Abstract:
A wireless communication system including: a first communication device including: a processor configured to: compensate a distortion to a input signal based on a determined distortion compensation coefficient set, a amplifier configured to amplify the input signal to which the distortion is compensated, and a first antenna configured to: transmit the amplified signal to a second communication device, and receive a first feedback signal from the second communication device, the first feedback signal relating to an error that is detected in a received signal by the second communication device, the received signal corresponding to the transmitted signal, wherein a plurality of distortion compensation coefficient included in the determined distortion compensation coefficient set are adjusted based on the first feedback signal relating to the error that is detected in the received signal by the second communication device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-152112, filed on Jul. 25, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a wireless communication system, a distortion compensation device, and a distortion compensation method. 
     BACKGROUND 
     A wireless transmission device in a wireless communication system includes an amplifier that amplifies the power of a transmission signal. In the wireless transmission device, the amplifier is typically operated in the vicinity of a saturation area of the amplifier in order to increase power efficiency of the amplifier. However, when the amplifier is operated in the vicinity of the saturation area, a non-linear distortion is increased. Therefore, the wireless transmission device includes a distortion compensation device that compensates such a non-linear distortion in order to suppress the non-linear distortion and reduce the adjacent channel leakage ratio (ACLR). 
     One of the distortion compensation schemes used for the distortion compensation device is referred to as a “predistortion (hereinafter may be referred to as “PD”) scheme”. The distortion compensation device of the PD scheme suppresses a distortion of output of the amplifier by multiplying, in advance, a transmission baseband signal before input to the amplifier by a distortion compensation coefficient that has the reverse characteristic of the non-linear distortion of the amplifier and increases the linearity of the output of the amplifier. The signal that has been obtained by multiplying the transmission baseband signal by the distortion compensation coefficient through the multiplier may be referred to as a “PD signal”. Therefore, the PD signal is a signal that has been distorted in advance in accordance with the reverse characteristic of the non-linear distortion of the amplifier before the signal is input to the amplifier. 
     For example, as in the distortion compensation device of the PD scheme in the related art, there is a device including a table that stores a plurality of distortion compensation coefficients, and reads, from the table, a distortion compensation coefficient corresponding to an address value in accordance with the power of a transmission baseband signal. In addition, the distortion compensation device adjusts the read distortion compensation coefficient using an “adjustment coefficient”, and outputs the adjusted distortion compensation coefficient to the multiplier. In addition, the distortion compensation device performs feedback of a part of an output signal of the amplifier in the wireless transmission device, and detects, from the “feedback signal”, a “distortion component” that appears outside the “transmission band (that is, a channel)” applied to the wireless transmission device. In addition, the distortion compensation device may adjust distortion compensation processing (for example, “correct (update) the above-described adjustment coefficient”), based on the detected “distortion component outside the transmission band”. 
     Japanese Laid-open Patent Publication No. 2009-303225 is the related art. 
     SUMMARY 
     According to an aspect of the invention, a wireless communication system comprising: a first communication device including: a memory configured to store a plurality of distortion compensation coefficient sets, each of the plurality of distortion compensation coefficient sets including a plurality of distortion compensation coefficients for compensating distortion that occurs in signals amplified by an amplifier, each of the plurality of distortion compensation coefficient sets being associated with each of powers of signals, a processor configured to: measure a power of an input signal, determine a distortion compensation coefficient set from the plurality of distortion compensation coefficient sets stored in the memory, based on the measured power of the input signal, and compensate the distortion to the input signal based on the determined distortion compensation coefficient set, the amplifier configured to amplify the input signal to which the distortion is compensated, and a first antenna configured to: transmit the amplified signal to a second communication device, and receive a first feedback signal from the second communication device, the first feedback signal relating to an error that is detected in a received signal by the second communication device, the received signal corresponding to the transmitted signal; and the second communication device including: a second antenna configured to: receive the transmitted signal from the first communication device, and transmit the first feedback signal to the first communication device, wherein the plurality of distortion compensation coefficient included in the determined distortion compensation coefficient set are adjusted based on the first feedback signal relating to the error that is detected in the received signal by the second communication device. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a wireless communication system according to a first embodiment; 
         FIG. 2  is a block diagram illustrating an example of a first communication device according to the first embodiment; 
         FIG. 3  is a block diagram illustrating an example of a distortion compensation device according to the first embodiment; 
         FIG. 4  is a diagram illustrating a compensation coefficient table; 
         FIG. 5  is a block diagram illustrating an example of an adjustment unit according to the first embodiment; 
         FIG. 6  is a diagram illustrating processing of the adjustment unit according to the first embodiment; 
         FIG. 7  is a block diagram illustrating an example of a second communication device according to the first embodiment; 
         FIG. 8  is a diagram illustrating calculation of a gap in the first embodiment; 
         FIG. 9  is a diagram illustrating an example of a wireless communication system according to a second embodiment; 
         FIG. 10  is a block diagram illustrating an example of a first communication device according to the second embodiment; 
         FIG. 11  is a block diagram illustrating an example of a distortion compensation device according to the second embodiment; 
         FIG. 12  is a block diagram illustrating an example of a second communication device according to the second embodiment; 
         FIG. 13  is a diagram illustrating an example of a wireless communication system according to a third embodiment; 
         FIG. 14  is a block diagram illustrating an example of a first communication device according to the third embodiment; 
         FIG. 15  is a block diagram illustrating an example of a distortion compensation device according to the third embodiment; 
         FIG. 16  is a block diagram illustrating an example of a second communication device according to the third embodiment; 
         FIG. 17  is a block diagram illustrating an example of a first communication device according to a fourth embodiment; 
         FIG. 18  is a block diagram illustrating an example of a distortion compensation device according to the fourth embodiment; 
         FIG. 19  is a block diagram illustrating an example of a first communication device according to a fifth embodiment; 
         FIG. 20  is a block diagram illustrating an example of a distortion compensation device according to the fifth embodiment; 
         FIG. 21  is a block diagram illustrating an example of a first communication device according to a sixth embodiment; 
         FIG. 22  is a block diagram illustrating an example of a distortion compensation device according to the sixth embodiment; and 
         FIG. 23  is a diagram illustrating a hardware configuration example of a distortion compensation device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     However, in the above-described distortion compensation device in the related art, the distortion compensation processing is adjusted based on the “distortion component outside the transmission band” included in the feedback signal in the wireless transmission device, so that an impact on the communication quality (for example, bit error rate (BER)) of a wireless reception device (that is, a communicating party of the wireless transmission device mounted on the distortion compensation device) due to the distortion compensation processing is not considered. Therefore, it is probable that the communication quality in the wireless reception device is reduced due to the distortion compensation processing. 
