Patent Publication Number: US-2010130134-A1

Title: Nonlinear distortion compensating apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of prior Japanese Patent Application No. 2008-299602, filed on Nov. 25, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Certain aspects of the present invention discussed herein are related to a nonlinear distortion compensating apparatus and compensating method. 
     BACKGROUND 
     In general wireless communication apparatuses, high transmission power attains high communication quality. However, when output power comes close to saturation power of an amplifying circuit, nonlinear distortion occurs. A wireless communication apparatus, for example, which performs digital wireless communication maps a digital signal on a plurality of certain signal points using a transmitter so as to perform modulation and transmission. Therefore, a technique of compensating for nonlinear distortion when a receiver demodulates the modulated signal or when the transmitter performs the mapping so as to modulate the signal has been developed (for example, Japanese Laid Open Patent Publication Nos. 2004-172921 and 08-163198). 
     However, a technique of correcting distortion in the related art has following disadvantages. (First Disadvantage) In a method for performing distortion compensation in accordance with a mathematical expression obtained by performing mathematization on an input-output characteristic of an amplifying circuit to be subjected to nonlinear distortion compensation in a transmitter, since a characteristic of the amplifying circuit which is obtained in advance is used, it is difficult to perform appropriate control in accordance with change of a state of the amplifying circuit. (Second Disadvantage) In a method for detecting a difference (deviation) between a position of a signal point which has been amplified by the amplifying circuit and a regular position and correcting the difference, the deviation of the position of the passing signal point represents considerable distortion due to deterioration. Therefore, a period of time in which moving and passing among signal points which exhibits the maximum power is not reflected, and the method is not sufficient for correcting deterioration of spectrum which occurs at a transmission antenna terminal. 
     (Third Disadvantage) In a method for detecting distortion using a reception signal in a receiver, distortion detection fails due to fading in a wireless transmission path, deterioration of a waveform due to rainfall, superposing of noise, and signal error due to such deterioration of communication quality. (Fourth Disadvantage) If an adaptive distortion compensation circuit, which perform mathematization on the input-output characteristic of the amplifying circuit and which updates coefficients included in the mathematical expression using a distortion compensating device in accordance with the mathematical expression, independently controls the coefficients, complicated control is induced. (Fifth Disadvantage) If a compensating amount at the beginning of control is considerably different from a state of generated distortion, or if a state of generation of the distortion is considerably changed and therefore the state becomes considerably different from the compensating amount, the difference can be made smaller in short time when an amount of change of the compensating amount which is changed by one control update is large. However, control is converged while the amount of change stays large, and operation is not stable in a state in which the difference is small. 
     SUMMARY 
     Accordingly, in an aspect, an object of the invention is to easily control nonlinear distortion compensation, and in another aspect, to control the nonlinear distortion compensation with high accuracy. 
     According to a certain aspect of the invention, a distortion compensating apparatus includes a distortion detector configured to detect nonlinear distortion by reproducing a reception signal and output information on the detected nonlinear distortion as control information to a distortion compensating unit which compensates for the nonlinear distortion and which is included in a transmitter. 
     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, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration of a nonlinear distortion compensating apparatus according to a first embodiment; 
         FIG. 2  illustrates a configuration of a transmitter; 
         FIG. 3  illustrates a flowchart of processing for controlling distortion compensation according to the first embodiment; 
         FIG. 4  illustrates a state of conversion performed by a converting unit of an input-output characteristic; 
         FIG. 5  illustrates a configuration of a nonlinear distortion compensating apparatus according to a second embodiment; 
         FIG. 6  illustrates an example of an eye pattern representing coordinates of passing signal points on a signal space when a reception signal moves among the signal points; 
         FIG. 7  illustrates a flowchart of processing for controlling distortion compensation according to the second embodiment; 
         FIG. 8  illustrates a state of calculation of differences; 
         FIG. 9  illustrates a configuration of a nonlinear distortion compensating apparatus according to a third embodiment; 
         FIG. 10  illustrates examples of passing signal points when a reception signal moves among signal points; 
         FIG. 11  illustrates examples of reception signals detected in various units of the apparatus; 
         FIG. 12  illustrates a flowchart of processing for controlling distortion compensation according to the third embodiment; 
         FIG. 13  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fourth embodiment; 
         FIG. 14  illustrates a flowchart of processing for controlling distortion compensation according to a fourth embodiment; 
         FIG. 15  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fifth embodiment; 
         FIG. 16  illustrates a state of movement of specified signal points detected by a specific-signal movement detector; 
         FIG. 17  illustrates a state of charged power of a reception signal which meets a specific-signal movement condition; 
         FIG. 18  illustrates a flowchart of processing for controlling distortion compensation according to the fifth embodiment; 
         FIG. 19  illustrates an example of a timing chart of signals; 
         FIG. 20  illustrates a timing chart representing a concrete example of determination of movement of specific signals; 
         FIG. 21  illustrates a configuration of a nonlinear distortion compensation apparatus according to a sixth embodiment; 
         FIG. 22  illustrates a state of charged power of a reception signal which meets a specific-signal movement condition; 
         FIG. 23  illustrates switch of a state of detection of movement of specified signal points detected by a specific-signal movement detector; 
         FIG. 24  illustrates a flowchart of processing for controlling distortion compensation according to the sixth embodiment; 
         FIG. 25  illustrates a configuration of a nonlinear distortion compensation apparatus according to a seventh embodiment; 
         FIG. 26  illustrates a flowchart of switching of a specific-signal movement condition; 
         FIG. 27  illustrates a change of the specific-signal movement condition (in a case of timer operation); 
         FIG. 28  illustrates a change of the specific-signal movement condition (in a case of control loop monitoring); 
         FIG. 29  illustrates a configuration of a nonlinear distortion compensation apparatus according to an eighth embodiment; 
         FIG. 30  illustrates states of detection of movement of specified signal points detected by the specific-signal movement detectors; 
         FIG. 31  illustrates a flowchart of processing for detecting matching of a specific-signal movement condition; 
         FIG. 32  illustrates a flowchart of the processing for detecting matching of the specific-signal movement condition; 
         FIG. 33  illustrates a configuration of a nonlinear distortion compensation apparatus according to a ninth embodiment; 
         FIG. 34  illustrates a state of detection of movement of specified signal points detected by first and second specific-signal movement detectors. 
         FIG. 35  illustrates a flowchart of processing for detecting matching of a specific-signal movement condition; 
         FIG. 36  illustrates a flowchart of the processing for detecting matching of a specific-signal movement condition; 
         FIG. 37  illustrates a configuration of a nonlinear distortion compensation apparatus according to a tenth embodiment; 
         FIG. 38  illustrates an example of detection performed by a signal-point error detector; 
         FIG. 39  illustrates a flowchart of processing for controlling distortion compensation according to the tenth embodiment; 
         FIG. 40  illustrates a configuration of a nonlinear distortion compensation apparatus according to an eleventh embodiment; 
         FIG. 41  illustrates an example of detection performed by a reception-level deterioration detector; 
         FIG. 42  illustrates a flowchart of processing for controlling distortion compensation according to the eleventh embodiment; 
         FIG. 43  illustrates a configuration of a nonlinear distortion compensation apparatus according to a twelfth embodiment; 
         FIG. 44  illustrates an example of detection of an error ratio detected by a digital-signal processing unit; 
         FIG. 45  illustrates a flowchart of processing for controlling distortion compensation according to the twelfth embodiment; 
         FIG. 46  illustrates a configuration of a nonlinear distortion compensation apparatus according to a thirteenth embodiment; 
         FIG. 47  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fourteenth embodiment; 
         FIG. 48  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fifteenth embodiment; 
         FIG. 49  illustrates a flowchart of processing for controlling distortion compensation according to the fifteenth embodiment; 
         FIG. 50  illustrates a configuration of a nonlinear distortion compensation apparatus according to a sixteenth embodiment; 
         FIG. 51  illustrates a flowchart of processing for controlling distortion compensation according to the sixteenth embodiment; and 
         FIG. 52  illustrates a state of generation of distortion detection results. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments for carrying out the present invention are described with reference to the drawings. 
     Nonlinear distortion compensating apparatuses according to the embodiments are basically configured so as to have a control loop in which a compensation signal is generated in accordance with a signal actually received by a receiver and the compensation signal is transmitted to a distortion compensating unit included in a transmitter. 
     First Embodiment 
       FIG. 1  illustrates a configuration of a nonlinear distortion compensating apparatus  100  according to a first embodiment. The nonlinear distortion compensating apparatus  100  includes a transmitter  101  and a receiver  105 . The transmitter  101  includes a distortion compensating unit  102 , and the receiver  105  includes a receiving unit  106  and an input-output-characteristic converting unit  107 . The converting unit of input-output characteristic  107  detects nonlinear distortion as described below. Furthermore,  FIG. 2  illustrates an internal configuration of the transmitter  101 . A digital signal is supplied to the transmitter  101 , the base-band digital signal is mapped on a plurality of predetermined signal points in an orthogonal signal space which is separated into I and Q by the signal mapping unit  111 . The signal subjected to distortion compensation in the distortion compensating unit  102  is further subjected to predetermined linear conversion such as PSK (phase shift keying) and QAM (quadrature amplitude modulation) in a modulation unit  113 , is up-converted in a frequency conversion unit  114 , is amplified in an amplifying circuit  115 , and is transmitted as a wireless signal. 
     The input-output-characteristic converting unit  107  calculates an input-output characteristic of the amplifying circuit  115  included in the transmitter  101  using the wireless signal received by the receiving unit  106  and a digital signal which has been received and reproduced. Data of the calculated input-output characteristic is transmitted as a compensation signal D 1  which is a wireless signal, for example, from the receiver  105  to the distortion compensating unit  102  included in the transmitter  101 . The distortion compensating unit  102  included in the transmitter  101  performs distortion compensation in accordance with the data of the input-output characteristic represented by the supplied compensation signal D 1  so as to address nonlinearity of amplitude performed by the amplifying circuit  115 . 
