Abstract:
A distortion compensating apparatus including: a processor to generate a compensated signal by performing distortion compensation on an input signal, based on a distortion compensation coefficient depending on the input signal, to separates the compensated signal into a first signal and a second signal that have constant amplitude and that have a phase difference based on amplitude of the compensated signal, to generate a third signal by multiplying the first signal by a first coefficient, and to generate a fourth signal by multiplying the second signal by a second coefficient, and a combiner to generate a seventh signal by combining the fifth signal and the sixth signal which are generated by amplifying the third signal and the fourth signal, wherein the processor is further configured to calculate the distortion compensation coefficient, the first coefficient, and the second coefficient, based on the third signal, the fourth signal and the seventh signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-147336 filed on Jun. 29, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a distortion compensating apparatus and a distortion compensating method. 
     BACKGROUND 
     High-frequency amplifying circuits that include an amplifier based on linear amplification with nonlinear components (LINC) are known as units for implementing a high-efficiency liner amplifier. 
       FIG. 1  is a diagram illustrating an example of a LINC-based amplifier. In the LINC-based amplifier, a LINC signal generating unit separates an input modulation signal Sin(t) into a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) and outputs the pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ). The phase difference between the phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) corresponds to the amplitude of the input modulation signal Sin(t). For example, the input modulation signal Sin(t) is a modulation signal exhibiting amplitude modulation and phase modulation (angular modulation) and the pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) are constant-envelope, constant-amplitude phase modulation signals. Here, the input modulation signal Sin(t) and the pair of the phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) may be baseband signals or intermediate frequency (IF) signals. The LINC signal generating unit outputs the pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) as digital signals. 
     Here, the signals Sin(t), Sc 1 ( t ), and Sc 2 ( t ) are represented, for example, by Expression 1. 
     
       
         
           
             
               
                 
                   
                     
                       Sin 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           a 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         · 
                         cos 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Sc 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                       ⁢ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         a 
                         max 
                       
                       · 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             
                               θ 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             + 
                             
                               ψ 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Sc 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         a 
                         max 
                       
                       · 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             
                               θ 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             - 
                             
                               ψ 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       ψ 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         cos 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             a 
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           
                             2 
                             · 
                             
                               a 
                               max 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 1, “a(t)” represents the amplitude component of the input modulation signal Sin(t), “θ(t)” represents the phase component of the input modulation signal Sin(t). Phase modulation is provided so that a phase difference of 2×ψ(t), which corresponds to the amplitude a(t), is generated. Furthermore, “a max ” represents the maximum value of the amplitude a(t) and is a constant. The signals Sc 1 ( t ) and Sc 2 ( t ) are constant envelope signals. That is, the amplitude of the signals Sc 1 ( t ) and Sc 2 ( t ) is fixed. 
     The signal Sc 1 ( t ), one of the pair of phase modulation signals output from the LINC signal generating unit, is converted from a digital signal into an analog signal by a digital-to-analog converter (DAC). Furthermore, when the converted analog signal passes through a low-pass filter, a component corresponding to a frequency band of the phase modulation signal Sc 1 ( t ) is extracted, and the other frequency components are suppressed. Similarly, the signal Sc 2 ( t ), the other of the pair of phase modulation signals, is converted from a digital signal to an analog signal by a DAC. Furthermore, when the converted analog signal passes through a low-pass filter, a component corresponding to the frequency band of the phase modulation signal Sc 2 ( t ) is extracted, and other frequency components are suppressed. 
     In the LINC-based amplifier, a quadrature modulator performs quadrature modulation on the phase modulation signal Sc 1 ( t ) which has passed through the corresponding low-pass filter. A frequency converter generates, using a high-frequency signal (oscillation signal) output from an oscillator, a signal S 1 ( t ), which is one of a pair of high-frequency signals that are radio-frequency (RF) signals, and outputs the generated high-frequency signal S 1 ( t ). Similarly, a quadrature modulator performs quadrature modulation on the phase modulation signal Sc 2 ( t ) which has passed through the corresponding low-pass filter. A frequency converter generates, using a high-frequency signal output from an oscillator, a signal S 2 ( t ), which is the other of the pair of high-frequency signals that are RF signals, and outputs the generated high-frequency signal S 2 ( t ). 
     The high-frequency signals S 1 ( t ) and S 2 ( t ) are represented by Expression 2, where “fc” represents a radio frequency (the frequency of the oscillator).
 
 S 1( t )= a   max ·cos(2π· fc·t +θ( t )+ψ( t ))
 
 S 2( t )= a   max ·cos(2π ·fc·t +θ( t )−ψ( t ))  [Expression 2]
 
     A pair of amplifiers include two amplifiers arranged in parallel to each other. The gain and phase characteristics of the two amplifiers are substantially the same. The amplifiers each amplify a high-frequency signal output from the corresponding frequency converter. A portion from the DAC to the amplifier (amplifier A or B) inclusive is also called one branch. 
     A combiner combines the pair of high-frequency signals amplified by the pair of amplifiers together, and outputs the combined signal as a high-frequency signal Sout(t). The signal Sout(t) output from the combiner is represented by Expression 3, where “G” represents the gain of the amplifiers. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Sout 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             G 
                             · 
                             
                               a 
                               max 
                             
                             · 
                             
                               cos 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       π 
                                       · 
                                       fc 
                                       · 
                                       t 
                                     
                                   
                                   + 
                                   
                                     θ 
                                     ⁡ 
                                     
                                       ( 
                                       t 
                                       ) 
                                     
                                   
                                   + 
                                   
                                     ψ 
                                     ⁡ 
                                     
                                       ( 
                                       t 
                                       ) 
                                     
                                   
                                   + 
                                   ϕ 
                                 
                                 ) 
                               
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           G 
                           · 
                           
                             a 
                             max 
                           
                           · 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   2 
                                   ⁢ 
                                   
                                     π 
                                     · 
                                     fc 
                                     · 
                                     t 
                                   
                                 
                                 + 
                                 
                                   θ 
                                   ⁡ 
                                   
                                     ( 
                                     t 
                                     ) 
                                   
                                 
                                 - 
                                 
                                   ψ 
                                   ⁡ 
                                   
                                     ( 
                                     t 
                                     ) 
                                   
                                 
                                 + 
                                 ϕ 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           2 
                           ⁢ 
                           
                             G 
                             · 
                             
                               a 
                               max 
                             
                             · 
                             
                               cos 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     
                                       π 
                                       · 
                                       fc 
                                       · 
                                       t 
                                     
                                   
                                   + 
                                   
                                     θ 
                                     ⁡ 
                                     
                                       ( 
                                       t 
                                       ) 
                                     
                                   
                                   + 
                                   ϕ 
                                 
                                 ) 
                               
                             
                           
                           ⁢ 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               
                                 ψ 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           2 
                           ⁢ 
                           
                             G 
                             · 
                             
                               a 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             · 
                             
                               cos 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     
                                       π 
                                       · 
                                       fc 
                                       · 
                                       t 
                                     
                                   
                                   + 
                                   
                                     θ 
                                     ⁡ 
                                     
                                       ( 
                                       t 
                                       ) 
                                     