     In addition, in the above-described distortion compensation device in the related art, the distortion compensation processing is adjusted based on the “distortion component outside the transmission band”, so that it is probable that the accuracy of the “distortion compensation processing in the transmission band” is not improved. As a result, the communication quality in the wireless reception device may be reduced. It is probable that such a trend has been increasing due to the recent bandwidth widening. 
     The technology discussed herein is made in view of the above-described problem, and an object of the technology discussed herein is to provide a wireless communication system, a distortion compensation device, and a distortion compensation method by which the communication quality is improved. 
     Embodiments of a wireless communication system, a distortion compensation device, and a distortion compensation method discussed herein are described below in detail with reference to drawings. The wireless communication system, the distortion compensation device, and the distortion compensation method discussed herein are not limited to the embodiments. In addition, in the embodiments, the same symbol is assigned to configurations having the same function, and a repeated description is omitted herein. 
     First Embodiment 
     Outline of Wireless Communication System 
       FIG. 1  is a diagram illustrating an example of a wireless communication system according to a first embodiment. In  FIG. 1 , a wireless communication system  1  includes a communication device  10  that amplifies the power of a transmission signal through an amplifier and transmits the signal using a “transmission band”, and a communication device  50  that receives the signal that has been transmitted from the communication device  10 . For example, one of the communication device  10  and the communication device  50  may be a wireless base station and the other communication device may be a wireless terminal device, and both of the communication devices may be wireless terminal devices. Hereinafter, the communication device  10  may be referred to as the “first communication device”, and the communication device  50  may be referred to as the “second communication device”. 
     For example, the communication device  10  has a “compensation coefficient table” that stores a plurality of distortion compensation coefficients that respectively correspond to a plurality of address values. In addition, the communication device  10  calculates an address value in accordance with a power value of a transmission baseband signal. In addition, the communication device  10  reads a distortion compensation coefficient corresponding to the calculated address value from the “compensation coefficient table”. In addition, the communication device  10  adjusts the read distortion compensation coefficient using an “adjustment coefficient”. In addition, the communication device  10  obtains a “PD signal” by multiplying the adjusted distortion compensation coefficient by the above-described transmission baseband signal, up-converts and amplifies the PD signal, and performs transmission of the PD signal. 
     The communication device  50  receives the signal that has been transmitted using a “transmission band” from the communication device  10 , and calculates the “gap (that is, an error)” between the “reception signal point” of the received signal and the “expected signal point” coming from the received signal. In addition, the communication device  50  transmits information on the calculated “gap”, to the communication device  10 , as a “feedback signal”. 
     The communication device  10  corrects (updates) the above-described “adjustment coefficient”, based on the “feedback signal” from the communication device  50 . 
     As described above, the communication device  10  may adjust the “adjustment coefficient”, based on the “feedback signal” from the reception side device (that is, the communication device  50 ), obtained from the signal that has been actually transmitted using the “transmission band”, so that the adjustment coefficient may be adjusted by reflecting the communication quality in the reception side device. In addition, the communication device  10  may adjust the “adjustment coefficient” based on the “feedback signal” from the reception side device (that is, the communication device  50 ), obtained from the signal that has been actually transmitted using the “transmission band”, so that the accuracy of the “distortion compensation processing in the transmission band” may be improved. 
     [Configuration Example of First Communication Device] 
       FIG. 2  is a block diagram illustrating an example of the first communication device according to the first embodiment. In  FIG. 2 , the communication device  10  includes a baseband unit  11 , a distortion compensation device  12 , a known signal output unit  13 , a wireless transmission unit  14 , a circulator  15 , a wireless reception unit  16 , and a reception processing unit  17 . In addition, the wireless transmission unit  14  includes a digital-to-analog converter (DAC)  21 , an up-converter  22 , and a power amplifier (PA)  23 . In addition, the wireless reception unit  16  includes a down-converter  25  and an analog-to-digital converter (ADC)  26 . 
