Patent Publication Number: US-2018054170-A1

Title: Distortion compensation device and coefficient update method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2018-161554, filed on Aug. 19, 2016 and Japanese Patent Application No. 2017-107127, filed on May 30, 2017, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a distortion compensation device and a coefficient update method. 
     BACKGROUND 
     In a radio transmission device, a power amplifier that amplifies power of a transmission signal is provided. In the radio transmission device, in general, in order to increase the power efficiency of the power amplifier, the power amplifier is operated in the vicinity of the saturation region of the power amplifier. However, when the power amplifier is operated in the vicinity of the saturation region, nonlinear distortion in the power amplifier is increased. If the nonlinear distortion is increased, the signal quality, such as the ratio of power leakage into an adjacent channel (adjacent channel leakage ratio: hereinafter, referred to as ACLR), or the like, is degraded. Thus, in order to reduce this nonlinear distortion, in the radio transmission device, a distortion compensation device that compensates nonlinear distortion is provided. 
     There is a “digital predistortian scheme” as one of the distortion compensation schemes used in a distortion compensation device. In a distortion compensation device that uses the digital predistortion scheme, a distortion compensation coefficient that has the inverse characteristic of the nonlinear distortion in the power amplifier is previously multiplied by the transmission signal and then a transmission signal in which the distortion compensation coefficient has been multiplied is input to the power amplifier. Consequently, the nonlinear distortion in the output signal that is output from the power amplifier is canceled out. The distortion compensation coefficients are stored in a look up table (LUT) by being associated with the addresses calculated from the transmission signal. 
     Furthermore, it is known that the phenomenon called memory effect occurs in the power amplifier with high power efficiency. The memory effect is a phenomenon in which an output with respect to an input to the power amplifier at a certain time point is affected by an input that is received at a time point in a past. To reduce the memory effect, distortion compensation is performed by also using a transmission signal that is present before by a predetermined number of samples. The distortion compensation, coefficients in the LUT are sequentially updated such that a difference between the signal that is obtained by feeding back the output signal sent from the power amplifier and the transmission signal that has not been subjected to the distortion compensation becomes small. As an update method of the distortion compensation coefficients, a method of, for example, normalized least-mean-square (NLMS), or the like, is known. 
     However, in the signal that is fed back from the power amplifier, a noise component, such as thermal noise of, for example, an analog-to-digital converter (ADC), or the like, is included. Thus, if the power of the fed back signal is small, a signal to noise ratio (hereinafter, referred to as an SN ratio) becomes small and the influence of the noise component becomes large. Thus, if the fed back signal is small, the value of an update amount of the distortion compensation coefficients calculated based on the fed back signal may sometimes be greatly different from a desired value. 
     To avoid this problem, there is a known technology in which, if the value of the address calculated from the transmission signal that has not been subjected to the distortion compensation is less than a threshold, the distortion compensation coefficient associated with the address having a value less than the threshold is not used by clipping the address by using the threshold. Prior art examples are disclosed in International Publication Pamphlet No. WO 2003/103163 and International Publication Pamphlet No. WO 2003/103167. 
     The quality of the signal, such as ACLR, or the like, in a case where the amplitude of the transmission signal is small is improved to some extent by clipping the address associated with the transmission signal that has not been subjected to the distortion compensation by using a predetermined threshold; however, the quality of the signal is still low. Consequently, the quality of the signal needs to be further improved. 
     SUMMARY 
     According to an aspect of an embodiment, a distortion compensation device compensates distortion generated in a power amplifier. The distortion compensation device includes a distortion compensation unit, a calculating unit, a clip processing unit, and an updating unit. The distortion compensation unit generates a distortion compensation signal by performing a predetermined arithmetic operation on a transmission signal by using a distortion compensation coefficient and that inputs the generated distortion compensation signal to the power amplifier. The calculating unit calculates a feedback coefficient based on an output signal output from the power amplifier. The clip processing unit outputs, when absolute value of the feedback coefficient calculated by the calculating unit is equal to or less than a threshold, the feedback coefficient calculated by the calculating unit and that outputs, when the absolute value of the feedback coefficient calculated by the calculating unit is greater than the threshold, the feedback coefficient of which absolute value is equal to or less than the threshold. The updating unit updates the distortion compensation coefficient by using an error between the transmission signal and the output signal, a predetermined step coefficient, and the feedback coefficient output from the clip processing unit. 
     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 block diagram illustrating an example of a distortion compensation device according to a first embodiment; 
         FIG. 2  is a schematic diagram illustrating an example of a threshold according to the first embodiment; 
         FIG. 3  is a schematic diagram illustrating the relationship between the addresses and distortion compensation coefficients; 
         FIG. 4  is a schematic diagram illustrating an example of a convergence course of a feedback signal; 
         FIG. 5  is a flowchart illustrating an example of a coefficient updating process according to the first embodiment; 
         FIG. 6  is a block diagram illustrating an example of a distortion compensation device according to a second embodiment; 
         FIG. 7  is a schematic diagram illustrating an example of the threshold according to the second embodiment; 
         FIG. 8  is a flowchart illustrating an example of a coefficient updating process according to the second embodiment; 
         FIG. 9  is a schematic diagram illustrating an example of calculation timing of a threshold according to a third embodiment; 
         FIG. 10  is a schematic diagram illustrating another example of calculation timing of the threshold according to the third embodiment; 
         FIG. 11  is a block diagram illustrating an example of a distortion compensation device according to a fourth embodiment; 
         FIG. 12  is a schematic diagram illustrating an example of distribution of feedback coefficients according to the fourth embodiment; 
         FIG. 13  is a flowchart illustrating an example of a coefficient updating process according to the fourth embodiment; 
         FIG. 14  is a block diagram illustrating an example of a distortion compensation device according to a fifth embodiment; 
         FIG. 15  is a schematic diagram illustrating an example of distribution of the product of the absolute value of a feedback coefficient and a step coefficient according to the fifth embodiment; 
         FIG. 16  is a flowchart illustrating an example of a coefficient updating process according to the fifth embodiment; 
         FIG. 17  is a block diagram illustrating an example of a distortion compensation device according to a sixth embodiment; 
         FIG. 18  is a schematic diagram illustrating an example of distribution of the product of the absolute value of a feedback coefficient and a step coefficient according to the sixth embodiment; 
         FIG. 19  is a flowchart illustrating an example of a coefficient updating process according to the sixth embodiment; 
         FIG. 20  is a block diagram illustrating an example of a distortion compensation device according to a seventh embodiment; 
         FIG. 21  is a block diagram illustrating an example of a distortion compensation device according to an eighth embodiment; and 
         FIG. 22  is a block diagram illustrating an example of hardware the distortion compensation device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invent ion will be explained with reference to accompanying drawings. Furthermore, the embodiments, described below do not limit the disclosed technology. Furthermore, each of the embodiments can be used in any appropriate combination as long as the processes do not conflict with each other. 
     [a] First Embodiment 
     Configuration of a Distortion Compensation Device  10   
       FIG. 1  is a block diagram illustrating an example of the distortion compensation device  10  according to a first embodiment. The distortion compensation device  10  according to the embodiment includes a Radio Frequency (RF) digital unit  20 , an RF analog unit  30 , and an antenna  40 . The RF digital unit  20  includes a distortion compensation unit  50  and a coefficient updating unit  60 . 
     The RF analog unit  30  includes a digital-to-analog converter (DAC)  31 , a mixer  32 , an oscillator  33 , a power amplifier  34 , a coupler  35 , a mixer  36 , and an ADC  37 . 
     The DAC  31  converts, from a digital signal to an analog signal, the transmission signal that is output from the distortion compensation unit  50  and that has been subjected to distortion compensation. Then, the DAC  31  outputs the signal that has been converted to the analog signal to the mixer  32 . The mixer  32  modulates and up converts, by using the local oscillator signal output from the oscillator  33 , the signal output from the DAC  31 . Then, the mixer  32  outputs the processed signal to the power amplifier  34 . The power amplifier  34  amplifies the signal output from the mixer  32  by a predetermined amplification factor. The signal amplified by the power amplifier  34  is transmitted from the antenna  40 . 
     A part of the signal amplified by the power amplifier  34  is fed back via the coupler  35 . The mixer  36  down converts, by using the signal output from the oscillator  33 , the signal that has been fed back via the coupler  35 . The ADC  37  converts, from an analog signal to a digital signal, the signal that has been subjected to demodulation or the like by the mixer  36 . Then, the ADC  37  outputs the feedback signal converted to the digital signal to the coefficient updating unit  60 . The feedback signal output from the ADC  37  is defined as Fb(t). The feedback signal Fb(t) is an example of an output signal that has been output from the power amplifier  34 . 
     The distortion compensation unit  50  includes a distortion compensation processing unit  51 , an address creating unit  52 , and a look up table (LUT)  53 . The address creating unit  52  generates, based on the baseband transmission signal Tx(t) generated by a base band signal (BB) processing unit, a plurality of transmission signals Tx(t-j) each having a different amount of delay. Then, the address creating unit  52  creates the address for each of the transmission signals Tx(t-j) that have a plurality of different amounts of delay and that include the transmission signal Tx(t) having the amount of delay of zero. Furthermore, regarding the transmission signals Tx(t-j), j represents an amount of delay and takes a value of 0 to N. Then, the address creating unit  52  outputs the address created for each of the transmission signals Tx(t-j) and outputs the addresses to the LUT  53  and the coefficient updating unit  60 . In the embodiment, the address creating unit  52  creates the address in accordance with the amplitude of each of the transmission signals Tx(t-j). The amplitude of the transmission signal Tx(t-j) is an example of the magnitude of the transmission signal Tx(t-j). Namely, the value of the addresses created by the address creating unit  52  are values that are in accordance with the magnitude of the corresponding transmission signals Tx(t-j). Furthermore, as another example, the address creating unit  52  may also create the address in accordance with the magnitude of the power of the delay signal. 
     The LUT  53  stores therein the distortion compensation coefficients that are associated with the addresses for each of the transmission signals Tx(t-j) having different amounts of delay. The LUT  53  outputs, to the distortion compensation processing unit  51  for each of the transmission signals Tx(t-j), the distortion compensation coefficient associated with the address output from the address creating unit  52 . Each of the distortion compensation coefficients in the LUT  53  is updated by the coefficient updating unit  60  as needed. The LUT  53  is an example of a table. 
     The distortion compensation processing unit  51  generates, based on the transmission signal Tx(t) output from the BB processing unit, the transmission signals Tx(t-j) having a plurality of different amounts of delay. Then, for each of the transmission signals Tx(t-j) that have a plurality of different amounts of delay and that include the transmission signal Tx(t) having the amount of delay of zero, the distortion compensation processing unit  51  multiplies the distortion compensation coefficient output from the LUT  53  by the transmission signal Tx(t-j). Then, by adding the transmission signal Tx(t-j) in which the distortion compensation coefficient is multiplied, the distortion compensation processing unit  51  generates the transmission signal Tx′(t) that has been, subjected to distortion compensation. The transmission signal Tx′(t) that has been subjected to the distortion compensation is output to the DAC  31 . 
     The coefficient updating unit  60  calculates an update amount of a distortion compensation coefficient for each of the plurality of the transmission signals Tx(t-j) each having a different amount of delay and then updates the distortion compensation coefficients in the LUT  53  by using the calculated update amount. The updated distortion compensation coefficient h j (p) related to the transmission signal Tx(t-j) that is delayed by j samples is calculated based on, for example, Equation (1) below. 
         h   j ( p )= h′   j ( p )+μ× e ( t )× C   j    (1)
 