                                   
                                   + 
                                   ϕ 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 3, “φ” represents the transmission phase of the pair of high-frequency signals S 1 ( t ) and S 2 ( t ). 
     Related arts are disclosed in, for example, Japanese National Publication of International Patent Application No. 2009-533947 and Japanese Laid-open Patent Publication Nos. 2003-298361, 2003-152464, 5-37263, 9-74320, and 2006-33988. 
     SUMMARY 
     According to an aspect of the invention, a distortion compensating apparatus including: a processor configured to generate a compensated signal by performing distortion compensation on an input signal, based on a distortion compensation coefficient depending on the input signal, to separates the compensated signal into a first signal and a second signal that have constant amplitude and that have a phase difference based on amplitude of the compensated signal, to generate a third signal by multiplying the first signal by a first coefficient, and to generate a fourth signal by multiplying the second signal by a second coefficient, and a first amplifier configured to generate a fifth signal by amplifying the third signal, a second amplifier configured to generate a sixth signal by amplifying the fourth signal, and a combiner configured to generate a seventh signal by combining the fifth signal and the sixth signal, wherein the processor is further configured to calculate the distortion compensation coefficient, the first coefficient, and the second coefficient, based on the third signal, the fourth signal and the seventh signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a LINC-based amplifier; 
         FIG. 2  is a diagram illustrating an example of the constellation of a digital signal sequence when an input modulation signal has two tones; 
         FIG. 3  is a diagram illustrating an example of the constellation after the signal illustrated in  FIG. 2  is converted into an analog signal by a DAC; 
         FIG. 4  illustrates an example of the constellation of output from an amplifier A illustrated in  FIG. 1 ; 
         FIG. 5  illustrates an example of the constellation of output from an amplifier B illustrated in  FIG. 1 ; 
         FIG. 6  is a diagram illustrating an example of the constellation of output from a combiner obtained by combing the signal illustrated in  FIG. 4  and the signal illustrated in  FIG. 5 ; 
         FIG. 7  is a diagram illustrating an example of the configuration of a distortion compensating apparatus according to a first embodiment; 
         FIG. 8  is a diagram illustrating an example of the operation flow of a distortion compensating apparatus; 
         FIG. 9  is a diagram illustrating an example of the hardware configuration of the distortion compensating apparatus according to the first embodiment; 
         FIG. 10  is a diagram illustrating an example of the configuration of a distortion compensating apparatus according to a second embodiment; and 
         FIG. 11  is a diagram illustrating an example of the operation flow of a distortion compensating apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     When a signal whose carrier polarity is inverted, such as a PSK signal, is input as an input modulation signal to a high-frequency amplifying circuit that includes a LINC-based amplifier, there is a point at which the phase is inverted by 180 degrees in a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ), which are generated by a LINC signal generating unit, and the frequency band of the signals enlarges. 
       FIG. 2  is a diagram illustrating an example of the constellation of a digital signal sequence when an input modulation signal has two tones.  FIG. 2  represents a signal Sc 1 , for example. In the example of  FIG. 2 , the signal is a constant envelope signal. Furthermore, in the example of  FIG. 2 , the phase is inverted by 180 degrees between a point represented by (I,Q)=(0, −1) and a point represented by (I,Q)=(0,1). 
     However, the pair of phase modulation signals Sc 1 ( t ) and Sc 1 ( t ), which are digital signals, are capable of expressing only half the sampling frequency, owing to the Nyquist theorem (Nyquist-Shannon sampling theorem). Thus, the digital signals are converted into analog signals by DACs, and then return components of the signals are removed by low-pass filters. Large ringing occurs in such signals, which are different from constant envelope signals. 
       FIG. 3  is a diagram illustrating an example of a constellation after the signal illustrated in  FIG. 2  is converted into an analog signal by a DAC and a high-frequency component of the signal is removed by a low-pass filter.  FIG. 3  illustrates, for example, a signal (analog signal) obtained after the signal Sc 1  is converted into analog by a DAC. 
     The amplitude component of the analog signal changes in response to ringing. That is, the amplitude component of the analog signal is not constant. When such analog signals are amplified by a pair of amplifiers, the analog signals are affected by the AM/AM characteristics and AM/PM characteristics (AM/AM distortion and AM/PM distortion) of the pair of amplifiers. Due to such distortion, a combined output high-frequency signal Sout(t) deteriorates. That is, distortion occurs in the output high-frequency signal Sout(t). AM/AM characteristics represent the amplitude of an output signal relative to the amplitude of an input signal. AM/AM distortion represents distortion caused by AM/AM characteristics. AM/PM characteristics represent the phase rotation of an output signal relative to the amplitude of an input signal. AM/PM distortion represents distortion caused by AM/PM distortion. 
     The amplitude component of such an analog signal changes over time; and meanwhile, the digital signal input to a DAC is a constant envelope signal. That is, the amplitude component of the digital signal input to the DAC does not change over time. Thus, it is difficult to perform digital predistortion processing on a digital signal to be input to a DAC in order to compensate for the nonlinearity of an amplifier. 
       FIG. 4  illustrates an example of the constellation of output from the amplifier A illustrated in  FIG. 1 .  FIG. 4  represents output from the amplifier A when the signal illustrated in  FIG. 3  is input to the amplifier A. The output from the amplifier A is affected by AM/AM distortion and AM/PM distortion. 
       FIG. 5  illustrates an example of the constellation of output from the amplifier B illustrated in  FIG. 1 .  FIG. 5  represents output from the amplifier B when a signal that forms a pair of signals together with the signal illustrated in  FIG. 3  is input to the amplifier B. The output from the amplifier B is affected by AM/AM distortion and AM/PM distortion. 
       FIG. 6  is a diagram illustrating an example of the constellation of output from a combiner that combines the signal illustrated in  FIG. 4  and the signal illustrated in  FIG. 5 . If there is no influence of distortion in an amplifier, the output from the combiner forms a straight line extending from a point represented by (I,Q)=(−2,0) and a point represented by (I,Q)=(2,0). However, as illustrated in  FIG. 6 , due to AM/AM distortion and AM/PM distortion, the output from the combiner has been rotated around the origin. 
     A typical combiner used in an amplifier is, for example, a Wilkinson power combiner with excellent linearity. Furthermore, with a power amplifier a Chireix power combiner may be used in order to increase efficiency. With the use of a Chireix power combiner, linearity decreases but efficiency increases. 
     When a combiner having low linearity, such as a Chireix power combiner, is used in a LINC-based amplifier, the phase difference between branches and the linearity of combined power decrease, and distortion occurs in high-frequency output. 
     Furthermore, when the imbalance between branches is compensated for by using the signal output from the combiner, due to the influence of the nonlinearity of the combiner, error occurs in the compensation for the imbalance between the branches, and high-frequency output may deteriorate. 
     The technique disclosed in the embodiments aims to provide a distortion compensating apparatus that suppresses the deterioration of an output high-frequency signal obtained by combining. 
     Hereinafter, the embodiments will be described with reference to the drawings. The configuration of the embodiments is merely an example, and the disclosed configuration is not limited to a specific configuration of the disclosed embodiments. For implementation of the disclosed embodiments, a specific configuration corresponding to an embodiment may be adopted in an appropriate manner. Individual embodiments may be implemented by combining them as long as no contradiction arises. 
     First Embodiment 
     Configuration Example 
       FIG. 7  is a diagram illustrating an example of the configuration of a distortion compensating apparatus according to a first embodiment. A distortion compensating apparatus  100  includes a digital predistorter (DPD)  102 , a LINC signal generating unit  104 , a phase amplitude adjusting unit  114 , a digital-to-analog converter (DAC)  116 , a low-pass filter (LPF)  118 , a quadrature modulator  120 , a frequency converter  122 , an amplifier  124 , and an inverse calculating unit  126 . The distortion compensating apparatus  100  also includes a phase amplitude adjusting unit  134 , a DAC  136 , a LPF  138 , a quadrature modulator  140 , a frequency converter  142 , an amplifier  144 , and an inverse calculating unit  146 . The distortion compensating apparatus  100  also includes a combiner  152 , a frequency converter  154 , a quadrature demodulator  156 , an analog-to-digital converter (ADC)  158 , a modeling unit  180 , and an inverse characteristics calculating unit  182 . 
     The DPD  102  performs distortion compensation processing for an input modulation signal Sin(t). The distortion compensation processing performed is processing to multiply a signal that has not been amplified by a LINC-based amplifier by a predistortion coefficient to generate distortion in advance so as to cancel distortion generated in the LINC-based amplifier. The predistortion coefficient is a distortion compensation coefficient for compensating for distortion in an amplifier. The predistortion coefficient, which is multiplied with an input modulation signal, depends on the input modulation signal Sin(t). The predistortion coefficient is a coefficient for performing compensation such that the amplitude of an output signal from an amplifier is proportional to the amplitude of an input signal to the amplifier and that the phase difference between the input signal and the output signal is 0. The input signal and the predistortion coefficient are represented by, for example, an in-phase (I) component and a quadrature (Q) component. The predistortion coefficient is calculated by the inverse characteristics calculating unit  182 . The DPD  102  outputs the processed signal to the LINC signal generating unit  104  and the modeling unit  180 . The input modulation signal Sin(t) is, for example, a baseband signal. The input modulation signal Sin(t) may be an intermediate frequency (IF) signal. The DPD  102  is an example of a distortion compensating unit. The predistortion coefficient depends on the size of an input signal. 
     The LINC signal generating unit  104  separates the output from the DPD  102  into a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) that have a phase difference corresponding to the amplitude of the output from the DPD  102 , and outputs the pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ). The signal Sc 1 ( t ) is output to the phase amplitude adjusting unit  114 . The signal Sc 2 ( t ) is output to the phase amplitude adjusting unit  134 . 
     The phase amplitude adjusting unit  114  multiplies the signal Sc 1 ( t ) output from the LINC signal generating unit  104  by a value calculated by the inverse calculating unit  126 . Here, the value does not depend on the size of the signal Sc 1 ( t ). Furthermore, the value may depend on time. The phase amplitude adjusting unit  114  is an example of an adjusting unit. 
     The DAC  116  converts a digital signal, which is output from the phase amplitude adjusting unit  114 , into an analog signal. 
     The low-pass filter  118  cuts off a high-frequency component of the signal output from the DAC  116 . 
     The quadrature modulator  120  performs quadrature modulation on a signal output from the low-pass filter  118  and outputs the quadrature-modulated signal. 
     The frequency converter  122  includes an oscillator. The frequency converter  122  up-converts the signal that has been quadrature-modulated by the quadrature modulator  120  into a radio frequency (RF). The frequency converter  122  generates a high-frequency signal using a high-frequency signal (oscillation signal) output from the oscillator and outputs the generated high-frequency signal. 
     The amplifier  124  power-amplifies the signal output from the frequency converter  122 . The amplifier  124  outputs the amplified signal to the combiner  152 . 
     The phase amplitude adjusting unit  134  and the DAC  136  are similar to the phase amplitude adjusting unit  114  and the DAC  116 , respectively. The LPF  138 , the quadrature modulator  140 , the frequency converter  142 , and the amplifier  144  are similar to the LPF  118 , the quadrature modulator  120 , the frequency converter  122 , and the amplifier  124 , respectively. 
     The combiner  152  combines the output from the amplifier  124  and the output from the amplifier  144  together and outputs the combined signal as a signal Sout(t). The signal output from the combiner  152  is transmitted via an antenna or the like. 
     The frequency converter  154  down-converts the output signal from the combiner  152  from a radio frequency into a baseband frequency. The quadrature demodulator  156  demodulates the output from the frequency converter  154  into an in-phase signal and a quadrature signal. The ADC  158  converts the signals output from the quadrature demodulator  156  from an analog signal into a digital signal. 
     The modeling unit  180  receives the digital signal output from the DPD  102 , the digital signal output from the phase amplitude adjusting unit  114 , the digital signal output from the phase amplitude adjusting unit  134 , and the digital signal output from the ADC  158 . The modeling unit  180  receives digital signals until the number of received digital signals reaches a specific number of samples. The modeling unit  180  simulates output from the combiner  152  every time digital signals corresponding to the specific number of samples are received. 
     Here, “x i ” represents the signal output from the DPD  102 , “p i ” represents the digital signal output from the phase amplitude adjusting unit  114 , “q i ” represents the digital signal output from the phase amplitude adjusting unit  134 , and “r i ” represents the digital signal output from the ADC  158 . The subscript “i” added to a signal represents that the signal is the ith digital signal received by the modeling unit  180 . Here, “N” represents a specific number of samples. For example, the value of N may be 100 or 1000. However, the value of N is not limited to the above examples. 
     The modeling unit  180  determines the value of a coefficient c n  so that ε1 represented by Expression 4 is 0 (that is, the absolute value of ε1 is minimum). Any method may be used to determine the coefficient c n . The modeling unit  180  calculates the coefficient c n  for each N samples. Here, “x i ” represents an input signal corresponding to a digital signal r i . 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           N 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           { 
                           