     The known signal output unit  13  outputs a “known signal” at “certain timing”. Here, the “known signal” includes “certain signal points” using the symbol s (“s” is a natural number) on a constellation diagram in accordance with the modulation scheme of the transmission baseband signal. In addition, the “known signal” has an amplitude corresponding to a non-linear domain of the PA  23 . Thus, when the known signal is amplified in the PA  23 , a non-linear distortion is superimposed over the known signal. In addition, the above-described “certain timing” corresponds to, for example, a sub-frame that has been defined in a frame in advance. That is, the known signal output unit  13  inserts the known signal into the transmission signal at the certain timing. Hereinafter, the above-described “known signal” may be referred to as a “calibration bit”. 
     The baseband unit  11  executes baseband processing such as coding processing and modulation processing for the input transmission data, generates a transmission baseband signal In (t), and output the generated transmission baseband signal In (t) (may be referred as to “input signal”) to the distortion compensation device  12 . 
     The distortion compensation device  12  is a distortion compensation device of the PD scheme, and has a “compensation coefficient table” that stores a plurality of distortion compensation coefficients that respectively correspond to a plurality of addresses that respectively correspond to a plurality of power ranges. The distortion compensation device  12  reads a distortion compensation coefficient from the distortion compensation table, based on an address that has been generated in accordance with the power of the transmission baseband signal, with reference to the compensation coefficient table. In addition, the distortion compensation device  12  adjusts the read distortion compensation coefficient using the “adjustment coefficient”, generates a PD signal Out (t) obtained by multiplying the adjusted distortion compensation coefficient by the transmission baseband signal, and outputs the generated PD signal Out (t) to the DAC  21 . In addition, the distortion compensation device  12  corrects (updates) the above-described “adjustment coefficient”, based on the “feedback signal” from the communication device  50 . 
     The DAC  21  converts the input signal (known signal or PD signal) from the digital signal into an analog signal, and outputs the converted signal to the up-converter  22 . 
     The up-converter  22  up-converts the analog signal that has been received from the DAC  21 , and outputs the up-converted signal to the PA  23 . 
     The PA  23  amplifies the power of the up-converted signal, and transmits the signal having the amplified power through the circulator  15  and an antenna. 
     A wireless signal that has been received through the antenna and the circulator  15  is input to the down-converter  25 . In addition, the down-converter  25  down-converts the input wireless signal, and outputs the down-converted signal to the ADC  26 . 
     The ADC  26  converts the down-converted signal from the analog signal to a digital signal, and outputs the converted digital signal to the reception processing unit  17 . 
     The reception processing unit  17  executes certain reception processing (demodulation, decoding, and the like) for the digital signal that has been received from the ADC  26 , and outputs a “feedback signal” included in the obtained received data, to the distortion compensation device  12 . Such a “feedback signal” is transmitted from the communication device  50  as described above. 
     [Configuration Example of Distortion Compensation Device] 
       FIG. 3  is a block diagram illustrating an example of the distortion compensation device according to the first embodiment. In  FIG. 3 , the distortion compensation device  12  includes an address calculation unit  31 , a reading unit  32 , a table storage unit  33 , an adjustment unit  34 , a correction unit  35 , and a multiplication unit  36 . 
     The address calculation unit  31  calculates an address Adr (t) in accordance with a power value of the transmission baseband signal In (t), and outputs the calculated address Adr (t) to the reading unit  32 . That is, the address calculation unit  31  calculates an amplitude (that is, a modular value) of the transmission baseband signal In (t) that is a complex signal, as the address Adr (t). 
     The reading unit  32  reads a distortion compensation coefficient corresponding to the address Adr (t) that has been calculated in the address calculation unit  31 , from the “compensation coefficient table” stored in the table storage unit  33 . 
     The table storage unit  33  has a “compensation coefficient table” that stores the plurality of distortion compensation coefficients that respectively corresponds to the plurality of address values. 
     For example, the table storage unit  33  stores N number of compensation coefficient tables that are orthogonal to each other. That is, the inner product of distortion compensation coefficient vectors of any two of the compensation coefficient tables becomes zero.  FIG. 4  is a diagram illustrating a compensation coefficient table. In  FIG. 4 , the horizontal axis indicates an address value, and the vertical axis indicates a distortion compensation coefficient. For example, when an address value that has been calculated in the address calculation unit  31  is set as “x i ”, the reading unit  32  reads distortion compensation coefficients y 1 , y 2 , . . . and, y N  (may be referred to as “distortion compensation coefficient set”) corresponding to the address value in each compensation coefficient table. In  FIG. 4 , “X in ” includes amplitude values x 1  to x L  that are typical of the transmission baseband signal In (t). In addition, “X” is an essential point vector, and the elements x 01  to x 0m  are selected from “X in ”. 
     The adjustment unit  34  adjusts the distortion compensation coefficient that has been read in the reading unit  32  using an “adjustment coefficient”. The adjusted distortion compensation coefficient is output to the multiplication unit  36 . 
       FIG. 5  is a block diagram illustrating an example of the adjustment unit according to the first embodiment.  FIG. 6  is a diagram illustrating processing of the adjustment unit according to the first embodiment. In  FIG. 5 , the adjustment unit  34  includes an adjustment coefficient calculation unit  41 , a distortion compensation parameter storage unit  42 , a multiplication unit  43 , and an addition unit  44 . 