     Here, in Equation (1) above, h′ j (p) represents the distortion compensation, coefficient that is before the update and μ represents a step coefficient. Furthermore, in Equation (1) above, the error e(t) is calculated, by using the transmission signal Tx(t) and the feedback signal Fb(t), based on, for example, Equation (2) below. 
         e ( t )= Tx ( t )− Fb ( t )   (2)
 
     Furthermore, in Equation (1) above, the feedback coefficient C j  is calculated based on, for example, Equation (3) below by using each of the feedback signals Fb(t-j) associated with the transmission signals Tx(t-j) delayed by j samples. 
     
       
         
           
             
               
                 
                   
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                              
                             
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     In Equation (3) above, Fb*(t-j) is a conjugate complex number of Fb(t-j). 
     In particular, the feedback coefficient C 0  calculated from the feedback signal Fb(t) with respect to transmission signal Tx(t) having an amount of delay of zero (i.e., j=0) is represented by, for example, Equation (4) below. 
     
       
         
           
             
               
                 
                   
                     C 
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                              
                             
                               Fb 
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     In a process of updating the distortion compensation coefficients, the coefficient updating unit  60  according to the embodiment performs a clip process, for each of the transmission signals Tx(t-j), such that the absolute value of the feedback coefficient C j  becomes equal to or less than a predetermined threshold C th . In the following, the coefficient updating unit  60  according to the embodiment will be described in detail below. 
     The coefficient updating unit  60  according to the embodiment includes, for example, as illustrated in  FIG. 1 , an updating unit  61 , a clip processing unit  62 , a holding unit  63 , a threshold creating unit  64 , an absolute value calculating unit  65 , a feedback coefficient calculating unit  66 , and a subtracter  67 . 
     The feedback coefficient calculating unit  66  calculates the feedback coefficient C j  for each of the transmission signals Tx(t-j) associated with the corresponding feedback signals Fb(t-j) by performing the arithmetic operation based on Equation (3) described above by using the feedback signal Fb(t-j) output from the ADC  37 . Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the clip processing unit  62  and the absolute value calculating unit  65 . The feedback coefficient calculating unit  66  is an example of a calculating unit. 
     The absolute value calculating unit  65  calculates, for each of the transmission signals Tx(t-j), the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66 . Then, the absolute value calculating unit  65  outputs the absolute value |C j | calculated for each of the transmission signals Tx(t-j) to the clip processing unit  62  and the threshold creating unit  64 . 
     The holding unit  63  stores therein the threshold C th  for each of the transmission signals Tx(t-j). The threshold creating unit  64  creates, for each of the transmission signals Tx(t-j), the threshold C th  based on the address output from the address creating unit  52  and based on the absolute value |C j | output from the absolute value calculating unit  65 . For example, the threshold creating unit  64  performs, at each predetermined timing, the following process regarding a predetermined number of samples counted from the top (for example, 100 samples) of each of the transmission signals Tx(t-j). 
     First, the threshold creating unit  64  initializes the value of the threshold C th  of each of the transmission signals Tx(t-j) in the holding unit  63  to zero. Then, the threshold creating unit  64  refers to the address from the address creating unit  52  for each of the transmission signals Tx(t-j) and determines whether the absolute value |C j | is the absolute value |C j | that is calculated from the feedback signal Fb(t-j) associated with the transmission signal Tx(t-j) having the address greater than the threshold A th . The threshold A th  of the address is previously set in the threshold creating unit  64  by an administrator or the like of the distortion compensation device  10 . 
     If the absolute value |C j | is the absolute value |C j | that is calculated from the feedback signal Fb(t-j) associated with the transmission signal Tx(t-j) having the address greater than the threshold A th , the threshold creating unit  64  compares, for each of the transmission signals Tx(t-j), the subject absolute value |C j | with the threshold C th  stored in the holding unit  63 . If the value of the absolute value |C j | is greater than the value of the threshold C th  stored in the holding unit  63 , the threshold creating unit  64  stores the value of the absolute value |C j | as the threshold C th  in the holding unit  63 . Consequently, if the determination about the predetermined number of samples of each of the transmission signals Tx(t-j) has been completed, for example, the threshold C th  for each of the transmission signals Tx(t-j) illustrated in  FIG. 2  is stored in the holding unit  63 . 
       FIG. 2  is a schematic diagram illustrating an example of a threshold according to the first embodiment.  FIG. 2  illustrates an example of the distribution of the absolute value |C 0 | calculated from the feedback signal Fb(t) associated with the transmission signal Tx(t) having the amount of delay of zero. Furthermore, regarding the absolute value |C j | calculated from the feedback signals Fb(t-j) associated with the transmission signals Tx(t-j) having another amount of delay, the same distribution as that illustrated in  FIG. 2  is obtained. In the embodiment, for example, as illustrated in  FIG. 2 , the value of the maximum value with the absolute value |C 0 | (for example, the absolute value |C 0 | indicated by a point  70  illustrated in  FIG. 2 ) is used as the value of the threshold C th  from among the absolute values |C 0 | associated with the addresses greater than the threshold A th . 
     In the following, an example of a method of deciding the threshold A th  of the address will be described.  FIG. 3  is a schematic diagram illustrating the relationship between the addresses and distortion compensation coefficients. In the feedback signal Fb(t) that is fed back from the power amplifier  34 , a noise component generated due to, for example, thermal noise of the ADC  37 , or the like is included. In the power amplifier  34 , because the transmission signal Tx′(t) that has been subjected to distortion compensation is amplified at a predetermined amplification factor, if the amplitude of the transmission signal Tx(t) is small, i.e., if the value of the address created from the transmission signal Tx(t) is small, the power of the feedback signal Fb(t) becomes small. If the power of the feedback signal Fb(t) is small, the SN ratio becomes small and the influence of the noise component becomes large. 
     The power amplifier  34  generally exhibits a nonlinear characteristic in a saturation region in which the amplitude of an input signal is large, whereas, the power amplifier  34  generally exhibits a linear characteristic in a region in which the amplitude of an input signal is small. Consequently, ideally, for example, as indicated by the broken line illustrated in  FIG. 3 , in the region in which the amplitude of the input signal is small, the distortion compensation coefficient becomes a constant value (for example 1). 
     However, if the amplitude of the transmission signal Tx(t) is small, i.e., if the value of the address of the transmission signal Tx(t) is small, because the influence of the noise component included in the feedback signal Fb(t) becomes large, for example, as indicated by the solid line illustrated in  FIG. 3 , the distortion compensation coefficient is updated to the value different from an ideal value. In the embodiment, if the value of the address of the transmission signal Tx(t) is made small, for example, the value of the address in which the distortion compensation coefficient starts to shift from the ideal value is previously decided sis the threshold A th . As an example of a specific value, for example, the upper limit of the addresses present in the range of about 40% of the address having a smaller value out of the entire range of the addresses may also be used as the threshold A th . For example, if the entire range of the address is 1 to 100, the value of the address of 40 may also be used as the threshold A th . 
     The clip processing unit  62  receives, for each of the transmission signals Tx(t-j), the feedback coefficient C j  from the feedback coefficient calculating unit  66  and receives the absolute value |C j | of the feedback coefficient C j  from the absolute value calculating unit  65 . Then, the clip processing unit  62  compares, for each of the transmission signals Tx(t-j), the absolute value |C j | received from the absolute value calculating unit  65  with the threshold C th  that is stored in the holding unit  63 . If the value of the absolute value |C j | is equal to or less than the value of the threshold C th , the clip processing unit  62  outputs the feedback coefficient C j  received from the feedback coefficient calculating unit  66  to the updating unit  61 . 
     In contrast, if the value of the absolute value |C j | is greater than the value of the threshold C th , the clip processing unit  62  performs a clip process that calculates a feedback coefficient C j ′ based on Equation (5) below. 
     