                             
                               r 
                               i 
                             
                             - 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   i 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       = 
                       0 
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 x 
                                 ) 
                               
                             
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   c 
                                   1 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               
                                 
                                   c 
                                   3 
                                 
                                 ⁢ 
                                 
                                   
                                      
                                     x 
                                      
                                   
                                   2 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               … 
                               + 
                               ɛ1 
                             
                           
                         
                       
                       
                         
                           
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   ∑ 
                                   
                                     n 
                                     = 
                                     0 
                                   
                                   M 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     c 
                                     
                                       
                                         2 
                                         ⁢ 
                                         n 
                                       
                                       + 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                        
                                       x 
                                        
                                     
                                     
                                       2 
                                       ⁢ 
                                       n 
                                     
                                   
                                   ⁢ 
                                   x 
                                 
                               
                               + 
                               
                                 ɛ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 4, “M” is a constant and a natural number. As the value of M increases, output from the combiner  152  may be simulated more accurately. For example, the value of M may be 1 or 2. However, the value of M is not limited to the above examples. A function “f” represents a first- or higher-order term, and “f(x i ) is obtained when the ith output from the combiner  152  is simulated by the modeling unit  180 . 
     Furthermore, the modeling unit  180  determines the values of a coefficient “a” and a coefficient “b” in such a manner that ε2 represented by Expression 5 is 0 (that is, the absolute value of ε2 is minimum). Any method may be used to determine the coefficients a and b. The modeling unit  180  calculates the coefficients a and b for each N samples. 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       N 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       { 
                       
                         
                           r 
                           i 
                         
                         - 
                         
                           ( 
                           
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   i 
                                 
                                 ) 
                               
                             
                             - 
                             
                               
                                 c 
                                 1 
                               
                               ⁢ 
                               
                                 x 
                                 i 
                               
                             
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               ap 
                               i 
                             
                             + 
                             
                               bq 
                               i 
                             
                             + 
                             
                               ɛ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                       
                       } 
                     
                   
                   = 
                   0 
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     The modeling unit  180  outputs the calculated coefficient a to the inverse calculating unit  126 . The modeling unit  180  outputs the calculated coefficient b to the inverse calculating unit  146 . The modeling unit  180  outputs the calculated coefficient c n  to the inverse characteristics calculating unit  182 . 
     The inverse characteristics calculating unit  182  calculates a predistortion coefficient. The inverse characteristics calculating unit  182  calculates the predistortion coefficient based on the function f output from the modeling unit  180 . The inverse characteristics calculating unit  182  outputs the calculated predistortion coefficient to the DPD  102 . The inverse characteristics calculating unit  182  may transmit a table representing the correspondence between an input signal and a predistortion coefficient to the DPD  102  at specific time intervals. When such a table is transmitted from the inverse characteristics calculating unit  182  to the DPD  102 , the DPD  102  stores therein the table. 
     The inverse calculating unit  126  calculates the inverse of the coefficient a (1/a). The inverse calculating unit  126  outputs the calculated value to the phase amplitude adjusting unit  114 . 
     The inverse calculating unit  146  calculates the inverse of the coefficient b (1/b). The inverse calculating unit  146  outputs the calculated value to the phase amplitude adjusting unit  134 . 
     On the basis of the coefficients a and b, the imbalance between the branches of the LINC-based amplifier is corrected. 
     The modeling unit  180  may output the coefficients a and b to the inverse calculating unit  146 . At this point, the inverse calculating unit  146  calculates the inverse of the coefficient b (1/b). Furthermore, the inverse calculating unit  146  calculates a value obtained by normalizing the calculated inverse by using the inverse of the coefficient a (1/a). That is, the value represented by a/b is calculated by the inverse calculating unit  146 . The inverse calculating unit  146  outputs the calculated value to the phase amplitude adjusting unit  134 . At this point, the inverse calculating unit  126  outputs the value 1 to the phase amplitude adjusting unit  114 . The value 1 is obtained by normalizing the inverse of the coefficient a (1/a) by the inverse of the coefficient a (1/a). 
     Furthermore, the modeling unit  180  may output the coefficients a and b to the inverse calculating unit  126 . At this point, the inverse calculating unit  126  calculates the inverse of the coefficient a (1/a). Furthermore, the inverse calculating unit  126  calculates a value obtained by normalizing the calculated inverse by the inverse of the coefficient b (1/b). That is, the value represented by b/a is calculated by the inverse calculating unit  126 . The inverse calculating unit  126  outputs the calculated value to the phase amplitude adjusting unit  114 . At this point, the inverse calculating unit  146  outputs the value 1 to the phase amplitude adjusting unit  134 . The value 1 is a value obtained by normalizing the inverse of the coefficient b (1/b) by the inverse of the coefficient b (1/b). 
     The inverse calculating unit  126 , the inverse calculating unit  146 , and the inverse characteristics calculating unit  182  may be included in the modeling unit  180 . That is, the modeling unit  180  may operate as the inverse calculating unit  126 , the inverse calculating unit  146 , and the inverse characteristics calculating unit  182 . 
     (Operation of Distortion Compensating Apparatus) 
     Operation of the distortion compensating apparatus  100  will be explained below. 
       FIG. 8  illustrates an example of the operation flow of the distortion compensating apparatus  100 . 
     The DPD  102  of the distortion compensating apparatus  100  receives a digital signal Sin(t) to be transmitted. The signal used here is a complex signal. The DPD  102  multiplies the digital signal Sin(t) by a predistortion coefficient and outputs the processed signal (S 101 ). The predistortion coefficient is a coefficient for compensating for distortion generated in a range from the LINC signal generating unit  104  to the combiner  152  inclusive. The predistortion coefficient is calculated by the inverse characteristics calculating unit  182 . The DPD  102  provides, by means of the predistortion coefficient, an input signal with characteristics inverse to distortion characteristics provided by the LINC-based amplifier. The predistortion coefficient depends on the input signal. The DPD  102  may store therein the correspondence between a signal and a predistortion coefficient, which is calculated by the inverse characteristics calculating unit  182 , as a table. The DPD  102  outputs a signal obtained by multiplying the input signal by the predistortion coefficient to the LINC signal generating unit  104 . 
     The LINC signal generating unit  104  receives the signal output from the DPD  102 . The LINC signal generating unit  104  separates the digital signal into a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) that have a phase difference corresponding to the amplitude of the digital signal (S 102 ). The LINC signal generating unit  104  outputs the signal Sc 1 ( t ) to the phase amplitude adjusting unit  114 . The LINC signal generating unit  104  outputs the signal Sc 2 ( t ) to the phase amplitude adjusting unit  134 . The signals output from the LINC signal generating unit  104  are expressed, for example, by amplitude and phase. Furthermore, the signals output from the LINC signal generating unit  104  may be expressed by an in-phase (I) component and a quadrature phase (Q) component. 
     The phase amplitude adjusting unit  114  multiplies the digital signal output from the LINC signal generating unit  104  by a specific coefficient and outputs the processed digital signal (S 103 ). The specific coefficient is calculated by the inverse calculating unit  126 . The phase amplitude adjusting unit  114  adjusts the phase and amplitude of the signal in accordance with the specific coefficient. The phase amplitude adjusting unit  114  performs adjustment using the specific coefficient so that the output from the amplifier  124  and the output from the amplifier  144  are not imbalanced. 
     The DAC  116  converts the digital signal output from the phase amplitude adjusting unit  114  into an analog signal (S 104 ). 
     The LPF  118  cuts off a high-frequency component of the analog signal converted by the DAC  116  (S 105 ). After the high-frequency component is cut off, the processed signal is output to the quadrature modulator  120 . 
     The quadrature modulator  120  performs quadrature modulation on the signal output from the LPF  118 . The frequency converter  122  converts, using a high-frequency signal output from the oscillator, the frequency of the signal output from the quadrature modulator  120  into a radio frequency and outputs the processed signal (S 106 ). 
     The amplifier  124  amplifies the signal output from the frequency converter  122  (S 107 ). The signal to be amplified by the amplifier  124  is subjected to distortion compensation processing in advance by the DPD  102  and the like. 
     Similar to the signal Sc 1 ( t ), the signal Sc 1 ( t ) output from the LINC signal generating unit  104  is processed by the phase amplitude adjusting unit  134 , the DAC  136 , the LPF  138 , the quadrature modulator  140 , the frequency converter  142 , and the amplifier  144 . 
     The combiner  152  combines the signal output from the amplifier  124  and the signal output from the amplifier  144  together and outputs the combined signal (S 108 ). The output signal is transmitted to an external apparatus via an antenna or the like. Furthermore, a portion of the output signal is input to the frequency converter  154 . 
     The frequency converter  154  down-converts the output signal from the combiner  152  from a radio frequency to a baseband frequency. The quadrature demodulator  156  demodulates the output from the frequency converter  154  into an in-phase signal and a quadrature signal. The ADC  158  converts the signals output from the quadrature demodulator  156  into a digital signal. 
     The modeling unit  180  receives the digital signal output from the DPD  102 , the digital signal output from the phase amplitude adjusting unit  114 , the digital signal output from the phase amplitude adjusting unit  134 , and the digital signal output from the ADC  158 . The modeling unit  180  calculates a function f(x) which simulates output from the combiner  152  every time digital signals corresponding to a specific number of samples are received. Furthermore, the modeling unit  180  calculates a coefficient a and a coefficient b for correcting the imbalance between the branches of the LINC-based amplifier (S 109 ). The modeling unit  180  outputs the calculated coefficient a to the inverse calculating unit  126 . The modeling unit  180  outputs the calculated coefficient b to the inverse calculating unit  146 . The modeling unit  180  outputs the calculated coefficient c n  (or function f(x)) to the inverse characteristics calculating unit  182 . 
     The inverse calculating unit  126  calculates the inverse of the coefficient a output from the modeling unit  180  and outputs the calculated value to the phase amplitude adjusting unit  114 . The inverse calculating unit  146  calculates the inverse of the coefficient b output from the modeling unit  180  and outputs the calculated value to the phase amplitude adjusting unit  134 . 
     The inverse characteristics calculating unit  182  calculates a predistortion coefficient (S 109 ). The inverse characteristics calculating unit  182  receives the coefficient c n  (or function f(x)) output from the modeling unit  180 . The inverse characteristics calculating unit  182  calculates, based on the coefficient c n , a predistortion coefficient for compensating for AM/AM distortion and AM/PM distortion. The inverse characteristics calculating unit  182  outputs the calculated predistortion to the DPD  102 . 
     The inverse characteristics calculating unit  182  calculates the predistortion coefficient as described below, for example. The inverse characteristics calculating unit  182  normalizes the function f(x) by using the sum of the coefficients c n . The function obtained by normalizing the function f(x) by using the sum of the coefficients c n  is defined as a function f N (x). 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       N 
                     
