Patent Publication Number: US-2018054263-A1

Title: Radio apparatus and detection method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-161639, filed on Aug. 22, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein relate to a radio apparatus and a detection method. 
     BACKGROUND 
     In the field of mobile networks, there is discussion on a mechanism of a new mobile network (a heterogeneous network) that improves the throughput by arranging small cells inside a macro cell covering a wide area. However, since many small cells are arranged in a macro cell in a heterogeneous network, inter-cell interference is easily caused between small cells. For this reason, beamforming has recently attracted attention, as a technique for effectively avoiding the inter-cell interference. 
     Beamforming is a technique for controlling the directivity of a beam by using a plurality of antennas so that the power is oriented in a certain direction. The directivity of a beam is controllable by adjusting the amplitude and the phase of a radio wave outputted from an individual antenna and orienting the direction in which the radio waves reinforce each other to a certain direction. In addition, by adjusting these amplitudes and the phases, the direction (NULL) in which the radio waves cancel each other is also controllable. Thus, by orienting the direction (NULL) to users in neighboring cells, the inter-cell interference is effectively avoided. 
     However, if an error is caused in the amplitude or phase adjustment, the beam or the direction NULL is shifted from its desired direction. Thus, a radio apparatus performing beamforming is adjusted in advance so that the amplitudes and the phases of the signals transmitted from the individual antennas are accurately controlled. 
     A signal outputted from a transmitting circuit (a TX circuit) is inputted to an antenna through a signal path connected to the antenna and is next outputted to the air via the antenna. In contrast, a signal received via an antenna is inputted to a receiving circuit (an RX circuit) through a signal path connected to the antenna. The amplitude and phase of a signal change along an aerial propagation channel, through which the signal propagates, and a signal path including an antenna. Thus, the shift amounts of the amplitude and the phase could change along the signal path as the signal path changes over time, for example. Namely, errors could be caused in the signal amplitude and phase afterward. 
     If such an error is caused, the beam or the direction NULL could be shifted. In addition, such an error could spread the side lobes and reduce the inter-cell interference prevention effect. However, there have been proposed methods of correcting errors in the amplitudes and phases in a radio communication system in which signals are transmitted by using a plurality of antennas. 
     One of the proposed methods is a method operating in a communications station for calibrating the communications station, the communications station including an antenna array of antenna elements included in a transmit apparatus chain and a receiver apparatus chain. In this method, the transmit apparatus chain associated with a certain antenna element transmits a signal, and the receiver apparatus chain not associated with the antenna element receives the signal. The communications station is calibrated by determining calibration factors of the individual antenna elements on the basis of the transfer functions associated with the respective transmit and receiver apparatus chains. 
     There has also been proposed a method in which wireless communication is performed by using an antenna for wireless communication and calibration processing is performed by using an antenna for calibration processing. In this method, these antennas are switched. 
     In addition, there has been proposed a multiple-input and multiple-output (MIMO) communication system in which transmit and receive chains are calibrated in advance by using a combination of a single receiver unit and N transmitter units and a combination of a single transmitter unit and N receiver units. 
     In addition, there has been proposed a radio communication device that detects time periods other than a downlink signal transmission time period as candidate time periods and starts calibration in a time period selected from the candidate time periods. In addition, there has been proposed a radio communication device based on a time division duplex (TDD) method. This communication device includes a plurality of antennas and realizes calibration by causing these antennas to transmit and receive a test signal among them. This communication device includes amplifiers and attenuators whose attenuation rate is variable. When attenuators attenuate a test signal, the communication device controls the attenuation rates so that the attenuated test signal is used as a usable power signal. See, for example, the following documents. 
     Japanese National Publication of International Patent Application No. 2002-530998 
     Japanese Laid-open Patent Publication No. 2005-130323 
     Japanese National Publication of International Patent Application No. 2007-531467 
     Japanese Laid-open Patent Publication No. 2010-041269 
     International Publication Pamphlet No. WO00/008777 
     Japanese Laid-open Patent Publication No. 2009-182441 
     According to the above methods, an error caused in a signal path (a transmission path) connected to a TX circuit and an error caused in a signal path (a reception path) connected to an RX circuit are not distinguished from each other. If it is possible to detect an error caused in the transmission path and reflect the error on the control on the amplitude and the phase in the TX circuit, the error could be reduced more effectively. Likewise, if it is possible to reflect an error caused in the reception path on the control on the amplitude and the phase in the RX circuit, the error could be reduced more effectively. 
     SUMMARY 
     According to one aspect, there is provided a radio apparatus including: a radio transceiver configured to transmit and receive a signal via a plurality of antennas; and a processor configured to control the radio transceiver so that a signal transmitted by a first antenna among the plurality of antennas is received by second and third antennas neighboring the first antenna and detect at least one of difference between amplitudes of signals received by the respective second and third antennas and difference between phases of the signals received by the respective second and third antennas. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of a radio apparatus according to a first embodiment; 
         FIG. 2  illustrates an example of a radio apparatus according to a second embodiment; 
         FIG. 3  illustrates an example of functions of an FPGA according to the second embodiment; 
         FIG. 4  illustrates an amplitude and phase correction method performed when the number of antennas is an odd number; 
         FIG. 5  illustrates an RX correction method according to the second embodiment (when the number of antennas is an odd number); 
         FIG. 6  is a flowchart illustrating RX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an odd number); 
         FIG. 7  illustrates a TX correction method according to the second embodiment (when the number of antennas is an odd number); 
         FIG. 8  is a flowchart illustrating a TX correction method performed by the radio apparatus according to the second embodiment (when the number of antennas is an odd number); 
         FIG. 9  illustrates an amplitude and phase correction method performed when the number of antennas is an even number; 
         FIG. 10  illustrates antenna groups and inter-group correction; 
         FIGS. 11 to 13  are flowcharts illustrating RX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an even number); 
         FIGS. 14 to 16  are flowcharts illustrating TX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an even number); 
         FIG. 17  illustrates comparison between radiation patterns obtained before and after amplitude and phase errors in RF circuits are corrected; 
         FIG. 18  illustrates correction timing control according to the second embodiment; and 
         FIG. 19  illustrates power control performed when the amplitude and phase correction according to the second embodiment is performed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. In the description and drawings, when elements have substantially the same function, these elements will be denoted by the same reference character, and redundant description thereof will be omitted as needed. 
     1. First Embodiment 
     A first embodiment will be described with reference to  FIG. 1 . The first embodiment relates to a radio apparatus including a plurality of antennas and to a method of detecting amplitude and phase errors caused in a reception path of an individual antenna.  FIG. 1  illustrates an example of a radio apparatus according to the first embodiment. This radio apparatus  10  illustrated in  FIG. 1  is an example of the radio apparatus according to the first embodiment. 
     As illustrated in  FIG. 1 , the radio apparatus  10  includes antennas  11  to  13 , a radio unit  14 , a control unit  15 , and a storage unit  16 . A radio transceiver is an example of the radio unit  14 . The radio transceiver may be referred to as a radio transceiver circuit, a radio transmitting/receiving circuit, radio transmitting/receiving circuitry, radio communication circuitry, radio circuitry, etc. 
     The radio unit  14  includes a plurality of RF (radio frequency) circuits that are connected to antennas  11  to  13 , respectively. An individual RF circuit includes a transmitting circuit (TX circuit) transmitting a signal and a receiving circuit (RX circuit; a reference character R in  FIG. 1 ) receiving a signal. An individual TX circuit controls the amplitude and phase of a signal, and an individual RX circuit detects the amplitude and phase of a signal. Hereinafter, as needed, a signal path between a TX circuit and an antenna will be referred to as a transmission path, and a signal path between an antenna and an RX circuit will be referred to as a reception path. 
     The control unit  15  is a processor such as a central processing unit (CPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA). The storage unit  16  is a volatile storage device such as a random access memory (RAM) or a non-volatile storage device such as a hard disk drive (HDD) or a flash memory. 
