DESCRIPTION DISTRIBUTED ANTENNA BASE STATION, MOBILE STATION, AND METHOD OF DISTANCE MEASUREMENT 5 Field [0001] The present invention relates to a technique for measuring a distance between a radio base station and a mobile terminal, and more particularly, to a technique of 10 distance measurement performed by a radio base station including a plurality of distributed antennas. Background [0002] There has been known a radio communication system 15 configured in assuming that a base station performs radio communication with a mobile terminal (mobile station) included in a device that travels on a predetermined route, such as a train and an automobile. As a method of configurating such a radio communication system, there has 20 been proposed a method of configuring a communicable range (cell) in an elongated shape along a traveling route with a plurality of directional beams using a distributed antenna base station to which a plurality of directional antennas, which is distributed along the traveling route of a mobile 25 device (mobile station), is connected (Non Patent Literature 1). When the shape of the cell is formed to be elongated along the traveling route, a cell length can be increased while interference with another radio communication system is suppressed. Such a distributed 30 antenna system can be implemented in a form in which a plurality of antennas is arranged from one communication modem using a transmission medium such as an optical fiber. [0003] Meanwhile, there has been known a method of measuring (ranging) a distance between a base station and a mobile station as a method of detecting a location of a mobile device, such as detection of a traveling location of a train. As such a method of measuring a distance, there 5 has been known a method of measuring a distance from the time required for communication between a radio device of the distance measuring side (e.g., base station) and a radio device of the distance measured side (e.g., mobile station) (Non Patent Literature 2). 10 Citation List Non Patent Literature [0004] Non Patent Literature 1: Hiroshi NISHIMOTO, et al., "A Proposal for Millimeter-wave Linear Cell Concept 15 Enabling High-speed Land-mobile Communications", Institute of Electronics, Information and Communication Engineers (IEICE) General Conference, B-5-77, March 2015 Non Patent Literature 2: Mitsuyoshi FUKUDA, et al., "Development of Train Location Detection Methods for 20 Signaling", Railway Technical Research Institute (RTRI) Report, Vol. 27, No. 9, September 2013 Summary Technical Problem 25 [0005] However, when the distributed antenna base station described above measures the distance to the mobile station using the conventional method of distance measurement mentioned above, there is a problem in that measurement cannot be performed accurately. When the 30 distributed antenna base station transmits signals to a mobile station using a plurality of antennas, an amount of transmission delay between the base station and each of the antennas is adjusted such that each of the signals transmitted from the plurality of antennas reaches the mobile station substantially simultaneously. Here, "substantially simultaneously" indicates reaching within the range of the difference in maximum delay of a multipath 5 allowed by the mobile station. For example, when orthogonal frequency division multiplexing (OFDM) is used as a communication method, the amounts of transmission delay are adjusted such that the signals arrive within the range of guard interval time. 10 [0006] In such a case, in the mobile station, a shift in the difference in maximum delay occurs between the timing of the signal received at the earliest (preceding wave timing) and the timing of the signal received at the latest (delay wave timing). In the case where distance 15 measurement is performed at the preceding wave timing and in the case where distance measurement is performed at the delay wave timing, a difference expressed as a difference in maximum delay x a light speed is generated in a result of the distance measurement, whereby accurate measurement 20 is difficult. For example, when the difference in maximum delay is 10 microseconds, the difference in the distance measurement result is about 3,000 m at a maximum. [0007] The present invention has been made in view of the above, and an object of the present invention is to 25 provide a distributed antenna base station and a mobile station capable of configuring a distributed radio antenna system in which a distance to a mobile station can be more accurately measured. 