       FIG. 3  illustrates a flowchart of processing for controlling the distortion compensation according to the first embodiment.  FIG. 4  illustrates a state of conversion performed by the input-output-characteristic converting unit  107 . The receiver  105  performs processes included in step S 200  of  FIG. 3  and the transmitter  101  performs a process in step S 206 . First, when the transmitter  101  transmits a signal which has not been subjected to distortion compensation, the receiving unit  106  included in the receiver  105  wirelessly receives the signal (in step S 201 ). Then, the reception signal is reproduced (in step S 202 ). Here, regular signal-point coordinates are calculated (in step S 203 ). It is assumed that white circles denote the regular signal-point coordinates, and black circles denote signal-point coordinates detected as reception signals (in a graph on the left side of  FIG. 4 ). 
     The input-output-characteristic converting unit  107  receives the regular signal-point coordinates and outputs reception-signal-point coordinates so as to convert the regular signal-point coordinates into an input-output characteristic of the amplifying circuit  115  included in the transmitter  101  (in step S 204 ). In  FIG. 4 , an axis of abscissa denotes an input characteristic and an axis of ordinate denotes an output characteristic (in a graph on the right side of  FIG. 4 ). In an example shown in  FIG. 4 , signal-point coordinates of 64 QAM are shown. Specifically, the input-output-characteristic converting unit  107  calculates differences Δ between coordinates in the signal space, which are obtained by calculation using a reproduction signal obtained by reproducing the reception signal and which are passed when the reproduced signal moves among signal points and coordinates of the detected reception signals. Then, the obtained differences are converted into an input-output characteristic f(x) which is a certain function normalized by a distance between signal points (Euclidean distance). 
     Then, data representing the input-output characteristic f(x) is transmitted as a compensation signal D 1  to the distortion compensating unit  102  included in the transmitter  101  (in step S 205 ). The distortion compensating unit  102  included in the transmitter  101  performs compensation by cancelling nonlinearity of the data f(x) representing the input-output characteristic (in step S 206 ). For example, the distortion compensating unit  102  performs control in order to transmit a wireless signal having a reverse characteristic of the input-output characteristic f(x) using a linear input-output characteristic shown in  FIG. 4  as a reference. 
     The input-output-characteristic converting unit  107  may constantly or periodically transmit the compensation signal D 1 . Furthermore, the distortion compensating unit  102  may store the received compensation signal D 1  in a storage unit, not shown, so as to use the compensation signal D 1  for the distortion compensation described above. In this case, data stored in the storage unit is updated every time the compensation signal D 1  is received. 
     With this configuration, the receiver  105  receives an actual wireless signal and performs mathematization on the input-output characteristic of the amplifying circuit  115  included in the transmitter  101 , which is to be subjected to nonlinear distortion compensation. As described above, since a control loop (closed loop) is generated by receiving a signal by the receiver  105  and transmitting the compensation signal D 1  used for nonlinear distortion compensation from the receiver  105  to the transmitter  101 , change of a state of the amplifying circuit  115 , for example, change of temperature or variation of the amplifying circuit  115  may be appropriately compensated for. 
     Second Embodiment 
       FIG. 5  illustrates a configuration of a nonlinear distortion compensating apparatus according to a second embodiment. A distortion detector  402  included in a receiver  401  according to the second embodiment includes a reference value generator  403  which generates reference coordinates of signal points, a passing coordinate extracting unit  404  which extracts passing coordinates of a detected reception signal, and a comparing unit  405  which compares the reference coordinates of the reference value generator  403  with the passing coordinates. 
       FIG. 6  illustrates an example of an eye pattern representing coordinates of passing signal points in a signal space when a reception signal moves among signal points. An axis of abscissa denotes time and an axis of ordinate denotes amplitude. As shown in  FIG. 6 , when a reception signal moves among a plurality of signal points S, passing coordinates T are generated. For example, the distortion detector  402  obtains reference coordinates to be passed, which are determined in accordance with the signal points in which the reception signal passes and which are located among the signal points S from the reference value generator  403 . Specifically, the reference coordinates are determined by discriminating signals which are synchronously detected by a receiving unit  106  by a discriminating unit, not shown, included in the reference value generator  403 , reproducing a digital signal, and obtaining a state of movement among signal points generated by an internal calculation circuit in accordance with the reproduced digital signal. 
     Furthermore, a passing coordinate detected as a reception signal is obtained from the passing coordinate extracting unit  404 . For example, the passing coordinates shown as points T in  FIG. 6  correspond to points of maximum power. When distortion occurs, the passing coordinates T of the detected reception signal are deviated relative to positions of the reference coordinates. The comparing unit  405  obtains differences between the reference coordinates and the passing coordinates. The distortion detector  402  transmits data representing the differences as a compensation signal D 1  to a distortion compensating unit  102  of a transmitter  101 . The distortion compensating unit  102  performs distortion compensation in accordance with the data representing the detected difference. 
       FIG. 7  illustrates a flowchart of processing for controlling distortion compensation according to the second embodiment. In  FIG. 7 , the receiver  401  performs processes included in step S 600  and the transmitter  101  performs processes included in step S 601 . First, the transmitter  101  transmits a signal which has not been subjected to distortion compensation, and the receiving unit  106  included in the receiver  401  wirelessly receives the signal (in step S 611 ). Then, the reception signal is reproduced (in step S 612 ). 
     At this time, the reference value generator  403  included in the distortion detector  402  calculates reference coordinates when movement among signal points S is performed (in step S 613 ). Thereafter, the comparing unit  405  included in the distortion detector  402  calculates differences e between passing coordinates obtained by monitoring the reception signal using the passing coordinate extracting unit  404  and the reference coordinates calculated using the reference value generator  403  (in step S 614 ). The distortion detector  402  transmits data representing the differences e to the distortion compensating unit  102  included in the transmitter  101  (in step S 615 ). 
       FIG. 8  illustrates a state of calculation of the differences in detail. An axis of abscissa denotes time. Passing coordinates are shown in a top line, S denotes passing coordinates in signal points, and T denotes coordinates (passing coordinates) in movement among the signal points S. Reference coordinates calculated using the reference value generator  403  are shown in a middle line. When |passing coordinate|≦|reference coordinate| is satisfied, data representing 0 is output as the difference e, and |passing coordinate|&lt;|reference coordinate| is satisfied, data representing 1 is output as the difference e. 
     Referring back to  FIG. 7 , when receiving the data representing the differences e, the distortion compensating unit  102  included in the transmitter  101  performs distortion compensation in accordance with the data representing the differences e (in step S 619 ). Here, it is determined whether a compensating amount is large or small using the data representing the differences e. When “1” representing shortage of the compensating amount is determined (in step S 616 : 1 ), the compensating amount is increased (in step S 617 ). On the other hand, when “0” representing overcompensating amount is determined, (in step S 616 : 0 ), the compensating amount is reduced (in step S 618 ). 
     With this configuration, the receiver  401  receives an actual wireless signal, detects differences (deviation) between positions of signal points which have been amplified using the amplifying circuit  115  (shown in  FIG. 2 ), which is included in the transmitter  101  and which is an object to be subjected to linear distortion compensation with regular positions, and controls the differences to be compensated for. In this way, the distortion compensation can be performed taking a state of moving and passing between signal points having maximum power into consideration using deviation of positions of passing signal points generated due to considerable distortion of deterioration of the amplifying circuit  115 . Accordingly, spectrum deterioration which occurs at an antenna terminal can be compensated for. 
     Third Embodiment 
       FIG. 9  illustrates a configuration of a nonlinear distortion compensating apparatus according to a third embodiment. In the third embodiment, an internal configuration of a distortion detector  402  which corresponds to that of the second embodiment ( FIG. 5 ) will be described. In the third embodiment, a digital filter (FIR filter)  803  having a function similar to that of a band-limiting filter included in the transmitter  101  is also included in the reference value generator  403  of the receiver  401 . Then, a signal reproduced by the receiver  401  is supplied to the digital filter (FIR filter)  803  having the function similar to that of the band-limiting filter used in the transmitter  101 , and a signal output from the digital filter is determined as a reference value (a coordinate of a signal to be detected by the receiver  401 ) of distortion detection. With this reference value, distortion is detected. 
     In  FIG. 9 , an A/D converter  801  is included in a receiving unit  106  and converts an analog reception signal into a digital signal. The reference value generator  403  includes a discriminating-and-judging unit  802  and the FIR filter  803 . The discriminating-and-judging unit  802  reproduces a transmission signal after performing logical determination on the reception signal which has been subjected to the A/D conversion. The FIR filter  803  has an input-output characteristic the same as that of the band-limiting filter included in the transmitter  101 . Furthermore, the passing coordinate extracting unit  404  extracts passing coordinates positioned among signal points. The passing coordinate extracting unit  404  includes a delay unit which delays a signal to be output by a delay time that is the same as processing time of the FIR filter  803  included in the reference value generator  403 . A comparing unit  405  compares, as with the second embodiment, reference coordinates with the passing coordinates. 
       FIG. 10  illustrates examples of passing signal points when a reception signal moves among signal points. An axis of ordinate denotes amplitude, and an axis of abscissa denotes time. Black circles denote reception signal points, white circles denote positions where signal points generate, and white squares denote middle points of the movement among the signal points. As shown in  FIG. 10 , a signal which has been synchronously detected by the receiver  401  is discriminated using the discriminating-and-judging unit  802  included in the reference value generator  403 . 
       FIG. 11  illustrates examples of reception signals detected in various units included in the apparatus. As a signal a output from the A/D converter  801 , a digital signal X(m) [m: N−3, N−2, N−1, N, N+1, N+2, and N+3] is reproduced. As a signal b output from the discriminating-and-judging unit  802 , a signal X(m) [m: N−3, N−1, N+1, and N+3] is reproduced. When the reproduced digital signal is supplied to the FIR filter  803  which performs band limitation, as a signal c output from the FIR filter  803 , a coordinate R(n) [n: N−2, N, and N+2] which is passed at a time of movement among output signal points is reproduced. The coordinate R(n) which is passed at the time of movement among output signal points is determined as a reference value (reference coordinate). 