       
         
           
             
               
                 
                   
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                         C 
                         j 
                       
                       
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                           C 
                           j 
                         
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                       C 
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     Regarding the feedback coefficient C j ′ calculated based on Equation (5) above, the absolute value |C j ′| that is the magnitude of the feedback coefficient C j ′ is equal to the threshold C th  and the phase is the same as that of the original feedback coefficient C j . Then, the clip processing unit  62  outputs the feedback coefficient C j ′ that has been subjected to the clip process to the updating unit  61 . 
     Consequently, example, as illustrated in FIG.  2 , the feedback coefficient C 0  (for example, a point  71 , or the like) having the value of the absolute value |C 0 | greater than the value of the threshold C th  is clipped such that the absolute value is equal to the threshold C th  while maintaining the phase of the feedback coefficient C 0 . 
     The subtracter  67  calculates an error e(t) by performing the arithmetic operation indicated by Equation (2) described above. Then, the subtracter  67  outputs the calculated error e(t) to the updating unit  61 . 
     The updating unit  61  receives the feedback coefficient C j  from the clip processing unit  62 , receives the error e(t) from the subtracter  67 , and reads the distortion compensation coefficient h′ j (p) that is before an update from the LUT  53 . Then, the updating unit  61  calculates an updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above. Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p). Furthermore, in the embodiment, step coefficient μ is previously set in the updating unit  61  by an administrator of the distortion compensation device  10  or the like. Furthermore, if the updating unit  61  receives the feedback coefficient C j ′ from the clip processing unit  62 , the updating unit  61  calculates an updated distortion compensation coefficient h j (p) by using, instead of the feedback coefficient C j , the feedback coefficient C j ′ in Equation (1) described above. 
     Here, for example, as illustrated in  FIG. 4 , in the process of updating the distortion compensation coefficient, regarding a feedback signal  80 , the distortion compensation coefficient is updated so as to approach a transmission signal  81  on the IQ plane.  FIG. 4  is a schematic diagram illustrating an example of a convergence course of the feedback signal  80 . If the SN ratio of the feedback signal  80  is small, for example, as illustrated in  FIG. 4 , the feedback signal  80  varies in a range  83  centered on the transmission signal  81 . Consequently, when the feedback signal  80  is viewed at a certain moment, the feedback signal  80  is sometimes present at the position away from the transmission signal  81  that corresponds to the correct solution. 
     In contrast, in the embodiment, in the process of updating the distortion compensation coefficient, the clip process is performed such that the absolute value |C j | of the feedback coefficient C j  is equal to or less than a predetermined threshold C th . Consequently, for example, as illustrated in  FIG. 4 , the feedback signal  80  varies within a range  82 , which is narrower than the range  83 , centered on the transmission signal  81 . Consequently, when the feedback signal  80  is viewed at a certain moment, the feedback signal  80  is present at the position closer to the range  83 . Consequently, the feedback signal  80 , i.e., a distortion component included in the signal output from the power amplifier  34 , is decreased and the characteristic of the ACLR or the like is improved. 
     Coefficient Updating Process 
       FIG. 5  is a flowchart illustrating an example of a coefficient updating process according to the first embodiment. The distortion compensation device  10  performs the coefficient updating process illustrated in  FIG. 5  at each predetermined timing. For example, if the distortion compensation device  10  transmits a downlink (DL) signal in the mobile communication system, such as long term evolution (LTE), or the like, the distortion compensation device  10  performs the coefficient updating process illustrated in  FIG. 5  for each, for example, single frame. Furthermore, in the following flowchart, a description will be given of the transmission signal Tx(t-j) delayed by j samples and given of the feedback signal Fb(t-j); however, the same process is also performed on each of the delay signals delayed by j represented by 0 to N. 
     First, the feedback coefficient calculating unit  66  initializes the variable s that counts the pieces of sampling data of the transmission signal Tx(t-j) to zero (Step S 100 ). Furthermore, the threshold creating unit  64  initializes the value of the threshold C th  in the holding unit  63  to zero (Step S 100 ). 
     Then, the feedback coefficient calculating unit  66  selects the sampling data of the feedback signal Fb(t-j) that is associated with the sampling data of the s th  transmission signal Tx(t-j) (Step S 101 ). Then, by performing the arithmetic operation indicated by Equation (3) described above by using the sampling data of the feedback signal Fb(t-j) selected at Step S 101 , the feedback coefficient calculating unit  66  calculates the feedback coefficient C j  (Step S 102 ). Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the absolute value calculating unit  65 . 
     Then, the absolute value calculating unit  65  calculates the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66  (Step S 103 ). Then, the absolute value calculating unit  65  outputs the calculated absolute value |C j | to the clip processing unit  62  and the threshold creating unit  64 . 
     Then, the clip processing unit  62  and the threshold creating unit  64  determine whether the value of the variable s is equal to or less than the reference value s num  (Step S 104 ). In the embodiment, the reference value s num  for example, 100. If the value of the variable s is equal to or less than the reference value s num  (Yes at Step S 104 ), the threshold creating unit  64  determines whether the value A of the address of the s th  transmission signal Tx(t-j) is greater than the value of the threshold A th  of the address (Step S 105 ). If the value A of the address of the s th  transmission signal Tx(t-j) is equal to or less than the value of the threshold A th  of the address (No at Step S 105 ), the clip processing unit  62  performs the process indicated at Step S 108 . 
     In contrast, if the value A of the address of the s th  transmission signal Tx(t-j) is greater than the value of the threshold A th  of the address (Yes at Step S 105 ), the threshold creating unit  64  reads the threshold C th  from the holding unit  63 . Then, the threshold creating unit  64  determines whether the value of the absolute value |C j | of the feedback coefficient C j  output from the absolute value calculating unit  65  is greater than the value of the threshold C th  (Step S 106 ). If the value of the absolute value |C j | is equal to or less than the value of the threshold C th  (No at Step S 106 ), the updating unit  61  performs the process indicated at Step S 108 . 
     In contrast, if the value of the absolute value |C j | is greater than the value of the threshold C th  (Yes at Step S 106 ), the threshold creating unit  64  substitutes the value of the threshold C th  in the holding unit  63  for the value of the absolute value |C j | of the feedback coefficient C j  output from the absolute value calculating unit  65  (Step S 107 ). 
     Then, the clip processing unit  62  outputs, to the updating unit  61 , the feedback coefficient C j  that is output from the feedback coefficient calculating unit  66 . By performing the arithmetic operation indicated by Equation (1) described above by using the feedback coefficient C j  output from the clip processing unit  62 , the updating unit  61  calculates the updated distortion compensation coefficient h j (p). Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  to the calculated distortion compensation coefficient h j (p) (Step S 108 ). 
     Then, the feedback coefficient calculating unit  66  increments the variable s by 1 (Step S 109 ). Then, the feedback coefficient calculating unit  66  determines whether the value of the variable s is greater than s max  that is the maximum value of the variable s (Step S 110 ). In the embodiment, s max  is the number of samples in a single frame and is, for example, 1000. If the value of the variable s is equal to or less than the value of s max  (No at Step S 110 ), the feedback coefficient calculating unit  66  again performs the process indicated at Step S 101 . In contrast, if the value of the variable s is greater than the value of s max  (Yes at Step S 110 ), the distortion compensation device  10  ends the process illustrated in the subject flowchart. 
     At Step S 104 , if the value of the variable s is greater than the reference value s num  (No at Step S 104 ), the clip processing unit  62  determines whether the value A of the address of the s th  transmission signal Tx(t-j) is less than the value of the threshold A th  of the address (Step S 111 ). If the value A of the address of the s th  transmission signal Tx(t-j) is equal to or greater than the value of the threshold A th  of the address (No at Step S 111 ), the clip processing unit  62  performs the process indicated at Step S 108 . 
     In contrast, if the value A of the address of the s th  transmission signal Tx(t-j) is less than the value of the threshold A th  of the address (Yes at Step S 111 ), the clip processing unit  62  reads the threshold C th  from the holding unit  63 . Then, the clip processing unit  62  determines whether the value of the absolute value |C j | of the feedback coefficient C j  output from the absolute value calculating unit  65  is greater than the value of the threshold C th  (Step S 112 ). If the value of the absolute value |C j | is equal to or less than the value of the threshold C th  (No at Step S 112 ), the clip processing unit  62  performs the process indicated at Step S 108 . 
     In contrast, if the value of the absolute value |C j | is greater than the value of the threshold C th  (Yes at Step S 112 ), the clip processing unit  62  performs the arithmetic operation indicated by Equation (5) described above (Step S 113 ). Consequently, the feedback coefficient C j ′ is created by being clipped such that the absolute value becomes the threshold C th  while maintaining the phase of the feedback coefficient C j . Then, the clip processing unit  62  outputs the feedback coefficient C j ′ to the updating unit  61 . 
     Then, the updating unit  61  calculates the updated distortion compensation, coefficient h j (p) indicated by Equation (1) described above by using the feedback coefficient C j ′ output from the clip processing unit  62 . Then, the updating unit  61  updates the distortion compensation coefficients h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 114 ). Then, the threshold creating unit  64  and the feedback coefficient calculating unit  66  performs the process indicated at Step S 109 . 
     Effects of the First Embodiment 
     As is clear from the description above, the distortion compensation device  10  according to the embodiment includes the LUT  53 , the feedback coefficient calculating unit  66 , the clip processing unit  62 , and the updating unit  61 . The LUT  53  stores therein the distortion compensation coefficients. The feedback coefficient calculating unit  66  calculates the feedback coefficient C j  based on the output signal from the power amplifier  34 . If the absolute value |C j | of the feedback coefficient C j  calculated by the feedback coefficient calculating unit  66  is equal to or less than the threshold C th , the clip processing unit  62  outputs the feedback coefficient C j  calculated by the feedback coefficient calculating unit  66 . Furthermore, if the absolute value |C j | of the feedback coefficient C j  calculated by the feedback coefficient calculating unit  66  is greater than the threshold C th , the clip processing unit  62  outputs the feedback coefficient C j ′ of which absolute value is equal to or less than the threshold C th . The updating unit  61  updates the distortion compensation coefficients in the LUT  53  by using the error between the transmission signal that has not been subjected to distortion compensation and the output signal that is output from the power amplifier  34 , by using a predetermined step coefficient, and by using the feedback coefficient output from the clip processing unit  62 . Consequently, the distortion compensation device  10  can improve the quality of the signal transmitted from the distortion compensation device  10 . 
     Furthermore, in the distortion compensation device  10  according to the embodiment, if the absolute value |C j | of the feedback coefficient C j  calculated by the feedback coefficient calculating unit  66  is greater than the threshold C th , regarding the subject feedback coefficient C j , the clip processing unit  62  calculates, by performing the clip process, the feedback coefficient of which absolute value is the threshold C th . The clip process in the embodiment is the process of, for example, multiplying the threshold C th  by the value that is obtained by dividing the feedback coefficient C j  calculated by the feedback coefficient calculating unit  66  by the absolute value |C j | of the subject feedback coefficient C j . Consequently, continuity of the phase of the feedback coefficient C j ′ is maintained even after the clip process and thus it is possible to suppress the degradation of the quality of the signal. 
     Furthermore, in the distortion compensation device  10  according to the embodiment, the clip processing unit  62  uses, as the threshold C th , the maximum value of the absolute value |C j | of the feedback coefficient C j  calculated based on the output signal that is associated with the transmission signal Tx(t-j) related to the address that is greater than the threshold A th  from among the samples of a predetermined number of the transmission signals Tx(t-j). Consequently, the distortion compensation device  10  can improve the quality of the signal transmitted from the distortion compensation device  10 . 
     [b] Second Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 6  is a block, diagram illustrating an example of the distortion compensation device  10  according to a second embodiment. In the distortion, compensation device  10  according to the embodiment, the configuration of the coefficient updating unit  60  is different that of the distortion compensation device  10  according to the first embodiment. Furthermore, the blocks illustrated in  FIG. 6  having the same reference numerals as those illustrated in  FIG. 1  have the same configuration as the blocks illustrated in  FIG. 1  except for the following points described below; therefore, descriptions thereof will be omitted. 