30 Solution to Problem [0008] A distributed antenna base station according to the present invention, to which a plurality of distributed antennas installed along a traveling route of a mobile station is connected, which communicates with the mobile station in a communicable range formed by the plurality of antennas, includes: a signal transmitter that generates a signal indicating execution of distance measurement and 5 transmits the signal to the plurality of antennas; a combiner that adds a plurality of received signals that are radio signals including a response signal to the signal indicating execution of distance measurement from the mobile station and each of which is correspondingly 10 received by each of the antennas; a timing detector that detects a timing at which reception power for a signal added by the combiner is maximized and determines a reception timing of the response signal; a base station time calculator that determines a round trip time from 15 transmission of the signal indicating execution of distance measurement to reception of the response signal on the basis of the reception timing and a timing of the transmission of the signal indicating execution of distance measurement; a power detector that identifies one of the 20 antennas that has received a received signal with highest reception power among the plurality of received signals; and a distance calculator that calculates a distance between the distributed antenna base station and the mobile station on the basis of the round trip time, a delay in 25 signal transmission to the antenna identified by the power detector, and a distance between the distributed antenna base station and the identified antenna. A mobile station according to the present invention to be used in a distributed antenna radio system in which, in 30 a communicable range formed by a plurality of distributed antennas installed along a traveling route of the mobile station, a base station connected to the plurality of antennas communicates with the mobile station, includes: a timing detector that detects a timing at which reception power for received signals that are obtained by receiving signals, which are transmission signals from the base station including a signal indicating execution of distance 5 measurement and wirelessly transmitted by the plurality of antennas, is maximized and that detects reception of the signal indicating execution of distance measurement; and a signal transmitter that transmits, upon detection of the reception of the signal indicating execution of distance 10 measurement, a response signal in response to the detected signal. A method of distance measurement according to the present invention for measuring, in a distributed antenna radio system in which a distributed antenna base station 15 communicates with a mobile station in a communicable range formed by a plurality of distributed antennas installed along a traveling route of the mobile station, a distance between the distributed antenna base station and the mobile station, by the distributed antenna base station being 20 connected to the plurality of antennas, the method including: a step of generating a signal indicating execution of distance measurement and transmitting the signal to the plurality of antennas; a step of receiving a plurality of received signals that are radio signals 25 including a response signal to the signal indicating execution of distance measurement from the mobile station and each of which is correspondingly received by each of the antennas; a step of adding the plurality of received signals; a step of detecting a timing at which reception 30 power is maximized from a signal obtained by adding the plurality of received signals; a step of determining a round trip time from transmission of the signal indicating execution of distance measurement to reception of the response signal on the basis of the timing at which the reception power is maximized and a timing of the transmission of the signal indicating execution of distance measurement; a step of identifying one of the antennas that 5 has received a received signal with highest reception power among the plurality of received signals; and a step of calculating a distance between the distributed antenna base station and the mobile station on the basis of the round trip time, a delay in signal transmission to the identified 10 antenna, and a distance between the distributed antenna base station and the identified antenna. Advantageous Effects of Invention [0009] According to the present invention, the 15 distributed antenna base station can more accurately measure the distance to the mobile station. Brief Description of Drawings [0010] FIG. 1 is a block diagram illustrating an 20 exemplary configuration of a distributed antenna radio system according to a first embodiment of the present invention. FIG. 2 is a block diagram illustrating an exemplary functional configuration of a base station according to the 25 first embodiment of the present invention. FIG. 3 is a block diagram illustrating an exemplary functional configuration of a mobile station according to the first embodiment of the present invention. FIG. 4 is a block diagram illustrating an exemplary 30 hardware configuration of the base station according to the first embodiment of the present invention. FIG. 5 is a flowchart illustrating an exemplary procedure of distance measurement between the base station and the mobile station in the distributed antenna radio system according to the first embodiment of the present invention. 