     The radio unit  14  transmits and receives a signal Sig via the antennas  11  to  13 . The control unit  15  controls the radio unit  14  so that the signal Sig transmitted from a first antenna, which is one of the antennas  11  to  13 , is received by second and third antennas neighboring the first antenna. 
     In the example in  FIG. 1 , the antennas  11  to  13  are arranged in this order at regular intervals. For example, the control unit  15  selects an antenna  11  as the first antenna (TX), as illustrated in (A) of  FIG. 1 . In addition, the control unit  15  selects the antennas  12  and  13  neighboring the antenna  11  as the second antenna (RX) and the third antenna (RX). Next, the control unit  15  causes the radio unit  14  to transmit a signal having an amplitude A 1  and a phase P 1  via the antenna  11  and to receive this signal via the antennas  12  and  13 . 
     Since the distance between the antennas  11  and  12  is the same as the distance between the antennas  11  and  13 , the shift amounts of the amplitude and phase caused between the antennas  11  and  12  are approximately equal to the shift amounts of the amplitude and phase caused between the antennas  11  and  13 . In addition, since both the antennas  12  and  13  receive the same signal transmitted from the antenna  11 , the shift amounts of the amplitude and phase caused in the transmission path are the same between the signals received by the antennas  12  and  13 . 
     Thus, the difference between the amplitude and phase of a signal received by the antenna  12  and the amplitude and phase of a signal received by the antenna  13  corresponds to the difference between the shift amounts of the amplitude and phase caused in the reception path of the antenna  12  and the shift amounts of the amplitude and phase caused in the reception path of the antenna  13 . 
     The control unit  15  detects the difference between the amplitude and phase of a signal received by the radio unit  14  via the antenna  12  and the amplitude and phase of a signal received by the radio unit  14  via the antenna  13 . When an error in the amplitude or phase is negligible, the control unit  15  may detect the difference in the corresponding one of the amplitude and phase. The control unit  15  stores information about the detected difference in the storage unit  16 . 
     Assuming that the amplitudes of signals detected by the RX circuits connected to the antennas  12  and  13  are denoted by A 2  and A 3 , respectively, in the case of the example in (A) of  FIG. 1 , a value represented by the difference dA 23  (dA 23 =A 2 −A 3 ) is stored in the storage unit  16 . Likewise, assuming that the phases corresponding to the antennas  12  and  13  are denoted by P 2  and P 3 , respectively, in the case of the example in (A) of  FIG. 1 , a value represented by the difference dP 23  (dP 23 =P 2 −P 3 ) is stored in the storage unit  16 . 
     In the example in (B) of  FIG. 1 , the antenna  12  is used as the transmitting antenna (TX), and the antennas  11  and  13  are used as the receiving antennas (RX). Assuming that the amplitude of a signal detected by the RX circuit connected to the antenna  11  is denoted by A 1 , in the case of the example in (B) of  FIG. 1 , a value represented by the difference dA 13  (dA 13 =A 1 −A 3 ) is stored in the storage unit  16 . Likewise, assuming that the phase corresponding to the antenna  11  is denoted by P 1 , in the case of the example in (B) of  FIG. 1 , a value represented by the difference dP 13  (dP 13 =P 1 −P 3 ) is stored in the storage unit  16 . 
     The control unit  15  refers to the storage unit  16 , adds the difference dA 23  to the amplitude A 2  detected by the RX circuit connected to the antenna  12 , and adds the difference dP 23  to the phase P 2  (see (C) of  FIG. 1 ). Through this processing, the error caused by the reception paths of the antennas  12  and  13  is corrected. In addition, the control unit  15  refers to the storage unit  16 , adds the difference dA 13  to the amplitude A 3  detected by the RX circuit connected to the antenna  13 , and adds the difference dP 13  to the phase P 3  (see (C) of  FIG. 1 ). Through this processing, the error caused by the reception paths of the antennas  11  and  13  is corrected. 
     In  FIG. 1 , A 2m  and A 3m  denote corrected amplitudes and P 2m  and P 3m  denote corrected phases. Through the above correction, when a signal having the same amplitude and the same phase is transmitted from a transmission point equally distanced from the antennas  11  to  13 , the RX circuits connected to the antennas  11  to  13  receive the same amplitude and phase corrected. 
     As described above, the radio apparatus  10  is able to easily correct amplitude and phase errors caused by, for example, reception paths changed over time, without using any external antenna (an antenna arranged outside the radio apparatus  10 ). In addition, the radio apparatus  10  distinguishes a transmission path and a reception path from each other, detects an error caused in the reception path, and reflects the detected error on RX correction. As a result, the error caused in the reception path is effectively corrected. 
     The first embodiment has thus been described. 
     2. Second Embodiment 
     Next, a second embodiment will be described. The second embodiment relates to a radio apparatus including a plurality of antennas and to a method of correcting amplitude and phase errors caused in a reception path of an individual antenna. 
     [2-1. Radio Apparatus] 
     First, as an example of a radio apparatus according to a second embodiment, a radio apparatus  100  will be described with reference to  FIG. 2 .  FIG. 2  illustrates an example of a radio apparatus according to the second embodiment. 
     As illustrated in  FIG. 2 , for example, the radio apparatus  100  is a base station apparatus such as a remote radio head (RRH) and is connected to a control apparatus  50  such as a centralized base band unit (C-BBU). The C-BBU is an example of a control apparatus that controls a plurality of RRHs in a centralized manner. 
     The radio apparatus  100  includes antennas  101   a  to  101   h , RF circuits  102   a  to  102   h , and an FPGA  103 . While the radio apparatus  100  in the example in  FIG. 2  includes eight antennas for convenience of the description, the number of antennas of the radio apparatus  100  may be any number equal to 3 or more. The antennas  101   a  to  101   h  are connected to the respective RF circuits  102   a  to  102   h . Each of the RF circuits  102   a  to  102   h  includes the same elements. 
     (A) of  FIG. 2  illustrates main elements of the RF circuit  102   h . For example, the RF circuit  102   h  includes, as its main elements, an RX circuit that detects the amplitude and phase of a signal received by the antenna  101   h , a TX circuit that controls the phase of a signal transmitted by the antenna  101   h , and a power amplifier (PA). In addition, the RF circuit  102   h  includes a circulator that switches a reception path connecting the antenna  101   h  and the RX circuit and a transmission path connecting the antenna  101   h  and the TX circuit. 
     In addition, the RF circuit  102   h  includes a digital-to-analog converter (DAC) that converts a digital signal outputted from the FPGA  103  to the TX circuit into an analog signal. In addition, the RF circuit  102   h  includes an analog-to-digital convertor (ADC) that converts an analog signal outputted from the RX circuit to the FPGA  103  into a digital signal. In addition, the RF circuit  102   h  includes a branch path or the like for acquiring a feedback signal used for negative feedback control. 
     The radio apparatus  100  includes beamforming functions, for example. In this case, the FPGA  103  controls each of the RF circuits  102   a  to  102   h , adjusts the amplitudes and phases of signals simultaneously transmitted and received by the antennas  101   a  to  101   h , and controls the directivity of an individual beam formed by the antennas  101   a  to  101   h.    
     The transmission paths and the reception paths connected to the antennas  101   a  to  101   h  may have different lengths, depending on the design or the like. Thus, adjustment values of the RF circuits  102   a  to  102   h  are set in advance so that the amplitudes and phases of the signals outputted by the antennas  101   a  to  101   h  are appropriately adjusted by the RF circuits  102   a  to  102   h  and a beam is accurately oriented in a desired direction. However, for example, as these transmission and reception paths deteriorate over time, amplitude and phase errors could be caused. Thus, the FPGA  103  according to the second embodiment includes a function of correcting these errors. 