     The coefficient updating unit  60  according to the embodiment includes the updating unit  61 , the clip processing unit  62 , the threshold creating unit  64 , the absolute value calculating unit  65 , the feedback coefficient calculating unit  66 , and the subtracter  67 . The threshold creating unit  64  creates the threshold C th  based on the absolute value |C j | that is output from the absolute value calculating unit  65 . 
     Specifically, regarding the predetermined number of samples counted from the top (for example, 100 samples) of each of the transmission signals Tx(t-j), the threshold creating unit  64  calculates, at each predetermined timing, the average value C ave  by using the absolute value |C j | that is calculated from the feedback signal Fb(t-j). Then, the threshold creating unit  64  calculates, for each of the transmission signals Tx(t-j), for example, as illustrated in  FIG. 7 , the threshold C th  by adding a predetermined offset C off  to the calculated average value C ave .  FIG. 7  is a schematic diagram illustrating an example of the threshold according to the second embodiment. Then, the threshold creating unit  64  outputs the threshold C th  calculated for each of the transmission signals Tx(t-j) to the clip processing unit  62 . 
     Furthermore, the offset C off  is set to the value in which, for example, in the standard environment, the threshold C th  of each of the transmission signals Tx(t-j) becomes the maximum value of the absolute value |C j | that is calculated from the feedback signal Fb(t-j) associated with the transmission signal Tx(t-j) having the address equal to or greater than the threshold A th . The value of the offset C off  is previously set in the threshold creating unit  64  by an administrator of the distortion compensation device  10 , or the like. 
     Here, in each of the transmission signals Tx(t-j), from among the feedback coefficients C j , there may sometimes be the feedback coefficient C j  having a temporarily greater value of the absolute value |C j | due to instantaneous noise. In such a case, if it is assumed that the maximum value of the absolute value |C j | associated with the value of the address equal to or greater than the threshold A th  is decided as the threshold C th , the absolute value |C j | that temporarily becomes a great value due to instantaneous noise is decided as the threshold C th . In such a case, the threshold C th  is maintained as a fixed large value until the subsequent calculation of the threshold C th  is performed. If the threshold C th  is maintained as the fixed large value, the absolute value |C j | of the feedback coefficient C j  obtained after the clip process does not particularly become small and thus the quality of the signal transmitted from the distortion compensation device  10  is not so improved. 
     In contrast, in the distortion compensation device  10  according to the embodiment, regarding the predetermined number of samples counted from the top of each of the transmission signals Tx(t-j), the threshold creating unit  64  calculates the average value C ave  about the absolute value |C j | calculated from the feedback signal Fb(t-j) associated with the transmission signal Tx(t-j). Then, the threshold creating unit  64  calculates the threshold C th  by adding the predetermined offset C off  to the calculated average value C ave . Consequently, in the process of calculating the threshold C th , the variation in the threshold C th  due to the influence of the absolute value |C j | that temporarily becomes a greater value due to instantaneous noise, can be kept low. Consequently, the quality of the signal transmitted from the distortion compensation device  10  can be more stably improved. 
     Coefficient Updating Process 
       FIG. 8  is a flowchart illustrating an example of a coefficient updating process according to the second embodiment. The distortion compensation device  10  performs, at each predetermined timing, the coefficient updating process illustrated in  FIG. 8 . For example, if the distortion compensation device  10  transmits a DL signal in the mobile communication system, such as LTE, or the like, the distortion compensation device  10  performs, for example, for each frame, the coefficient updating process illustrated in  FIG. 8 . Furthermore, in the following flowchart, a description will be given of the transmission signal Tx(t-j) delayed by j samples and the feedback signal Fb(t-j); however, the same process is also performed on each of the delay signals delayed by j represented by 0 to N. 
     First, the feedback coefficient calculating unit  66  initializes the variable s that counts the pieces of sampling data of the transmission signal Tx(t-j) to zero (Step S 200 ). Then, the feedback coefficient calculating unit  66  selects the sampling data of the feedback signal Fb(t-j) that is associated with the sampling data of the s th  transmission signal Tx(t-j) (Step S 201 ). Then, the feedback coefficient calculating unit  66  determines whether the value of the variable s is less than the reference value s num  (Step S 202 ). In the embodiment, the reference value s num  is, for example, 100. 
     If the value of the variable s is less than the reference value s num  (Yes at Step S 202 ), the feedback coefficient calculating unit  66  performs arithmetic operation indicated by Equation (3) described above by using the sampling data of the feedback signal Fb(t-j) selected at Step S 201 . Consequently, the feedback coefficient C j  is calculated (Step S 203 ). Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the absolute value calculating unit  65 . 
     Then, the absolute value calculating unit  65  calculates the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66  (Step S 204 ). Then, the absolute value calculating unit  65  outputs the calculated absolute value |C j | to the threshold creating unit  64 . The threshold creating unit  64  holds the absolute value |C j | output from the absolute value calculating unit  65 . 
     Then, the clip processing unit  62  outputs, the updating unit  61 , the feedback coefficient C j  output from the feedback coefficient calculating unit  66 . The updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above by using the feedback coefficient C j  output from the clip processing unit  62 . Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 205 ). 
     Then, the feedback coefficient calculating unit  66  increments the variable s by 1 (Step S 206 ). Then, the feedback coefficient calculating unit  66  determines whether the value of the variable s is greater than s max  that is the maximum value of the variable s (Step S 207 ). In the embodiment, s max  is the number of samples in a single frame and is, for example, 1000. If the value of the variable s is equal to or less than the value of s max  (No at Step S 207 ), the feedback coefficient calculating unit  66  again performs the process indicated at Step S 201 . In contrast, if the value of the variable s is greater than the value of s max  (Yes at Step S 207 ), the distortion compensation device  10  ends the process illustrated in the subject flowchart. 
     At Step S 202 , if the value of the variable s is equal to or greater than the reference value s num  (No at Step S 202 ), the feedback coefficient calculating unit  66  determines whether the value of the variable s is equal to the reference value s num  (Step S 208 ). If the value of the variable s is equal to the reference value s num  (Yes at Step S 208 ), the feedback coefficient calculating unit  66  performs the arithmetic operation indicated by Equation (3) described above by using the sampling data of the feedback signal Fb(t-j) selected at Step S 201 . Consequently, the feedback coefficient C j  is calculated (Step S 209 ). Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the absolute value calculating unit  65 . 
     Then, the absolute value calculating unit  65  calculates the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66  (Step S 210 ). Then, the absolute value calculating unit  65  outputs the calculated absolute value |C j | to the clip processing unit  62  and the threshold creating unit  64 . 
     Then, the threshold creating unit  64  calculates the average value C ave  of the absolute values |C j | by using the absolute value |C j | output from the absolute value calculating unit  65  and by using the holding absolute value |C j | (Step S 211 ). Then, the threshold creating unit  64  calculates the threshold C th  by adding the offset C off  to the average value C ave  (Step S 212 ). Then, the threshold creating unit  64  outputs the calculated threshold C th  to the clip processing unit  62 . 
     Then, the clip processing unit  62  determines whether the value of the absolute value |C j | of the feedback coefficient C j  output from the absolute value calculating unit  65  is greater than the value of the threshold C th  output from the threshold creating unit  64  (Step S 213 ). If the value of the absolute value |C j | is equal to or less than the value of the threshold C th  (No at Step S 213 ), the clip processing unit  62  performs the process indicated at Step S 205 . 
     In contrast, if the value of the absolute value |C j | is greater than the value of the threshold C th  (Yes at Step S 213 ), the clip processing unit  62  performs the arithmetic operation indicated by Equation (5) described above (Step S 214 ). Consequently, the clip process of clipping is performed, while maintaining the phase of the feedback coefficient C j , such that the absolute value of the feedback coefficient C j  becomes the threshold C th . Then, the clip processing unit  62  outputs the feedback coefficient ty that is clipped at the threshold C th  to the updating unit  61 . 
     Then, the updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above by using the feedback coefficient C j ′ output from the clip processing unit  62 . Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 215 ). Then, the threshold creating unit  64  and the feedback coefficient calculating unit  66  performs the process indicated at Step S 206 . 
     Effects of the Second Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, regarding the predetermined number of samples of the transmission signals, the clip processing unit  62  uses, as the threshold C th , the value obtained by adding the predetermined offset C off  to the average value C ave  of the absolute values of the feedback coefficients calculated based on the output signal that is associated with the transmission signal. Consequently, it is possible to more stably improve the quality of the signal transmitted from the distortion compensation device  10 . 
     [c] Third Embodiment 
     In the first and the second embodiments described above, as described by using, for example,  FIG. 5 or 8 , the threshold C th  is calculated for each first period, such as the period of a single frame, or the like, by using samples in the beginning of a second period in a first period. Then, in the first period and in the remaining period after the second period has elapsed, the clip process is performed by using the threshold C th  that is calculated in the second period. In contrast, in the third embodiment, the threshold C th  calculated in the beginning of the second period in the first period is used for the clip process until the threshold C th  is calculated in the beginning of the second period in the first period. 
       FIG. 9  is a schematic diagram illustrating an example of calculation timing of a threshold according to a third embodiment. In the third embodiment, for example, as illustrated in  FIG. 9 , first, the threshold C th  is calculated by using the samples that are present in the beginning of a second period b in a first period a. The calculated threshold C th  is used for the clip process performed in a period c during which the threshold C th  is calculated in the beginning of a second period b′ in a subsequent first period a′. Then, the threshold C th  that is calculated by using the samples in the beginning of the second period b′ in the first period a′ is used for the clip process in the period c′ during which the threshold C th  is calculated in the beginning of a second period b″ in a subsequent first period a″. 
     Furthermore, in the third embodiment, the first period a and the second period b are arbitrarily set. For example, in an environment in which communication traffic sharply varies, the first period a may also be set shorter with respect to the second period b. Consequently, the threshold C th  can be updated as needed in accordance with the variation in the communication environment. In contrast, in an environment in which communication traffic does not vary so much, the first period a may also be set longer with respect to the second period b. Consequently, the frequency of updating the threshold C th  is reduced and the processing load of the distortion compensation device  10  is reduced. 
     Furthermore, for example, as illustrated in  FIG. 10 , the second period b may also be overlapped with another second period b.  FIG. 10  is a schematic diagram illustrating another example of calculation timing of the threshold according to the third embodiment. For example, as illustrated in  FIG. 10 , the threshold C th  calculated in the second period b 1  is used for the clip process performed in the period c 1  during which the threshold C th  is calculated in a subsequent second period b 2 . Similarly, the threshold C th  calculated in the second period b 2  is used for the clip process in the period c 2  during which the threshold C th  is calculated in a subsequent second period b 3 . 
     Effects of the Third Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, the threshold C th  calculated in the beginning of the second period in the first period is used for the clip process during which the threshold C th  is calculated in the beginning of the second period in the first period. Consequently, it is possible to more stably improve the quality of the signal transmitted from the distortion compensation device  10 . 
     [d] Fourth Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 11  is a block diagram illustrating an example of the distortion compensation device  10  according to a fourth embodiment. The distortion compensation device  10  according to the embodiment differs from the distortion compensation device  10  according to the first embodiment in that, if the value of the address of the transmission signal Tx(t) is equal to or less than the threshold A th , the feedback coefficient C j  is clipped by using the threshold C th  that is calculated based on the magnitude of the transmission signal Tx(t). Furthermore, the blocks illustrated in  FIG. 11  having the same reference numerals as those illustrated in  FIG. 1  have the same configuration as the blocks illustrated in  FIG. 1  except for the following points described below; therefore, descriptions thereof will be omitted. 
     The threshold creating unit  64  determines whether the value of the address output from the address creating unit  52  is greater than the threshold A th . If the value of the address output from the address creating unit  52  is greater than the threshold A th , the threshold creating unit  64  outputs the maximum value to the clip processing unit  62  as the threshold C th . 
     If the value of the address output from the address creating unit  52  is equal to or less than the threshold A th , the threshold creating unit  64  creates the threshold C th  based on, for example, Equation (6) below. Then, the threshold creating unit  64  outputs the created threshold C th  to the clip processing unit  62 . 
     
