Abstract:
A step size parameter μ is adaptively varied when a distortion compensation coefficient is calculated in a distortion compensation apparatus, relation between transmission signal level and step size parameter μ is considered. The distortion compensation apparatus includes a memory storing distortion compensation coefficient in a designated write address, and outputting distortion compensation coefficient being stored in a designated readout address; a predistortion section performing distortion compensation processing onto a transmission signal, using the distortion compensation coefficient being output from memory; and a distortion compensation section calculating an update value of distortion compensation coefficient, based on error component existent between transmission signal before distortion compensation processing and transmission signal after being amplified by an amplifier. Further, the distortion compensation section modifies magnitude of step size parameter determining degree of effect of error component produced on the update value, when calculating the update value of the distortion compensation coefficient.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a predistortion compensation apparatus for performing distortion compensation processing in advance to a transmission signal before amplification. 
   2. Description of the Related Art 
   In recent years, high-efficient digital transmission has been adopted in the radio communication field. When multilevel phase modulation is adopted in the radio communication, a technique for reducing adjacent channel leak power becomes important, in which nonlinear distortion is restrained by linearizing the amplification characteristic of a power amplifier on the transmission side. 
   Also, to improve power efficiency even in case an amplifier having a degraded linearity is used, a technique for compensating nonlinear distortion for the degraded linearity is necessary. 
     FIG. 1  shows an exemplary block diagram of transmission equipment in the conventional radio equipment. A transmission signal generator  1  outputs a digital serial data sequence. Also, a serial-to-parallel (S/P) converter  2  converts the digital data sequence into two series, in-phase component (I-component) signals and quadrature component (Q-component) signals, by alternately distributing the digital data sequence on a bit-by-bit basis. 
   A digital-to-analog (D/A) converter  3  converts the respective I-signals and Q-signals into analog baseband signals, and inputs the signals into a quadrature modulator  4 . This quadrature modulator  4  performs orthogonal transformation and outputs signals by multiplying the input I-signals and Q-signals (transmission baseband signals) by a reference carrier wave  8  and a carrier wave phase-shifted therefrom by 90°, respectively, and adding the multiplied results. 
   A frequency converter  5  mixes the quadrature modulation signals with local oscillation signals, and converts the mixed signals into radio frequency. A transmission power amplifier  6  performs power amplification of the radio frequency signals output from frequency converter  5 , and radiates the signal to the air from an antenna  7 . 
   Here, in the mobile communication using W-CDMA, etc., transmission equipment power is substantially large, becoming as much as 10 mW to several tens of mW, and transmission power amplifier  6  has a nonlinear input/output characteristic having a distortion function f(p), as shown by the dotted line in  FIG. 2 . This non-linearity causes a non-linear distortion. As shown by the solid line (b) in  FIG. 3 , the frequency spectrum in the vicinity of a transmission frequency f 0  comes to have a raised sidelobe from the characteristic shown by the broken line (a). This leaks to adjacent channels and produces adjacent interference. Namely, due to the nonlinear distortion shown in  FIG. 2 , leak power of the transmission wave to the adjacent frequency channels becomes large, as shown in  FIG. 3 . 
   An ACPR (adjacent channel power ratio) is used to indicate the magnitude of leak power. ACPR is a ratio of leak power to adjacent channels to the power in the channel of interest, in other words, a ratio of the spectrum area in the adjacent channels sandwiched between the lines B and B′ in  FIG. 3  to the spectrum area between the lines A and A′. Such leak power affects other channels as noise, and degrades communication quality of the channels concerned. Therefore, a strict regulation has been established to the issue of leak power. 
   The leak power is substantially small in a linear region of, for example, a power amplifier (refer to a linear region I in  FIG. 2 ), but is large in a nonlinear region II. Accordingly, to obtain a high-output transmission power amplifier, the linear region I has to be widened. However, for this purpose, it becomes necessary to provide an amplifier having a larger capacity than is actually needed, which causes disadvantage in apparatus cost and size. As a measure to solve this problem, a distortion compensation function to compensate for transmission power distortion is added to radio equipment. 
     FIG. 4  shows the block diagram of transmission equipment having a digital nonlinear distortion compensation function by use of a DSP (digital signal processor). A digital data group (transmission signals) transmitted from transmission signal generator  1  is converted into two series, I-signals and Q-signals, in S/P converter  2 , and then the two series of signals are input to a distortion compensator  9 . 