     As illustrated in  FIG. 3 , the FPGA  103  includes, as main elements of the above error correction function, an operation unit  131 , a RAM  132 , a phase and amplitude detector  133 , switches (SWs)  134 ,  137 ,  138 , and  139 , and finite impulse responses (FIR)  135   a  to  135   h  and  136   a  to  136   h .  FIG. 3  illustrates an example of functions of the FPGA according to the second embodiment. 
     The operation unit  131  is a circuit element that performs operation processing of the FPGA  103 . The phase and amplitude detector  133  is a circuit element that detects the phases and amplitudes of the signals received by the antennas  101   a  to  101   h . The switch  134  switches paths connected to the FIRs  136   a  to  136   h  (corresponding to the RX circuits of the RF circuits  102   a  to  102   h ). 
     The switch  137  switches a path connected to one of the FIRs  135   a  to  135   h  (corresponding to the TX circuits of the RF circuits  102   a  to  102   h ) and a path connected to one of the FIRs  136   a  to  136   h  (corresponding to the RX circuits of the RF circuits  102   a  to  102   h ). The switch  138  switches paths connected to the FIRs  135   a  to  135   h  (corresponding to the TX circuits of the RF circuits  102   a  to  102   h ). The switch  139  switches paths connected to the FIRs  136   a  to  136   h  (corresponding to the RX circuits of the RF circuits  102   a  to  102   h ). 
     The FIRs  135   a  to  135   h  and  136   a  to  136   h  are circuit elements that change characteristics of the amplitudes and phases of signals passing therethrough. Each of the FIRs  135   a  to  135   h  changes the amplitude and phase of a passing signal on the basis of a control signal inputted by the operation unit  131  via the switches  137  and  138  (on the basis of a signal specifying shift amounts of the amplitude and phase). Each of the FIRs  136   a  to  136   h  changes the amplitude and phase of a passing signal on the basis of a control signal inputted by the operation unit  131  via the switches  137  and  139  (on the basis of a signal specifying shift amounts of the amplitude and phase). 
     When performing the above correction, the operation unit  131  selects a combination of transmitting and receiving antennas. For example, when correcting an error caused in a reception path, the operation unit  131  selects a single transmitting antenna and two receiving antennas neighboring the transmitting antenna. Next, the operation unit  131  transmits a signal by controlling the RF circuit connected to the transmitting antenna selected from the RF circuits  102   a  to  102   h.    
     In addition, the operation unit  131  switches the switch  134  so that the paths of the two selected receiving antennas are connected to the phase and amplitude detector  133 . The phase and amplitude detector  133  detects the amplitudes and phases of the signals inputted via the switch  134  and stores information about the detected amplitudes and phases in the RAM  132 . 
     The operation unit  131  detects the above errors from the amplitude and phase information stored in the RAM  132  and switches the switches  137  and  139  so that an FIR connected to a reception path on which error correction is performed is connected to the operation unit  131 . Next, the operation unit  131  outputs a control signal specifying the shift amounts of the amplitude and phase that have been set to cancel out the amplitude and phase errors. When receiving the control signal outputted from the operation unit  131 , the FIR connected to the operation unit  131  shifts the amplitude and phase of the signal by the respective shift amounts specified by the control signal. 
     In contrast, when correcting an error caused in a transmission path, the operation unit  131  selects a single receiving antenna and two transmitting antennas neighboring the receiving antenna. Next, the operation unit  131  transmits the same signal by controlling the RF circuits connected to the transmitting antennas selected from the RF circuits  102   a  to  102   h.    
     In addition, the operation unit  131  switches the switch  134  so that the path of the selected receiving antenna is connected to the phase and amplitude detector  133 . The phase and amplitude detector  133  detects the amplitudes and phases of the signals inputted via the switch  134  and stores information about the detected amplitudes and phases in the RAM  132 . 
     The operation unit  131  detects the above errors from the amplitude and phase information stored in the RAM  132  and switches the switches  137  and  138  so that an FIR connected to a transmission path on which error correction is performed is connected to the operation unit  131 . Next, the operation unit  131  outputs a control signal specifying the shift amounts of the amplitude and phase that have been set to cancel out the amplitude and phase errors. When receiving the control signal outputted from the operation unit  131 , the FIR connected to the operation unit  131  shifts the amplitude and phase of the signal by the respective shift amounts specified by the control signal. 
     Next, a method (RX correction) of correcting an error caused in a reception path and a method (TX correction) of correcting an error caused in a transmission path will be described in detail. 
     [2-2. Correction Method #1: When the Number of Antennas is an Odd Number] 
     First, RX correction and TX correction when the number of antennas is an odd number will be described with reference to  FIG. 4 .  FIG. 4  illustrates an amplitude and phase correction method performed when the number of antennas is an odd number. 
     In the example in  FIG. 4 , for convenience of the description, the radio apparatus  100  includes three antennas  101   a  to  101   c . In addition, the distance between the antennas  101   a  and  101   b , the distance between the antennas  101   b  and  101   c , and the distance between the antennas  101   c  and  101   a  are the same. 
     In addition, the phase shift amounts caused in the transmission and reception paths of the antenna  101   a  will be denoted by tP a  and rP a , respectively. Likewise, the phase shift amounts caused in the transmission and reception paths of the antenna  101   b  will be denoted by tP b  and rP b , respectively. Likewise, the phase shift amounts caused in the transmission and reception paths of the antenna  101   c  will be denoted by tP c  and rP c , respectively. 
     In addition, the phase shift amount caused between neighboring antennas (between the antennas  101   a  and  101   b , between the antennas  101   b  and  101   c , and between the antennas  101   c  and  101   a ) will be denoted by P sp . In addition, the difference between tP i  and tP j  (i, j=a, b, c; i≠j) will be denoted by ΔtP ij  (ΔtP ij =tP i −tP j ). Likewise, the difference between rP i  and rP j  (i, j=a, b, c; i≠j) will be denoted by ΔrP ij  (ΔrP i =rP i −rP j ). 
     (RX Correction) 
     First, RX correction will be described with reference to  FIG. 5 .  FIG. 5  illustrates an RX correction method according to the second embodiment (when the number of antennas is an odd number). 
     In the example in  FIG. 5 , first, as illustrated in (A) of  FIG. 5 , the antenna  101   a  is selected as the transmitting antenna (TX) from the antennas  101   a  to  101   c . In addition, the antennas  101   b  and  101   c  neighboring the antenna  101   a  are selected as the receiving antennas (RX). In this case, a signal transmitted by the antenna  101   a  is received by the antennas  101   b  and  101   c.    
     Between when the signal is outputted by the TX circuit connected to the antenna  101   a  and when the signal is inputted to the RX circuit connected to the antenna  101   b , the phase of the signal changes by tP a  in the transmission path, changes by P sp  in the air, and changes by rP b  in the reception path. Namely, the phase shift amount of the signal received by the antenna  101   b  is represented by (tP a +P sp +rP b ). Likewise, the phase shift amount of the signal received by the antenna  101   c  is represented by (tP a +P sp +rP c ). 
     Thus, the difference between the phases of the signals received by the antennas  101   b  and  101   c  is represented by (rP b −rP c ). Namely, ΔrP bc  is the difference between the phase shift amounts caused in the reception paths of the antennas  101   b  and  101   c . If there is no change over time in the reception paths of the antennas  101   b  and  101   c , ΔrP bc  is 0 (a negligible level). 
     Next, when an error between the phases in the reception paths of the antennas  101   a  and  101   b  is detected, as illustrated in (B) of  FIG. 5 , the antenna  101   c  is selected as the transmitting antenna (TX) from the antennas  101   a  to  101   c . In addition, the antennas  101   b  and  101   a  neighboring the antenna  101   c  are selected as the receiving antennas (RX). In this case, a signal transmitted by the antenna  101   c  is received by the antennas  101   b  and  101   a.    