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         f 
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           c 
                           1 
                         
                         + 
                         
                           c 
                           3 
                         
                         + 
                         … 
                       
                     
                     = 
                     
                       
                         f 
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             0 
                           
                           M 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           c 
                           
                             
                               2 
                               ⁢ 
                               j 
                             
                             + 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     The inverse characteristics calculating unit  182  acquires, based on the function f N , which is obtained by normalizing the function f, the characteristics inverse to the characteristics of the LINC-based amplifier, and calculates a predistortion coefficient. The DPD  102  performs distortion compensation for an input signal, in accordance with the predistortion coefficient calculated by the inverse characteristics calculating unit  182 . 
     Furthermore, for example, a method based on a least mean squares (LMS) algorithm, a method based on an exponential weighting recursive least square (RLS) algorithm, or the like may be adopted as a method to calculate a predistortion coefficient (distortion compensation coefficient). An algorithm that is used as a method to calculate a predistortion coefficient is not limited to the above examples. The predistortion coefficient is updated, for example, at specific time intervals. 
     The characteristics of a combiner and an amplifier may change in accordance with the passage of time, the operating temperature, the environmental temperature, an input signal, and the like. Thus, by updating the predistortion coefficient at specific time intervals, the predistortion coefficient is capable of following changes in the characteristics of the combiner and the amplifier. The predistortion coefficient may be updated (calculated) independently of this operation flow. 
     A series of processing operations may be performed by hardware or software. 
     Steps describing a program include not only processing operations performed in a time sequence manner in accordance with the written order but also processing operations performed in parallel or independently, the processing operations being not necessarily performed in a time sequence manner. 
     (Example of Hardware Configuration of Distortion Compensating Apparatus According to First Embodiment) 
       FIG. 9  is a diagram illustrating an example of the hardware configuration of a distortion compensating apparatus according to the first embodiment. A distortion compensating apparatus  1000  up-converts an input digital signal into a radio frequency, amplifies the processed signal, and outputs the amplified signal. The distortion compensating apparatus  1000  includes a processor  1002 , a storage device  1004 , a DAC  1102 , an LPF  1104 , an up-converter  1106 , and an amplifier  1108 . The distortion compensating apparatus  1000  also includes a DAC  1202 , an LPF  1204 , an up-converter  1206 , an amplifier  1208 , a combiner  1012 , and an antenna  1014 . The distortion compensating apparatus  100  is implemented by the hardware configuration of the distortion compensating apparatus  1000 , for example. 
     The processor  1002  is, for example, a central processing unit (CPU) or a digital signal processor (DSP). The processor  1002  controls the entire distortion compensating apparatus  1000 . An application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like may be used as the processor  1002 . 
     The storage device  1004  is, for example, a random access memory (RAM) or a read-only memory (ROM). Alternatively, the storage device  1004  is, for example, an erasable programmable read-only memory (EPROM) or a hard disk drive (HDD). A secondary storage device may include a removable medium, that is, a portable recording medium. The removable medium is, for example, a universal serial bus (USB) memory, or a disk recording medium, such as a compact disk (CD), a digital versatile disk (DVD), or the like. The storage device  1004  may store therein a correspondence table that represents the correspondence between an input signal and a predistortion coefficient or the like. 
     When the processor  1002  executes a program stored in the storage device  1004 , the distortion compensating apparatus  1000  implements the functions of the DPD  102 , the LINC signal generating unit  104 , the phase amplitude adjusting unit  114 , the inverse calculating unit  126 , the modeling unit  180 , the inverse characteristics calculating unit  182 , and the like. 
     The DAC  1102  converts a digital signal output from the processor  1002  to an analog signal. The DAC  1102  implements the function of the DAC  116 . 
     The LPF  1104  removes a high-frequency component from the analog signal output from the DAC  1102 . The LPF  1104  implements the function of the LPF  118 . 
     The up-converter  1106  implements the functions of the quadrature modulator  120  and the frequency converter  122 . 
     The amplifier  1108  amplifies an analog signal output from the up-converter  1106 . Various amplifiers may be used as the amplifier  1108 . However, it is desirable that an amplifier having the same characteristics as the characteristics of the amplifier  1208  is used as the amplifier  1108 . The amplifiers  1108  and  1208  implement the functions of the amplifiers  124  and  144 , respectively. 
     The DAC  1202 , the LPF  1204 , the up-converter  1206 , and the amplifier  1208  have functions similar to those of the DAC  1102 , the LPF  1104 , the up-converter  1106 , and the amplifier  1108 , respectively. 
     The combiner  1012  combines the output from the amplifier  1108  and the amplifier  1208 . The combiner  1012  implements the function of the combiner  152 . For example, a Chireix power combiner is used as the combiner  1012 . However, the combiner  1012  is not limited to a Chireix power combiner. 
     The antenna  1014  transmits the signal combined by the combiner  1012  to a separate apparatus. 
     (Variation) 
     Here, a variation of the modeling unit  180  will be described. Here, a signal r i  that is used to calculate a coefficient c n  (function f(x)) has a size greater than or equal to a specific threshold. Furthermore, a signal r i  that is used to calculate a coefficient a and a coefficient b has a size smaller than the specific threshold. Units other than the modeling unit  180  have the configuration as described above. 
     The modeling unit  180  receives the digital signal output from the DPD  102 , the digital signal output from the phase amplitude adjusting unit  114 , the digital signal output from the phase amplitude adjusting unit  134 , and the digital signal output from the ADC  158 . The modeling unit  180  receives digital signals until the number of received digital signals reaches a specific number of samples. The modeling unit  180  simulates output from the combiner  152  every time digital signals corresponding to the specific number of samples are received. 
     Here, “x i ” represents the signal output from the DPD  102 , “p i ” represents the digital signal output from the phase amplitude adjusting unit  114 , “q i ” represents the digital signal output from the phase amplitude adjusting unit  134 , and “r i ” represents the digital signal output from the ADC  158 . The subscript “i” added to a signal represents that the signal is the ith digital signal received by the modeling unit  180 . Here, “N” represents a specific number of samples. For example, the value of N may be 100 or 1000. However, the value of N is not limited to the above examples. Here, it is assumed that, among digital signals r i  (i=1 . . . N), the number of digital signals r i  having a size greater than or equal to a specific value S is J and the number of digital signals r i  having a size smaller than the specific value S is K. Here, the relationship among the values N, J, and K is represented by N=J+K. Furthermore, the digital signals r i  having a size greater than or equal to the specific value S are represented by r j  (j=1 . . . J). A signal output from the DPD  102  that corresponds to a digital signal r j  is represented by x j , a digital signal output from the phase amplitude adjusting unit  114  that corresponds to a digital signal r j  is represented by p j , and a digital signal output from the phase amplitude adjusting unit  134  that corresponds to a digital signal r j  is represented by q j . Furthermore, the digital signals r i  having a size smaller than the specific value S are represented by r k  (k=1 . . . K). A signal output from the DPD  102  that corresponds to a digital signal r k  is represented by x k , a digital signal output from the phase amplitude adjusting unit  114  that corresponds to a digital signal r k  is represented by p k , and a digital signal output from the phase amplitude adjusting unit  134  that corresponds to a digital signal r k  is represented by q k . On the basis of the assumption that the size of an output signal from the distortion compensating apparatus  100  is regarded as being proportional to the size of an input signal to the distortion compensating apparatus  100  when the size of a digital signal r i  is smaller than the specific value S, the specific value S is determined in advance. 
     The modeling unit  180  determines the value of the coefficient c n  so that ε1 represented by Expression 7 is 0 (that is, the absolute value of ε1 is minimum). Any method may be used to determine the coefficient c n . The modeling unit  180  calculates the coefficient c n  every N samples. Here, “x j ” represents an input signal corresponding to a digital signal r j . 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           J 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           { 
                           