   As shown in the lower part of  FIG. 4  in enlargement, distortion compensator  9  includes a distortion compensation coefficient storage  90  for storing a distortion compensation coefficient h(pi) corresponding to the power level pi (i=0-1023) of a transmission signal x(t); a predistortion portion  91  for performing a distortion compensation process (predistortion) onto the transmission signal, using the distortion compensation coefficient h(pi) corresponding to the transmission signal power level; and a distortion compensation coefficient calculator  92  for comparing the transmission signal x(t) with a demodulation signal (a feedback signal) y(t) demodulated in the quadrature detector which will be described later, and calculates and updates the distortion compensation coefficient h(pi) so that the difference between the transmission signal and the demodulation signal becomes zero. 
   The signal to which distortion process is performed in distortion compensator  9  is input into D/A converter  3 . D/A converter  3  converts the input I-signal and Q-signal into analog baseband signals, and inputs the converted signals into quadrature modulator  4 . Quadrature modulator  4  performs quadrature modulation by multiplying the input I-signal and Q-signal by a reference carrier wave  8  and a carrier wave being phase-shifted from carrier wave  8  by 90°. Quadrature modulator  4  then adds and outputs the multiplied result. 
   A frequency converter  5  mixes the quadrature modulation signal with a local oscillation signal, and performs frequency conversion. A transmission power amplifier  6  performs power amplification of the radio frequency signal output from frequency converter  5 , and radiates the signal to the air by an antenna  7 . 
   A portion of the transmission signal is input to a frequency converter  11  via a directional coupler  10 , and input into a quadrature detector  12  after being converted by the above frequency converter  11 . Quadrature detector  12  performs quadrature detection by multiplying the input signal by a reference carrier wave, and by a signal which is phase shifted by 90° from the reference signal, respectively. Thus, the baseband I-signal and Q-signal on the transmission side are reproduced, which are then input into an analog-to-digital (A/D) converter  13 . 
   A/D converter  13  converts the input I-signal and Q-signal into digital signals, and inputs into distortion compensator  9 . Through the adaptive signal processing, using an LMS (least-mean-square) algorithm, in distortion compensation coefficient calculator  92  of distortion compensator  9 , the pre-compensated transmission signal is compared with the feedback signal being demodulated in quadrature detector  12 . Then distortion compensator  9  calculates the distortion compensation coefficient h(p 1 ) so as to make the above difference zero. Then, distortion compensator  9  updates the above-obtained coefficient which has been stored in distortion compensation coefficient storage  90 . Through the repetition of calculations above, nonlinear distortion in transmission power amplifier  6  is restrained, and adjacent channel leak power is reduced. 
   By way of example, in the PCT International Publication WO 2003/103163, such a configuration as shown in  FIG. 5 , in which distortion compensation is performed using the adaptive LMS algorithm, is described as an embodiment of distortion compensator  9  shown in  FIG. 4 . 
   In  FIG. 5 , a multiplier  15   a  corresponds to a predistortion section  91  shown in  FIG. 4 , in which a transmission signal x(t) is multiplied by a distortion compensation coefficient h n-1 (p). Also, a distortion device  15   b  having a distortion function f(p) corresponds to a transmission power amplifier  6  shown in  FIG. 4 . 
   Further, as to the portion in  FIG. 4  including a frequency converter  11 , a quadrature detector  12  and an A/D converter  13 , in which the output signal being output from transmission power amplifier  15   b  is feedbacked, a feedback system  15   c  is shown in  FIG. 5 . 
   Moreover, in  FIG. 5 , a look-up table (LUT)  15   e  constitutes a distortion compensation coefficient storage  90  shown in  FIG. 4 . A distortion compensation coefficient calculation section  16  constitutes a distortion compensation coefficient calculation section  92  shown in  FIG. 4 , which generates an update value of the distortion compensation coefficient stored in look-up table  15   e.    
   In the distortion compensation apparatus having the configuration shown in  FIG. 5 , look-up table  15   e  has a distortion compensation coefficient for canceling the distortion produced in transmission power amplifier  6 , namely, distortion device  15   b , in a two-dimensional address location corresponding to each discrete power value of the transmission signal x(t). 