     Between when the signal is outputted by the TX circuit connected to the antenna  101   c  and when the signal is inputted to the RX circuit connected to the antenna  101   b , the phase of the signal changes by tP c  in the transmission path, changes by P sp  in the air, and changes by rP b  in the reception path. Namely, the phase shift amount of the signal received by the antenna  101   b  is represented by (tP c +P sp +rP b ). Likewise, the phase shift amount of the signal received by the antenna  101   a  is represented by (tP c +P sp +rP a ). 
     Thus, the difference between the phases of the signals received by the antennas  101   b  and  101   a  is represented by (rP b −rP a ). Namely, ΔrP ba  (−ΔrP ab ) is the difference between the phase shift amounts caused in the reception paths of the antennas  101   b  and  101   a . If there is no change over time in the reception paths of the antennas  101   b  and  101   a , ΔrP ba  is 0 (a negligible level). Likewise, by selecting a transmitting antenna and a pair of receiving antennas as illustrated in (C) of  FIG. 5 , the difference ΔrP ca  (−ΔrP ac ) between the phase shift amounts caused in the reception paths of the antennas  101   c  and  101   a  is obtained. 
     The antenna selection order is not limited to the above example. In addition, the operation unit  131  selects the antennas and calculates the difference ΔrP ij  (i, j=a, b, c; i≠j). In addition, when acquiring the difference ΔrP ij , the operation unit  131  controls the phase shift amount applied to an FIR connected to a receiving antenna so that the signal phase is shifted by the difference ΔrP ij  in the path corresponding to the receiving antenna. 
     When the number of antennas is an odd number, a pair of receiving antennas is sequentially selected in accordance with the above method, and the difference between phase shift amounts of an individual pair is detected. In this way, a phase error caused in the reception path of an individual antenna is corrected on the basis of the detected difference. The amplitude error is corrected in the same way by replacing the “phase shift amount” in the above description with “amplitude shift amount.” 
     Next, RX correction processing performed by the radio apparatus  100  will be described in more detail with reference to  FIG. 6 .  FIG. 6  is a flowchart illustrating RX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an odd number). 
     In the following example, for convenience of the description, the radio apparatus  100  includes N antennas (N is an odd number) and the n-th antenna will be referred to as ANT#n. In addition, ANT#m(m&gt;N) and ANT#(m−N) represent the same antenna. In addition, ANT#m(m&lt;1) and ANT#(m+N) represent the same antenna. In addition, ANT#1, ANT#2, . . . , ANT#N are arranged at regular intervals in the order of their indexes #1, #2, . . . , N. 
     (S 101 ) The operation unit  131  selects ANT#1 as a reference antenna and sets the index n to 1. Namely, error correction is performed so that, when a signal is transmitted from a transmission point equally distanced from ANT#1, #2, . . . , #N, the amplitudes and the phases of the signals received by ANT#2, . . . , #N will be equal to the amplitude and the phase of the signal received by ANT#1. 
     (S 102 ) The operation unit  131  selects ANT#n as the transmitting antenna (TX). 
     (S 103 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the receiving antennas (RX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the receiving antennas. 
     (S 104 ) The operation unit  131  causes the RF circuit connected to the transmitting antenna to transmit a signal via ANT#n and causes the RF circuits connected to the receiving antennas to receive the signal transmitted via ANT#n via ANT#(n−1) and ANT#(n+1). 
     (S 105 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals received by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔrP (n−1)(n+1)  between the phases of the signals received by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔrA (n−1)(n+1)  between the amplitudes in the same way. 
     The phase shift amount caused in the transmission path of ANT#k (k=1, 2, . . . , N) will be denoted by tP k , and the phase shift amount caused in the reception path of ANT#k will be denoted by rP k . In addition, the amplitude shift amount caused in the transmission path of ANT#k will be denoted by tA k , and the amplitude shift amount caused in the reception path of ANT#k will be denoted by rA k . When ANT#(k−1) and ANT#(k+1) are selected as the receiving antennas, the phase difference ΔrP (k−1)(k+1)  is represented by (rP (k−1) −rP (k+1) ), and the amplitude difference ΔrA (k−1)(k+1)  is represented by (rA (k−1) −rA (k+1) ). 
     (S 106 ) The operation unit  131  sets a phase shift amount in an FIR connected to an RX circuit connected to ANT#(n+1) so that the difference ΔrP (n−1)(n+1)  is added to the signal phase outputted from the RX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the RX circuit connected to ANT#(n+1) so that the difference ΔrA (n−1)(n+1)  is added to the signal amplitude outputted from the RX circuit. 
     (S 107 ) The operation unit  131  increments the index n by 2. 
     (S 108 ) The operation unit  131  determines whether RX correction has been performed on all the reception paths (correction on the signal amplitudes and phases outputted from all the RX circuits). For example, when all pairs of selectable receiving antennas have been selected, the operation unit  131  determines that the RX correction has been performed. When the RX correction has been performed, the operation unit  131  ends the processing in  FIG. 6 . Otherwise, the processing proceeds to S 102 . 
     The RX correction performed when the number of antennas is an odd number has thus been described. 
     (TX Correction) 
     Next, TX correction will be described with reference to  FIG. 7 .  FIG. 7  illustrates a TX correction method according to the second embodiment (when the number of antennas is an odd number). 
     In the example in  FIG. 7 , first, as illustrated in (A) of  FIG. 7 , the antenna  101   a  is selected as the receiving antenna (RX) from the antennas  101   a  to  101   c . In addition, the antennas  101   b  and  101   c  neighboring the antenna  101   a  are selected as the transmitting antennas (TX). In this case, signals transmitted by the antennas  101   b  and  101   c  are received by the antenna  101   a.    
     Between when the signal is outputted by the TX circuit connected to the antenna  101   b  and when the signal is inputted to the RX circuit connected to the antenna  101   a , the phase of the signal changes by tP b  in the transmission path, changes by P sp  in the air, and changes by rP a  in the reception path. Namely, the phase shift amount of the signal received by the antenna  101   a  is represented by (tP b +P sp +rP a ). Likewise, the phase shift amount of the signal transmitted by the antenna  101   c  is represented by (tP c +P sp +rP a ). 
     Thus, the difference between the phases of the signals transmitted by the antennas  101   b  and  101   c  is represented by (tP b −tP c ). Namely, ΔtP bc  is the difference between the phase shift amounts caused in the transmission paths of the antennas  101   b  and  101   c . If there is no change over time in the transmission paths of the antennas  101   b  and  101   c , ΔtP bc  is 0 (a negligible level). 
     Next, when an error between the phases in the transmission paths of the antennas  101   a  and  101   b  is detected, as illustrated in (B) of  FIG. 7 , the antenna  101   c  is selected as the receiving antenna (RX) from the antennas  101   a  to  101   c . In addition, the antennas  101   b  and  101   a  neighboring the antenna  101   c  are selected as the transmitting antennas (TX). In this case, signals transmitted by the antennas  101   b  and  101   a  are received by the antenna  101   c.    
     Between when the signal is outputted by the TX circuit connected to the antenna  101   b  and when the signal is inputted to the RX circuit connected to the antenna  101   c , the phase of the signal changes by tP b  in the transmission path, changes by P sp  in the air, and changes by rP c  in the reception path. Namely, the phase shift amount of the signal transmitted by the antenna  101   b  is represented by (tP b +P sp +rP c ). Likewise, the phase shift amount of the signal transmitted by the antenna  101   a  is represented by (tP a +P sp +rP c ). 
     Thus, the difference between the phases of the signals transmitted by the antennas  101   b  and  101   a  is represented by (tP b −tP a ). Namely, ΔtP ba  (−ΔtP ab ) is the difference between the phase shift amounts caused in the transmission paths of the antennas  101   b  and  101   a . If there is no change over time in the transmission paths of the antennas  101   b  and  101   a , ΔtP ba  is 0 (a negligible level). Likewise, by selecting a pair of transmitting antennas and a receiving antenna as illustrated in (C) of  FIG. 7 , the difference ΔtP ca  (−ΔtP ac ) between the phase shift amounts caused in the transmission paths of the antennas  101   c  and  101   a  is obtained. 