     The adjustment coefficient calculation unit  41  receives a “basic point vector” from each of the compensation coefficient tables. The hatched portion in  FIG. 6  indicates an element of the basic point vector. In addition, the adjustment coefficient calculation unit  41  obtains a “distortion compensation parameter” from the distortion compensation parameter storage unit  42 . Here, the “distortion compensation parameter” includes an “amplitude component compensation parameter” and a “phase component compensation parameter”. 
     An “amplitude component compensation parameter V am ” and a “phase component compensation parameter V pm ” are respectively represented, for example, by the following formulas (1) and (2). The superscript T in the formula indicates a transpose operation.
 
 V   am   =[a   1   ,a   2   , . . . , a   m ] T   (1)
 
 V   pm   =[p   1   ,p   2   , . . . , p   m ] i   (2)
 
     In addition, the adjustment coefficient calculation unit  41  calculates the “adjustment coefficient” by calculating the inner product between the “distortion compensation parameter” and each of the “basic point vectors”. That is, the “adjustment coefficient” is a scalar value. For example, as illustrated in  FIG. 6 , the adjustment coefficient calculation unit  41  obtains an amplitude adjustment coefficient  1  by calculating the inner product between the “basic point vector” that has been received from a compensation coefficient table  1  and the “amplitude component compensation parameter”. In addition, the adjustment coefficient calculation unit  41  obtains a phase adjustment coefficient  1  by calculating the inner product between the “basic point vector” that has been received from the compensation coefficient table  1  and the “phase component compensation parameter”. Amplitude adjustment coefficients  2  to N and phase adjustment coefficients  2  to N are calculated similarly. 
     The “adjustment coefficient” that has been calculated in the adjustment coefficient calculation unit  41  is output to the multiplication unit  43 . 
     The distortion compensation parameter storage unit  42  stores the above-described “distortion compensation parameter”. The stored “distortion compensation parameter” is corrected (updated) by the correction unit  35  described later. 
     The multiplication unit  43  multiplies the distortion compensation coefficient that has been read from each compensation coefficient table in the reading unit  32 , by the “adjustment coefficient” corresponding to the compensation coefficient table that is the read source of the distortion compensation coefficient, and outputs the multiplication result to the addition unit  44 . 
     The addition unit  44  calculates the total sum of the multiplication results that have been obtained in the multiplication unit  43  for the amplitude adjustment coefficients. The total sum that has been obtained by such calculation may be referred to as an “amplitude predistortion value”. The “amplitude predistortion value” corresponds to an amplitude component of the adjusted distortion compensation coefficient. In addition, the addition unit  44  calculates the total sum of the multiplication results that have been obtained in the multiplication unit  43  for the phase adjustment coefficients. The total sum that has been obtained by such calculation may be referred to as a “phase predistortion value”. The “phase predistortion value” corresponds to a phase component of the adjusted distortion compensation coefficient. 
     The multiplication unit  36  multiplies the adjusted distortion compensation coefficient by the above-described transmission baseband signal, and outputs the obtained “PD signal” to the DAC  21 . 
     The correction unit  35  corrects (updates) the “adjustment coefficient” used in the adjustment unit  34 , based on the “feedback signal” from the communication device  50 . For example, the correction unit  35  corrects (updates) the adjustment coefficient by correcting (updating) the distortion compensation parameter stored in the distortion compensation parameter storage unit  42 . As the correction method, for example, a gradient descent method may be used. When the gradient descent method is used, update of the “amplitude component compensation parameter” and update of the “phase component compensation parameter” may be respectively represented by the following formulas (3) and (4).
 
 V   am ( n+ 1)= V   am ( n )−μ a ·(∂ J ( n )/∂ V   am )  (3)
 
 V   pm ( n+ 1)= V   pm ( n )−μ p ·(∂ J ( n )/∂ V   pm )  (4)
 
     Here, “J(n)” is the value of a “feedback signal” of the n-th repetition (that is, a gap amount). In addition, “μ a ” and “μ p ” are step lengths to be updated. 
     Here, when the value of a “feedback signal” from the communication device  50  (that is, a gap amount) is less than a “threshold value”, the correction unit  35  does not execute the above-described correction (update), and when the value of the “feedback signal” from the communication device  50  is the “threshold value” or more, the correction unit  35  may execute the above-described correction (update). 
     [Configuration Example of Second Communication Device] 
       FIG. 7  is a block diagram illustrating an example of the second communication device according to the first embodiment. In  FIG. 7 , the communication device  50  includes a wireless reception unit  51 , a reception processing unit  52 , a gap calculation unit  53 , a transmission processing unit  54 , and a wireless transmission unit  55 . 
     The wireless reception unit  51  executes certain wireless reception processing (down-conversion, analog-to-digital conversion, and the like) for a signal that has been received through an antenna, and outputs the signal to the reception processing unit  52 . 
     The reception processing unit  52  executes certain reception processing (demodulation and the like) for the signal that has been received from the wireless reception unit  51 , and outputs the signal to the gap calculation unit  53 . Specifically, the reception processing unit  52  outputs the signal corresponding to the “known signal” that has been transmitted from the communication device  10 , to the gap calculation unit  53 . 