       
         
           
             
               
                 
                   
                     C 
                     th 
                   
                   = 
                   
                     β 
                     
                       α 
                       - 
                       
                          
                         
                           Tx 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In Equation (6) above, α and β are the predetermined constants. 
     The clip processing unit  62  receives, for each of the transmission signals Tx(t-j), the feedback coefficient C j  from the feedback coefficient calculating unit  66  and receives the absolute value |C j | of the feedback coefficient C j  from the absolute value calculating unit  65 . Then, the clip processing unit  62  compares, for each of the transmission signals Tx(t-j), the absolute value |C j | received from the absolute value calculating unit  65  with the threshold C th  output from the threshold creating unit  64 . If the value of the absolute value |C j | is equal to or less than the value of the threshold C th , the clip processing unit  62  outputs the feedback coefficient C j  received from the feedback coefficient calculating unit  66  to the updating unit  61 . 
     In contrast, if the value of the absolute value |C j | is greater than the value of the threshold C th , the clip processing unit  62  calculates the feedback coefficient C j ′ based on Equation (5) described above. Then, the clip processing unit  62  outputs the feedback coefficient C j ′ that has been subjected to the clip process to the updating unit  61 . 
     Consequently, the distribution of the feedback coefficients becomes the state illustrated in, for example,  FIG. 12 .  FIG. 12  is a schematic diagram illustrating an example of distribution of the feedback coefficients according to the fourth embodiment.  FIG. 12  illustrates an example of the distribution of the absolute values |C 0 | calculated from the feedback signals Fb(t) that is associated with the transmission signals Tx(t) with the amount of delay of zero. Furthermore, the same distribution as that illustrated in  FIG. 12  is also obtained regarding the absolute values |C j | calculated from the feedback signals Fb(t-j) associated with the transmission signals Tx(t-j) having another amount of delay. In the fourth embodiment, for example, as illustrated in  FIG. 12 , regarding the address having the value equal to or less than the threshold A th , the value of the absolute value |C j | of the feedback coefficient C j  is equal to or less than the threshold C th  and divergence of the feedback coefficient C j  is suppressed. Consequently, the degradation of the accuracy of distortion compensation in the address having a small value is suppressed. 
     Coefficient Updating Process 
       FIG. 13  is a flowchart illustrating an example of a coefficient updating process according to the fourth embodiment. The distortion compensation device  10  starts the coefficient updating process illustrated in FIG,  13  at a predetermined timing. For example, if the distortion compensation device  10  starts transmission of the DL signal in the mobile communication system, such as LTE, or the like, the distortion compensation device  10  starts the coefficient updating process illustrated in, for example,  FIG. 13 . Furthermore, in the following flowchart described, a description will be given of the transmission signal Tx(t-j) delayed by j samples and the feedback signal Fb(t-j); however, the same process is performed on each of the delay signals delayed by j represented by 0 to N. 
     First, the feedback coefficient calculating unit  66  calculates the feedback coefficient C j  by performing the arithmetic operation indicated by Equation (3) described above by using the sampling data of the feedback signal Fb(t-j) associated with the sampling data of the transmission signal Tx(t-j) (Step S 220 ). Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the clip processing unit  62 . 
     Then, the threshold creating unit  64  determines whether the value of the address output from the address creating unit  52  is greater than the threshold A th  (Step S 221 ). If the value of the address output from the address creating unit  52  is greater than the threshold A th  (Yes at Step S 221 ), the threshold creating unit  64  outputs the maximum value to the clip processing unit  62  as the threshold C th . Because the absolute value |C j | received from the absolute value calculating unit  65  is smaller than the threshold C th  output from the threshold creating unit  64 , the clip processing unit  62  outputs the feedback coefficient C j  received from the feedback coefficient calculating unit  66  to the updating unit  61 . 
     The updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above by using the feedback coefficient C j  output from the clip processing unit  62 . Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 222 ). Then, the feedback coefficient calculating unit  66  again performs the process indicated at Step S 220 . 
     In contrast, if the value of the address output from the address creating unit  52  is equal to or less than the threshold A th  (No at Step S 221 ), the threshold creating unit  64  creates the threshold C th  based on Equation (6) described above (Step S 223 ). Then, the threshold creating unit  64  outputs the created threshold C th  to the clip processing unit  62 . The clip processing unit  62  determines whether the absolute value |C j | received from the absolute value calculating unit  65  is greater than the threshold C th  output from the threshold creating unit  64  (Step S 224 ). If the absolute value |C j | is equal to or less than the threshold C th  (No at Step S 224 ), the clip processing unit  62  outputs the feedback coefficient C j  received from the feedback coefficient calculating unit  66  to the updating unit  61 . Then, the updating unit  61  performs the process indicated at Step S 222 . 
     In contrast, if the absolute value |C j | is greater than the threshold C th  (Yes at Step S 224 ), the clip processing unit  62  calculates the feedback coefficient C j ′ based on Equation (5) described above (Step S 225 ). Then, the clip processing unit  62  outputs the feedback coefficient C j ′ to the updating unit  61 . The updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above by using the feedback coefficient C j ′ output form the clip processing unit  62 . Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 226 ). Then, the feedback coefficient calculating unit  66  again performs the process indicated at Step S 220 . 
     Effects of the Fourth Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, if the value of the address of the transmission signal Tx(t) is equal to or less than the predetermined value, the feedback coefficient C j  is clipped by using the threshold C th  that is calculated based on the magnitude of the transmission signal Tx(t). Consequently, it is possible to more stably improve the quality of the signal transmitted from the distortion compensation device  10 . 
     [e] Fifth Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 14  is a block diagram illustrating an example of the distortion compensation device  10  according to a fifth embodiment. The distortion compensation device  10  according to the embodiment differs from the distortion compensation device  10  according to the first embodiment in that, instead of the process of clipping the feedback coefficient C j , the process of switching a step coefficient μ is performed in accordance with the value of the address of the transmission signal Tx(t-j). Furthermore, the blocks illustrated in  FIG. 14  having the same reference numerals as those illustrated in  FIG. 1  have the same configuration as the blocks illustrated in  FIG. 1  except for the following points described below; therefore, descriptions thereof will be omitted. 
     The coefficient updating unit  60  according to the embodiment includes the updating unit  61 , the feedback coefficient calculating unit  66 , the subtracter  67 , and a step coefficient switching unit  68 . The feedback coefficient calculating unit  66  calculates the feedback coefficient C j  for each of the transmission signals Tx(t-j) by performing the arithmetic operation based on Equation (3) described above by using the feedback signal Fb(t) output from the ADC  37 . Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the updating unit  61 . The subtracter  67  calculates the error e(t) by performing the arithmetic operation indicated by Equation (2) described above and outputs the calculated error e(t) to the updating unit  61 . 
     The step coefficient switching unit  68  acquires, for each of the transmission signals Tx(t-j), the address created by the address creating unit  52 . Then, the step coefficient switching unit  68  determines, for each of the transmission signals Tx(t-j), whether the value of the address is greater than the predetermined threshold A th . Namely, the step coefficient switching unit  68  determines, for each of the transmission signals Tx(t-j) each having a different amount of delay, whether the amplitude of the transmission signal Tx(t-j) is greater than the predetermined value. Furthermore, because the threshold A th  is derived from the noise in the section from the amplifier to the ADC, the threshold A th  is set based on the measured value of the magnitude of the noise of this portion. 