     The antenna selection order is not limited to the above example. In addition, the operation unit  131  selects the antennas and calculates the difference ΔtP ij  (i, j=a, b, c; i≠j). In addition, when acquiring the difference ΔtP ij , the operation unit  131  controls the phase shift amount applied to an FIR connected to a transmitting antenna so that the signal phase is shifted by the difference ΔtP ij  in the path corresponding to the transmitting antenna. 
     When the number of antennas is an odd number, a pair of transmitting antennas is sequentially selected in accordance with the above method, and the difference between phase shift amounts of an individual pair is detected. In this way, a phase error caused in the transmission path of an individual antenna is corrected on the basis of the detected difference. The amplitude error is corrected in the same way by replacing the “phase shift amount” in the above description with “amplitude shift amount.” 
     Next, TX correction processing performed by the radio apparatus  100  will be described in more detail with reference to  FIG. 8 .  FIG. 8  is a flowchart illustrating TX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an odd number). 
     In the following example, for convenience of the description, the radio apparatus  100  includes N antennas (N is an odd number) and the n-th antenna will be referred to as ANT#n. In addition, ANT#m(m&gt;N) and ANT#(m−N) represent the same antenna. In addition, ANT#m(m&lt;1) and ANT#(m+N) represent the same antenna. In addition, ANT#1, ANT#2, . . . , ANT#N are arranged at regular intervals in the order of their indexes #1, #2, . . . , N. 
     (S 111 ) The operation unit  131  selects ANT#1 as a reference antenna and sets the index n to 1. Namely, error correction is performed so that, when the same signals are transmitted to a reception point equally distanced from ANT#1, #2, . . . , #N, the amplitudes and the phases of the signals transmitted by ANT#2, . . . , #N will be equal to the amplitude and the phase of the signal transmitted by ANT#1 at the reception point. 
     (S 112 ) The operation unit  131  selects ANT#n as the receiving antenna (RX). 
     (S 113 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the transmitting antennas (TX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the transmitting antennas. 
     (S 114 ) The operation unit  131  causes the RF circuits connected to the respective transmitting antennas to transmit the same signal via ANT#(n−1) and ANT#(n+1) and causes the RF circuit connected to the receiving antenna to receive the signals transmitted via ANT#(n−1) and ANT#(n+1) via ANT#n. 
     (S 115 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals transmitted by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔtP (n−1)(n+1)  between the phases of the signals transmitted by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔtA (n−1)(n+1)  between the amplitudes in the same way. 
     The phase shift amount caused in the transmission path of ANT#k (k=1, 2, . . . , N) will be denoted by tP k , and the phase shift amount caused in the reception path of ANT#k will be denoted by rP k . In addition, the amplitude shift amount caused in the transmission path of ANT#k will be denoted by tA k , and the amplitude shift amount caused in the reception path of ANT#k will be denoted by rA k . When ANT#(k−1) and ANT#(k+1) are selected as the transmitting antennas, the phase difference ΔtP (k−1)(k+1)  is represented by (tP (k−1) −tP (k+1) ), and the amplitude difference ΔtA (k−1)(k+1)  is represented by (tA (k−1) −tA (k+1) ). 
     (S 116 ) The operation unit  131  sets a phase shift amount in an FIR connected to a TX circuit connected to ANT#(n+1) so that the difference ΔtP (n−1)(n+1)  is added to the signal phase outputted from the TX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the TX circuit connected to ANT#(n+1) so that the difference ΔtA (n−1)(n+1)  is added to the signal amplitude outputted from the TX circuit. 
     (S 117 ) The operation unit  131  increments the index n by 2. 
     (S 118 ) The operation unit  131  determines whether TX correction has been performed on all the transmission paths (correction on the signal amplitudes and phases outputted from all the TX circuits). For example, when all pairs of selectable transmitting antennas have been selected, the operation unit  131  determines that the TX correction has been performed. When the TX correction has been performed, the operation unit  131  ends the processing in  FIG. 8 . Otherwise, the processing proceeds to S 112 . 
     The TX correction performed when the number of antennas is an odd number has thus been described. 
     [2-3. Correction Method #2: When the Number of Antennas is an Even Number] 
     Next, RX and TX correction performed when the number of antennas is an even number will be described with reference to  FIG. 9 .  FIG. 9  illustrates an amplitude and phase correction method performed when the number of antennas is an even number. 
     In the example in  FIG. 9 , for convenience of the description, the radio apparatus  100  includes four antennas  101   a  to  101   d . In addition, the distance between the antennas  101   a  and  101   b , the distance between the antennas  101   b  and  101   c , the distance between the antennas  101   c  and  101   d , and the distance between the antennas  101   d  and  101   a  are the same. 
     In addition, the phase shift amounts caused in the transmission and reception paths of the antenna  101   a  will be denoted by tP a  and rP a , respectively. Likewise, the phase shift amounts caused in the transmission and reception paths of the antenna  101   b  will be denoted by tP b  and rP b , respectively. Likewise, the phase shift amounts caused in the transmission and reception paths of the antenna  101   c  will be denoted by tP c  and rP c , respectively. Likewise, the phase shift amounts caused in the transmission and reception paths of the antenna  101   d  will be denoted by tP d  and rP d , respectively. 
     In addition, the phase shift amount caused between neighboring antennas (between the antennas  101   a  and  101   b , between the antennas  101   b  and  101   c , between the antennas  101   c  and  101   d , and between the antennas  101   d  and  101   a ) will be denoted by P sp . In addition, the difference between tP i  and tP j  (i, j=a, b, c, d; i≠j) will be denoted by ΔtP ij  (ΔtP ij =tP i −tP j ). Likewise, the difference between rP i  and rP j  (i, j=a, b, c, d; i≠j) will be denoted by ΔrP ij  (ΔrP ij =rP i −rP j ). 
     When the number of antennas is an odd number, the operation unit  131  selects a single transmitting antenna and two receiving antennas neighboring the transmitting antenna while sequentially changing the transmitting antenna. In this way, the operation unit  131  selects an individual pair of receiving antennas used to correct errors in the respective reception paths. However, when the number of antennas is an even number, the above selection method produces two antenna groups on which error correction has separately been performed. Namely, error correction between the antenna groups has not been performed. 
     For example, in the case of RX correction on the phase in the example of  FIG. 9 , when the antenna  101   b  is selected as the transmitting antenna, the antennas  101   a  and  101   c  are selected as the receiving antennas. With this selection, the error ΔrP ac  between the antennas  101   a  and  101   c  is corrected. When the antenna  101   c  is selected as the transmitting antenna, the antennas  101   b  and  101   d  are selected as the receiving antennas. With this selection, the error ΔrP bd  between the antennas  101   b  and  101   d  is corrected. 
     Likewise, when the antenna  101   d  is selected as the transmitting antenna, the antennas  101   c  and  101   a  are selected as the receiving antennas. When the antenna  101   a  is selected as the transmitting antenna, the antennas  101   d  and  101   b  are selected as the receiving antennas. With these selections, the errors ΔrP ac (−ΔrP ca ) and ΔrP bd (−ΔrP db ) are corrected. 
     Namely, while the error ΔrP ac  between the antennas  101   a  and  101   c  and the error ΔrP bd  between the antennas  101   b  and  101   d  are corrected, the error ΔrP ab  between the antennas  101   a  and  101   b  is not corrected. In this case, the antennas  101   a  and  101   c  form one antenna group, and the antennas  101   a  and  101   b  form the other antenna group. Under such circumstances, when the number of antennas is an even number, the radio apparatus  100  includes a function of correcting the error between two antenna groups (inter-group correction function). 