     The gap calculation unit  53  calculates the “gap” between the “reception signal point” of the signal corresponding to the “known signal”, which has been received from the reception processing unit  52 , and the “expected signal point” coming from the signal. Here, as described above, the “known signals” correspond to “certain signal points” using the symbol s (“s” is a natural number) on a constellation diagram in accordance with the modulation scheme of the transmission baseband signal. That is, the “known signals” correspond to symbols, for example, “11” symbols on the constellation diagram in accordance with quadrature phase shift keying (QPSK). In this case, the “reception signal points” are “11”. 
     For example, the gap calculation unit  53  may calculate, as the “gap”, an “error vector” in which the “expected signal point” is used as a starting point, and the “reception signal point” is used as an ending point. That is, the “error vector” is represented by an angle made by a reference line and a line segment extending from the “expected signal point” to the “reception signal point”, and the length of the line segment. Alternatively, the gap calculation unit  53  may calculate, as the “gap”, a “modulation error ratio (MER)” based on the “error vector” in which the “expected signal point” is used as the starting point, and the “reception signal point” is used as the ending point.  FIG. 8  is a diagram illustrating the calculation of a gap in the first embodiment. In the left diagram of  FIG. 8 , a constellation diagram of QPSK is illustrated. The right diagram in  FIG. 8  is a diagram in which the first quadrant of the left diagram of in  FIG. 8  has been enlarged. In the constellation diagram illustrated in the right diagram of  FIG. 8 , the position indicated by a cross indicates the position of the “expected signal point”. As descried above, the gap calculation unit  53  may calculate, as the “gap”, an “error vector” between the k-th “reception signal point” illustrated in the right diagram of  FIG. 8  and the “expected signal point”. Alternatively, the gap calculation unit  53  may calculate, as the “gap”, a “MER” based on the “error vector group” between the “expected signal point” and s “reception signal points” illustrated in the right diagram of  FIG. 8 , and the following formula (5). 
     
       
         
           
             
               
                 
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     The transmission processing unit  54  executes certain transmission processing (coding, modulation, and the like) for the feedback signal including information on the “gap” that has been calculated in the gap calculation unit  53 , and outputs the signal to the wireless transmission unit  55 . 
     The wireless transmission unit  55  executes certain wireless transmission processing (digital-to-analog conversion, up-conversion, and the like) for the feedback signal that has been received from the transmission processing unit  54 , and transmits the feedback signal through the antenna. 
     [Operation Example of Wireless Communication System] 
     A processing operation example of the wireless communication system  1  including the above-described configuration is described below. 
     As described above, the communication device  10  amplifies the “known signal (that is, a calibration bit)” in the PA  23  at “certain timing”, and transmits the known signal to the communication device  50 . 
     When the communication device  50  receives the known signal that has been transmitted from the communication device  10 , the communication device  50  calculates the gap between the reception signal point of the received known signal and the expected signal point coming from the known signal. In addition, the communication device  50  transmits a feedback signal including information on the calculated gap, to the communication device  10 . 
     When the communication device  10  receives the feedback signal from the communication device  50 , the communication device  10  corrects (updates) the “adjustment coefficient” used in the adjustment unit  34 , based on the feedback signal, in the correction unit  35 . 
     As described above, in the embodiment, in the distortion compensation device  12 , the correction unit  35  corrects (updates) the adjustment coefficient used in the adjustment unit  34 , based on the feedback signal sent back from the communication device  50  that has received the signal transmitted from the communication device  10 . The feedback signal includes information on the gap between the reception signal point of the known signal that has been transmitted from the communication device  10 , and the expected signal point coming from the known signal. 
     Due to such a configuration of the distortion compensation device  12 , the “adjustment coefficient” may be adjusted based on the “feedback signal” from the reception side device (that is, the communication device  50 ), obtained from the signal that has been actually transmitted using the “transmission band”, so that the adjustment coefficient may be adjusted by reflecting the communication quality in the reception side device. In addition, due to such a configuration of the distortion compensation device  12 , the “adjustment coefficient” may be adjusted based on the “feedback signal” from the reception side device (that is, the communication device  50 ), obtained from the signal that has been actually transmitted using the “transmission band”, so that the accuracy of the “distortion compensation processing in the transmission band” may be improved. In addition, it may be avoided that a feedback path is provided in the communication device  10 , so that an increased in the circuit scale of the communication device  10  may be avoided. 
     In addition, the known signal (that is, a calibration bit) has an amplitude corresponding to the non-linear domain of the PA  23 . Therefore, a non-linear distortion component may be appropriately superimposed over the known signal output from the PA  23 . 
     In addition, in the communication device  50 , the gap calculation unit  53  calculates the “gap” between the “reception signal point” of the signal corresponding to the received “known signal”, and the “expected signal point” coming from the signal. The gap calculation unit  53  may calculate, as the “gap”, an “error vector” in which the “expected signal point” is used as a starting point, and the “reception signal point” is used as an ending point. Alternatively, the gap calculation unit  53  may calculate, as the “gap”, a “MER” based on the “error vector” in which the “expected signal point” is used as the starting point, and the “reception signal point” is used the ending point. 
     Second Embodiment 
     In the first embodiment, information on the gap between the reception signal point of a known signal transmitted from the communication device on the transmission side including the distortion compensation device, and the expected signal point is used as a feedback signal. On the contrary, in a second embodiment, information on the gap between the reception signal point of a regular data signal (that is, the transmission baseband signal in the first embodiment) transmitted from a transmission side device including a distortion compensation device and the expected signal point is used as a feedback signal. 