   When the transmission signal x(t) is input, an address generation circuit  15   d  calculates the power p (=x 2 (t)) of the transmission signal x(t), and generates an address of one dimensional direction, for example the X-axis direction, which uniquely corresponds to the above-calculated power p (=x 2 (t)) of the transmission signal x(t). At the same time, address generation circuit  15   d  obtains a difference ΔP of the power P 1  (=x 2 (t- 1 )) of the transmission signal x(t- 1 ) of the previous time point (t- 1 ) having been stored in address generation circuit  15   d , and generates an address of the other dimensional direction, for example, the Y-axis direction, which uniquely corresponds to the above difference ΔP. 
   Thus, from address generation circuit  15   d , a store location in look-up table  15   e , which is specified by the address P in the X-axis direction and the address ΔP in the Y-axis direction, is read out. The readout address is output as address designation information (AR). 
   Then, a distortion compensation coefficient h n-1 (p) stored in the above readout address is read out from look-up table  15   e , so as to be used in the distortion compensation processing performed by multiplier  15   a.    
   Meanwhile, an update value for updating the distortion compensation coefficient having been stored in look-up table  15   e  is calculated in a distortion compensation coefficient calculation section  16 . Namely, distortion compensation coefficient calculation section  16  is constituted of a conjugate complex calculation section  16  and multipliers  15   h - 15   j . A subtractor  15   g  outputs a difference e(t) between the transmission signal x(t) and the feedback demodulation signal y(t). Multiplier  15   i  multiplies the distortion compensation coefficient h n-1 (p) by y*(t), so as to obtain an output u*(t) (= n-1 (p)y*(t)). Multiplier  15   h  multiplies the difference e(t) being output from subtractor  15   g  by u*(t). Multiplier  15   j  multiplies the output of multiplier  15   h  by a step size parameter μ. 
   Next, an adder  15   k  adds the distortion compensation coefficient h n-1 (p) to the output μe(t)u*(t) being output from multiplier  15   j , and obtains an update value of look-up table  15   e . This update value is to be stored in a write address (AW), consisting of the X-axis direction address and the Y-axis direction address, being specified by address generation circuit  15   d  as the address corresponding to the transmission signal power p (=x 2 (t)). 
   Here, the aforementioned write address (AW) is the same address as the readout address (AR). However, because of a calculation time, etc. needed to obtain the update value, the readout address is used as the write address after the readout address is delayed in a delay section  15   m.    
   Delay portions  15   m ,  15   n ,  15   p  add to the transmission signal x(t), the delay time D, which is the period from the input of the transmission signal x(t) to the feed back decoded signal y(t) input to the subtractor  15   g.    
   The delay time D being set by the delay portions  15   m ,  15   n ,  15   p  is determined so as to satisfy D=D 0 +D 1 , where D 0  is the delay time in transmission power amplifier  15   b , and D 1  is the delay time in feedback system  15   c.    
   Using the above configuration, the following calculations are performed.
 
 h   n ( p )= h   n-1 ( p )+μ e ( t ) u *( t )
 
 e ( t )= x ( t )− y ( t )
 
 y ( t )= h   n-1 ( p )×( t ) f ( p )
 
 u *( t )= x ( t ) f ( p )= h   n-1 ( p ) y *( t )
 
 p=|x ( t )| 2  
 
Here, x, y, f, h, u, e are complex numbers, and * denotes a conjugate complex number.
 
   Through the above calculation processing, the distortion compensation coefficient h(p) is updated so as to minimize the differential signal e(t) between the transmission signal x(t) and the feedbacked demodulation signal y(t). Finally, the value converges to an optimal distortion compensation coefficient, so that the distortion of the transmission power amplifier is compensated. 
   Now, in the above calculation, the step size parameter μ determines a degree of effect of an error component e(t) between the transmission signal x(t), i.e. the reference signal, and the feedback demodulation signal y(t), i.e. the feedback signal, on the update value of the distortion compensation coefficient. In the conventional system, the value of the step size parameter μ is set to a fixed value. 
   In the configuration of the distortion compensation apparatus shown in  FIG. 5 , the inventor of the present invention has observed the output of the distortion compensation apparatus by inputting the outputs of transmission power amplifier  15   b , the distortion device, into a spectrum analyzer with sweep frequencies.  FIGS. 6A through 7B  are the results obtained at those times. In the examples shown in  FIGS. 6A through 7B , the observations have been made with different transmission signal levels in four frequency bands (channels). 