     Next, antenna groups and inter-group correction will be described in more detail with reference to  FIG. 10 .  FIG. 10  illustrates antenna groups and inter-group correction. For convenience of the description, in  FIG. 10 , eight antennas are used, which are distinguished by their respective indexes #1, #2, . . . , #8. In addition, an antenna having an index#k (k=1, 2, . . . , 8) will be denoted by ANT#k. As illustrated in  FIG. 10 , ANT#1, ANT#2, . . . , ANT#8 are arranged at regular intervals in the order of their indexes. 
     In the case of RX correction, a single transmitting antenna and two receiving antennas neighboring the transmitting antenna are selected. For example, when ANT#1 is selected as the transmitting antenna, ANT#8 and ANT#2 are selected as the receiving antennas. Thus, the error ΔrP 28  between ANT#8 and ANT#2 is corrected. 
     Likewise, when ANT#k (k=2, . . . , 8) is selected as the transmitting antenna, ANT#(k−1) and ANT#(k+1) are selected as the receiving antennas. ANT#m(m&lt;1) and ANT#(m+8) represent the same antenna, and ANT#m(m&gt;8) and ANT#(m−8) represent the same antenna. Thus, the error ΔrP (k−1)(k+1)  between ANT#(k−1) and ANT#(k+1) is corrected. 
     One antenna group is a group of antennas having odd-numbered indexes (an odd-numbered antenna group; ANT#1, ANT#3, ANT#5, and ANT#7). By applying the above correction method, errors caused in the reception paths of the antennas in the odd-numbered antenna group are corrected. Namely, the amplitudes and the phases are accurately corrected among a group of antennas (the odd-numbered antenna group) connected by a chain line in  FIG. 10 . 
     The other antenna group is a group of antennas having even-numbered indexes (an even-numbered antenna group; ANT#2, ANT#4, ANT#6, and ANT#8). By applying the above correction method, errors caused in the reception paths of the antennas in the even-numbered antenna group are corrected. Namely, the amplitudes and the phases are accurately corrected among a group of antennas (the even-numbered antenna group) connected by a solid line in  FIG. 10 . 
     To correct the amplitude and phase errors between the odd-numbered antenna group and the even-numbered antenna group, the operation unit  131  selects a single transmitting antenna and three receiving antennas. For example, when the transmitting antenna belongs to the odd-numbered antenna group, the operation unit  131  selects two antennas equally distanced from the transmitting antenna as two of the receiving antennas from the even-numbered antenna group. In addition, the operation unit  131  selects a single antenna equally distanced from the two receiving antennas as the other receiving antenna from the odd-numbered antenna group. 
     Next, the operation unit  131  causes the RF circuit connected to the selected transmitting antenna to transmit a signal via the selected transmitting antenna and causes RF circuits connected to the selected three receiving antennas to receive the signal via the selected three receiving antennas. For example, the operation unit  131  selects ANT#1 as the transmitting antenna and selects ANT#4, ANT#5, and ANT#6 as the receiving antennas. When the phase of a signal received by the RX circuit connected to ANT#k (k=4, 5, 6) is denoted by gPr k , the phase error ΔgPr (inter-group error) between the odd-numbered antenna group and the even-numbered antenna group is represented by {(gPr 5 −gPr 4 )−(gpr 5 −gPr 6 )}. 
     Namely, by obtaining the phase error between a receiving antenna selected from the antenna group to which the transmitting antenna belongs and the receiving antennas selected from the antenna group to which the transmitting antenna does not belong, the inter-group error is obtained. In accordance with the above method, the operation unit  131  detects the inter-group error ΔgPr and shifts the signal phase received by an antenna(s) belonging to an antenna group by the inter-group error ΔgPr. For example, the operation unit  131  controls the FIRs connected to the RX circuits connected to ANT#2, #4, #6, and #8 so that the phases of the signals outputted by the RX circuits are shifted by ΔgPr. 
     The amplitude error correction is performed in the same way. When TX correction is performed, antenna groups of transmitting antennas are formed. Thus, by reading the “transmitting antenna” in the above description as “receiving antenna,” TX correction is performed in the same way. 
     (RX Correction) 
     Next, RX correction processing performed by the radio apparatus  100  when the number of antennas is an even number will be described in more detail with reference to  FIGS. 11 to 13 . 
       FIGS. 11 to 13  are flowcharts illustrating RX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an even number); 
     In the following example, for convenience of the description, the radio apparatus  100  includes N antennas (N is an even number) and the n-th antenna will be referred to as ANT#n. In addition, ANT#m(m&gt;N) and ANT#(m−N) represent the same antenna. In addition, ANT#m(m&lt;1) and ANT#(m+N) represent the same antenna. In addition, ANT#1, ANT#2, . . . , ANT#N are arranged at regular intervals in the order of their indexes #1, #2, . . . , #N. 
     (S 201 ) The operation unit  131  selects ANT#1 as a reference antenna and sets the index n to 1. 
     (S 202 ) The operation unit  131  selects ANT#n as the transmitting antenna (TX). 
     (S 203 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the receiving antennas (RX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the receiving antennas. 
     (S 204 ) The operation unit  131  causes the RF circuit connected to the transmitting antenna to transmit a signal via ANT#n and causes the RF circuits connected to the receiving antennas to receive the signal transmitted via ANT#n via ANT#(n−1) and ANT#(n+1). 
     (S 205 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals received by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔrP (n−1)(n+1)  between the phases of the signals received by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔrA (n−1)(n+1)  between the amplitudes in the same way. 
     The phase shift amount caused in the transmission path of ANT#k (k=1, 2, . . . , N) will be denoted by tP k , and the phase shift amount caused in the reception path of ANT#k will be denoted by rP k . In addition, the amplitude shift amount caused in the transmission path of ANT#k will be denoted by tA k , and the amplitude shift amount caused in the reception path of ANT#k will be denoted by rA k . When ANT#(k−1) and ANT#(k+1) are selected as the receiving antennas, the phase difference ΔrP (k−1)(k+1)  is represented by (rP (k−1) −rP (k+1) , and the amplitude difference ΔrA (k−1)(k+1)  is represented by (rA (k−1) −rA (k+1) ). 
     (S 206 ) The operation unit  131  sets a phase shift amount in an FIR connected to an RX circuit connected to ANT#(n+1) so that the difference ΔrP (n−1)(n+1)  is added to the signal phase outputted from the RX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the RX circuit connected to ANT#(n+1) so that the difference ΔrA (n−1)(n+1)  is added to the signal amplitude outputted from the RX circuit. 
     (S 207 ) The operation unit  131  increments the index n by 2. 
     (S 208 ) The operation unit  131  determines whether RX correction has been performed on half of the reception paths (correction on the signal amplitudes and phases outputted from half of the RX circuits). 
     Since the index n is set to 1 in S 201 , an antenna having an odd-numbered index is selected as the transmitting antenna in S 202 . Thus, from S 202  to S 207 , RX correction is performed on the reception paths of the antennas belonging to the even-numbered antenna group. 
     For example, when all pairs of selectable receiving antennas have been selected (n&gt;N), the operation unit  131  determines that the RX correction has been performed on half of the reception paths. When the RX correction has been performed on half of the reception paths, the processing proceeds to S 209 . Otherwise, the processing returns to S 202 . 
     (S 209 ) The operation unit  131  sets the index n to 2. Namely, the operation unit  131  performs RX correction on the reception path of an antenna belonging to the odd-numbered antenna group. 
     (S 210 ) The operation unit  131  selects ANT#n as the transmitting antenna (TX). 
     (S 211 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the receiving antennas (RX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the receiving antennas. 
     (S 212 ) The operation unit  131  causes the RF circuit connected to the transmitting antenna to transmit a signal via ANT#n and causes the RF circuits connected to the receiving antennas to receive the signal transmitted via ANT#n via ANT#(n−1) and ANT#(n+1). 