     Furthermore, the passing coordinate extracting unit  404  delays a reception signal by the delay time which is the same as the processing time of the reference value generator  403 , and extracts a passing coordinate S(n) [n: N−2, N, and N+2] at a time the signal serving as an output d moves among signals. The comparing unit  405  compares the passing coordinate S(n) with the reference coordinate R(n), and outputs information C(n) [n: N−2, N, and N+2] of a coordinate difference serving as an output e. The difference e includes information on a distortion overcompensating amount and information on shortage of the distortion compensating amount. The distortion detector  402  transmits data of the difference e as a compensation signal D 1  to the distortion compensating unit  102  included in the transmitter  101 . The distortion compensating unit  102  performs distortion compensation in accordance with distortion using the data of the detected difference e. 
       FIG. 12  illustrates a flowchart of processing for controlling distortion compensation according to the third embodiment. The receiver  401  performs processes included in step S 1100  shown in  FIG. 12 , and the transmitter  101  performs a process of step S 1107  shown  FIG. 12 . First, the transmitter  101  transmits a signal which has not been subjected to distortion compensation, and the receiving unit  106  of the receiver  401  wirelessly receives the signal (in step S 1101 ). Next, the reception signal is reproduced (in step S 1102 ). 
     Then, the reception reproduction signal reproduced by the discriminating-and-judging unit  802  is supplied to the FIR filter  803  having a characteristic the same as that of the band-limiting filter included in the transmitter  101  (in step S 1103 ). By this, passing coordinates at a time of movement among signal points are calculated using a signal output from the FIR filter  803  (in step S 1104 ). The passing coordinates correspond to reference values (reference coordinates). Then, the comparing unit  405  calculates differences e between the reference coordinates and the passing coordinates of the reception signal extracted by the passing coordinate extracting unit  404  (in step S 1105 ). The distortion detector  402  transmits data representing the differences e serving as a compensation signal D 1  to the distortion compensating unit  102  included in the transmitter  101  (in step S 1106 ). Then, the distortion compensating unit  102  included in the transmitter  101  performs distortion compensation in accordance with the differences e represented by the supplied compensation signal D 1  (in step S 1107 ). 
     With this configuration, since the FIR filter  803  having a function the same as that of the band-limiting filter is included in the receiver  401 , a state in which distortion occurs is detected using the transmitter  101  and the receiver  401 , and a signal output from the FIR filter  803  is used as a reference value of the distortion detection. In this way, using deviation of positions of the passing signal points generated in the amplifying circuit  115  due to generation of distortion which has been considerably deteriorated, distortion compensation can be performed taking a state of moving and passing between signal points having maximum power into consideration using the deviation of the positions of the passing signal points generated due to considerable distortion of deterioration of the amplifying circuit  115 . Accordingly, spectrum deterioration which occurs at an antenna terminal can be compensated for. 
     Fourth Embodiment 
       FIG. 13  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fourth embodiment. In the fourth embodiment, a receiver  1200  generates reference values (reference coordinates) used for distortion detection using a reception signal which has been subjected to waveform equalization, interference compensation, and error correction so as to accurately perform the distortion detection taking a characteristic of a wireless transmission path into consideration. 
     A signal which is synchronously detected by a receiving unit  106  is supplied to a waveform equalizing unit  1201 . The waveform equalizing unit  1201  equalizes linear distortion in the wireless transmission path. The waveform equalizing unit  1201  is connected to an interference compensating unit  1202  which compensates for interference of different polarization and the interference compensating unit  1202  is connected to an error correcting unit  1203  which corrects a discrimination error which occurs due to waveform deterioration, interference, and thermal noise. A reproduction digital signal obtained after such signal processing is supplied to the reference value generator  403 . A passing coordinate extracting unit  404  receives the reception signal detected by the receiving unit  106 . 
       FIG. 14  illustrates a flowchart of processing for controlling the distortion compensation according to the fourth embodiment. The receiver  1200  performs processes included in step S 1300 , and a transmitter  101  performs a process of step S 1307 . First, the transmitter  101  transmits a signal which has not been subjected to distortion compensation, and the receiving unit  106  included in the receiver  1200  wirelessly receives the signal (in step S 1301 ). Next, the reception signal is reproduced (in step S 1302 ). 
     Then, the signal output from the receiving unit  106  is supplied to the waveform equalizing unit  1201 , the interference compensating unit  1202 , and the error correcting unit  1203  so that influence of linear distortion in the wireless transmission path is removed (in step S 1303 ). The reception signal from which the influence of the linear distortion is removed is supplied to the reference value generator  403  included in a distortion detector  402 . As described in the foregoing embodiment, the reference value generator  403  calculates reference coordinates when the reception signal moves among signal points (in step S 1304 ). Here, passing coordinates correspond to reference values. Then, the comparing unit  405  calculates differences e between the reference coordinates and the passing coordinates extracted by the passing coordinate extracting unit  404  (in step S 1305 ). The distortion detector  402  transmits data representing the differences e as a compensation signal D 1  to the distortion compensating unit  102  included in the transmitter  101  (in step S 1306 ). Then, the distortion compensating unit  102  included in the transmitter  101  performs distortion compensation in accordance with the differences e represented by the input compensation signal D 1  (in step S 1307 ). 
     According to the configuration described above, the receiver detects distortion using a reception signal. However, since the reference values are obtained by removing fading in the wireless transmission path, deterioration of a waveform due to rainfall, superposing of noise, and signal error due to such deterioration of communication quality. Accordingly, accuracy of calculation of coordinates which are passed when the reception signal moves among signal points can be improved. In this way, accuracy of determination as to whether overcompensation of a distortion amount or shortage of the distortion compensating amount occurs is improved, and accuracy of the distortion compensating amount can be improved. 
     Fifth Embodiment 
       FIG. 15  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fifth embodiment. In the fifth embodiment, movement among specific signal points in which power suitable for detection of distortion is generated is detected, a result of the distortion detection when a reception signal moves among the specific signal points is determined to be effective, and distortion compensation is performed. 
     In the fifth embodiment, a receiving unit  106  outputs a signal to a specific-signal movement detector  1401 , and the specific-signal movement detector  1401  transmits an effective/ineffective signal D 2  of distortion compensation to a distortion compensating unit  102  included in a transmitter  101 . The specific-signal movement detector  1401  detects a plurality of signal points (for example, successive four signal points N−3, N−1, N+1, and N+3). When it is determined that the four signal points in which a signal has moved are specific signal points, the specific-signal movement detector  1401  outputs an effective signal D 2  representing that the result of the detection of the distortion is effective. The specific-signal movement detector  1401  stores in a storing unit, not shown in  FIG. 15 , a pattern of movement among the specific signal points (N−3, N−1, N+1, and N+3) in which power suitable for the distortion detection is generated. 
       FIG. 16  illustrates a state of movement among the specified signal points detected by the specific-signal movement detector  1401 . When a reception signal moves among signal points as shown in an upper portion of  FIG. 16 , the maximum power is generated at a sampled reception signal N. In this case, as with the third embodiment, the distortion detector  402  outputs information C(n) [n: N−2, N, N+2] on differences e as a compensation signal D 1 . Furthermore, the specific-signal movement detector  1401  detects a state in which a movement pattern (from +3, −3, −3, to +3) in which power distribution of passing coordinates (N−3, N−1, N+1, N+3) of the reception signal becomes close to the maximum power is generated, and outputs an effective signal D 2  which represents that a result of distortion detection of the information C(N) of the differences e is effective. The distortion compensating unit  102  of the transmitter  101  performs the distortion compensation using the information C(N) of the differences e only when the effective/ineffective signal D 2  is output. A specific-signal movement condition using the four signals is not limited to the condition shown in  FIG. 16 , that is, the pattern (from +3, −3, −3, to +3), and a reverse pattern (from −3, +3, +3, to −3) may be employed. 
       FIG. 17  illustrates a state of charged power of a reception signal which satisfies the specific-signal movement condition. An axis of abscissa denotes power of the signal and an axis of ordinate denotes a generation probability (accumulation value). The electric power of the reception signal N which satisfies the specific-signal movement condition is obtained by accumulating the power as shown in  FIG. 17 . Here, a reference value of the distortion detector  402  is set so as to be equal to a portion P 1  shown in  FIG. 17 . The portion P 1  is located in the middle of an average power and the maximum power. By this, distortion of the reception signal N which satisfies the specific-signal movement condition can be compensated for by performing the distortion compensation. If the receiving unit  106  receives the reception signal having appropriate electric power at the portion P 1  as the result of the distortion compensation, a rate of output of a value “1” to output of a value “0” from the comparing unit  405  is balanced (refer to  FIG. 7 ), and control of the distortion compensation is converged. 
       FIG. 18  illustrates a flowchart of processing for controlling distortion compensation according to the fifth embodiment. In  FIG. 18 , the receiver  1200  performs processes included in step S 1700 , and the transmitter  101  performs processes included in step S 1710 . First, the transmitter  101  transmits a signal which has not been the distortion compensation, and the receiving unit  106  of the receiver  1200  wirelessly receives the signal (in step S 1701 ). Then, the reception signal is reproduced (in step S 1702 ). Although not shown, as with the third embodiment, the distortion detector  402  outputs information C(N) on differences e which is a result of the distortion detection as a compensation signal D 1 . 
     The specific-signal movement detector  1401  determines whether the reception signal moves among specific signal points (in step S 1703 ). When the determination is affirmative (“Yes” is selected in step S 1703 ), an effective signal D 2  (EN=1) representing that a compensation signal D 1  is effective is transmitted to the distortion compensating unit  102  included in the transmitter  101  (in step S 1704 ). By this, the distortion compensating unit  102  updates a distortion compensating amount (in step S 1711 ). Here, the distortion compensating unit  102  obtains the received compensation signal D 1  as a new distortion compensating amount. 
     On the other hand, when the determination is negative (“No” is selected in step S 1703 ), an ineffective signal D 2  (EN=0) representing that the compensation signal D 1  is ineffective is transmitted to the distortion compensating unit  102  included in the transmitter  101  (in step S 1705 ). By this, the distortion compensating unit  102  does not update the distortion compensating amount and holds a previous compensating amount (in step S 1712 ). Here, the distortion compensating unit  102  does not obtain the compensation signal D 1  even when the compensation signal D 1  is supplied. 