     If the value of the address is greater than the predetermined threshold A th , the step coefficient switching unit  68  outputs a step coefficient μ 0  that is a first value to the updating unit  61 . In contrast, if the value of the address is equal to or less than the predetermined threshold A th , the step coefficient switching unit  68  outputs, to the updating unit  61 , a step coefficient μ 1  that is a second value smaller than the first value. Furthermore, the values of the step coefficients μ 0  and μ 1  are previously stored in a memory of the distortion compensation device  10  by an administrator of the distortion compensation device  10 , or the like. 
     The updating unit  61  receives the feedback coefficient C j  from the clip processing unit  62 , receives the error e(t) from the subtracter  67 , and receives the step coefficient μ 0  or μ 1  from the step coefficient switching unit  68 . Furthermore, the updating unit  61  reads, from the LUT  53 , the distortion, compensation coefficient h′ j (p) that is before the update. Then, the updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation indicated by Equation (1) described above. Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p). 
     As described above, in the embodiment, in the process of updating the distortion compensation coefficient, regarding the transmission signal having the value of the address equal to or less than the threshold A th , the step coefficient μ 1  having the value smaller than that of the step coefficient μ 0  that is applied to the transmission signal having the value of the address greater than the threshold is used. Consequently, for example, as illustrated in  FIG. 15 , in the transmission signal Tx(t) having the value of the address equal to or less than the threshold A th , the value of the product of the absolute value |C 0 | of the feedback coefficient C 0  and the step coefficient μ becomes small.  FIG. 15  is a schematic diagram illustrating an example of distribution of the products of the absolute values |C 0 | of the feedback coefficients C 0  and the step coefficient μ according to the fifth embodiment. 
     Consequently, the update amount of the distortion compensation coefficient with respect to the transmission signal Tx(t-j) having the value of the address equal to or less than the threshold A th , i.e., the transmission signal Tx(t-j) having a small amplitude, is calculated as a small value. Consequently, in the update process of the distortion compensation coefficient performed on the transmission signal Tx(t-j) having the small amplitude, the influence of noise can be kept low. Consequently, the distortion compensation device  10  can improve the quality of the signal transmitted from the distortion compensation device  10 . 
     Coefficient Updating Process 
       FIG. 16  is a flowchart illustrating an example of a coefficient updating process according to the fifth embodiment. The distortion compensation device  10  performs, at each predetermined timing, the coefficient updating process illustrated in  FIG. 16 . For example, if the distortion compensation device  10  transmits a DL signal in the mobile communication system, such as LTE, or the like, the distortion compensation device  10  performs, for each, for example, single frame, the coefficient updating process illustrated in  FIG. 16 . Furthermore, regarding the following flowchart, the transmission signal Tx(t-j) delayed by j samples and the feedback signal Fb(t-j) will be described, the same process is also performed on each of the delay signals delayed by j represented by 0 to N. 
     First, the feedback coefficient calculating unit  66  initializes the variable s that counts the sampling data of the transmission signal Tx(t-j) to zero (Step S 300 ). Then, the feedback coefficient calculating unit  66  selects the sampling data of the feedback signal Fb(t-j) associated with the sampling data of the s th  transmission signal Tx(t-j) (Step S 301 ). Then, the feedback coefficient calculating unit  66  calculates the feedback coefficient C j  by performing the arithmetic operation indicated by Equation (3) described above by using the sampling data of the feedback signal Fb(t-j) selected at Step S 301  (Step S 302 ). Then, the feedback coefficient calculating unit  66  outputs the calculated feedback coefficient C j  to the updating unit  61 . 
     Then, the step coefficient switching unit  68  refers to the value of the address created by the address creating unit  52  and determines whether the value A of the subject address is greater than the predetermined threshold A th  (Step S 303 ). If the value A of the address is greater than the predetermined threshold A th  (Yes at Step S 303 ), the step coefficient switching unit  68  outputs, to the updating unit  61  as the step coefficient μ, the step coefficient μ 0  that is the first value (Step S 304 ). In contrast, if the value A of the address is equal to or less than the predetermined threshold A th  (No at Step S 303 ), the step coefficient switching unit  68  outputs, to the updating unit  61  as the step coefficient μ, the step coefficient μ 1  that is the second value and that is smaller than the step coefficient μ 0  that is the first value (Step S 305 ). 
     Then, the updating unit  61  receives the feedback coefficient, C j  from the feedback coefficient calculating unit  66 , receives the error e(t) from the subtracter  67 , and receives the step coefficient μ from the step coefficient switching unit  68 . Furthermore, the updating unit  61  reads, from the LUT  53 , the distortion compensation coefficient h′ j (p) that is before the update. Then, the updating unit  61  calculates the updated distortion compensation coefficient h j (p) by performing the arithmetic operation based on Equation (1) described above. Then, the updating unit  61  updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p) (Step S 306 ). 
     Then, the feedback coefficient calculating unit  66  increments the variable s by  1  (Step S 307 ). Then, the feedback coefficient calculating unit  66  determines whether the value of the variable s is greater than the maximum value s max  of the variable s (Step S 308 ). In the embodiment, s max  is, for example, 1000. If the value of the variable s is equal to or less than the value of s max  (No at Step S 308 ), the feedback coefficient calculating unit  66  again performs the process indicated at Step S 301 . In contrast, if the value of the variable s is greater than the value of s max  (Yes at Step S 308 ), the distortion compensation device  10  ends the process illustrated in the flowchart. 
     Effect of the Fifth Embodiment 
     As is clear from the description above, the distortion compensation device  10  according to the embodiment includes the LUT  53 , the feedback coefficient calculating unit  66 , and the updating unit  61 . The LUT  53  stores therein the distortion compensation coefficients. The feedback coefficient calculating unit  66  calculates the feedback coefficient based on the output signal from the power amplifier  34 . The updating unit  61  updates the distortion compensation coefficients in the LUT  53  by using the error between the transmission signal that has not been subjected to distortion compensation and the output signal output from the power amplifier  34 , by using the predetermined step coefficient, and by using the feedback coefficient output from the feedback coefficient calculating unit  66 . Furthermore, when the updating unit  61  updates the distortion compensation coefficients associated with the transmission signal having the value equal to or less than the predetermined value, the updating unit  61  updates the distortion compensation coefficients by using the step coefficient μ that is the value smaller than that of the step coefficient μ 0  that is used to update the distortion compensation coefficients associated with the transmission signal having the value greater than the predetermined value. Consequently, the distortion compensation device  10  can improve the quality of the signal transmitted from the distortion compensation device  10 . 
     [f] Sixth Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 17  is a block diagram illustrating an example of the distortion compensation device  10  according to a sixth embodiment. The distortion compensation device  10  according to the embodiment differs from the distortion compensation device  10  according to the fifth embodiment in that the distortion compensation coefficients are updated by using the step coefficient that is in accordance with the value of the address of the transmission signal. Furthermore, the blocks illustrated in  FIG. 17  having the same reference numerals as those illustrated in  FIG. 14  have the same configuration as the blocks illustrated in  FIG. 14  except for the following points described below; therefore, descriptions thereof will be omitted 
     The coefficient updating unit  60  includes the updating unit  61 , the feedback coefficient calculating unit  66 , the subtracter  67 , and a step coefficient calculating unit  69 . The step coefficient calculating unit  69  acquires, for each of the transmission signals Tx(t-j) each having a different amount of delay, the address created by the address creating unit  52 . Then, the step coefficient calculating unit  69  determines, for each of the transmission signals Tx(t-j), whether the value of the address is greater than the predetermined threshold A th . Namely, the step coefficient calculating unit  69  determines, for each of the transmission signals Tx(t) each having a different amount of delay, whether the amplitude of the transmission signal Tx(t) is greater than the predetermined value, 
     If the value of the address is greater than the predetermined threshold A th , the step coefficient calculating unit  69  outputs the step coefficient μ 0  to the updating unit  61 . In contrast, if the value of the address is equal to or less than the predetermined threshold A th , the step coefficient calculating unit  69  calculates the step coefficient μ 1  based on Equation (7) below and outputs the calculated step coefficient μ 1  to the updating unit  61 . 
     