                             
                               r 
                               j 
                             
                             - 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   j 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       = 
                       0 
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 x 
                                 ) 
                               
                             
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   c 
                                   1 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               
                                 
                                   c 
                                   3 
                                 
                                 ⁢ 
                                 
                                   
                                      
                                     x 
                                      
                                   
                                   2 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               … 
                               + 
                               ɛ1 
                             
                           
                         
                       
                       
                         
                           
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   ∑ 
                                   
                                     n 
                                     = 
                                     0 
                                   
                                   M 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     c 
                                     
                                       
                                         2 
                                         ⁢ 
                                         n 
                                       
                                       + 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                        
                                       x 
                                        
                                     
                                     
                                       2 
                                       ⁢ 
                                       n 
                                     
                                   
                                   ⁢ 
                                   x 
                                 
                               
                               + 
                               ɛ1 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 7, “M” is a constant and a natural number. As the value of M increases, output from the combiner  152  is simulated more accurately. For example, the value of M may be 1 or 2. However, the value of M is not limited to the above examples. A function “f” represents a first- or higher-order term, and “f(x j )” is obtained when output r j  from the combiner  152  is simulated by the modeling unit  180 . Furthermore, f(x) is calculated by using a digital signal r j  having a size greater than or equal to the specific value S. The coefficient c n  is calculated by using a digital signal r j  for which the size of a corresponding output signal from the distortion compensating apparatus  100  is regarded as not being proportional to the size of a corresponding input signal to the distortion compensating apparatus  100 . 
     Furthermore, the modeling unit  180  determines the values of the coefficients a and b in so that ε2 represented by Expression 8 is 0 (that is, the absolute value of ε2 is minimum). Any method may be used to determine the coefficients a and b. The modeling unit  180  calculates the coefficients a and b every N samples. 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         1 
                       
                       K 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       { 
                       
                         
                           r 
                           k 
                         
                         - 
                         
                           ( 
                           
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   k 
                                 
                                 ) 
                               
                             
                             - 
                             
                               
                                 c 
                                 1 
                               
                               ⁢ 
                               
                                 x 
                                 k 
                               
                             
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               ap 
                               k 
                             
                             + 
                             
                               bq 
                               k 
                             
                             + 
                             ɛ2 
                           
                           ) 
                         