       FIG. 19  illustrates a timing chart of signals. At a time point t 1 , the specific-signal movement detector  1401  detects generation of the movement of a reception signal having the pattern (from +3, −3, −3, to +3) in which power distribution of the passing coordinates (N−3, N−1, N+1, and N+3) of the reception signal is close to the maximum power. By this, the effective/ineffective signal D 2  corresponding to the result C(N) of the distortion detection serving as the compensation signal D 1  is output, and the distortion compensating unit  102  performs distortion compensation using a compensating amount in accordance with the result of the distortion detection. The effective signal D 2  is output only during the specific pattern of movement among the signal points (in a period from the time point t 1  to a time point t 2 ). Accordingly, the compensating amount updated at the time point t 1  is used in distortion compensation performed after the time point t 2 . 
       FIG. 20  illustrates a timing chart representing a concrete example of the detection of movement among specific signals. Passing coordinates of a reception signal is the same as those of  FIG. 8 . In the fifth embodiment, an effective signal D 2  is obtained only when the specific pattern (from +3, −3, −3, to +3) in which power distribution of signal points S of the passing coordinates becomes close to the maximum power is generated, and a reference coordinate (−3.5) at this time is transmitted as a detection result C(N). In other periods before and after the generation of the specific pattern, when compared with the case of  FIG. 8 , the specific-signal movement pattern is not detected, and therefore, an ineffective signal D 2  is transmitted. 
     With this configuration, the distortion compensating amount is updated when distortion occurs due to the maximum power by using the specific signal movement pattern which attains the maximum power and detecting the movement of the reception signal among the specific signal points, and on the other hand, signal points in which power is low and probability of generation of distortion is low and a distortion compensating amount at a time of movement among signal points is not used. Therefore, accuracy of the distortion compensation is improved. Accordingly, distortion compensation can be performed taking a state at a time when the reception signal moves among signal points which attains the maximum power into consideration. Consequently, deterioration of spectrum at a terminal of a transmission antenna can be corrected. 
     Sixth Embodiment 
       FIG. 21  illustrates a configuration of a nonlinear distortion compensation apparatus according to a sixth embodiment. In the sixth embodiment, as with the fifth embodiment, movement among specific signal points in which power suitable for distortion detection is generated is detected, a result of the distortion detection when a reception signal moves among the specific signal points is determined to be effective, and distortion compensation is performed. Then, as a specific condition of movement among the signal points, a first condition in which probability of generation of events which satisfy the first condition is high or a second condition in which probability of generation of events which satisfy the second condition is low but accuracy of distortion detection is high is selected so that convergence of a distortion compensation factor or accuracy of distortion detection is appropriately prioritized. 
     As shown in  FIG. 21 , the sixth embodiment is different from the fifth embodiment in that a switching controller  1901  is added to the configuration of the fifth embodiment ( FIG. 15 ). Furthermore, a distortion detector  402  determines whether distortion occurs when a reception signal moves among, in addition to the four successive signal points (N−3, N−1, N+1, and N+3) described in the fifth embodiment, six successive signal points (N−5, N−3, N−1, N+1, N+3, and N+5). Furthermore, the specific-signal movement detector  1401  also detects movement among the specific four successive signal points and movement among the specific six signal points. Then, the switching controller  1901  determines whether a method for detecting distortion and a method for detecting movement of specific signals is performed using (1) the four successive signal points or (2) the six successive signal points by outputting a switching signal SW 1 . When the four successive signal points are used, control is converged fast, but accuracy of the distortion detection is low (first specific-signal movement condition). When the six successive signal points are used, the control is slowly converged, but high accuracy of the distortion detection is attained (second specific-signal movement condition). 
       FIG. 22  illustrates a state of charged power of a reception signal which satisfies a specific-signal movement condition. When the second specific-signal movement condition is selected, the specific-signal movement detector  1401  uses the six signal points. In this case, as shown in  FIG. 22 , a center P 2  of distribution of passing coordinates is closer to the maximum power relative to a center P 1  of distribution of passing coordinates when the four signal points are used (first specific-signal movement condition). Accordingly, distortion detection can be performed with high electric power, and distortion compensation can be performed even for small distortion. 
       FIG. 23  illustrates switching of a state of detection of movement among specified signal points detected by the specific-signal movement detector  1401 . In a case where a reception signal moves among signal points shown in an upper portion of  FIG. 23 , the maximum power is generated at a sampled reception signal N (similarly to the description of the fifth embodiment ( FIG. 16 )). Under the first specific-signal movement condition, as with the case of  FIG. 16 , an effective signal D 2  representing that a result of distortion detection of information C(N) of differences e is effective is output. EN 1  denotes a result of the first specific-signal movement condition, and a value 0 denotes “ineffective” and a value 1 denotes “effective”. EN 2  denotes a result of the second specific-signal movement condition, and a value 0 denotes “ineffective” and a value 1 denotes “effective”. 
     On the other hand, under the second specific-signal movement condition, compensation can be performed even for small distortion. However, due to the strict condition, the number of generations of movement among signal points which satisfy the second specific-signal movement condition is reduced. In addition, time required for convergence of a control loop of distortion compensating control is larger than that of the first specific-signal movement condition. As shown in  FIG. 23 , the effective signal D 2  is output in response to the information C(N) of the differences e under the first specific-signal movement condition whereas an ineffective signal D 2  is output in response to the information C(N) of the differences e under the second specific-signal movement condition. 
       FIG. 24  illustrates a flowchart of processing for controlling distortion compensation according to the sixth embodiment. In  FIG. 24 , the receiver  1200  performs processes included in step S 2200 , and the transmitter  101  performs processes included in step S 2220 .  FIG. 24  shows control processing performed in accordance with selection of a specific-signal movement condition. Furthermore, although not shown, when distortion compensation is started, as described in the fifth embodiment ( FIG. 18 ), for example, the transmitter  101  transmits a signal which has not been subjected to distortion compensation and the receiving unit  106  of the receiver  1200  wirelessly receives the signal and reproduces the reception signal. Furthermore, the distortion detector  402  outputs information C(N) of differences e which is a result of distortion detection as a compensation signal D 1 . 
     First, a specific-signal movement condition is set (in step S 2201 ). When it is determined that the convergence of the control loop is prioritized, the first specific-signal movement condition is selected (in step S 2202 ) and the following processing is performed. The specific-signal movement detector  1401  determines whether movement of the reception signal matches the movement among signal points of the first specific-signal movement condition (in step S 2203 ). When the determination is affirmative (“Yes” is selected in step S 2203 ), an effective signal D 2  which is a result of the distortion detection is transmitted (in step S 2204 ), and the distortion compensating unit  102  of the transmitter  101  updates a compensating amount so that the distortion compensation is performed (in step S 2221 ). When the determination is negative (“No” is selected in step S 2203 ), an ineffective signal D 2  which is a result of the distortion detection is transmitted (in step S 2205 ), and the distortion compensating unit  102  of the transmitter  101  does not update the distortion compensating amount and performs the distortion compensation using a previous compensating amount (in step S 2222 ). Thereafter, it is determined whether the current specific-signal movement condition is to be changed (in step S 2206 ). When the determination is negative (“No” is selected in step S 2206 ), the process returns to step S 2203  and the processing from step S 2203  onwards is performed. On the other hand, when the determination is affirmative (“Yes” is selected in step S 2206 ), the second specific-signal movement condition is selected (the process proceeds to step S 2210 ). 
     When it is determined that accuracy of the distortion detection is prioritized in step S 2201 , the second specific-signal movement condition is selected (in step S 2210 ), and the following processing is performed. The specific-signal movement detector  1401  determines whether movement of the reception signal matches the movement among signal points of the second specific-signal movement condition (in step S 2211 ). When the determination is affirmative (“Yes” is selected in step S 2211 ), an effective signal D 2  which is a result of the distortion detection is transmitted (in step S 2212 ), and the distortion compensating unit  102  of the transmitter  101  updates a distortion compensating amount so that the distortion compensation is performed (in step S 2221 ). When the determination is negative (“No” is selected in step S 2211 ), an ineffective signal D 2  which is a result of the distortion detection is transmitted (in step S 2213 ), and the distortion compensating unit  102  of the transmitter  101  does not update the distortion compensating amount and performs the distortion compensation using a previous compensating amount (in step S 2222 ). Thereafter, it is determined whether the current specific-signal movement condition is to be changed (in step S 2214 ). When the determination is negative (“No” is selected in step S 2214 ), the process returns to step S 2211  and the processing from step S 2211  onwards is performed. On the other hand, when the determination is affirmative (“Yes” is selected in step S 2214 ), the first specific-signal movement condition is selected (the process proceeds to step S 2202 ). 
     With this configuration, the distortion compensation can be performed taking a state of moving and passing among signal points having maximum power into consideration. Accordingly, spectrum deterioration which occurs at a transmission antenna terminal can be compensated for. In addition, since a plurality of patterns of specific signal movement in which the maximum power is generated are used, the first specific-signal movement condition in which control is converged fast but accuracy of the distortion detection is low and the second specific-signal movement condition in which the control is slowly converged but the accuracy of the detection of distortion is high can be switched from one to another. Accordingly, time required for convergence of a control loop and the accuracy of the distortion detection are selected so as to attain optimum control. Furthermore, in the foregoing example, the two conditions to be selected are provided. However, a specific-signal movement condition which uses a larger number of successive signal points and which attains higher detection accuracy may be added. In this case, a larger number of conditions to be selected may be provided. 
     Seventh Embodiment 
       FIG. 25  illustrates a configuration of a nonlinear distortion compensation apparatus according to a seventh embodiment. The seventh embodiment is a modification of the sixth embodiment, and a switching controller  1901  which is the same as that described in the sixth embodiment is included in a transmitter  101 . 
     The switching controller  1901  included in the transmitter  101  transmits a switching signal SW 1  used for switching between the first and second specific-signal movement conditions from a distortion compensating unit  102  to a distortion detector  402 . 