5 Description of Embodiments [0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited by this embodiment. The same or corresponding 10 portions are denoted by the same reference signs in the drawings referred to in the following descriptions. [0012] First Embodiment FIG. 1 is a block diagram illustrating a configuration of a radio system 6 to which a base station (distributed 15 antenna base station) according to a first embodiment of the present invention is applied. The radio system 6 is a distributed antenna radio system in which N (N is an integer of two or more) antennas 300i to 300N are connected to a base station 100, and a mobile station 400 20 communicates with the base station 100. Each of the antennas 300i to 300N is connected to the base station 100 via each of N signal lines 200i to 200N. In the following descriptions, in the case where individual pieces of the N antennas 300i to 300N do not need to be distinguished, they 25 may be indicated as an antenna 300 in some cases. The same will be applied to a signal line 200. [0013] Each antenna 300 exchanges radio signals with an antenna 403 included in the mobile station 400. The frequency of the radio signals transmitted by each antenna 30 300 to the antenna 403 is the same. A traveling path of the mobile station 400 is defined, and the antenna 300 is installed along the traveling path of the mobile station 400. A communicable range (cell) of the base station 100 is formed in a shape along the traveling path of the mobile station 400 according to the communicable range of each antenna 300. The antenna 300 may be an antenna having no directivity, or may be a directional antenna forming 5 directivity along the traveling path of the mobile station 400. [0014] The signal line 200 may be any medium, such as a coaxial cable and an optical fiber, as long as it is a wired cable capable of transmitting a signal. The signal 10 transmitted through the signal line 200 may be an analog signal or a digital signal. It is assumed that a delay in signal transmission between the base station 100 and each antenna 300 is adjusted such that the wirelessly transmitted signal reaches the antenna 403 of the mobile 15 station 400 simultaneously. Here, "simultaneously" indicates that the difference in arrival time of the radio signal to the antenna is within the range of the difference in maximum delay in which the base station 100 and the mobile station 400 can be equalized. 20 [0015] FIG. 2 is a block diagram illustrating an exemplary functional configuration of the base station 100 according to the first embodiment of the present invention. Note that the configuration used for normal radio communication with the mobile station 400 (configuration of 25 digital-analog conversion, frequency conversion, etc.), which is not related to distance measurement, is not illustrated in FIG. 1 to simplify description. The base station 100 includes a signal transmitter 101, a combiner 102, a power detector 103, a timing detector 104, a base 30 station time calculator 105, a demodulator 106, a delay amount table 108, a location table 109, and a distance calculator 110. [0016] In FIG. 2, each antenna 300 wirelessly transmits a signal output from the base station 100, and receives a radio signal from the mobile station 400, which is a counterpart communication device, to input it to the base station 100. If a value "n" is an integer of one or more 5 and N or less, each antenna 300n is connected to the base station 100 using a signal line 200n via a delay device 201n for adjusting an amount of transmission delay of a signal to be transmitted. Note that it can be configured without using a delay device 201 by adjusting the amount of 10 delay of the transmitted signal by changing a line length of the signal line 200. FIG. 1 illustrates a system configuration without using the delay devices 201, and the configuration without including the delay devices is assumed in the following descriptions. 15 [0017] The signal transmitter 101 generates a ranging signal indicating execution of distance measurement according to a determined distance measurement timing, modulates the signal into a signal for radio transmission, and outputs it. Here, the ranging signal is a signal 20 including a known signal (e.g., bit pattern), and the known signal may be a predetermined signal indicating that it is a ranging signal, may be a reference signal for equalization (pilot signal), or may be a unique word for slot detection. The modulated ranging signal output from 25 the signal transmitter 101 is subject to processing (not illustrated) necessary for transmitting radio signals, such as digital-analog conversion and frequency conversion, and then input to each antenna 300 via the signal line path 200. Note that the ranging signals input to antennas 300 are all 30 the same signals. [0018] The signal transmitter 101 outputs the timing (time) at which the ranging signal is output, to the base station time calculator 105 as a transmission timing. In the present embodiment, the ranging signal is transmitted via a dedicated transmission slot. Note that, the ranging signal may be used as a ranging signal by adding, to a part of other transmission data, an index indicating that it is 5 a ranging signal, or all transmission signals may be treated as ranging signals. In the former case, the signal transmitter 101 performs processing of adding an index indicating that it is a ranging signal to a part of transmission data, and outputs the timing at which the 10 processing is performed as a transmission timing. In the latter case, the signal transmitter 101 only needs to output the transmission timing. Here, the index indicating that it is a ranging signal can be obtained by setting one to a specific bit, for example. 