmitted by a direct wave, by using a triangulation method or a least square method. Note that in a case that a reflection wave is used, the number of parameters to be estimated is increased compared to a case where a direct wave is used, so the reliability is reduced. Accordingly, the position measurement unit  42  may estimate a temporary position in consideration of the reliability depending on the direct wave and the reflection wave. 
     In this manner, the position measurement unit  42  according to the present embodiment determines the base station apparatus in which the signal is transmitted by a reflection wave, by using base station direction information, estimates the reflection position of the reflection wave, and measures the position of the terminal apparatus  20 , based on the determination result. As a result, a decrease in the measurement accuracy of the position can be suppressed in a case that a signal between any of the base station apparatuses  10  and the terminal apparatus  20  is transmitted by a reflection wave. 
     The base station apparatus  10 , the terminal apparatus  20 , and the location server  40  in  FIG. 2  may be realized by recording a program to realize functions (or some functions) of each of the base station apparatus  10 , the terminal apparatus  20 , and the location server  40  on a computer-readable recording medium and causing a computer system to read the program recorded on the recording medium for execution. The “computer system” here includes an OS and hardware components such as a peripheral device. 
     Furthermore, the “computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, and the like, and a storage apparatus such as a hard disk built into the computer system. Moreover, the “computer-readable recording, medium” may include a medium that dynamically retains the program for a short period of time, such as a communication line that is used to transmit the program over a network such as the Internet or over a communication circuit such as a telephone circuit, and a medium that retains, in that case, the program for a fixed period of time, such as a volatile memory within the computer system which functions as a server or a client. Furthermore, the program may be configured to realize some of the functions described above, and also may be configured to be capable of realizing the functions described above in combination with a program already recorded in the computer system. 
     Further, each functional block of the base station apparatus  10 , the terminal apparatus  20 , the location server  40  described above in  FIG. 2  may be individually realized as chips, or may be partially or completely integrated into a chip. The circuit integration technique is not limited to LSI, and the integrated circuits for the functional blocks may be realized as dedicated circuits or a multi-purpose processor. It may be either hybrid or monolithic. Some functions may be realized as hardware and other may be realized as software. 
     Furthermore, in a case where with advances in semiconductor technology, a circuit integration technology with which an LSI is replaced and the like appears, it is also possible to use an integrated circuit based on the technology. 
     The embodiments of the present invention have been described in detail above referring to the drawings, but the specific configuration is not limited to the embodiments and includes, for example, an amendment to a design that falls within the scope that does not depart from the gist of the present invention. 
     INDUSTRIAL APPLICABILITY 
     An aspect of the present invention can be utilized, for example, in a communication system, communication equipment (for example, a cellular phone apparatus, a base station apparatus, a radio LAN apparatus, or a sensor device), an integrated circuit (for example, a communication chip), or a program. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  10   a,    10   b,    10   c,    10   d  Base station apparatus 
           20  Terminal apparatus 
           30  Core network 
           40  Location server 
           21  Array antenna 
           22  Receiver 
           23  Controller 
           24  Transmitter 
           41  Information acquisition unit 
           42  Position measurement unit 
           43  Location information storage unit 
           221  Combiner unit 
           222  Radio unit 
           223  Reception processing unit