     [Configuration Example of Wireless Communication System] 
       FIG. 9  is diagram illustrating an example of a wireless communication system according to the second embodiment. In  FIG. 9 , a wireless communication system  2  includes a communication device  110  that amplifies the power of a transmission signal through an amplifier, and transmits the signal using the “transmission band”, and a communication device  150  that receives the signal that has been transmitted from the communication device  110 . For example, one of the communication device  110  and the communication device  150  may be a wireless base station, and the other communication device may be a wireless terminal device, and both of the communication devices may be wireless terminal devices. Hereinafter, the communication device  110  may be referred to as the “first communication device”, and the communication device  150  may be referred to as the “second communication device”. 
     The communication device  110  does not perform transmission of the above-described “known signal”, differently from the communication device  10  according to the first embodiment. 
     Similar to the communication device  50  according to the first embodiment, the communication device  150  receives the signal that has been transmitted from the communication device  110  using a “transmission band”, and calculates the “gap” between the “reception signal point” of the received signal and the “expected signal point” coming from the received signal. In addition, the communication device  150  transmits information on the calculated “gap”, to the communication device  110 , as a “feedback signal”. 
     However, in the second embodiment, the received signal used for the calculation of a gap is not a known signal, so that a symbol (signal point) on a constellation diagram, which is the closest to the “reception signal point” of the received signal, is used as the “expected signal point”. In addition, the communication device  150  uses each symbol on the constellation diagram as the expected signal point, and calculates the “gap” for each of the expected signal points. Similar to the first embodiment, the “gap” that is a calculation target may be an error vector, and may be an MER. 
     The communication device  150  causes information on the “gaps” that have been calculated for the expected signal points to be included in the feedback signal, and transmits the feedback signal to the communication device  110 . 
     The communication device  110  performs weighted average on the plurality “gaps” included in the feedback signal, using a weighting factor. The weighting factor for the expected signal point having a larger distance (namely, being farther) from the origin on the constellation diagram becomes greater. 
     In addition, similar to the first embodiment, the communication device  110  controls execution of correction (update) of the “adjustment coefficient” based on a magnitude relation between the weighted average value and “threshold value”. 
     Here, the weighted average value is calculated in the communication device  110 , but the embodiment is not limited to such an example, and a weighted average value may be calculated in the communication device  150 , and the obtained weighted average value may be used as a feedback signal. 
     As described above, an effect similar to that of the first embodiment may be obtained even using a regular data signal. 
     [Configuration Example of First Communication Device] 
       FIG. 10  is a block diagram illustrating an example of the first communication device according to the second embodiment. In  FIG. 10 , the communication device  110  includes a distortion compensation device  112 . 
       FIG. 11  is a block diagram of an example of the distortion compensation device according to the second embodiment. In  FIG. 11 , the distortion compensation device  112  includes a correction unit  135 . 
     The correction unit  135  corrects (updates) the “adjustment coefficient” used in the adjustment unit  34 , based on the “feedback signal” from the communication device  150 . As described above, the feedback signal includes information on the “gaps” that have been calculated for the expected signal points. For example, the correction unit  135  performs weighted average on the plurality of “gaps” included in the feedback signal, using a weighting factor, and corrects (updates) the adjustment coefficient based on the obtained weighted average value, using, for example, the above-described gradient descent method. 
     [Configuration Example of Second Communication Device] 
       FIG. 12  is a block diagram illustrating an example of the second communication device according to the second embodiment. In  FIG. 12 , the communication device  150  includes a gap calculation unit  153 . 
     The gap calculation unit  153  uses each symbol on the constellation diagram as the expected signal point and calculates the “gap” for each of the expected signal points. Similar to the first embodiment, the “gap” that is a calculation target may be an error vector, and may be an MER. 
     As described above, in the embodiment, in the communication device  150 , the gap calculation unit  153  uses each of the symbols on the constellation diagram as the expected signal point, and calculates the “gap” for each of the expected signal points. In addition, the transmission processing unit  54  and the wireless transmission unit  55  feed back information on the gaps that have been calculated in the gap calculation unit  153 , to the communication device  110 . 
     Due to such a configuration of the communication device  150 , it may be avoided that an output function of a “known signal” is provided in the communication device  110 , so that the device scale of the communication device  110  may be downsized as compared with the communication device  10  according to the first embodiment. 
     As described above, in the communication device  150 , a weighted average value is calculated, and information on the calculated weighted average value may be used as a feedback signal. Therefore, a data amount of the feedback signal may be reduced. 
     Third Embodiment 
     In the first and second embodiments, the gap between the reception signal point and the expected signal point is used as the feedback signal in the communication device on the reception side. On the contrary, in a third embodiment, an “error rate” of a received signal in a communication device on the reception side is used as a feedback signal. 
     [Configuration Example of Wireless Communication System] 
       FIG. 13  is diagram illustrating an example of a wireless communication system according to the third embodiment. In  FIG. 13 , a wireless communication system  3  includes a communication device  210  that amplifies the power of a transmission signal through an amplifier, and transmits the signal using a “transmission band”, and a communication device  250  that receives the signal that has been transmitted from the communication device  210 . For example, one of the communication device  210  and the communication device  250  may be a wireless base station, and the other communication device may be a wireless terminal device, and both of the communication devices may be wireless terminal devices. Hereinafter, the communication device  210  may be referred to as the “first communication device”, and the communication device  250  may be referred to as the “second communication device”. 