15 [0019] When each antenna 300 transmits the ranging signal output from the base station 100 and the mobile station 400 receives the ranging signal, the mobile station 400 wirelessly transmits a response signal, as described later. The response signal includes a predetermined index 20 indicating that it is a response to the ranging signal. Then, each antenna 300 receives the signal wirelessly transmitted by the mobile station 400, and inputs the received signal to the base station 100 via the signal line 200. 25 [0020] The combiner 102 adds a plurality of received signals input from the antennas 300 to the base station via the signal lines 200, and outputs the added received signals to the timing detector 104 and the demodulator 106. Note that, in the addition processing performed by the 30 combiner 104, the received signal to be added may be an analog signal, or may be a digital signal having been subject to the analog-digital conversion (not illustrated). When the addition processing is performed in the state of the analog signal, the analog-digital conversion may be performed on the signal after the addition. Further, the received signal to be subject to the addition processing by the combiner 102 may be limited to a signal in which power 5 of the received signal is equal to or more than a predetermined threshold value. [0021] The timing detector 104 detects a timing (time) at which the reception power becomes maximum based on the received signal after the addition that is input from the 10 combiner 102. This timing detection processing may be performed using a known method such as a method in which a replica (known signal replica) of a known signal included in the received signal is held in the timing detector 104, and cross correlation computing between the received signal 15 after the addition and the replica of the known signal is performed to detect a correlative power peak. According to this processing, it becomes possible to detect the timing at which the base station 100 receives the response signal received by the antenna 300 corresponding to the cell in 20 which the mobile station is located. The timing detector 104 outputs the timing at which the power of the detected received signal becomes maximum to the base station time calculator 105 as a reception timing (base station reception timing). 25 [0022] The base station time calculator 105 subtracts the time indicated by the transmission timing of the ranging signal input from the signal transmitter 101 from the time indicated by the reception timing input from the timing detector 104, thereby calculating the base station 30 time TA, which is the round trip time from the transmission of the ranging signal transmitted by the base station 100 to the reception of the response signal transmitted by the mobile station 400. The base station time calculator 105 outputs the base station time TA to the distance calculator 110. The demodulator 106 demodulates the received signal after the addition input from the combiner 102, and outputs it to the distance calculator 110. Note that the received 5 signal includes mobile station time TB, which will be described later. [0023] The power detector 103 measures the reception power for each of the N received signals received by the antennas 300, and identifies the antenna that has received 10 the received signal with the maximum reception power. Then, an antenna identifier k indicating the antenna is output to the distance calculator 110. Here, it is assumed that N antennas are numbered from 1 to N, and the number given to the identified antenna is output as the antenna identifier. 15 Note that the antenna identifier is not necessarily numeric, and may be another kind of information as long as each antenna 300 can be identified. Measurement of the reception power performed by the power detector 103 may be performed on an analog signal, or may be performed on a 20 digital signal. The measurement of the reception power may be performed by each antenna 300, and a result of the measurement may be input to the power detector 103 via the signal line 200, whereby the power detector 103 identifies the antenna 300 that has received the signal with the 25 highest power. The measurement of the reception power may be performed using a publicly known method. [0024] In the following descriptions, the antenna 300 corresponding to the antenna identifier k is referred to as an antenna 300k. The delay amount table 108 is a table 30 that holds a time required for signal transmission between the base station 100 and each antenna 300. This table can be prepared in advance by performing measurement at the time of installing the base station 100 and each antenna 300. Note that propagation time of a signal of each signal line 200 may be periodically measured to update the delay amount table 108. The distance calculator 110 searches the delay amount table 108 using the antenna identifier k 5 output from the power detector 103 as a search key, thereby obtaining the transmission time between the base station 100 and the antenna 300k. [0025] Meanwhile, the location table 109 is a table that holds a distance between the base station 100 and each 10 antenna 300. Here, the distance between the base station 100 and each antenna 300 is assumed as a length (mobile station path length) of traveling path of the mobile station in the case where the mobile station 400 communicating with the base station 100 travels from the 15 location of the base station 100 to the location of the antenna 