     FIGS. 6A ,  6 B represent the output spectrum waveforms of the spectrum analyzer when the transmission signal level is large (43 dB).  FIG. 6A  shows the spectrum waveform when the step size parameter μ is set to 1/1024, while  FIG. 6B  shows the spectrum waveform when the step size parameter μ is set to 1/16. 
   Also,  FIGS. 7A ,  7 B represent the output spectrum waveforms of the spectrum analyzer when the transmission signal level is small (27 dB).  FIG. 7A  shows the spectrum waveform when the step size parameter μ is set to 1/1024, while  FIG. 7B  shows the spectrum waveform when the step size parameter μ is set to 1/16. 
   From these  FIGS. 6A through 7B , it has been found out that the relation between the transmission signal level and the step size parameter μ produces an effect on the distortion compensation coefficient. In  FIGS. 6A ,  6 B, if the step size parameter μ is set larger when the transmission signal level is large (refer to  FIG. 6B ), external disturbance (phase rotation, quantization error in an A/D converter, etc.) affects greater, resulting in a larger number of rise pulses being produced. By this, the compensation coefficient tends to diverge at the time of calculating the error. 
   On the contrary, if the step size parameter μ is set smaller when the transmission signal level is small, a minute error having been detected is canceled, which produces a problem of preventing proper update of the compensation coefficient (refer to  FIG. 7A ). 
   SUMMARY OF THE INVENTION 
   Accordingly, on the basis of the above results, it is an object of the present invention to provide a distortion compensation apparatus in which further improvement of the distortion compensation characteristic is intended. 
   As a first aspect of the distortion compensation apparatus according to the present invention to attain the aforementioned object, the distortion compensation apparatus includes: a memory storing a distortion compensation coefficient in a designated write address, and outputting a distortion compensation coefficient being stored in a designated readout address; a predistortion section performing distortion compensation processing onto a transmission signal, using the distortion compensation coefficient being output from the memory; and a distortion compensation section calculating an update value of the distortion compensation coefficient, based on the error component existent between the transmission signal before the distortion compensation processing and the transmission signal after being amplified by an amplifier. The distortion compensation section further includes a function of controlling to modify a degree of effect of the error component produced on the update value, when calculating the update value of the distortion compensation coefficient. 
   As a second aspect of the distortion compensation apparatus according to the present invention to attain the aforementioned object, in the above first aspect, the modification of the degree of effect is realized by modifying the magnitude of the parameter to be multiplied by the error component. The distortion compensation section includes: a memory storing a parameter in advance, corresponding to the integral value of the transmission signal before the distortion compensation processing or the transmission signal after being amplified by the amplifier; an integrator generating an integral value of the transmission signal before the distortion compensation processing or the transmission signal after being amplified by the amplifier; and a control means for reading out the parameter corresponding to the output of the integrator from the memory. The parameter corresponding to the integral value is being set to a smaller value as the integral value becomes larger. 
   As a third aspect of the distortion compensation apparatus according to the present invention to attain the aforementioned object, in the above second aspect, when the integral value is not larger than a predetermined value, the control means sets the parameter value to zero. 
   As a fourth aspect of the distortion compensation apparatus according to the present invention to attain the aforementioned object, in the first aspect, the modification of the degree of effect is realized by modifying the magnitude of the parameter to be multiplied by the error component. The distortion compensation apparatus includes: a fast Fourier transform circuit performing fast Fourier transform of the transmission signal after being amplified by the amplifier; and a control means setting an optimal parameter value based on the output of the fast Fourier transform circuit. When the magnitude of the parameter is expressed by ½ n , the control means varies the n value by the step of ±1 in the direction of decreasing the power amount of the noise floor which is produced in the output of the fast Fourier transform circuit after the distortion compensation coefficient is updated. 
   As a fifth aspect of the distortion compensation apparatus according to the present invention to attain the aforementioned object, in the fourth aspect, when the power amount of the noise floor is not varied, the control means selects a smaller value out of the parameter values. 
   Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of exemplary transmission equipment in the conventional radio equipment. 
       FIG. 2  shows a diagram illustrating the input/output characteristic (having a distortion function f(p)) of a transmission power amplifier. 
       FIG. 3  shows a diagram illustrating nonlinear distortions produced by the nonlinear characteristic. 
       FIG. 4  shows a block diagram of transmission equipment having a digital nonlinear distortion compensation function by use of a DSP (digital signal processor). 
       FIG. 5  shows an exemplary configuration of the embodiment of a distortion compensator  9  shown in  FIG. 4 . 