     Similar to the communication device  50  according to the first embodiment, the communication device  250  receives the signal that has been transmitted from the communication device  210  using a “transmission band”. In addition, the communication device  250  calculates an “error rate (for example, a BER)” of the received signal. In addition, the communication device  250  transmits information on the calculated error rate, to the communication device  210 , as a “feedback signal”. 
     Similar to the first embodiment, the communication device  210  controls execution of correction (update) of the “adjustment coefficient”, based on a magnitude relation between the “error rate” included in the feedback signal and a “threshold value”. 
     [Configuration Example of First Communication Device] 
       FIG. 14  is a block diagram illustrating the first communication device according to the third embodiment. In  FIG. 14 , the communication device  210  includes a distortion compensation device  212 . 
       FIG. 15  is a block diagram illustrating the distortion compensation device according to the third embodiment. In  FIG. 15 , the distortion compensation device  212  includes a correction unit  235 . 
     The correction unit  235  corrects (updates) the “adjustment coefficient” used in the adjustment unit  34 , based on the “feedback signal” from the communication device  250 . The feedback signal includes the information on the “error rate” as described above. For example, the correction unit  235  corrects (updates) the adjustment coefficient, based on the “error rate”, using the above-described gradient descent method. 
     [Configuration Example of Second Communication Device] 
       FIG. 16  is a block diagram illustrating an example of the second communication device according to the third embodiment. In  FIG. 16 , the communication device  250  includes an error rate calculation unit  253 . The error rate calculation unit  253  calculates an error rate of the signal that has been transmitted from the communication device  210 . 
     As described above, in the embodiment, in the communication device  250 , the error rate calculation unit  253  calculates the error rate of the signal that has been transmitted from the communication device  210 . In addition, the transmission processing unit  54  and the wireless transmission unit  55  feed back the information on the error rate that has been calculated in the error rate calculation unit  253 , to the communication device  210 . 
     Due to such a configuration of the communication device  250 , it may be avoided that an output function of a “known signal” is provided in the communication device  210 , so that the device scale of the communication device  210  may be downsized as compared with the communication device  10  according to the first embodiment. 
     Fourth Embodiment 
     In a fourth embodiment, an existing “feedback path in a device” is added the configuration of the first communication device on the transmission side according to the first embodiment. 
     [Configuration Example of First Communication Device] 
       FIG. 17  is a block diagram illustrating an example of a first communication device according to the fourth embodiment. In  FIG. 17 , a communication device  10 M includes a distortion compensation device  12 M. In addition, the communication device  10 M includes a coupler  18 , a down-converter  19 , and an ADC  20 . That is, the communication device  10 M includes a feedback path in the device. 
     The coupler  18  outputs a part of an output signal of the PA  23 , to the down-converter  19 , that is, the feedback path in the device. 
     The down-converter  19  down-converts the output signal of the PA  23 , which has been received through the coupler  18 , and outputs the down-converted signal to the ADC  20 . 
     The ADC  20  converts the down-converted signal from the analog signal to a digital signal, and outputs the converted digital signal (hereinafter may be referred to as a “feedback signal in the device”) to the distortion compensation device  12 M. 
       FIG. 18  is a block diagram illustrating an example of the distortion compensation device according to the fourth embodiment. In  FIG. 18 , the distortion compensation device  12 M includes a selection unit  37 . 
     The selection unit  37  (or selector  37 ) selects one of a feedback signal from the communication device  50  (hereinafter may be referred to as a “feedback signal outside the device”) and the above-described feedback signal in the device, and outputs the selected signal to the correction unit  35 . That is, the selection unit  37  performs switching between the feedback path outside the device and the feedback path in the device. 
     For example, in a state in which communication between the communication device  10 M and the communication device  50  is not established (for example, the state before the communication starts), the selection unit  37  switches the path to the feedback path in the device. In addition, in a state in which the communication between the communication device  10 M and the communication device  50  has been established (for example, the state after the communication has started), the selection unit  37  switches the path to the feedback path outside the device. 
     In addition, when the communication quality level of the communication path from the communication device  50  to the communication device  10 M is lower than a certain level in the state in which the selection unit  37  has switched the path to the feedback path outside the device, the selection unit  37  switches the feedback path outside the device to the feedback path in the device. Here, the “case in which the communication quality level of the communication path from the communication device  50  to the communication device  10 M is lower than the certain level” is, for example, a case in which the communication between the communication device  50  and the communication device  10 M has been disconnected, or a case in which an error rate (for example, a BER) of the transmission signal from the communication device  50  to the communication device  10 M is lower than a certain value. 
     As described above, in the embodiment, the communication device  10 M includes the feedback path in the device, in addition to the configuration similar to that of the communication device  10 . In addition, in the distortion compensation device  12 M, the selection unit  37  outputs, to the correction unit  35 , a feedback signal sent back from the communication device  50  that has received a signal transmitted from the communication device  10 , or a feedback signal in the device. 
     Due to such a configuration of the distortion compensation device  12 M, even in the state in which the communication between the communication device  10 M and the communication device  50  is not established, the adjustment coefficient may be corrected (updated). 