     (S 213 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals received by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔrP (n−1)(n+1)  between the phases of the signals received by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔrA (n−1)(n+1)  between the amplitudes in the same way. 
     (S 214 ) The operation unit  131  sets a phase shift amount in an FIR connected to an RX circuit connected to ANT#(n+1) so that the difference ΔrP (n−1)(n+1)  is added to the signal phase outputted from the RX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the RX circuit connected to ANT#(n+1) so that the difference ΔrA (n−1)(n+1)  is added to the signal amplitude outputted from the RX circuit. 
     (S 215 ) The operation unit  131  increments the index n by 2. 
     (S 216 ) The operation unit  131  determines whether RX correction has been performed on the other half of the reception paths (correction on the signal amplitudes and phases outputted from the other half of the RX circuits). 
     Since the index n is set to 2 in S 209 , an antenna having an even-numbered index is selected as the transmitting antenna in S 210 . Thus, from S 210  to S 215 , RX correction is performed on the reception paths of the antennas belonging to the odd-numbered antenna group. 
     For example, when all pairs of selectable receiving antennas have been selected (n&gt;N), the operation unit  131  determines that the RX correction has been performed on the other half of the reception paths. When the RX correction has been performed on the other half of the reception paths, the processing proceeds to S 217 . Otherwise, the processing returns to S 210 . 
     (S 217 ) To perform inter-group correction, the operation unit  131  selects ANT#1 as the transmitting antenna (TX). ANT#1 belongs to the odd-numbered antenna group. 
     (S 218 ) The operation unit  131  selects ANT#4, ANT#5, and ANT#6 as the receiving antennas (RX). ANT#4 and ANT#6 are a pair of antennas that belong to the even-numbered antenna group, which is different from the odd-numbered antenna group to which ANT#1 belongs, and that are equally distanced from ANT#1. ANT#5 is an antenna that belongs to the odd-numbered antenna group to which ANT#1 also belongs and is equally distanced from ANT#4 and #6. 
     (S 219 ) The operation unit  131  controls the relevant RF circuits so that a signal transmitted by ANT#1 is received by ANT#4, ANT#5, and ANT#6. The amplitude gAr 4  and phase gPr 4  received by the RX circuit connected to ANT#4, the amplitude gAr 5  and phase gPr a  received by the RX circuit connected to ANT#5, and the amplitude gAr 6  and phase gPr 6  received by the RX circuit connected to ANT#6 are detected by the phase and amplitude detector  133 . Information about the detected amplitudes and phases is stored in the RAM  132 . 
     (S 220 ) The operation unit  131  refers to the RAM  132  and acquires the information about the amplitudes and phases (gAr 4 , gPr 4 ), (gAr 5 , gPr 5 ), and (gAr 6 , gPr 6 ). Next, the operation unit  131  calculates an inter-group phase error ΔgPr and an inter-group amplitude error ΔgAr on the basis of the following expressions (1) and (2). 
       Δ gPr =( gPr   5   −gPr   4 )−( gPr   5   −gPr   6 )  (1)
 
       Δ gAr =( gAr   5   −gAr   4 )−( gAr   5   −gAr   6 )  (2)
 
     (S 221 ) The operation unit  131  controls relevant FIRs so that ΔgPr (inter-group difference) is added to the signal phases outputted by the RX circuits connected to all ANT#k (k is an odd number) belonging to the odd-numbered antenna group. In addition, the operation unit  131  controls relevant FIRs so that ΔgAr (inter-group difference) is added to the signal amplitudes outputted by the RX circuits connected to all ANT#k (k is an odd number) belonging to the odd-numbered antenna group. Namely, the operation unit  131  corrects the inter-group errors by using ΔgPr and ΔgAr. 
     After S 221 , the operation unit  131  ends the processing illustrated in  FIGS. 11 to 13 . Inter-group errors are corrected by applying the above method. Even when the number of antennas is an even number, RX correction is achieved on all the reception paths. 
     (TX Correction) 
     Next, TX correction processing performed by the radio apparatus  100  when the number of antennas is an even number will be described in detail with reference to  FIGS. 14 to 16 . 
       FIGS. 14 to 16  are flowcharts illustrating TX correction processing performed by the radio apparatus according to the second embodiment (when the number of antennas is an even number). 
     (S 231 ) The operation unit  131  selects ANT#1 as a reference antenna and sets the index n to 1. 
     (S 232 ) The operation unit  131  selects ANT#n as the receiving antenna (RX). 
     (S 233 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the transmitting antennas (TX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the transmitting antennas. 
     (S 234 ) The operation unit  131  causes the RF circuits connected to the respective transmitting antennas to transmit a signal via ANT#(n−1) and ANT#(n+1) and causes the RF circuit connected to the receiving antenna to receive the signals transmitted via ANT#(n−1) and ANT#(n+1) via ANT#n. 
     (S 235 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals transmitted by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔtP (n−1)(n+1)  between the phases of the signals transmitted by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔtA (n−1)(n+1)  between the amplitudes in the same way. 
     The phase shift amount caused in the transmission path of ANT#k (k=1, 2, . . . , N) will be denoted by tP k , and the phase shift amount caused in the reception path of ANT#k will be denoted by rP k . In addition, the amplitude shift amount caused in the transmission path of ANT#k will be denoted by tA k , and the amplitude shift amount caused in the reception path of ANT#k will be denoted by rA k . When ANT#(k−1) and ANT#(k+1) are selected as the transmitting antennas, the phase difference ΔtP (k−1)(k+1)  is represented by (tP (k−1) −tP (k+1) ), and the amplitude difference ΔtA (k−1)(k+1)  is represented by (tA (k−1) −tA (k+1) ). 
     (S 236 ) The operation unit  131  sets a phase shift amount in an FIR connected to a TX circuit connected to ANT#(n+1) so that the difference ΔtP (n−1)(n+1)  is added to the signal phase outputted from the TX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the TX circuit connected to ANT#(n+1) so that the difference ΔtA (n−1)(n+1)  is added to the signal amplitude outputted from the TX circuit. 
     (S 237 ) The operation unit  131  increments the index n by 2. 
     (S 238 ) The operation unit  131  determines whether TX correction has been performed on half of the transmission paths (correction on the signal amplitudes and phases outputted from half of the TX circuits). 
     Since the index n is set to 1 in S 231 , an antenna having an odd-numbered index is selected as the receiving antenna in S 232 . Thus, from S 232  to S 237 , TX correction is performed on the transmission paths of the antennas belonging to the even-numbered antenna group. 
     For example, when all pairs of selectable transmitting antennas have been selected (n&gt;N), the operation unit  131  determines that the TX correction has been performed on half of the transmission paths. When the TX correction has been performed on half of the transmission paths, the processing proceeds to S 239 . Otherwise, the processing returns to S 232 . 
     (S 239 ) The operation unit  131  sets the index n to 2. Namely, the operation unit  131  performs TX correction on the transmission path of an antenna belonging to the odd-numbered antenna group. 
     (S 240 ) The operation unit  131  selects ANT#n as the receiving antenna (RX). 
     (S 241 ) The operation unit  131  selects ANT#(n−1) and ANT#(n+1) as the transmitting antennas (TX). Namely, the operation unit  131  selects the two antennas neighboring ANT#n as the transmitting antennas. 
     (S 242 ) The operation unit  131  causes the RF circuits connected to ANT#(n−1) and ANT#(n+1) to transmit a signal and causes the RF circuit connected to ANT#n to receive the signal transmitted via ANT#(n−1) and ANT#(n+1) via ANT#n. 