       FIG. 26  illustrates a flowchart of switching of a specific-signal movement condition. The switching controller  1901  included in the transmitter  101  transmits the switching signal SW 1  used for switching of a distortion detection condition to a receiver  1200  when power is supplied or when a state in which output is stopped is changed to a state in which output is started, for example (transmission is started: in step S 2401 ). It is assumed that, first, the switching controller  1901  instructs selection of a first specific-signal movement condition (in step S 2402 ). By this, a specific-signal movement detector  1401  included in a distortion detector  402  of the receiver  1200  selects the first specific-signal movement condition (in step S 2410 ), detects distortion of a reception signal in accordance with the first specific-signal movement condition, and transmits a result of the distortion detection to the distortion compensating unit  102  included in the transmitter  101  (in step S 2411 ). 
     Thereafter, if switching of the distortion detection condition is performed by the switching controller  1901  (after “No” is selected in step S 2403 , “Yes” is selected in step S 2403 ), the switching signal SW 1  used to switch the first specific-signal movement condition to the second specific-signal movement condition is transmitted (in step S 2404 ). Then, the specific-signal movement detector  1401  included in the distortion detector  402  of the receiver  1200  selects the second specific-signal movement condition (in step S 2412 ), detects distortion of the reception signal in accordance with the second specific-signal movement condition, and transmits a result of the distortion detection to the distortion compensating unit  102  included in the transmitter  101  (in step S 2413 ). The processing described above performed by the transmitter  101  is continued until the transmission is stopped (in step S 2405 ). For example, if distortion is not compensated for even though the distortion compensation is performed in accordance with the first or second specific-signal movement condition, transmission may be stopped for maintenance. 
       FIGS. 27 and 28  illustrate concrete examples of timings at which the specific-signal movement condition is changed. In the process for changing a condition (in step S 2403 ) shown in  FIG. 26 , the convergence of the control loop is monitored and the switching signal SW 1  is automatically transmitted after a predetermined period of time.  FIG. 27  shows a timing chart for switching of a condition by timer operation. When the transmission is started, the transmitter  101  first transmits a switching signal SW 1  for selecting the first specific-signal movement condition, and after a setting period T 1 , transmits a switching signal SW 1  for selecting the second specific-signal movement condition. 
       FIG. 28  shows a timing chart for switching of a condition by monitoring the control loop. When the transmission is started, the transmitter  101  first transmits a switching signal SW 1  for selecting the first specific-signal movement condition, and continues the control in accordance with the first specific-signal movement condition during a period T 2  in which the control of the distortion compensation performed by the distortion compensating unit  102  is not converged. Thereafter, when the control of the distortion compensation performed by the distortion compensating unit  102  is converged, the transmitter  101  transmits a switching signal SW 1  for selecting the second specific-signal movement condition, and the control performed in accordance with the first specific-signal movement condition is changed to control performed in accordance with the second specific-signal movement condition. 
     With this configuration, the distortion compensation can be performed taking a state of moving and passing among signal points having maximum power into consideration. Accordingly, spectrum deterioration which occurs at a transmission antenna terminal can be compensated for. In addition, the control loop can be quickly converged from an initial state such as a state when power is supplied, and accuracy of the distortion compensation after the convergence can be improved. Furthermore, the configuration described above can be employed in a case where a circuit of the distortion compensating unit  102  and a circuit of the distortion detector  402  are disposed on different circuit substrates. Also in this case, the distortion compensation is quickly performed. 
     Eighth Embodiment 
       FIG. 29  illustrates a configuration of a nonlinear distortion compensation apparatus according to an eighth embodiment. The eighth embodiment is a modification of the sixth embodiment ( FIG. 21 ), and is configured such that it is determined whether a reception signal satisfies a first or second specific-signal movement condition. 
     A distortion detector  402  compares a reference values for four successive signal points (N−3, N−1, N+1, and N+3) and a reference value for six successive signal points (N−5, N−3, N−1, N+1, N+3, and N+5) with a signal passing coordinate (N) so as to determine whether distortion occurs. Results of the comparisons between the two reference value with the passing coordinate (N) are transmitted to a distortion compensating unit  102  as compensation signals D 1  which have been converted into serial signals (C, C 2 ). 
     Examples of a specific-signal movement condition in which a result of distortion detection is enabled include a first specific-signal movement condition in which convergence of a control loop is prioritized and a second specific-signal movement condition in which accuracy of distortion compensation is prioritized. A receiver  1200  includes a first-specific-signal-movement-condition detector  1401   a  which determines whether a reception signal satisfies the first specific-signal movement condition and a second-specific-signal-movement-condition detector  1401   b  which determines whether the reception signal satisfies the second specific-signal movement condition. The first-specific-signal-movement-condition detector  1401   a  determines whether four successive signal points satisfy a specific condition. The second-specific-signal-movement-condition detector  1401   b  determines whether six signal points satisfy a specific condition. Signals output from the two specific-signal movement detectors ( 1401   a  and  1401   b ) are converted into a single serial signal by a P/S converting circuit  2601 , and the serial signal is transmitted to the distortion compensating unit  102  as an effective/ineffective signal D 2 . 
       FIG. 30  illustrates states of detection of movement among specific signal points detected by the specific-signal movement detectors  1401   a  and  1401   b . As shown in  FIG. 30 , the first-specific-signal-movement-condition detector  1401   a  detects movement of a certain pattern (from +3, −3, −3, to +3) and movement of a reverse pattern (from −3, +3, +3, to −3) which are included in the first specific-signal movement condition. The second-specific-signal-movement-condition detector  1401   b  detects a movement of a certain pattern (from −3, +3, −3, −3, +3, to −3) shown in  FIG. 30  and movement of a reverse pattern (from +3, −3, +3, +3, −3, +3) which are included in the second specific-signal movement condition. Then, each of the specific-signal-movement-condition detectors  1401   a  and  1401   b  transmits an effective/ineffective signal D 2  at a timing when a movement detection result EN 1  for the first specific-signal movement condition or a movement detection result EN 2  for the second specific-signal movement condition is output. 
       FIG. 31  illustrates a flowchart of processing for detecting matching of a specific-signal movement condition. The receiver  1200  performs this processing. When the receiver  1200  receives a signal (in step S 2801 ), the first-specific-signal-movement-condition detector  1401   a  and the second-specific-signal-movement-condition detector  1401   b  simultaneously determine whether movement of a reception signal satisfies the respective conditions. The first-specific-signal-movement-condition detector  1401   a  determines whether the movement of the reception signal satisfies the first specific-signal movement condition (in step S 2802 ). When the determination is affirmative (“Yes” is selected in step S 2802 ), “1” is assigned to a value EN 1  as matching information (in step S 2803 ), a detection result C of the distortion detector  402  is updated (in step S 2804 ), and the process proceeds to step S 2810 . When the determination is negative (“No” in step S 2802 ), “0” is assigned to the value EN 1  (in step S 2805 ), and the process proceeds to step S 2810 . 
     The second-specific-signal-movement-condition detector  1401   b  determines whether the movement of the reception signal satisfies the second specific-signal movement condition (in step S 2806 ). When the determination is affirmative (“Yes” is selected in step S 2806 ), “1” is assigned to a value EN 2  as matching information (in step S 2807 ), a detection result C 2  of the distortion detector  402  is updated (in step S 2808 ), and the process proceeds to step S 2810 . When the determination is negative (“No” is selected in step S 2806 ), “0” is assigned to the value EN 2  (in step S 2809 ), and the process proceeds to step S 2810 . 
     In step S 2810 , the matching information EN 1  and EN 2  are converted into serial signals (in step S 2810 ), and the effective/ineffective signal D 2  and the compensation signal D 1  are transmitted to the distortion compensating unit  102  included in the transmitter  101  (in step S 2811 ). 
       FIG. 32  illustrates a flowchart of the processing for detecting matching of the specific-signal movement condition. The transmitter  101  performs the processing. After transmission is started (in step S 2821 ), the distortion compensating unit  102  included in the transmitter  101  receives the matching information EN 1  or EN 2  (effective/ineffective signal D 2 ) and the distortion detection result C or C 2  (compensation signal D 1 ) (in step S 2822 ). The distortion compensating unit  102  selects the first specific-signal movement condition (in step S 2823 ) when a control loop is in an initial state, for example, immediately after power is supplied, and enhances convergence of the control loop using the detection result C(N) in which the number of generation of movement among signal points which matches the first specific-signal movement condition is large. Here, when the value EN 1  is “1” representing “effective” (“Yes” is selected in step S 2824 ), a distortion compensating amount is updated in accordance with the distortion detection result C (in step S 2825 ). On the other hand, when the value EN 1  is “0” representing “ineffective” (“No” is selected in step S 2824 ), the distortion compensation is performed using a previous distortion compensating amount instead of the distortion detection result C represented by the received compensation signal D 1  (in step S 2826 ). 
     Thereafter, it is determined whether the condition is to be changed (in step S 2827 ). As is described in the seventh embodiment, the distortion compensating unit  102  does not change the condition until a predetermined period of time has passed or while the control loop is not converged (“No” is selected in step S 2827 ), and the process returns to step S 2824 . On the other hand, after the predetermined period of time or after the control loop is converged, the condition is changed (“Yes” is selected in step S 2827 ). 
     As described above, after the convergence of the control loop progresses, distortion compensation is performed using the detection result C 2 (N) which is obtained with high detection accuracy. First, the second specific-signal movement condition is selected (in step S 2828 ). When the value of EN 2  is “1” representing “effective” (“Yes” is selected in step S 2829 ), the distortion compensating amount is updated in accordance with the distortion detection result C 2  (in step S 2830 ). On the other hand, when the value of EN 2  is “0” representing “ineffective” (“No” is selected in step S 2829 ), distortion compensation is performed using a previous distortion compensating amount instead of the distortion detection result C 2  represented by the received compensation signal D 1  (in step S 2831 ). 