                       
                       } 
                     
                   
                   = 
                   0 
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     The modeling unit  180  outputs the calculated coefficient a to the inverse calculating unit  126 . The modeling unit  180  outputs the calculated coefficient b to the inverse calculating unit  146 . The modeling unit  180  outputs the calculated coefficient c n  to the inverse characteristics calculating unit  182 . The coefficients a and b are calculated by using a digital signal r k  having a size smaller than the specific value S. The coefficients a and b are calculated by using a digital signal r k  for which the size of a corresponding output signal from the distortion compensating apparatus  100  is regarded as being proportional to the size of a corresponding input signal to the distortion compensating apparatus  100 . 
     The output signal may be deteriorated due to imbalance between the branches of the LINC-based amplifier and the nonlinearity of the combiner. When the size of the output signal is small, the output signal deteriorates due to the influence of the imbalance between the branches of the LINC-based amplifier. When the size of the output signal is large, the output signal deteriorates due to the influence of the nonlinearity of the combiner. Thus, the modeling unit  180  according to this variation adjusts the nonlinearity of the combiner by using a digital signal for which the size of a corresponding output signal is greater than or equal to a specific value, and corrects the imbalance between the branches of the LINC-based amplifier by using a digital signal for which the size of a corresponding output signal is smaller than the specific value. 
     (Operation and Effects of First Embodiment) 
     The distortion compensating apparatus  100  simulates the characteristics of an amplifier, a combiner, and the like in accordance with output from the combiner  152  by using a polynomial expression. The distortion compensating apparatus  100  compensates for distortion in a LINC-based amplifier by performing distortion compensation for an input signal before the input signal is separated into LINC signals. A predistortion coefficient (distortion compensation coefficient) for compensating for distortion in a LINC-based amplifier is calculated based on the output signal. Furthermore, the distortion compensating apparatus  100  adjusts the balance between branches of the LINC-based amplifier by multiplying separated LINC signals by specific coefficients. The specific coefficients are calculated based on the difference between an output signal and a three- or higher-order term of a function f obtained by simulating the LINC-based amplifier. The distortion compensating apparatus  100  is capable of compensating for the characteristics of a combiner, the distortion characteristics of an amplifier, and the imbalance between the amplifier  124  and the like for processing a signal Sc 1  and the amplifier  144  and the like for processing a signal Sc 2 . Furthermore, the distortion compensating apparatus  100  suppresses the deterioration of output from the combiner  152 . 
     The distortion compensating apparatus  100  is capable of suppressing the deterioration of high-frequency output by distortion compensation for a LINC-based amplifier and by correcting the imbalance between the branches of the LINC-based amplifier. 
     Since the distortion compensating apparatus  100  compensates for distortion based on output from the combiner  152 , a combiner having a low linearity may be used as the combiner  152 . That is, in the distortion compensating apparatus  100 , a combiner having a low linearity but good efficiency may be used. 
     The characteristics of the combiner  152 , the amplifier  124 , and the amplifier  144  may change due to a deterioration over time, the operating environment (temperature and the like), and the like. Even when the characteristics of an amplifier and the like change, the distortion compensating apparatus  100  is capable of achieving more appropriate distortion compensation by calculating a predistortion coefficient at specific time intervals by using output from a DPD, output from a phase amplitude adjusting unit, and output from a combiner. 
     Second Embodiment 
     A second embodiment will now be described. The second embodiment and the first embodiment have common characteristics. Thus, differences between the second embodiment and the first embodiment will be mainly explained, and the explanation of the common points will be omitted. 
     (Example of Configuration) 
       FIG. 10  is a diagram illustrating an example of the configuration of a distortion compensating apparatus according to the second embodiment. A distortion compensating apparatus  200  includes a DPD  202 , a LINC signal generating unit  204 , a first LPF (LPF)  212 , a DPD  214 , a DAC  216 , a second LPF  218 , a quadrature modulator  220 , a frequency converter  222 , an amplifier  224 , and an inverse characteristics calculating unit  226 . The distortion compensating apparatus  200  also includes a first LPF  232 , a DPD  234 , a DAC  236 , a second LPF  238 , a quadrature modulator  240 , a frequency converter  242 , an amplifier  244 , and an inverse characteristics calculating unit  246 . The distortion compensating apparatus  200  also includes a combiner  252 , a frequency converter  254 , a quadrature demodulator  256 , an ADC  258 , a modeling unit  280 , and an inverse characteristics calculating unit  282 . 
     The DPD  202  performs distortion compensation processing for an input modulation signal Sin(t). The distortion compensation processing performed here is processing to multiply a signal that has not been amplified by a LINC-based amplifier by a predistortion coefficient to generate distortion in advance and cancel distortion generated in the LINC-based amplifier. The predistortion coefficient is a distortion compensation coefficient for compensating for distortion in an amplifier. The predistortion coefficient that is multiplied with an input modulation signal depends on the input modulation signal Sin(t). The predistortion coefficient is a coefficient for performing compensation so that the amplitude of an output signal from an amplifier is proportional to the amplitude of an input signal to the amplifier and that the phase difference between the input signal and the output signal is 0. The predistortion coefficient is calculated by the inverse characteristics calculating unit  282 . The DPD  202  outputs the processed signal to the LINC signal generating unit  204  and the modeling unit  280 . The input modulation signal Sin(t) is, for example, a baseband signal. The input modulation signal Sin(t) may be an intermediate frequency (IF) signal. The DPD  202  is an example of a distortion compensating unit. 
     The LINC signal generating unit  204  separates the output from the DPD  202  into a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) that have a phase difference corresponding to the amplitude of the output from the DPD  202 . The signal Sc 1 ( t ) is output to the first LPF  212 . The signal Sc 2 ( t ) is output to the first LPF  232 . 
     The first LPF  212  cuts off a high-frequency component of the signal output from the LINC signal generating unit  204 . 
     The DPD  214  performs distortion compensation processing for output from the first LPF  212 . The distortion compensation processing performed here is processing to multiply a signal that has not been amplified by the amplifier  224  by a predistortion coefficient to generate distortion in advance and cancel distortion generated in the amplifier  224 . The predistortion coefficient is a distortion compensation coefficient for compensating for distortion in an amplifier. The predistortion coefficient that is multiplied with the output from the first LPF  212  depends on the output from the first LPF  212 . The predistortion coefficient is a coefficient for performing compensation so that the amplitude of an output signal is proportional to the amplitude of an input signal and that the phase difference between the input signal and the output signal is 0. The predistortion coefficient is calculated by the inverse characteristics calculating unit  226 . The DPD  214  is an example of a distortion compensating unit. 
     The DAC  216  converts a digital signal that is output from the DPD  214  into an analog signal. 
     The second LPF  218  cuts off a high-frequency component of the signal output from the DAC  216 . 
     The quadrature modulator  220  performs quadrature modulation on the signal output from the second LPF  218  and outputs the processed signal. 
     The frequency converter  222  includes an oscillator. The frequency converter  222  up-converts the signal that has been quadrature-modulated by the quadrature modulator  220  into a radio frequency (RF). The frequency converter  222  generates a high-frequency signal by using a high-frequency signal (oscillation signal) output from the oscillator and outputs the generated high-frequency signal. 
     The amplifier  224  power-amplifies the signal output from the frequency converter  222 . The amplifier  224  outputs the amplified signal to the combiner  252 . 
     The inverse characteristics calculating unit  226  calculates a predistortion coefficient. The inverse characteristics calculating unit  226  calculates the predistortion coefficient based on a function g output from the modeling unit  280 . The relationship between the output from the DPD  214  and the output from the amplifier  224  may be understood from the function g. With the use of the function g, the inverse characteristics calculating unit  226  calculates the predistortion coefficient. The inverse characteristics calculating unit  226  outputs the calculated predistortion coefficient to the DPD  214 . The inverse characteristics calculating unit  226  may transmit a table representing the correspondence between an input signal and a predistortion coefficient to the DPD  214  at specific time intervals. When a table is transmitted from the inverse characteristics calculating unit  226  to the DPD  214 , the DPD  214  stores therein the transmitted table. 
     The first LPF  232 , the DPD  234 , and the DAC  236  are similar to the first LPF  212 , the DPD  214 , and the DAC  216 , respectively. The second LPF  238 , the quadrature modulator  240 , the frequency converter  242 , the amplifier  244 , and the inverse characteristics calculating unit  246  are similar to the second LPF  218 , the quadrature modulator  220 , the frequency converter  222 , the amplifier  224 , and the inverse characteristics calculating unit  226 , respectively. 
     The combiner  252  combines the output from the amplifier  224  and the output from the amplifier  244  together and outputs the combined signal as a signal Sout(t). The signal output from the combiner  252  is transmitted via an antenna or the like. 
     The frequency converter  254  down-converts the output signal from the combiner  252  from an RF frequency to a baseband frequency. The quadrature demodulator  256  demodulates the output from the frequency converter  254  into an in-phase signal and a quadrature signal. The ADC  258  converts the signal output from the quadrature demodulator  256  from an analog signal into a digital signal. 
     The modeling unit  280  receives the digital signal output from the DPD  202 , the digital signal output from the DPD  214 , the digital signal output from the DPD  234 , and the digital signal output from the ADC  258 . The modeling unit  280  receives digital signals until the number of received digital signals reaches a specific number of samples. The modeling unit  280  simulates output from the combiner  252  every time receiving digital signals corresponding to the specific number of samples. 
     Here, “x i ” represents the signal output from the DPD  202 , “p i ” represents the digital signal output from the DPD  214 , “q i ” represents the digital signal output from the DPD  234 , and “r i ” represents the digital signal output from the ADC  258 . The subscript “i” added to a signal represents that the signal is the ith digital signal received by the modeling unit  280 . Here, “N” represents a specific number of samples. The value of N may be, for example, 100 or 1000. However, the value of N is not limited to the above examples. Here, it is assumed that, among digital signals r i  (i=1 . . . N), the number of digital signals r i  having a size greater than or equal to a specific value S is J and the number of digital signals r i  having a size smaller than the specific value S is K. Here, the relationship between the values N, J, and K is represented by N=J+K. Furthermore, the digital signals r i  having a size greater than or equal to the specific value S are represented by r j  (j=1 . . . J). A signal output from the DPD  202  that corresponds to a digital signal r j  is represented by x j , a digital signal output from the DPD  214  that corresponds to a digital signal r j  is represented by p j , and a digital signal output from the DPD  234  that corresponds to a digital signal r j  is represented by q j . Furthermore, the digital signals r j  having a size smaller than the specific value S are represented by r k  (k=1 . . . K). A signal output from the DPD  202  that corresponds to a digital signal r k  is represented by x k , a digital signal output from the DPD  214  that corresponds to a digital signal r k  is represented by p k , and a digital signal output from the DPD  234  that corresponds to a digital signal r k  is represented by q k . On the basis of the assumption that the size of an output signal from the distortion compensating apparatus  200  is regarded as being proportional to the size of an input signal to the distortion compensating apparatus  200  when the size of a digital signal r i  is smaller than the specific value S, the specific value S is determined in advance. 
     The modeling unit  280  determines the value of a coefficient c n  so that ε1 represented by Expression 9 is 0 (that is, the absolute value of ε1 is minimum). Any method may be used to determine the coefficient c n . The modeling unit  280  calculates the coefficient c n  every N samples. Here, “x j ” represents an input signal that corresponds to the digital signal r j . 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           J 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           { 
                           
                             
                               r 
                               j 
                             
                             - 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   j 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       = 
                       0 
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 x 
                                 ) 
                               
                             
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   c 
                                   1 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               
                                 
                                   c 
                                   3 
                                 
                                 ⁢ 
                                 
                                   
                                      
                                     x 
                                      
                                   
                                   2 
                                 
                                 ⁢ 
                                 x 
                               
                               + 
                               … 
                               + 
                               ɛ1 
                             
                           
                         
                       
                       
                         
                           
                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   ∑ 
                                   
                                     n 
                                     = 
                                     0 
                                   
                                   M 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     c 
                                     
                                       
                                         2 
                                         ⁢ 
                                         n 
                                       