       
         
           
             
               
                 
                   
                     μ 
                     1 
                   
                   = 
                   
                     β 
                     
                       α 
                       - 
                       
                          
                         
                           Tx 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In Equation (7) above, α and β are a predetermined constant and are previously set in the step coefficient calculating unit  69  by an administrator of the distortion compensation device  10  or the like. Furthermore, regarding α and β, for example, in the transmission signal Tx(t) in which the value of the address is equal to or less than the threshold A th , the value in which the value of the step coefficient μ 1  is smaller than the value of the step coefficient μ 0  is selected. 
     The updating unit  61  receives the feedback coefficient C j  from the feedback coefficient calculating unit  66 , receives the error e(t) from the subtracter  67 , and receives the step coefficient μ from the step coefficient calculating unit  69 . Furthermore, the updating unit  61  reads, from the LUT  53 , the distortion compensation coefficient h′ j (p) that is before the update. Then, the updating unit  61  calculates the updated distortion compensation coefficient h j (p) based on Equation (1) described above and updates the distortion compensation coefficient h′ j (p) in the LUT  53  by using the calculated distortion compensation coefficient h j (p). 
     In this way, in the embodiment, in the process of updating the distortion compensation coefficient, regarding the transmission signal having the value of the address equal to or less than the threshold A th , the step coefficient μ 1  calculated based on Equation (7) described above is used. Consequently, for example, as illustrated in  FIG. 18 , in the transmission signal Tx(t) having the address equal to or less than the threshold A th , the value of the product of the absolute value |C 0 | of the feedback coefficient C 0  and the step coefficient μ becomes small.  FIG. 18  is a schematic diagram illustrating an example of distribution of the products of the absolute values |C 0 | of feedback coefficients C 0  and step coefficient μ according to the sixth embodiment. 
     Consequently, an update amount of the distortion compensation coefficient with respect to the transmission signal Tx(t-j) having the value of the address equal to or less than the threshold A th , i.e., the transmission signal Tx(t-j) with a small amplitude, is calculated as a small value. Thus, in the process of updating the distortion compensation coefficient with respect to the transmission signal Tx(t-j) with a small amplitude, the influence of noise can be kept low. Consequently, the distortion compensation device  10  can improve the quality of the signal transmitted from the distortion compensation device  10 . 
     Coefficient Updating Process 
       FIG. 19  is a flowchart illustrating an example of a coefficient updating process according to the sixth embodiment. The distortion compensation device  10  performs, at each predetermined timing, the coefficient updating process illustrated in  FIG. 19 . Furthermore, the processes illustrated in  FIG. 19  having the same reference numerals as those illustrated in  FIG. 16  have the same processes as those illustrated in  FIG. 16  except for the following points described below; therefore, descriptions thereof will be omitted. 
     At Step S 303 , the step coefficient calculating unit  69  refers to the value of the address created by the address creating unit  52  determines whether the value A of the subject address is greater than the predetermined threshold A th  (Step S 303 ). If the value A of the address is greater than the predetermined threshold A th  (Yes at Step S 303 ), the step coefficient calculating unit  69  outputs the step coefficient μ 0  to the updating unit  61  as the step coefficient μ (Step S 304 ). In contrast, if the value A of the address is equal to or less than the predetermined threshold A th  (No at Step S 303 ), the step coefficient calculating unit  69  outputs, to the updating unit  61  as the step coefficient μ, the step coefficient μ 1  that is calculated based on Equation (7) described above (Step S 310 ). Then, the updating unit  61  performs the process indicated at Step S 306 . 
     Effect of the Sixth Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, when the updating unit  61  updates the distortion compensation coefficients associated with the transmission signal having the value equal to or less than the predetermined value, the updating unit  61  updates the distortion compensation coefficients by using the step coefficients calculated based on the magnitude of the transmission signal. Consequently, in the process of updating the distortion compensation coefficient associated with the transmission signal having a small amplitude, the influence of noise can be kept low and the quality of the signal transmitted from the distortion compensation device  10  can be improved. 
     [g] Seventh Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 20  is a block diagram illustrating an example of the distortion compensation device  10  according to a seventh embodiment. The distortion compensation device  10  according to the embodiment differs from the distortion compensation device  10  according to the sixth embodiment in that, if the value of the address of the transmission signal Tx(t) is equal to or less than the threshold A th , the step coefficient μ is changed based on the ratio of the absolute value |C j | of the feedback coefficient C j  to the threshold C th . Furthermore, the blocks illustrated in  FIG. 20  having the same reference numerals as those illustrated in  FIG. 1 or 17  have the same configuration as the blocks illustrated in  FIG. 1 or 17  except for the following points described below; therefore, descriptions thereof will be omitted 
     In the holding unit  63 , the threshold C th  for each of the transmission signals Tx(t-j) is previously stored. The absolute value calculating unit  65  calculates, for each of the transmission signals Tx(t-j), the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66  and outputs the calculated absolute value |C j | to the step coefficient calculating unit  69 . 
     The step coefficient calculating unit  69  acquires, for each of the transmission signals Tx(t-j) each having a different amount of delay, the address created by the address creating unit  52 . Then, the step coefficient calculating unit  69  determines, for each of the transmission signals Tx(t-j), whether the value of the address is greater than the predetermined threshold A th . If the value of the address is greater than the predetermined threshold A th , the step coefficient calculating unit  69  outputs the step coefficient μ 0  to the updating unit  61 . 
     In contrast, if the value of the address is equal to or less than the predetermined threshold A th , the step coefficient calculating unit  69  calculates the step coefficient μ 1  based on, for example, Equation (8) below by using both the threshold C th  in the holding unit  63  and the absolute value |C j | output from the absolute value calculating unit  65 . Then, the step coefficient calculating unit  63  outputs the calculated step coefficient μ 1  to the updating unit  61 . 
     
       
         
           
             
               
                 
                   
                     μ 
                     1 
                   
                   = 
                   
                     
                       μ 
                       0 
                     
                      
                     
                       
                         C 
                         th 
                       
                       
                          