     With this configuration, the distortion compensation can be performed taking a state of moving and passing among signal points having maximum power into consideration. Accordingly, spectrum deterioration which occurs at a transmission antenna terminal can be compensated for. In addition, since matching with the specific-signal movement conditions can be simultaneously detected, oversight of condition matching which occurs in a configuration in which the detection conditions are switched from one to another can be prevented. Accordingly, the control loop can be quickly converged from the initial state, and in addition, high accuracy of the distortion compensation after the convergence can be ensured. Furthermore, as a modification of the configuration described above, the distortion detector  402  may supply the compensation signal D 1  to the P/S converting circuit  2601 , and the compensation signal D 1  and the effective/ineffective signal D 2  may be collectively converted into serial signals. 
     Ninth Embodiment 
       FIG. 33  illustrates a configuration of a nonlinear distortion compensation apparatus according to a ninth embodiment. The ninth embodiment is a modification of the eighth embodiment ( FIG. 29 ) and is configured such that it is determined whether a reception signal matches a first or second specific-signal movement condition and a method for transmitting a detection result is changed depending on the condition. 
     Examples of a specific-signal movement condition in which a result of distortion detection is enabled include, as with the eighth embodiment, a first specific-signal movement condition in which convergence of a control loop is prioritized and a second specific-signal movement condition in which accuracy of distortion compensation is prioritized. In addition, the second specific-signal movement condition includes content of the first specific-signal movement condition so that the second specific-signal movement condition includes a large number of condition items (in a range represented by a dotted line shown in  FIG. 34 ). Accordingly, when movement of a reception signal satisfies the first specific-signal movement condition, a detection result corresponding to the first specific-signal movement condition is transmitted. On the other hand, when the movement of the reception signal satisfies the second specific-signal movement condition, a detection result corresponding to the second specific-signal movement condition is transmitted. By this, a distortion compensating unit  102  performs distortion compensation in accordance with the first specific-signal movement condition until the movement of the reception signal satisfies the second specific-signal movement condition, and after the second specific-signal movement condition is selected, distortion compensation in accordance with the second specific-signal movement condition is performed. 
     In the configuration shown in  FIG. 33 , a selection circuit (SEL)  2901  selects and outputs a detection result (effective/ineffective signal D 2 ) of the matched first or second specific-signal movement condition and a distortion detection result (compensation signal D 1 ) corresponding to the specific-signal movement condition. 
       FIG. 34  illustrates a state of detection of movement of specified signal points detected by first and second specific-signal movement detectors  1401   a  and  1401   b . A state in which a reception signal moves among signal points is the same as that of the eighth embodiment ( FIG. 30 ). When the second-specific-signal-movement-condition detector  1401   b  determines that the movement of the reception signal among signal points satisfies the second specific-signal movement condition, the first specific-signal movement condition is also satisfied (the range represented by the dotted line shown in  FIG. 34 ). When only the first specific-signal movement condition is satisfied, the selection circuit  2901  transmits a distortion detection result C corresponding to the first specific-signal movement condition as a compensation signal D 1 . When the second specific-signal movement condition is satisfied, the selection circuit  2901  transmits a distortion detection result C 2  corresponding to the second specific-signal movement condition as a compensation signal D 1  to the distortion compensating unit  102 . Simultaneously, when the second specific-signal movement condition is satisfied (EN 1 =1 and EN 2 =1 in  FIG. 34 ), the selection circuit  2901  transmits EN 1 =1 and EN 2 =1 to the distortion compensating unit  102  as an effective/ineffective signal D 2 . 
       FIG. 35  illustrates a flowchart of processing for detecting matching of a specific-signal movement condition. A receiver  1200  performs this processing. When the receiver  1200  receives a signal (in step S 3101 ), the first-specific-signal-movement-condition detector  1401   a  and the second-specific-signal-movement-condition detector  1401   b  simultaneously determine whether movement of a reception signal satisfies the respective conditions. The first-specific-signal-movement-condition detector  1401   a  determines whether the movement of the reception signal satisfies the first specific-signal movement condition (in step S 3102 ). When the determination is affirmative (“Yes” is selected in step S 3102 ), a detection result C of the distortion detector  402  is updated (in step S 3103 ), and the process proceeds to step S 3105 . When the determination is negative (“No” is selected in step S 3102 ), “0” is assigned to the values EN 1  and EN 2  (representing an ineffective distortion detection result) and the values EN 1  and EN 2  are transmitted to a transmitter  101  (in step S 3104 ). 
     In step S 3105 , the second-specific-signal-movement-condition detector  1401   b  determines whether the second specific-signal movement condition is satisfied. When the determination is affirmative (“Yes” is selected in step S 3105 ), the detection result C of the distortion detector  402  is updated to the detection result C 2  (in step S 3106 ). Then, the second-specific-signal-movement-condition detector  1401   b  assigns “1” to the values of EN 1  and EN 2  (the first and second specific-signal movement conditions are satisfied) (in step S 3107 ), and transmits the distortion detection result C 2  to the transmitter  101  (in step S 3107 ). On the other hand, when the determination is negative (“No” is selected in step S 3105 ), “1” is assigned to the value EN 1  of the matching information and “0” is assigned to the value EN 2  (representing that only the first specific-signal movement condition is satisfied), and the distortion detection result C is transmitted to the transmitter  101  (in step S 3108 ). 
       FIG. 36  illustrates a flowchart of the processing for detecting matching of a specific-signal movement condition. The transmitter  101  performs this processing. After transmission is started (in step S 3111 ), the distortion compensating unit  102  included in the transmitter  101  receives the matching information EN 1  and EN 2  (effective/ineffective signal D 2 ) and the distortion detection result C (compensation signal D 1 ) transmitted from the receiver  1200  (in step S 3112 ). The distortion compensating unit  102  selects the first specific-signal movement condition when a control loop is in an initial state, for example, immediately after power is supplied (in step S 3113 ), and progresses convergence of the control loop using the detection result C(N) in which a large number of movements among signal points which satisfy the first specific-signal movement condition are generated. Next, the values which have been assigned to the EN 1  and EN 2  are determined (in step S 3114 ). When the “1” representing “effective” has been assigned to the “EN 1 ” (the value of EN 2  is negligible) (EN 1 =1 in step S 3114 ), a distortion compensating amount is updated in accordance with the distortion detection result C (in step S 3115 ). On the other hand, if the values of EN 1  and EN 2  are “0” representing “ineffective” (EN 1 =0 and EN 2 =0 in step S 3114 ), distortion compensation is performed while a previous compensating amount is maintained without using the distortion detection result C represented by the received compensation signal D 1  (in step S 3116 ). 
     Thereafter, it is determined whether the condition is to be changed (in step S 3117 ). As is described in the seventh embodiment, the distortion compensating unit  102  does not change the condition until a predetermined period of time has passed or while the control loop is not converged (“No” is selected in step S 3117 ), and the process returns to step S 3114 . On the other hand, after the predetermined period of time or after the control loop is converged, the condition is changed (“Yes” is selected in step S 3117 ). 
     As described above, after the convergence of the control loop progresses, distortion compensation is performed using the detection result C 2 (N) which is obtained with high detection accuracy. First, the second specific-signal movement condition is selected (in step S 3118 ). When the value of EN 2  is “1” representing “effective” (EN 2 =1 in step S 3119 ), the distortion compensating amount is updated in accordance with the distortion detection result C 2  (in step S 3120 ). On the other hand, when the value of EN 2  is “0” representing “ineffective” (EN 2 =0 in step S 3119 ), distortion compensation is performed using a previous distortion compensating amount instead of the distortion detection result C 2  represented by the received compensation signal D 1  (in step S 3121 ). 
     With this configuration, the distortion compensation can be performed taking a state of moving and passing among signal points having maximum power into consideration. Accordingly, spectrum deterioration which occurs at a transmission antenna terminal can be compensated for. In addition, since matching with the specific-signal movement conditions can be simultaneously detected, oversight of condition matching which occurs in a configuration in which the detection conditions are switched from one to another can be prevented. Furthermore, when the control loop is in an initial state, the convergence of the control loop is enhanced using the detection result in which a large number of movements among signal points corresponding to the first specific-signal movement condition are generated, and after the convergence of the loop progresses, distortion compensation can be performed with high accuracy using only the detection result which has high accuracy and which corresponds to the second specific-signal movement condition. 
     Tenth Embodiment 
       FIG. 37  illustrates a configuration of a nonlinear distortion compensation apparatus according to a tenth embodiment. In the tenth embodiment, a distortion detection result is ineffective when a difference between a coordinate of a received signal point and a regular coordinate is larger than a predetermined threshold value. 
     In  FIG. 37 , a receiving unit  106  supplies a signal to a signal-point error detector  3201 . The signal-point error detector  3201  detects a distance (difference) between a signal-point coordinate of the reception signal and a regular signal-point coordinate. When the difference is larger than a reference value, an ineffective signal D 2  is output. A transmitter  101  includes a compensating-amount controller  3202 . The compensating-amount controller  3202  receives a compensation signal D 1  and an effective/ineffective signal D 2 . When receiving the ineffective signal D 2 , the compensating-amount controller  3202  outputs an ineffective compensation signal D 1  which is ineffective to a distortion compensating unit  102 . The compensating-amount controller  3202  may be included in the receiver  1200 . 
       FIG. 38  illustrates an example of detection performed by the signal-point error detector  3201 . An axis of abscissa denotes time and an axis of ordinate denotes an error (difference). A threshold value of a predetermined range from a point smaller than the center between the coordinate of the received signal point and the regular coordinate to a point larger than the center is set to the signal-point error detector  3201 . The signal-point error detector  3201  enables a distortion detection result while the signal-point coordinate of the reception signal is included in the range of the threshold value with the regular signal-point coordinate as the center whereas the signal-point error detector  3201  disables the distortion detection result while the signal-point coordinate of the reception signal is out of the range of the threshold value, that is, during a period T 3 . When fading is generated, the signal-point coordinate of the reception signal is out of the range of the threshold value. Accordingly, a distortion detection result including detection of distortion due to fading can be made ineffective. 