300. For example, in the case where the mobile station 400 is a train, the distance held in the location table 109 is the difference in kilometrage between the base station 100 and the antenna 300. This table can be 20 prepared in advance by performing measurement at the time of installing the base station 100 and each antenna 300. The distance calculator 110 searches the location table 109 using the antenna identifier k output from the power detector 103 as a search key, thereby obtaining a distance 25 RDk between the antenna 300k and the base station 100. [0026] The distance calculator 110 calculates, using a formula (1) set out below, Rl corresponding to a linear distance between the mobile station 400 and the antenna 300k from a light speed (denoted by c), the base station 30 time TA input from the base station time calculator 105, the mobile station time TB included in the response signal from the mobile station 400 input from the demodulator 106, and the transmission time (assumed to be TCk) between the base station and the antenna 300k obtained from the delay amount table 108. [0027] [Formula 1] Rl = {TA - (2 x TCk) + TB}x c + 2 ^ (1) 5 [0028] Note that the calculation of Rl may be performed using the following formula (2) in consideration of a delay amount TRF in the antenna 300k. [0029] [Formula 2] Rl = {TA - (2 x TCk) + TB + 2 x TRF} x c + 2 ^ (2) 10 [0030] In a case where a mobile station traveling path included in a communication area of a certain antenna 300 is a curve, the distance calculator 110 may hold a conversion table from which a traveling path length between the mobile station 400 and the antenna 300 can be obtained 15 from a linear distance between the mobile station 400 and the antenna 300, and may update Rl using the traveling path length between the mobile station and the antenna 300 obtained by referring to the conversion table on the basis of Rl calculated according to the formulae (1) and (2). 20 Note that this conversion table can also be obtained and held in advance in a similar manner to the location table and the like. [0031] As will be described later, when the mobile station 400 and the base station share a fixed mobile 25 station time TB and the response signal transmitted from mobile station 400 does not include TB, the base station 100 performs a calculation of the formula (1) or the formula (2) using the mobile station time TB that is shared with the mobile station 400. Although the accuracy of the 30 distance measurement is lowered, it is also conceivable to simplify the process by assuming that the mobile station time TB is zero. [0032] The distance calculator 110 calculates, using the following formula (3), a distance R2 between the base station 100 and the mobile station 400 from the calculated Rl and the RDk that is obtained from the location table 109. 5 [0033] [Formula 3] R2 = Rl + RDk (3) [0034] FIG. 3 is a block diagram illustrating an exemplary functional configuration of the mobile station 400 according to the first embodiment of the present 10 invention. Note that only functional blocks specific to the present invention are illustrated in FIG. 3, and the radio communication functions of a normal mobile terminal is not illustrated in a similar manner to FIG. 2. The mobile station 400 includes a timing detector 401, a signal 15 transmitter 402, and the antenna 403. [0035] The antenna 403 receives a ranging signal wirelessly transmitted from the base station 100. Note that the signal received by the antenna 403 is a signal transmitted from one or more antennas among the plurality 20 of antennas 300i to 300N connected to the base station 100, which is a mixed wave in which a preceding wave and a delay wave are mixed. The received signal received by the antenna 403 is subject to processing of receiving radio signals such as frequency conversion (not illustrated) and 25 analog-digital conversion (not illustrated), and then input to the timing detector 401. In a similar manner to the timing detector 104 included in the base station 100, the timing detector 401 outputs, to the signal transmitter 402, the timing (time) at which the reception power becomes the 30 highest as a reception timing of a ranging signal on the basis of cross correlation computing on a replica of a known signal included in the ranging signal. [0036] When the notification of the reception timing of the ranging signal is received, the signal transmitter 402 generates and outputs a response signal. In a similar manner to the ranging signal, the response signal includes 5 an index indicating that the signal is a response signal. Here, when outputting the response signal, the signal transmitter 402 sets, in a predetermined area of the response signal to be transmitted, a difference between the time at which the signal is output and the time indicated 10 by the reception timing input from the timing detector 401 as a mobile station time TB, and transmits the response signal. The mobile station time TB is a required time (difference in timing) from when the ranging signal is received by the mobile station 400 until when the response 15 signal is transmitted. [0037] Note that TB does not need to indicate time itself. For example, in a case where the difference in timing is Td, the difference in timing may be quantized and transmitted as a coded value, such as TB=0 in the case of 20 0