                                       + 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                        
                                       x 
                                        
                                     
                                     
                                       2 
                                       ⁢ 
                                       n 
                                     
                                   
                                   ⁢ 
                                   x 
                                 
                               
                               + 
                               ɛ1 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 9, “M” is a constant and a natural number. As the value of M increases, output from the combiner  252  is simulated more accurately. For example, the value of M may be 1 or 2. However, the value of M is not limited to the above examples. A function “f” represents a first- or higher-order term, and “f(x j )” is obtained when output r j  from the combiner  252  is simulated by the modeling unit  280 . Furthermore, f(x) is calculated by using a digital signal r j  having a size greater than or equal to the specific value S. The coefficient c n  is calculated by using a digital signal r j  for which the size of a corresponding output signal from the distortion compensating apparatus  200  is regarded as not being proportional to the size of a corresponding input signal to the distortion compensating apparatus  200 . 
     Furthermore, the modeling unit  280  determines the values for the function g (or coefficient u n ) and a function h (or coefficient v n ) in such a manner that ε2 represented by Expression 10 is 0 (that is, the absolute value of ε2 is minimum). Any method may be used to determine the functions g and h. The modeling unit  280  calculates the functions g and h every N samples. Here, the functions g and h simulate the amplitude, phase, and nonlinearity of an amplifier. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         K 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         { 
                         
                           
                             r 
                             k 
                           
                           - 
                           
                             ( 
                             
                               
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                                   1 
                                 
                                 ⁢ 
                                 
                                   x 
                                   k 
                                 
                               
                             
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                                 ⁡ 
                                 
                                   ( 
                                   
                                     p 
                                     k 
                                   
                                   ) 
                                 
                               
                               + 
                               
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                                 ⁡ 
                                 
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                                     k 
                                   
                                   ) 
                                 
                               
                               + 
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                             ) 
                           
                         
                         } 
                       
                     
                     = 
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                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         p 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           
                             u 
                             1 
                           
                           ⁢ 
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                         + 
                         
                           
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                             3 
                           
                           ⁢ 
                           
                             
                                
                               p 
                                
                             
                             2 
                           
                           ⁢ 
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                         + 
                         … 
                       
                       = 
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             0 
                           
                           
                             M 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
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                                 2 
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                               + 
                               1 
                             
                           
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                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       h 
                       ⁡ 
                       
                         ( 
                         q 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           
                             v 
                             1 
                           
                           ⁢ 
                           q 
                         
                         + 
                         
                           
                             v 
                             3 
                           
                           ⁢ 
                           
                             
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                               q 
                                
                             
                             2 
                           
                           ⁢ 
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                         + 
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                       = 
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             0 
                           
                           
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
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                             v 
                             
                               
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                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression 10, M1 is a constant and a natural number. As the value of M1 increases, the functions g and h simulate the amplitude, phase, and nonlinearity of an amplifier more accurately. For example, the value of M1 may be 1 or 2. 
     The modeling unit  280  outputs the calculated coefficient u n  to the inverse characteristics calculating unit  226 . The modeling unit  280  outputs the calculated coefficient v n  to the inverse characteristics calculating unit  246 . The modeling unit  280  outputs the calculated coefficient c n  to the inverse characteristics calculating unit  282 . The coefficients u n  and v n  are calculated by using a digital signal r k  having a size smaller than a specific value S. The coefficients u n  and v n  are calculated by using a digital signal r k  for which the size of a corresponding output signal from the distortion compensating apparatus  200  is regarded as being proportional to the size of a corresponding input signal to the distortion compensating apparatus  200 . 
     The inverse characteristics calculating unit  282  calculates a predistortion coefficient. The inverse characteristics calculating unit  282  calculates the predistortion coefficient based on a function f output from the modeling unit  280 . The inverse characteristics calculating unit  282  outputs the calculated predistortion coefficient to the DPD  202 . The inverse characteristics calculating unit  282  may transmit a table representing the correspondence between an input signal and a predistortion coefficient to the DPD  202  at specific time intervals. When a table is transmitted from the inverse characteristics calculating unit  282  to the DPD  202 , the DPD  202  stores therein the transmitted table. 
     The inverse characteristics calculating unit  282  calculates the predistortion coefficient, for example, as described below. The inverse characteristics calculating unit  282  normalizes a function f(x) by using the sum of coefficients c n . The function obtained by normalizing the function f(x) by using the sum of the coefficients c n  is defined as a function f N (x). 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       N 
                     
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         f 
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           c 
                           1 
                         
                         + 
                         
                           c 
                           3 
                         
                         + 
                         … 
                       
                     
                     = 
                     
                       
                         f 
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             0 
                           
                           M 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           c 
                           
                             
                               2 
                               ⁢ 
                               j 
                             
                             + 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
             
           
         
       
     