     (S 243 ) The phase and amplitude detector  133  detects the amplitudes and phases of the signals transmitted by ANT#(n−1) and ANT#(n+1) and stores information about the detected amplitudes and phases in the RAM  132 . The operation unit  131  refers to the information stored in the RAM  132  and detects the difference ΔtP (n−1)(n+1)  between the phases of the signals transmitted by ANT#(n−1) and ANT#(n+1). The operation unit  131  detects the difference ΔtA (n−1)(n+1)  between the amplitudes in the same way. 
     (S 244 ) The operation unit  131  sets a phase shift amount in an FIR connected to a TX circuit connected to ANT#(n+1) so that the difference ΔtP (n−1)(n+1)  is added to the signal phase outputted from the TX circuit. In addition, the operation unit  131  sets an amplitude shift amount in the FIR connected to the TX circuit connected to ANT#(n+1) so that the difference ΔtA (n−1)(n+1)  is added to the signal amplitude outputted from the TX circuit. 
     (S 245 ) The operation unit  131  increments the index n by 2. 
     (S 246 ) The operation unit  131  determines whether TX correction has been performed on half of the transmission paths (correction on the signal amplitudes and phases outputted from the other half of the TX circuits). 
     Since the index n is set to 2 in S 239 , an antenna having an even-numbered index is selected as the receiving antenna in S 240 . Thus, from S 240  to S 245 , TX correction is performed on the transmission paths of the antennas belonging to the odd-numbered antenna group. 
     For example, when all pairs of selectable transmitting antennas have been selected (n&gt;N), the operation unit  131  determines that the TX correction has been performed on the other half of the transmission paths. When the TX correction has been performed on the other half of the transmission paths, the processing proceeds to S 247 . Otherwise, the processing returns to S 240 . 
     (S 247 ) To perform inter-group correction, the operation unit  131  selects ANT#1 as the receiving antenna (RX). ANT#1 belongs to the odd-numbered antenna group. 
     (S 248 ) The operation unit  131  selects ANT#4, ANT#5, and ANT#6 as the transmitting antennas (TX). ANT#4 and ANT#6 are a pair of antennas that belong to the even-numbered antenna group, which is different from the odd-numbered antenna group to which ANT#1 belongs, and that are equally distanced from ANT#1. ANT#5 is an antenna that belongs to the odd-numbered antenna group to which ANT#1 also belongs and is equally distanced from ANT#4 and #6. 
     (S 249 ) The operation unit  131  controls the relevant RF circuits so that a signal transmitted by ANT#4, ANT#5, and ANT#6 is received by ANT#1. The amplitude gAt 4  and phase gPt 4  transmitted by ANT#4 and received by the RX circuit connected to ANT#1, the amplitude gAt a  and phase gPt 5  transmitted by ANT#5 and received by the RX circuit connected to ANT#1, and the amplitude gAt 6  and phase gPt 6  transmitted by ANT#6 and received by the RX circuit connected to ANT#1 are detected by the phase and amplitude detector  133 . Information about the detected amplitudes and phases is stored in the RAM  132 . 
     (S 250 ) The operation unit  131  refers to the RAM  132  and acquires the information about the amplitudes and phases (gAt 4 , gPt 4 ), (gAt 5 , gPt 5 ), and (gAt 6 , gPt 6 ). Next, the operation unit  131  calculates an inter-group phase error ΔgPt and an inter-group amplitude error ΔgAt on the basis of the following expressions (3) and (4). 
       Δ gPt =( gPt   5   −gPt   4 )−( gPt   5   −gPt   6 )  (3)
 
       Δ gAt =( gAt   5   −gAt   4 )−( gAt   5   −gAt   6 )  (4)
 
     (S 251 ) The operation unit  131  controls relevant FIRs so that ΔgPt (inter-group difference) is added to the signal phases outputted by the TX circuits connected to all ANT#k (k is an odd number) belonging to the odd-numbered antenna group. In addition, the operation unit  131  controls relevant FIRs so that ΔgAt (inter-group difference) is added to the signal amplitudes outputted by the TX circuits connected to all ANT#k (k is an odd number) belonging to the odd-numbered antenna group. Namely, the operation unit  131  corrects the inter-group errors by using ΔgPt and ΔgAt. 
     After S 251 , the operation unit  131  ends the processing illustrated in  FIGS. 14 to 16 . Inter-group errors are corrected by applying the above method. Even when the number of antennas is an odd number, TX correction is achieved on all the transmission paths. 
     (Improvement of Beam Characteristics) 
     When the above technique according to the second embodiment is applied to correct errors in transmission and reception paths, the directivity of a beam is improved as illustrated in  FIG. 17 .  FIG. 17  illustrates comparison between radiation patterns obtained before and after amplitude and phase errors in RF circuits are corrected. 
     (A) of  FIG. 17  illustrates a radiation pattern obtained before amplitude and phase errors in RF circuits are corrected. The hatched area in (A) of  FIG. 17  represents beam spread. As illustrated in (A) of  FIG. 17 , when errors as described above are caused, the side lobes spread widely. As a result, more interference with neighboring cells is caused. (B) of  FIG. 17  illustrates a radiation pattern obtained after the amplitude and phase errors in the RF circuits are corrected. When the radiation patterns in (A) and (B) of  FIG. 17  are compared with each other, it is seen that the spread of the side lobes has been reduced in (B) of  FIG. 17 . Namely, it is seen that the above technique according to the second embodiment has an advantageous effect of reducing the inter-cell interference. 
     [2-4. Correction Timing and Power Control] 
     The above technique according to the second embodiment is applicable to various radio communication systems. For example, the technique is applicable to a radio communication system based on a TDD method. For example, in a TDD-LTE (Long Term Evolution) method, uplink and downlink communication timings are defined as illustrated in  FIG. 18 .  FIG. 18  illustrates correction timing control according to the second embodiment. 
     As illustrated in  FIG. 18 , one single frequency network (SFN) is divided into a plurality of subframes (subframes 0 to 9) including downlink subframes (D) in which downlink communication is allowed and uplink subframes (U) in which uplink communication is allowed. In addition, a period (switch point periodicity) is set, and a downlink subframe (D) and an uplink subframe (U) are switched on the basis of this period. 
     In addition, when a downlink subframe (D) and an uplink frame (U) are switched, a special subframe (S) is inserted therebetween. An individual special subframe (S) is divided into three periods (a downlink pilot time slot (DwPTS), GAP, an uplink PTS (UpPTS)). GAP is a period in which neither transmission nor reception is performed. DL and UL are for downlink and uplink communications, respectively. 
     In a TDD method, the same frequency is used for transmission and reception. Since transmission and reception units corresponding to all antennas are synchronized with each other, one unit does not receive a signal while another unit is transmitting a signal. Thus, when the correction method according to the second embodiment in which a plurality of antennas included in a single antenna array are allocated to transmission and reception is applied, it is suitable to detect errors by using the above GAP periods. 
     For example, when ANT#1, . . . , ANT#4 are used, as illustrated in  FIG. 19 , when to transmit and receive signals used for error detection is controlled so that ANT#1, ANT#2, etc. sequentially transmit signals and ANT#2, ANT#3, etc. sequentially receive the signals by using the GAP periods.  FIG. 19  illustrates power control performed when the amplitude and phase correction according to the second embodiment is performed. 
     In  FIG. 19 , MAX, OFF, and LOW represent when an antenna is transmitting maximum, minimum, and low power, respectively. In addition, ON represents when an antenna is receiving a signal, and DET represents when an antenna is detecting a signal used for error correction. In addition, an individual dashed-dotted line in  FIG. 19  represents a time mask of transmission power. An individual time mask defines change between an OFF state and an ON state (MAX) and the maximum activation time of a transmission signal. Since change to an OFF state is needed in a GAP period, the power used to transmit a signal for error correction is set to be lower than the time mask value in the GAP period, so as not to violate the radio law. 
     The second embodiment has thus been described. 
     Amplitude and phase errors caused in a reception path of an individual antenna are easily detected. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.