       FIG. 39  illustrates a flowchart of processing for controlling distortion compensation according to the tenth embodiment. Processing performed by the signal-point error detector  3201  is extracted and shown in  FIG. 39 . When a receiving unit  106  receives a signal (in step S 3301 ), a distance (difference e) between a regular signal-point coordinate and a signal-point coordinate of the reception signal after the reception signal is reproduced is calculated (in step S 3302 ). Next, an absolute value of the difference e is compared with a threshold value. Specifically, when a formula |e|≦(threshold value) is satisfied (“Yes” is selected in step S 3303 ), an effective signal D 2  which enables the distortion detection result is transmitted to the transmitter  101  (in step S 3304 ). On the other hand, when a formula |e|&gt;(threshold value) is satisfied (“No” is selected in step S 3303 ), an ineffective signal D 2  which disables the distortion detection result is transmitted to the transmitter  101  (in step S 3305 ). 
     With this configuration, irrespective of magnitude of power at a signal point, deviation (error) from a regular position of the signal point is monitored, and when the error is larger than the reference value, it is determined that fading, for example, is generated. Thereafter, the distortion detection result including detection of distortion due to the fading is disabled. By this, distortion compensation can be performed without influence of deterioration of a waveform which occurs due to fading, for example, generated in a wireless transmission path, and accuracy of compensation is prevented from being deteriorated. 
     Eleventh Embodiment 
       FIG. 40  illustrates a configuration of a nonlinear distortion compensation apparatus according to an eleventh embodiment. In the eleventh embodiment, reception power of a reception signal is detected, and a distortion detection result is disabled when the reception power is smaller than a predetermined threshold value. 
     In  FIG. 40 , a receiving unit  106  outputs a signal to a reception-level deterioration detector  3401 . The reception-level deterioration detector  3401  monitors the reception power, and when the reception power becomes smaller than the predetermined threshold value, an ineffective signal D 2  is output. A transmitter  101  includes, as with the tenth embodiment, a compensating-amount controller  3202 . The compensating-amount controller  3202  receives a compensation signal D 1  and an effective/ineffective signal D 2 . When receiving the ineffective signal D 2 , the compensating-amount controller  3202  outputs the ineffective compensation signal D 1  to a distortion compensating unit  102 . The compensating-amount controller  3202  may be included in a receiver  1200 . 
       FIG. 41  illustrates an example of detection performed by the reception-level deterioration detector  3401 . An axis of abscissa denotes time and an axis of ordinate denote the reception power. A threshold value which is determined on the basis of predetermined reception power is set to the reception-level deterioration detector  3401 . Then, the reception-level deterioration detector  3401  enables the distortion detection result while the power of the reception signal is equal to or larger than the threshold value, whereas the reception-level deterioration detector  3401  disables the distortion detection result in a period T 3  in which the power of the reception signal is smaller than the threshold value. The power of the reception signal becomes smaller than the threshold value due to fading or rainfall, for example, and a distortion detection result including detection of distortion due to the fading and the rainfall can be disabled. 
       FIG. 42  illustrates a flowchart of processing for controlling distortion compensation according to the eleventh embodiment. Processing performed by the reception-level deterioration detector  3401  is extracted and shown in  FIG. 42 . When the receiving unit  106  receives a signal (in step S 3501 ), a reception power Pr of the reception signal is detected (in step S 3502 ). Then, the reception power Pr is compared with the threshold value. Specifically, when the reception power Pr is equal to or larger than the threshold value (“Yes” in step S 3503 ), an effective signal D 2  which enables a distortion detection result is transmitted to the transmitter  101  (in step S 3504 ). On the other hand, when the reception power Pr is smaller than the threshold value (“No” in step S 3503 ), an ineffective signal D 2  which disables the distortion detection result is transmitted to the transmitter  101  (in step S 3505 ). 
     With this configuration, the reception power of the reception signal is detected, and when the reception power is smaller than the threshold value, it is determined that this is caused by fading or rainfall. Accordingly, a distortion detection result including detection of distortion due to fading and rainfall, for example, is disabled. By this, distortion compensation can be performed without influence of deterioration of a waveform due to deterioration of a signal to noise ratio (SNR) caused by deterioration of the reception power due to fading generated in a wireless transmission path or rainfall, and deterioration of accuracy of the compensation can be avoided. 
     Twelfth Embodiment 
       FIG. 43  illustrates a configuration of a nonlinear distortion compensation apparatus according to a twelfth embodiment. In the twelfth embodiment, deterioration of an error ratio of a reception reproduction signal is detected in digital signal processing, and when the error ratio becomes smaller than a predetermined threshold value, a distortion detection result is enabled. 
     In  FIG. 43 , a digital-signal processing unit  3601  which is connected to a receiving unit  106  reproduces a reception signal and performs certain processing such as parity check on a reproduction digital signal. When a result of the parity check is smaller than a predetermined threshold value, an ineffective signal D 2  is output. A transmitter  101  includes, as with the tenth embodiment, a compensating-amount controller  3202 . The compensating-amount controller  3202  receives a compensation signal D 1  and an effective/ineffective signal D 2 . When receiving the ineffective signal D 2 , the compensating-amount controller  3202  outputs the ineffective compensation signal D 1  which is a result of distortion detection and which is set to be ineffective to a distortion compensating unit  102 . The compensating-amount controller  3202  may be included in a receiver  1200 . 
       FIG. 44  illustrates an example of detection of an error ratio detected by the digital-signal processing unit  3601 . An axis of abscissa denotes time and an axis of ordinate denotes the error ratio. A threshold value which is determined on the basis of a predetermined error ration Pe is set to the digital-signal processing unit  3601 . While the error ratio is equal to or smaller than the threshold value, the digital-signal processing unit  3601  enables a distortion detection result. On the other hand, during a period T 3  in which the error ratio is larger than the threshold value, the digital-signal processing unit  3601  disables the result of the distortion detection. The error ratio exceeds the threshold value when a level of identification of a reception signal point is deteriorated, and the distortion detection result can be disabled. 
       FIG. 45  illustrates a flowchart of processing for controlling distortion compensation according to the twelfth embodiment. Processing performed by the digital-signal processing unit  3601  is extracted and shown in  FIG. 45 . A receiving unit  106  receives a signal (in step S 3701 ), and the digital-signal processing unit  3601  detects the error ratio Pe when a digital signal is reproduced (in step S 3702 ). Then the error ratio Pe is compared with the threshold value. Specifically, when the error ratio Pe is equal to or smaller than the threshold value (“Yes” is selected in step S 3703 ), an effective signal D 2  representing an effective distortion detection result is transmitted to the transmitter  101  (in step S 3704 ). On the other hand, when the error ratio Pe is larger than the threshold value (“No” is selected in step S 3703 ), an ineffective signal D 2  representing that the distortion detection result is ineffective is transmitted to the transmitter  101  (in step S 3705 ). 
     With this configuration, when the error ratio at a time of reproduction of the reception signal is detected, a level of identification of the reception signal point is deteriorated, and it is determined that a distortion detection result obtained using signal points or passing coordinates at a time of movement among signal points includes an error, the distortion detection result is disabled. By this, influence of specific-signal movement detection using the reception signal which is misidentified can be eliminated, and deterioration of accuracy of compensation can be avoided. 
     Thirteenth Embodiment 
       FIG. 46  illustrates a configuration of a nonlinear distortion compensation apparatus according to a thirteenth embodiment. In the thirteenth embodiment, when a distortion compensating unit  102  which performs distortion compensation on a signal output from an amplifying circuit  115  (refer to  FIG. 2 ) included in a transmitter  101  is controlled in accordance with an expression, control processing is facilitated. 
     The transmitter  101  includes a coefficient setting unit  3801  which controls the distortion compensating unit  102  by setting a coefficient in accordance with an input compensation signal D 1 . The distortion compensating unit  102  approximates an input-output characteristic x of the amplifying circuit  115  by an approximate expression F(x)=K 1 ·x+K 3 ·x 3 +K 5 ·x 5 +K 7 ·x 7  and performs distortion compensation in accordance with the approximate expression. The coefficient setting unit  3801  includes a coefficient controller  3802  and coefficient generators  3803 . The coefficient generators  3803  transmit respective coefficients K 1 , K 3 , K 5 , and K 7  to the distortion compensating unit  102 . The coefficient controller  3802  controls coefficient setting for the coefficients K 1 , K 3 , K 5 , and K 7  of the coefficient generators  3803  in accordance with an input compensation signal D 1 . The distortion compensating unit  102  performs distortion compensation suitable for a state of the amplifying circuit  115  in accordance with the set coefficients. 
     Then, among the coefficients K 1 , K 3 , K 5 , and K 7  which constitute the input-output characteristic of the amplifying circuit  115  which has been subjected to mathematization, the high-order coefficients K 5  and K 7  which less affect change of the input-output characteristic are fixed to initial values, and the low-order coefficients K 1  and K 3  which considerably affect is variably controlled using the coefficient controller  3802 . By this, complicated control of the distortion compensating unit  102  which controls a compensating amount in accordance with the expression can be avoided and distortion compensation can be controlled so as to be suitable for the state of the amplifying circuit  115 . 
     With this configuration, in a case where the input-output characteristic of the amplifying circuit  115  is subjected to mathematization so that distortion compensation is controlled, the distortion compensation is easily controlled by assigning fixed values to the high-order coefficients K 5  and K 7  which less affect a compensating amount. 
     Fourteenth Embodiment 
       FIG. 47  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fourteenth embodiment. In the fourteenth embodiment, when a distortion compensating unit  102  which performs distortion compensation on a signal output from an amplifying circuit  115  (refer to  FIG. 2 ) included in a transmitter  101  is controlled in accordance with an expression, control processing is facilitated. 
     As with the thirteenth embodiment, a coefficient controller  3802  sets coefficients K 1 , K 3 , K 5 , and K 7 . The coefficient controller  3802  receives a compensation signal D 1  which is a distortion detection result and an effective/ineffective signal D 2  which has been described in the foregoing embodiments. In accordance with the compensation signal D 1  and the effective/ineffective signal D 2 , the coefficient controller  3802  determines constants (A 1 , A 3 , A 5 , and A 7 ) having appropriate changing amounts and changing directions (increase and decrease, i.e., plus and minus) for the corresponding coefficient in advance, and outputs them. 