       FIGS. 6A &amp; 6B  show an output waveform of a spectrum analyzer when the transmission signal level is large (43 dB). 
       FIGS. 7A &amp; 7B  show an output waveform of a spectrum analyzer when the transmission signal level is small (27 dB). 
       FIG. 8  shows a configuration block diagram according to a first embodiment of the distortion compensation apparatus to which the present invention is applied. 
       FIG. 9  shows a configuration block diagram according to a second embodiment of the distortion compensation apparatus to which the present invention is applied. 
       FIG. 10  shows a configuration block diagram according to a third embodiment of the distortion compensation apparatus to which the present invention is applied. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The preferred embodiment of the present invention is described hereinafter referring to the charts and drawings. However, it is noted that the embodiments described are intended for better understanding of the invention, and therefore the scope of the present invention is not limited to the embodiments described below. 
   First Embodiment 
     FIG. 8  is an exemplary configuration block diagram according to a first embodiment of a distortion compensation apparatus to which the present invention is applied. The parts having the like function as in the conventional configuration shown in  FIG. 5  are referred to by the like reference numerals. Accordingly, further description of the like parts as shown in  FIG. 5  is omitted. In the following, the above may also be applied to other embodiments. 
   In  FIG. 8 , as a feature, a control block  30  is provided for adaptively and variably setting (with a function of controlling to modify) a step size parameter μ (a parameter for varying a degree of effect of the error component produced on the update value). 
   Control block  30  includes a CPU  32 , as a control means, and a nonvolatile memory  33  which are connected to a bus  31 . Further, a distortion compensation coefficient generation circuit  16  works similarly to the circuit shown in  FIG. 5 . In the configuration of the exemplary embodiment shown in  FIG. 8 , a step size parameter μ, which is multiplied by the output of multiplier  15   h  in multiplier  15   j , is supplied from control block  30 . 
   The step size parameter μ supplied from control block  30  is output from CPU  32 , after referring to a table stored in nonvolatile memory  33 , and selecting a value corresponding to the output of an integrator  20 . 
   Namely, integrator  20  integrates the transmission signal x(t), namely the reference signal, or the feedback signal y(t), and outputs as an integral value for each predetermined period (each calculation time of the distortion compensation coefficient). Meanwhile, in nonvolatile memory  33 , which is included in control block  30  and connected to bus  31 , step size parameter values μ corresponding to the integrated values obtained in integrator  20  are stored in advance and preserved. 
   Accordingly, CPU  32  reads the output of integrator  20 , reads out a step size parameter μ having a magnitude corresponding to the above output from the table retained in nonvolatile memory  33 , so as to input to the multiplier  15   j.    
   As to the magnitudes of the step size parameters μ retained in the table stored in nonvolatile memory  33 , the larger the level of either the feedback signal y(t) or the reference signal x(t) is, the smaller the magnitude is set. With this, it becomes possible to maintain the relation shown in  FIG. 6A . 
   On the contrary, if the output of integrator  20  becomes too small, namely, if the value of the feedback signal y(t) or the reference signal x(t) is small, a distortion compensation coefficient error becomes large because of the quantization error in the A/D converter (refer to A/D converter  13  shown in  FIG. 4 ), etc., and as a result, it becomes not possible to correctly update the distortion compensation coefficient. 
   Therefore, in this case, the step size parameter μ is set to “0”. By this, the output of multiplier  15   j  becomes “0”, and the output of adder  15   k  remains the distortion compensation coefficient h n-1 (p) having previously been read out unchanged. Thus, the distortion compensation coefficient stored in look-up table  15   e  is not updated. This includes the same meaning as that no update is necessary, because the distortion produced is small when the level of the transmission signal x(t) is small. 
   Second Embodiment 
     FIG. 9  is a configuration block diagram according to a second embodiment of the distortion compensation apparatus to which the present invention is applied. In contrast to the first embodiment shown in  FIG. 8 , a FFT (fast Fourier transform) circuit  21  is provided, in place of integrator  20 . 
   In the processing of the present embodiment, a method of determining an optimal value μ by varying the values of the step size parameter μ is applied. 
   Namely, when the value of the step size parameter μ is expressed by ½ n , CPU  32  varies the value of “n” periodically by +1 or −1. At the time of each variation, the power amount of a noise floor (for example, the range Δ5M enclosed by a circle shown in  FIG. 6A ) of the output of FFT circuit  21 . 