                         
                           C 
                           j 
                         
                          
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Effect of the Seventh Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, if the value of the address of the transmission signal Tx(t) is equal to or less than the threshold A th , the step coefficient calculating unit  69  changes the step coefficient μ based on the ratio of the absolute value |C j | of the feedback coefficient C j  to the threshold C th . Consequently, in the process of updating the distortion compensation coefficient with respect to the transmission signal having a small amplitude, the influence of noise can be kept low and the quality of the signal transmitted from the distortion compensation device  10  can be improved. 
     Eighth Embodiment 
     Configuration of the Distortion Compensation Device  10   
       FIG. 21  is a block diagram illustrating an example of the distortion compensation device  10  according to an eighth embodiment. The eighth embodiment is a combination of the first embodiment and the seventh embodiment. Namely, the distortion compensation device  10  according to the embodiment specifies, as the threshold C th , the maximum value of the absolute value |C j | of the feedback coefficient C j  associated with the address having the value greater than the threshold A th . Then, if the value of the address of the transmission signal Tx(t) is equal to or less than the threshold A th , the distortion compensation device  10  according to the embodiment changes the step coefficient μ based on the ratio of the absolute value |C j | of the feedback coefficient C j  to the threshold C th . Furthermore, the blocks illustrated in  FIG. 21  having the same reference numerals as those illustrated in  FIG. 1 or 17  have the same configuration as the blocks illustrated in  FIG. 1 or 17  except for the following points described below; therefore, descriptions thereof will be omitted. 
     The absolute value calculating unit  65  calculates, for each of the transmission signals Tx(t-j), the absolute value |C j | of the feedback coefficient C j  output from the feedback coefficient calculating unit  66  and then outputs the calculated absolute value |C j | to both the threshold creating unit  64  and the step coefficient calculating unit  69 . The threshold creating unit  64  creates, for each predetermined period, the threshold C th  by using the predetermined number of top samples included in the predetermined period related to the feedback coefficient C j  associated with the transmission signal Tx(t-j). Specifically, the threshold creating unit  64  creates, as the threshold C th , the maximum value from among the absolute values |C j | of the feedback coefficients C j  associated with the address having the value greater than the threshold A th . Then, the threshold creating unit  64  stores the created threshold C th  in the holding unit  63 . The holding unit  63  stores therein the threshold C th  created for each of the transmission signals Tx(t-j) by the threshold creating unit  64 . 
     The step coefficient calculating unit  69  acquires, for each of the transmission signals Tx(t-j) each having a different amount of delay, the address created by the address creating unit  52  and determines whether the value of the address is greater than the predetermined threshold A th . If the value of the address is greater than the predetermined threshold A th , the step coefficient calculating unit  69  outputs the step coefficient μ 0  to the updating unit  61 . 
     In contrast, if the value of the address is equal to or less than the predetermined threshold A th , the step coefficient calculating unit  69  calculates the step coefficient μ 1  based on, for example, Equation (8) described above by using both the threshold C th  in the holding unit  63  and the absolute value |C j | output from the absolute value calculating unit  65 . Then, the step coefficient calculating unit  69  outputs the calculated step coefficient μ 1  to the updating unit  61 . 
     Furthermore, similarly to the second embodiment described above, the threshold creating unit  64  may also create, as the threshold C th  for each of the transmission signals Tx(t-j), the value obtained by adding the predetermined offset C off  to the average value C ave  of the absolute values {C j } of the feedback coefficients C j  associated with the transmission signals Tx(t-j). 
     Effect of the Eighth Embodiment 
     As is clear from the description above, in the distortion compensation device  10  according to the embodiment, the threshold creating unit  64  creates, for each predetermined period, the threshold C th  by using the feedback coefficient C j . Furthermore, if the value of the address of the transmission signal Tx(t) is equal to or less than the threshold A th , the step coefficient calculating unit  69  changes the step coefficient μ based on the ratio of the absolute value |C j | of the feedback coefficient C j  to the threshold C th . Consequently, in the process of updating the distortion compensation coefficient with respect to the transmission signal having a small amplitude, the influence of noise can be kept low and the quality of the signal transmitted from the distortion compensation device  10  can be improved. 
     Hardware 
     The distortion compensation device  10  according to the first to the eight embodiments can be implemented by, for example, the hardware illustrated in  FIG. 22 .  FIG. 22  is a block diagram illustrating an example of hardware the distortion compensation device  10 . The distortion compensation device  10  includes, for example, as illustrated in  FIG. 22 , an interface circuit  11 , a memory  12 , a processor  13 , a radio circuit  14 , and the antenna  40 . 
     The interface circuit  11  is an interface for performing wired communication with the BB processing unit. The radio circuit  14  includes the power amplifier  34 , or the like. The radio circuit  14  performs a process, such as up-conversion, or the like, on the signal output from the processor  13 , amplifies the processed signal by using the power amplifier  34 , and transmits the signal from the antenna  40 . Furthermore, the radio circuit  14  performs a process, such as down-conversion, or the like, on a part of the signal amplified by the power amplifier  34  and feeds back the processed signal to the processor  13 . In the radio circuit  14 , for example, the DAC  31 , the mixer  32 , the oscillator  33 , the power amplifier  34 , the coupler  35 , the mixer  36 , the ADC  37 , and the like are included. 
     The memory  12  stores therein various kinds of programs, data, and the like for implementing the function of, for example, the distortion compensation unit  50  and the coefficient updating unit  60 . The processor  13  implements each of the functions of, for example, the distortion compensation unit  50  and the coefficient updating unit  60  by executing the programs read from the memory  12 . 
     Furthermore, in the distortion compensation device  10  illustrated in  FIG. 22  as an example, each of the single processor  13 , the radio circuit  14 , and the antenna  40  is provided; however, two or more of the processors  13 , the radio circuits  14 , and the antennas  40  may also be provided in the distortion compensation device  10 . 
     Furthermore, the programs, the data, or the like in the memory  12  do not need to be stored in the memory  12  from the beginning. For example, each program, the data, or the like may also be stored in a portable recording medium, such as a memory card, or the like, inserted in the distortion compensation device  10  and the distortion compensation device  10  may also acquire each of the programs, the data, or the like from the portable recording medium and executes the programs. Furthermore, the distortion compensation device  10  may also acquire each of the programs from another computer, a server device, or the like that stores therein each program, the data, or the like via a wireless communication line, a public circuit, the Internet, a LAN, a WAN, or the like. 
     Others 
     Furthermore, the technology disclosed in the present application is not limited to the embodiments described above and various modifications are possible as long as they do not depart from the spirit of the present application. 
     For example, in the first to the fourth and the eighth embodiments described above, the threshold C th  of the feedback coefficient C j  is created for each of the transmission signals Tx(t-j); however, the disclosed technology is not limited to this. As another example, the threshold that is created from the feedback coefficient C 0  with respect to the transmission signal Tx(t) having the amount of delay of zero may also be used as the threshold C th  of the transmission signal Tx(t-j) having another delay signal. Consequently, it is possible to reduce the processing load applied to create the threshold C th . 
     Furthermore, in each of the embodiments described above, a method of obtaining the distortion compensation coefficient for each magnitude of the amplitude or the power of the transmission signal and performing the distortion compensation (LUT method) by using the obtained distortion compensation coefficient has been described as an example; however, the disclosed technology is not limited to this. For example, instead of obtaining the distortion compensation coefficient for each magnitude of the amplitude or the power of the transmission signal, the disclosed technology can also be applied to a case of using a method (series method) of creating a distortion compensation signal based on a series expansion that uses the magnitude of the amplitude or the power of the transmission signal as an argument. In the series method, for example, the distortion compensation signal u(t) is created based on equation (9) below. 
     
       
         
           
             
               
                 
                   
                     u 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         1 
                       
                       k 
                     
                      
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           0 
                         
                         Q 
                       
                        
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             0 
                           
                           Q 
                         
                          
                         
                           
                             h 
                             
                               i 
                               , 
                               j 
                               , 
                               k 
                             
                           
                            
                           
                             
                                
                               
                                 x 
                                  
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     i 
                                   
                                   ) 
                                 
                               
                                
                             
                             
                               k 
                               - 
                               1 
                             
                           
                            
                           
                             x 
                              
                             
                               ( 
                               
                                 t 
                                 - 
                                 j 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     In Equation (9) above, h i, j, k  are examples of the distortion compensation coefficients and are updated by the coefficient updating unit  60  as needed. 
     Furthermore, in each of the embodiments described above, the feedback coefficient C j  is calculated based on Equation (3) described above; however, the disclosed technology is not limited to this. The feedback coefficient C j  may also be calculated based on, for example, Equation (10) or Equation (11) below. 
     
       
         
           
             
               
                 
                   
                     C 
                     j 
                   
                   = 
                   
                     
                       
                         Fb 
                         * 
                       
                        
                       
                         ( 
                         
                           t 
                           - 
                           j 
                         
                         ) 
                       
                     
                     
                       
                         1 
                         N 
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           N 
                         
                          
                         
                           
                              
                             
                               Tx 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     j 
                   
                   = 
                   
                     
                       
                         Tx 
                         * 
                       
                        
                       
                         ( 
                         
                           t 
                           - 
                           j 
                         
                         ) 
                       
                     
                     
                       
                         1 
                         N 
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           N 
                         
                          
                         
                           
                              
                             
                               Fb 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     Similarly, in each of the embodiments described above, the feedback coefficient C 0  is calculated based on Equation (4) described above; however, the disclosed technology is not limited to this. The feedback coefficient C 0  may also be calculated based on, for example, Equation (12) or Equation (13) below. 
     
       
         
           
             
               
                 
                   
                     C 
                     0 
                   
                   = 
                   
                     
                       
                         Fb 
                         * 
                       
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                     
                       
                         1 
                         N 
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           N 
                         
                          
                         
                           
                              
                             
                               Tx 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     C 
                     0 
                   
                   = 
                   
                     
                       
                         Tx 
                         * 
                       
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                     
                       
                         1 
                         N 
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           N 
                         
                          
                         
                           
                              
                             
                               Fb 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     Furthermore, in each of the embodiments described above, the threshold A th  of the address is the fixed value; however, the disclosed technology is not limited to this. For example, one of the thresholds A th  between two thresholds A th  having different values may also be selected in accordance with the power of the distortion compensation signal that is input to the power amplifier  34 . Specifically, if the value of the power of the distortion compensation signal is equal to or greater than the predetermined threshold P th , the threshold A th  having a greater value between the two thresholds A th  is selected, whereas, if the value of the power of the distortion compensation signal is less than the threshold P th , the threshold A th  having a smaller value is selected. The threshold P th  is set to, for example, the intermediate value between the maximum value of the power that can be input to the power amplifier  34  and the minimum value of the power of the transmission signal that is input to the power amplifier  34 , such as a half of (the maximum value-the minimum value). In a case of heavy communication traffic, the power of the transmission signal input to the power amplifier  34  becomes large, whereas, in a case of low communication traffic, the power of the transmission signal that is input to the power amplifier  34  becomes small. Consequently, the distortion compensation device  10  can switch the threshold A th  in accordance with the variation in communication traffic. 
     According to an aspect of an embodiment, it is possible to improve the quality of transmission signals. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such 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.