     Fifth Embodiment 
     In a fifth embodiment, an existing “feedback path in a device” is added to the configuration of the first communication device on the transmission side according to the second embodiment. 
     [Configuration Example of First Communication Device] 
       FIG. 19  is a block diagram illustrating an example of a first communication device according to the fifth embodiment. In  FIG. 19 , a communication device  110 M includes a distortion compensation device  112 M. In addition, the communication device  110 M includes a coupler  18 , a down-converter  19 , and an ADC  20 . That is, the communication device  110 M includes a feedback path in the device. 
       FIG. 20  is a block diagram illustrating an example of the distortion compensation device according to the fifth embodiment. In  FIG. 20 , the distortion compensation device  112 M includes a selection unit  137 . 
     The selection unit  137  selects one of a feedback signal from the communication device  150  (that is, a “feedback signal outside the device”) and a feedback signal in the device, and outputs the selected signal to the correction unit  135 . That is, the selection unit  137  performs switching between the feedback path outside the device and the feedback path in the device. The switching processing by the selection unit  137  may be executed similar to that of the selection unit  37  according to the fourth embodiment. 
     As described above, in the embodiment, the communication device  110 M includes the feedback path in the device, in addition to the configuration similar to that of the communication device  110 . In addition, in the distortion compensation device  112 M, the selection unit  137  outputs, to the correction unit  135 , a feedback signal sent back from the communication device  150  that has received a signal transmitted from the communication device  110 , or a feedback signal in the device. 
     Due to such a configuration of the distortion compensation device  112 M, even in the state in which the communication between the communication device  110 M and the communication device  150  is not established, the adjustment coefficient may be corrected (updated). 
     Sixth Embodiment 
     In a sixth embodiment, an existing “feedback path in a device” is added to the configuration of the first communication device on the transmission side according to the third embodiment. 
     [Configuration Example of First Communication Device] 
       FIG. 21  is a block diagram illustrating an example of a first communication device according to the sixth embodiment. In  FIG. 21 , a communication device  210 M includes a distortion compensation device  212 M. In addition, the communication device  210 M includes a coupler  18 , a down-converter  19 , and an ADC  20 . That is, the communication device  210 M includes a feedback path in the device. 
       FIG. 22  is a block diagram illustrating an example of the distortion compensation device according to the sixth embodiment. In  FIG. 22 , the distortion compensation device  212 M includes a selection unit  237 . 
     The selection unit  237  selects one of a feedback signal from the communication device  250  (that is, a “feedback signal outside the device”) and a feedback signal in the device, and outputs the selected signal to the correction unit  235 . That is, the selection unit  237  performs switching between the feedback path outside the device and the feedback path in the device. The switching processing by the selection unit  237  may be executed similar to that of the selection unit  37  according to the fourth embodiment. 
     As described above, in the embodiment, the communication device  210 M includes the feedback path in the device, in addition to the configuration similar to that of the communication device  210 . In addition, in the distortion compensation device  212 M, the selection unit  237  outputs, to the correction unit  235 , a feedback signal sent back transmitted from the communication device  250  that has received a signal transmitted from the communication device  210 , or a feedback signal in the device. 
     Due to such a configuration of the distortion compensation device  212 M, even in the state in which the communication between the communication device  210 M and the communication device  250  is not established, the adjustment coefficient may be corrected (updated). 
     Other Embodiments 
     The elements of the units described in the first embodiment to the sixth embodiment may not be configured as physically illustrated in the figures. That is, a specific form for division and integration of the units is not limited to the illustrated form, and all or some of the units may be functionally or physically divided or integrated in a certain unit in accordance with various loads, usage, and the like. 
     In addition, all or some of the various processing functions executed in the units may be executed on a central processing unit (CPU) (or a microcomputer such as a micro processing unit (MPU) or a micro controller unit (MCU)). In addition, all or some of the various processing functions may be executed on a program analyzed and executed by the CPU (or the microcomputer such as the MPU or the MCU), or on hardware by wired logic. 
     Each of the distortion compensation devices according to the first embodiment to the sixth embodiment may be achieved, for example, by the following hardware configuration. 
       FIG. 23  is a diagram illustrating a hardware configuration example of a distortion compensation device. As illustrated in  FIG. 23 , a distortion compensation device  300  includes a processor  301  and a memory  302 . As an example of the processor  301 , there are a CPU, a digital signal processor (DSP), a field programmable gate array (FPGA), and the like. In addition, as an example of the memory  302 , there are a random access memory (RAM) such as a synchronous dynamic random access memory (SDRAM), a read only memory (ROM), a flash memory, and the like. 
     In addition, the various processing functions executed in the distortion compensation devices according to the first embodiment to the sixth embodiment may be achieved by causing the processor to execute programs stored in the various memories such as a nonvolatile storage medium. That is, programs corresponding to the pieces of processing executed by the address calculation unit  31 , the reading unit  32 , the adjustment unit  34 , the correction units  35 ,  135 , and  235 , the multiplication unit  36 , and the selection units  37 ,  137 , and  237  are recorded to the memory  302 , and each of the programs may be executed by the processor  301 . In addition, the table storage unit  33  may be obtained by the memory  302 . 
     As described above, the various processing functions executed in the distortion compensation devices according to the first embodiment to the sixth embodiment are executed by the single processor  301 , but the embodiment is not limited to such an example, and the various processing functions may be executed by a plurality of processors. 
     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.