   Then, CPU  32  selects the direction in which the above power amount is reduced, that is, the variation direction of the value “n” to either +1 or −1. Further, when CPU  32  detects a state that the above-mentioned power amount of the noise floor does not vary virtually while varying the value “n”, in order to avoid divergence of the update coefficient, CPU  32  selects the step size parameter μ having a smaller value (since the step size parameter value μ is expressed by ½ n , the value after being varied is selected when varying the value “n” to +1 direction, while the value μ before being varied is selected when varying the value “n” to −1 direction). 
   Third Embodiment 
     FIG. 10  is a configuration block diagram according to a third embodiment of the distortion compensation apparatus to which the present invention is applied. The like parts as in the other embodiments are referred to by the like reference numerals. 
   In  FIG. 10 , as a feature, there is provided an error averaging circuit  22 , to which an error signal e(t), namely the difference between the transmission signal x(t), i.e. the reference signal obtained from subtractor  15   g , and the feedback signal y(t), is input. 
   Error averaging circuit  22  calculates a mean amplitude value of the error signal, when the error signal obtained from subtractor  15   g  is input. 
   Here, the error signal may be considered equivalent to noise. In general, the larger the transmission signal is, the larger the noise level becomes. Therefore, considering the relation between the transmission signal level and the step size parameter μ having been described in  FIGS. 6A through 7B , a mean noise level, namely the mean amplitude value of the error signal is calculated. Then, an inverse number corresponding to the mean amplitude value of the error signal is used as the step size parameter μ. 
   By this, the smaller the mean amplitude value is, the larger value the step size parameter becomes, or the larger the mean amplitude value is, the smaller value the step size parameter becomes. Thus, it becomes possible to avoid undesirable events such that the distortion compensation coefficient is not virtually updated because of the small amplitude of the error signal, or the distortion compensation coefficient diverges because of the large amplitude of the error signal. 
   For the above purpose, in nonvolatile memory  33  provided in control block  30 , inverse numbers of the values corresponding to the mean amplitude values of the error signal are prepared in advance in the form of table, as step size parameters μ. CPU  32  reads the mean amplitude value output from error averaging circuit  22 , reads out a step size parameter μ corresponding to the mean amplitude value output from the table retained in nonvolatile memory  33 , and inputs the readout step size parameter μ into multiplier  15   j.    
   With the method described in the above embodiment, it becomes possible to generate a distortion compensation coefficient to be updated, using the mean amplitude value of the error signal, namely the mean noise level, as a reference. Accordingly, it is possible to avoid divergence of the distortion compensation coefficient, and to prevent a minute error component from being regarded as “0”. 
   In each of the foregoing first through third embodiments, in general, at the time of the start of transmission, or the occurrence of a large fluctuation in the transmission output, the optimal value of the distortion compensation coefficient in the look-up table varies to a great extent. To cope with such cases, CPU  32  detects the start of transmission, or occurrence of a large fluctuation during the transmission, and sets the step size parameter μ to a large value for several hundred milliseconds from the time of the above detection. 
   By this operation, the update speed becomes fast, and the distortion compensation coefficient in the look-up table can be optimized at high speed. 
   Then, after a lapse of several hundred milliseconds, since updating of the distortion compensation coefficient in look-up table  15   e  is almost completed, the step size parameter μ is set to a smaller value. With this, divergence of the distortion compensation coefficient can be suppressed. 
   As having been described, in the aforementioned embodiments, with the provision of the means for varying and controlling the step size parameter value, it becomes possible to control a degree of effect of the error signal on the update value depending on the situations. For example, when the transmission signal level (the transmission signal level after being amplified) is high, or the error signal level is high, divergence of the distortion compensation coefficient can be suppressed by decreasing the step size parameter value. On the other hand, when the transmission signal level (the transmission signal level after being amplified) is low, or the error signal level is low, the distortion compensation coefficient can be updated effectively by increasing the step size parameter value. 
   According to the present invention, divergence of the distortion compensation coefficient can be avoided, and the distortion compensation characteristic can be improved. As a result, a highly reliable distortion compensation apparatus can be provided. 
   The foregoing description of the embodiments is not intended to limit the invention to the particular details of the examples illustrated. Any suitable modification and equivalents may be resorted to the scope of the invention. All features and advantages of the invention which fall within the scope of the invention are covered by the appended claims.