     Then, in accordance with information representing an effective signal D 2  or information representing an ineffective signal D 2 , the coefficient controller  3802  adds the constants A which have been weighted for individual coefficients to one another or subtracts the constants A from one another and outputs a result. The coefficients K 1 , K 3 , K 5 , and K 7  are integrated by integrating circuits  3803   a  included in the coefficient generators  3803  and are output. 
     When the distortion compensating unit  102  performs approximation by the approximate expression F(x)=K 1 ·x+K 3 ·x 3 +K 5 ·x 5 +K 7 ·x 7  so as to perform distortion compensation in accordance with the coefficients included in the approximate expression, appropriate changing amounts (A 1 , A 3 , A 5 , and A 7 ) and changing directions (increase and decrease, i.e., plus and minus) are determined in advance for individual coefficients. When the ineffective signal D 2  is transmitted, control is performed so as to add a value “0”. In the integrating circuits  3803   a , appropriate initial values corresponding to the coefficients K 1 , K 3 , K 5 , and K 7  are set. In this way, only by setting the appropriate initial values corresponding to the coefficients K 1 , K 3 , K 5 , and K 7  in the integrating circuits  3803   a , by instructing the coefficient controller  3802  to perform uniform addition (+) or uniform subtraction (−) to be performed on the constants A 1 , A 3 , A 5 , and A 7  to which magnitude and polarity±corresponding to the coefficients K 1 , K 3 , K 5 , and K 7  are set, and by controlling the addition of a value “0” when the ineffective signal D 2  is transmitted, the distortion compensating unit  102  can be easily controlled. Alternatively, the distortion compensating unit  102  can be controlled only using a distortion detection result of one bit represented by the compensation signal D 1  and a signal of one bit representing “effective”/“ineffective” of the distortion detection result represented by the effective/ineffective signal D 2 . 
     With the configuration described above, when the input-output characteristic of the amplifying circuit  115  is subjected to mathematization so that distortion compensation control is performed, the coefficients constituting the input-output characteristic of the amplifying circuit  115  which has been mathematization can be easily updated, and the distortion compensating unit  102  can be easily controlled. 
     Fifteenth Embodiment 
       FIG. 48  illustrates a configuration of a nonlinear distortion compensation apparatus according to a fifteenth embodiment. In the fifteenth embodiment, when a distortion compensating unit  102  which performs distortion compensation on a signal output from an amplifying circuit  115  (refer to  FIG. 2 ) included in a transmitter  101  is controlled in accordance with an expression, following capability and stability for changing of a distortion amount are attained. 
     A coefficient controller  3802  which is similar to that described in the fourteenth embodiment includes an integrating controller  4001  which performs control of addition/subtraction or addition using a value “0” in integrating circuits  3803   a , an absolute-value controller  4002  which controls absolute values of constants A 1 , A 3 , A 5 , and A 7  which are to be input in the integrating circuits  3803   a  in accordance with an input compensation signal D 1  and an input effective/ineffective signal D 2 , and multipliers  4004  which multiply the constants A 1 , A 3 , A 5 , and A 7  which have been set by constant setting units  4003  by an absolute-value control signal output from the absolute-value controller  4002  and which multiply the constants A 1 , A 3 , A 5 , and A 7  by an integrating control signal output from the integrating controller  4001 . 
     Each of the constants A 1 , A 3 , A 5 , and A 7  has positive or negative polarity. In accordance with a control signal output from the absolute-value controller  4002 , magnitudes of the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are controlled at a uniform ratio. Constants A 1   a , A 3   a , A 5   a , and A 1   a  obtained after controlling the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are multiplied by a control signal of ±1 and 0 supplied from the integrating controller  4001  and are supplied to the corresponding integrating circuits  3803   a.    
       FIG. 49  illustrates a flowchart of processing for controlling distortion compensation according to the fifteenth embodiment. Processing performed by the coefficient controller  3802  is extracted and shown in  FIG. 49 . When distortion compensation is started (in step S 4101 ), first, the absolute-value controller  4002  outputs an absolute value as an initial value K (in step S 4102 ). Thereafter, it is determined whether a condition is to be changed (in step S 4103 ). Here, as is described in the seventh embodiment, the distortion compensating unit  102  does not change the condition until a predetermined period of time has passed or while a control loop is not converged (the process proceeds to “No” and enters a loop in step S 4103 ). On the other hand, the distortion compensating unit  102  changes the condition after the predetermined period of time or after the control loop is converged (“Yes” is selected in step S 4103 ). 
     In this way, the absolute value to which the initial value is assigned is changed to be increased or reduced (in step S 4104 ). When a changing amount of a control signal is to become large, the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are increased. On the other hand, the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are reduced so that the changing amount of the control signal is reduced in order to attain stable operation. In this way, weighting of the constants is changed depending on a condition. 
     With this configuration, in a distortion compensation circuit which controls the coefficients by performing addition or subtraction on the constants which have been weighted for individual coefficients, when a large change of a compensating amount is required, appropriate constants are selected so that control speed is prioritized whereas when stability of the compensating amount is required, appropriate constants are selected so that the control stability is prioritized. In this way, following capability and stability for changing of a distortion amount are attained. 
     Sixteenth Embodiment 
       FIG. 50  illustrates a configuration of a nonlinear distortion compensation apparatus according to a sixteenth embodiment. The sixteenth embodiment which is a modification of the fifteenth embodiment detects convergence of a control loop and attains stable changing of constants before convergence to constants after convergence by changing absolute values of the constants in accordance with a result of the detection of the convergence. 
     In a configuration shown in  FIG. 50 , a compensation signal D 1  and an effective/ineffective signal D 2  are also supplied to a convergence detector  4201 . The convergence detector  4201  detects the convergence of the control loop in accordance with a result of a determination as to whether distortion has occurred using the compensation signal D 1  and the effective/ineffective signal D 2  and controls an absolute-value controller  4002 . Each of the constants A 1 , A 3 , A 5 , and A 7  has positive or negative polarity. In accordance with a control signal output from the absolute-value controller  4002 , magnitudes of the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are controlled at a uniform ratio. Constants A 1   a , A 3   a , A 5   a , and A 7   a  obtained after controlling the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are multiplied by a control signal of ±1 and 0 supplied from the integrating controller  4001  and are supplied to the corresponding integrating circuits  3803   a.    
       FIG. 51  illustrates a flowchart of processing for controlling distortion compensation according to the sixteenth embodiment.  FIG. 52  illustrates a state of generation of distortion detection results. An axis of abscissa denotes time and an axis of ordinate denotes a distortion detection result (compensation signal D 1 ). A value “1” of the distortion detection result (compensation signal D 1 ) represents that the result is larger than a reference value, that is, distortion is not detected or overcompensation whereas a value “0” represents that the result is smaller than the reference value, that is, distortion is generated. 
     When distortion compensation is started (in step S 4301 ), the absolute-value controller  4002  outputs an absolute value as an initial value K (in step S 4302 ). Thereafter, the convergence detector  4201  monitors a ratio of generation (appearance ratio) of the distortion detection result “1, 0” (in step S 4303 ). As shown in  FIG. 52 , for example, in a first stage, the value “1” frequently appears as the value of the distortion detection result (compensation signal D 1 ). 
     The convergence detector  4201  monitors until the ratio of generation of the distortion detection result “1, 0” is close to 50% (the process proceeds to “No” and enters a loop in step S 4304 ). When the generation ratio is close to 50% (“Yes” is selected in step S 4304 ) as shown in  FIG. 52 , since the control loop is converged, the convergence detector  4201  transmits convergence information to the absolute-value controller  4002 . The absolute-value controller  4002  changes the absolute values of the constants A 1 , A 3 , A 5 , and A 7  to be small in accordance with the convergence information (in step S 4305 ). Accordingly, high accuracy of a distortion compensating amount is attained. 
     Thereafter, the convergence detector  4201  continues to monitor the ratio of generation of the distortion detection result “1, 0” (proceeds to “Yes” and enters a loop in step S 4306  and step S 4307 ). When the generation ratio is out of 50% again as shown in  FIG. 52  (“No” is selected in step S 4307 ), an appropriate distortion compensating amount is not obtained due to change of a state of an amplifying circuit  115  after the convergence of the control loop, and the generation ratio of the values “1” and “0” of the distortion detection result is not balanced. Therefore, the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are increased (in step S 4308 ). Thereafter, the process returns to step S 4303  and the control is continued. 
     With this configuration, after the distortion compensation is started, the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are reduced so that the ratio of generation of the distortion detection result “1, 0” of 50% is attained. In this way, high accuracy of the distortion compensating amount and convergence of the control loop is attained. Furthermore, even after convergence of the control loop, if the distortion compensating amount becomes inappropriate due to change of the state of the amplifying circuit, for example, deviation of the ratio of generation of the distortion detection results “1” and “0” is monitored, and the absolute values of the constants A 1 , A 3 , A 5 , and A 7  are made larger. By this, the inappropriate distortion compensating amount is changed to an appropriate distortion compensating amount. As described above, in the distortion compensating circuit which controls the coefficients by addition or subtraction of the coefficients which have been weighted for individual coefficients, the absolute values of the constants to be subjected to addition or subtraction can be changed, the convergence of the control loop is detected, and the absolute values of the constants can be changed in accordance with the result of the detection of convergence. In this way, change of the constants before or after convergence can be made stable. 
     According to the nonlinear distortion compensating apparatus and the method for compensating for nonlinear distortion described above, nonlinear distortion compensation can be easily controlled with high accuracy, and therefore, quality of wireless communication may be improved. 
     Furthermore, according to the nonlinear distortion compensating apparatus and the method for compensating for nonlinear distortion described above, in a digital wireless communication, nonlinear distortion can be detected from a reception signal received by a receiver, information on the detected nonlinear distortion can be transmitted to a transmitter, and the transmitter can perform distortion compensation. Accordingly, nonlinear distortion generated in an amplifying circuit included in the transmitter can be cancelled in the transmitter serving as a transmission source. Consequently, communication quality may improved. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation 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 the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.