     The inverse characteristics calculating unit  282  calculates characteristics inverse to the characteristic of a LINC-based amplifier by using the function f N , which is obtained by normalizing the function f, and calculates the predistortion coefficient. The DPD  202  performs distortion compensation for an input signal by using the predistortion coefficient calculated by the inverse characteristics calculating unit  282 . 
     The inverse characteristics calculating unit  226  calculates a predistortion coefficient. The inverse characteristics calculating unit  226  calculates the predistortion coefficient based on the function g output from the modeling unit  280 . The inverse characteristics calculating unit  226  outputs the calculated predistortion coefficient to the DPD  214 . The inverse characteristics calculating unit  226  may transmit a table representing the correspondence between an input signal and a predistortion coefficient to the DPD  214  at specific time intervals. When a table is transmitted from the inverse characteristics calculating unit  226  to the DPD  214 , the DPD  214  stores therein the transmitted table. 
     The inverse characteristics calculating unit  246  calculates a predistortion coefficient. The inverse characteristics calculating unit  246  calculates the predistortion coefficient based on the function h output from the modeling unit  280 . The inverse characteristics calculating unit  246  outputs the calculated predistortion coefficient to the DPD  234 . The inverse characteristics calculating unit  246  may transmit a table representing the correspondence between an input signal and a predistortion coefficient at specific time intervals. When a table is transmitted from the inverse characteristics calculating unit  246  to the DPD  234 , the DPD  234  stores therein the transmitted table. 
     The inverse characteristics calculating unit  226  and the inverse characteristics calculating unit  246  may calculate predistortion coefficients by normalizing individual functions on the basis of the gain of the inverse characteristics of the function g or the function h. 
     The inverse characteristics calculating unit  226 , the inverse characteristics calculating unit  246 , and the inverse characteristics calculating unit  282  may be included in the modeling unit  280 . That is, the modeling unit  280  may operate as the inverse characteristics calculating unit  226 , the inverse characteristics calculating unit  246 , and the inverse characteristics calculating unit  282 . 
     (Operation of Distortion Compensating Apparatus) 
     Operation of the distortion compensating apparatus  200  will now be described. 
       FIG. 11  is a diagram illustrating an example of the operation flow of the distortion compensating apparatus  200 . 
     The DPD  202  of the distortion compensating apparatus  200  receives a digital signal Sin(t) to be transmitted. The signal used here is a complex signal. The DPD  202  multiples the digital signal Sin(t) by a predistortion coefficient and outputs the processed signal (S 201 ). The predistortion coefficient is a coefficient for compensating for distortion generated in a range from the LINC signal generating unit  204  to the combiner  252  inclusive. The predistortion coefficient is calculated by the inverse characteristics calculating unit  282 . The DPD  202  provides, by using the predistortion coefficient, an input signal with characteristics inverse to distortion characteristics provided by a LINC-based amplifier. The predistortion coefficient depends on an input signal. The DPD  202  may store therein the correspondence between a signal and a predistortion coefficient, which is calculated by the inverse characteristics calculating unit  282 , as a table. The DPD  202  outputs a signal obtained by multiplying the input signal by the predistortion coefficient to the LINC signal generating unit  204 . 
     The LINC signal generating unit  204  receives the signal output from the DPD  202 . The LINC signal generating unit  204  separates the digital signal into a pair of phase modulation signals Sc 1 ( t ) and Sc 2 ( t ) that have a phase difference corresponding to the amplitude of the digital signal (S 202 ). The LINC signal generating unit  204  outputs the signal Sc 1 ( t ) to the first LPF  212 . The LINC signal generating unit  204  outputs the signal Sc 2 ( t ) to the first LPF  232 . The signals output from the LINC signal generating unit  204  are expressed by, for example, amplitude and phase. Furthermore, the signals output from the LINC signal generating unit  204  may be expressed by an in-phase (I) component and a quadrature phase (Q) component. 
     The first LPF  212  cuts off a high-frequency component of the signal Sc 1 ( t ) (S 203 ). After the high-frequency component is cut off, the processed signal is input to the DPD  214 . The frequency band obtained by band limiting by the first LPF  212  is set to be similar to the frequency band of an analog signal by the second LPF  218  or narrower than the frequency band of an analog signal from the second LPF  218 . Ringing occurs in a signal transmitted through the first LPF  212 . That is, even if a signal input to the first LPF  212  is a constant envelope signal, an amplitude component of a signal output from the first LPF  212  is not constant. 
     The DPD  214  multiplies the digital signal output from the first LPF  212  by a specific coefficient and outputs the processed signal (S 204 ). The specific coefficient is calculated by the inverse characteristics calculating unit  226 . The DPD  214  adjusts the phase and amplitude of a signal by using the specific coefficient. The imbalance between the output from the amplifier  224  and the output from the amplifier  244  is corrected by using the specific coefficient. 
     The DAC  216  converts the digital signal output from the DPD  214  into an analog signal (S 205 ). 
     The second LPF  218  cuts off a high-frequency component of the analog signal converted by the DAC  216  (S 206 ). After the high-frequency component is cut off, the processed signal is output to the quadrature modulator  220 . 
     The quadrature modulator  220  performs quadrature modulation on the signal output from the second LPF  218 . The frequency converter  222  converts, using a high-frequency signal output from the oscillator, the frequency of the signal output from the quadrature modulator  220  into a radio frequency and outputs the processed signal (S 207 ). 
     The amplifier  224  amplifies the signal output from the frequency converter  222  (S 208 ). The signal to be amplified by the amplifier  224  has been subjected to distortion compensation processing in advance by the DPD  202  and the like. 
     Similar to the signal Sc 1 ( t ), the signal Sc 2 ( t ) output from the LINC signal generating unit  204  is processed by the first LPF  232 , the DPD  234 , the DAC  236 , the second LPF  238 , the quadrature modulator  240 , the frequency converter  242 , and the amplifier  244 . 
     The combiner  252  combines the signal output from the amplifier  224  and the signal output from the amplifier  244  and outputs the combined signal (S 209 ). The output signal is transmitted to an external apparatus via an antenna or the like. Furthermore, part of the output signal is input to the frequency converter  254 . 
     The frequency converter  254  down-converts the output signal from the combiner  252  from a radio frequency into a baseband frequency. The quadrature demodulator  256  demodulates the output from the frequency converter  254  into an in-phase signal and a quadrature signal. The ADC  258  converts the signals output from the quadrature demodulator  256  into a digital signal. 
     The modeling unit  280  receives the digital signal output from the DPD  202 , the digital signal output from the DPD  214 , the digital signal output from the DPD  234 , and the digital signal output from the ADC  258 . The modeling unit  280  receives digital signals until the number of received digital signals reaches a specific number of samples. The modeling unit  280  calculates a function f(x) which simulates output from the combiner  252  every time digital signals corresponding to the specific number of samples are received. Furthermore, the modeling unit  280  calculates functions g(p) and h(q) to correct the imbalance between the branches of the LINC-based amplifier and adjust the nonlinearity of the amplifier  224  and the amplifier  244  (S 210 ). The modeling unit  280  outputs the calculated coefficient u n  (function g(p)) to the inverse characteristics calculating unit  226 . The modeling unit  280  outputs the calculated coefficient v n  (or function h(q)) to the inverse characteristics calculating unit  226 . The modeling unit  280  outputs the calculated coefficient c n  (function f(x)) to the inverse characteristics calculating unit  282 . 
     The inverse characteristics calculating unit  282  calculates a predistortion coefficient. The inverse characteristics calculating unit  282  receives a coefficient c n  (function f(x)) output from the modeling unit  280 . The inverse characteristics calculating unit  282  calculates the predistortion coefficient for compensating for AM/AM distortion and AM/PM distortion by using the coefficient c n . The inverse characteristics calculating unit  282  outputs the calculated predistortion coefficient to the DPD  202 . The DPD  202  performs distortion compensation for an input signal in accordance with the predistortion coefficient calculated by the inverse characteristics calculating unit  282 . 
     The inverse characteristics calculating unit  226  calculates the characteristics inverse to the characteristics of the function g output from the modeling unit  280 , and outputs the calculated characteristics to the DPD  214 . The inverse characteristics calculating unit  246  calculates the characteristics inverse to the characteristics of the function h output from the modeling unit  280 , and outputs the calculated characteristics to the DPD  234 . 
     A series of processing operations may be performed by hardware or software. 
     Steps describing a program include not only processing operations performed in a time sequence manner in accordance with the written order but also processing operations performed in parallel or independently, the processing operations being not necessarily performed in a time sequence manner. 
     (Example of Hardware Configuration of Distortion Compensating Apparatus According to Second Embodiment) 
     The distortion compensating apparatus  200  according to the second embodiment is implemented with a hardware configuration similar to that of the distortion compensating apparatus  1000  according to the first embodiment. Hereinafter, an example of the hardware configuration of the distortion compensating apparatus  200  will be explained with reference to the distortion compensating apparatus  1000 . 
     When the processor  1002  executes a program stored in the storage device  1004 , the functions of the DPD  202 , the LINC signal generating unit  204 , the first LPF  212 , the DPD  214 , and the like are implemented. Furthermore, when the processor  1002  executes a program stored in the storage device  1004 , the functions of the inverse characteristics calculating unit  226 , the modeling unit  280 , the inverse characteristics calculating unit  282 , and the like are implemented. 
     In the storage device  1004 , information including a predistortion coefficient used by the processor  1002  is stored. 
     The DAC  1102  converts a digital signal output from the processor  1002  into an analog signal. The DAC  1102  implements the function of the DAC  216 . 
     The LPF  1104  removes a high-frequency component from the analog signal output from the DAC  1102 . The LPF  1104  implements the function of the second LPF  218 . 
     The up-converter  1106  implements the functions of the quadrature modulator  220  and the frequency converter  222 . 
     The amplifier  1108  amplifies an analog signal output from the up-converter  1106 . Various amplifiers may be used as the amplifier  1108 . However, it is desirable that an amplifier having the same characteristics as those of the amplifier  1208  is used as the amplifier  1108 . The amplifier  1108  and the amplifier  1208  implement the functions of the amplifier  124  and the amplifier  144 , respectively. 
     The DAC  1202 , the LPF  1204 , the up-converter  1206 , and the amplifier  1208  have functions similar to those of the DAC  1102 , the LPF  1104 , the up-converter  1106 , and the amplifier  1108 . 
     The combiner  1012  combines the output from the amplifier  1108  and the output from the amplifier  1208  together. The combiner  1012  implements the function of the combiner  152 . 
     The antenna  1014  transmits the signal combined by the combiner  1012  to a separate apparatus. 
     (Operation and Effect of Second Embodiment) 
     The distortion compensating apparatus  200  cuts off a high-frequency component of a constant envelope digital signal generated by the LINC signal generating unit  204 . Ringing occurs in the signal that has been subjected to cutting off of a high-frequency component. That is, the amplitude component of a signal that has been subjected to processing to cut off a high-frequency component from a constant envelope signal is not constant. The distortion compensating apparatus  200  performs distortion compensation processing to compensate for nonlinear distortion in an amplifier for a signal of which a high-frequency component has been cut-off. The distortion compensating apparatus  200  is capable of suppressing the deterioration of an output signal caused by ringing and distortion characteristics of an amplifier by performing distortion compensation processing for a signal in which ringing is generated. Furthermore, the distortion compensating apparatus  200  is capable of performing digital predistortion processing (distortion compensation processing) for a digital signal by performing frequency band limiting (cutting off a high-frequency component) for the digital signal. 
     A predistortion coefficient (distortion compensation coefficient) for compensating for distortion in the LINC-based amplifier is calculated based on an output signal. The predistortion coefficients (distortion compensation coefficients) for the amplifier  224  and the amplifier  244  are calculated based on the difference between an output signal and a third- or higher-order term of a function f which simulates the LINC-based amplifier. 
     An output signal may be deteriorated due to the imbalance between the branches of the LINC-based amplifier and the nonlinearity of the combiner. When the size of an output signal is small, the output signal deteriorates due to the influence of the imbalance between the branches of the LINC-based amplifier. When the size of an output signal is large, the output signal deteriorates due to the influence of the nonlinearity of the combiner. Thus, the modeling unit  280  adjusts the nonlinearity of the combiner by using a digital signal for which the size of a corresponding output signal is greater than or equal to a specific value, and corrects the imbalance between the branches of the LINC-based amplifier by using a digital signal for which the size of a corresponding output signal is smaller than the specific value. 
     Furthermore, the distortion compensating apparatus  200  is capable of compensating for the nonlinearity of the amplifier  224  and the amplifier  244  by identifying the amplifier  224  and the amplifier  244  by using a polynomial expression. 
     The distortion compensating apparatus  200  is capable of suppressing the deterioration of a high-frequency output by means of distortion compensation for the LINC-based amplifier and distortion compensation for amplifiers of the individual branches of the LINC-based amplifier. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.