Abstract:
Disclosed is a distortion-compensation apparatus that can reduce the storage space for storing coefficients required for distortion-compensation calculation, and can accurately execute distortion compensation. Distortion-compensation apparatus ( 100 ) compensates for distortion of an output signal from a predetermined circuit by predistortion in which an input signal is preliminarily multiplied by a coefficient. First multiplication section ( 200   c ) of distortion-compensation apparatus ( 100 ) multiplies an input signal by a first coefficient selected from coefficient candidates in accordance with the input signal; second multiplication section ( 201   c   1  to  201   c   m ) multiplies a delay signal of an input signal by a tap coefficient; and adding section ( 202 ) outputs a signal obtained by adding together an input signal multiplied by the compensation coefficient and the delay signal multiplied by the tap coefficient to the predetermined circuit.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a distortion-compensation apparatus and a distortion-compensation method that compensate for distortion of a signal output from a predetermined circuit. 
       BACKGROUND ART 
       [0002]    Conventionally, it is known that non-linear signal distortion is generated in an analog circuit, an RF (Radio Frequency) circuit and the like making up a transmitting system such as a radio communication device. To compensate for such signal distortion, a technique called adaptive digital predistortion has been developed. 
         [0003]    In this technique, the reverse characteristics of an analog circuit and an RF circuit are stored in a LUT (Look Up Table) in the form of compensation coefficients in accordance with the amplitude and the power of the input signal. With this configuration, distortion compensation is achieved by preliminarily applying to an input signal a compensation coefficient corresponding to the amplitude and the power. 
         [0004]    In addition, in this technique, the input signal and the transmission signal to which the compensation coefficients have been applied are compared with each other, and the compensation coefficients are adaptively updated such that the difference between the input signal and the transmission signal is reduced. In this manner, even in the case where the distortion characteristics are changed under the influence of temperature change, voltage change and the like, compensation of signal distortion can be effectively executed. 
         [0005]    For example, PTL 1 discloses a technique intended to compensate for signal distortion generated by an amplifier. In this technique, a compensation coefficient of compensation of signal distortion is generated by an adaptive algorithm based on a difference between an input signal and an output signal of the amplifier. 
         [0006]    It is known that signal distortion has a memory effect. Memory effect is a phenomenon in which signal distortion is dependent not only on a current input signal, but also on historical input signals. In the technique disclosed in PTL 1, however, memory effect is not taken into consideration, and an effect of limiting signal distortion cannot be sufficiently obtained. 
         [0007]    To solve such a problem, PTL 2 discloses a technique in which L×M compensation-coefficient candidates corresponding to possible L states of the power (or amplitude) of a current input signal and possible M states of the power (or amplitude) of historical input signals are preliminarily stored in a memory, and, from the candidates, one compensation coefficient corresponding to the power (or amplitude) of the current and historical input signals is read out, and then, the compensation coefficient is applied to the input signal. 
       CITATION LIST 
     Patent Literatures 
     PTL 1 
     Japanese Patent Application Laid-Open No. 9-69733 
     PTL 2 
     WO01/008320 
     SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the technique disclosed in PTL 2, however, L×M compensation-coefficient candidates are required to be stored in a memory, and therefore a large storage space is required. In a case which historical input signals (number of states N) are taken into consideration to more accurately evaluate the influence of the memory effect, the number of compensation-coefficient candidates is L×M×N, and thus a significantly large storage space is required in order to store the compensation coefficients. 
         [0009]    In addition, in the technique disclosed in PTL 2, a current input signal is multiplied by one compensation coefficient to compensate for distortion; however, with such a method, it is difficult to accurately perform distortion compensation. 
         [0010]    An object of the present disclosure is to provide a distortion-compensation apparatus and a distortion-compensation method which can reduce the storage space for storing coefficients required for distortion-compensation calculation, and can accurately execute distortion compensation. 
       Solution to Problem 
       [0011]    A distortion-compensation apparatus of an embodiment of the present disclosure compensates for distortion of an output signal from a predetermined circuit by predistortion in which an input signal is preliminarily multiplied by a coefficient, the distortion-compensation apparatus including: a first multiplication section that multiplies the input signal by a first coefficient selected from candidates of the first coefficient in accordance with the input signal; a second multiplication section that multiplies a delay signal of the input signal by a second coefficient; and an adding section that outputs a signal obtained by adding together an input signal multiplied by the first coefficient and a delay signal multiplied by the second coefficient to the predetermined circuit. 
         [0012]    A distortion-compensation method of an embodiment of the present disclosure is intended for compensating for distortion of an output signal from a predetermined circuit by predistortion in which an input signal is preliminarily multiplied by a coefficient, the method including: multiplying the input signal by a first coefficient selected from candidates of the first coefficient in accordance with the input signal; multiplying a delay signal of the input signal by a second coefficient; and outputting a signal obtained by adding together an input signal multiplied by the first coefficient and a delay signal multiplied by the second coefficient to the predetermined circuit. 
       Advantageous Effects of Invention 
       [0013]    According to the present disclosure, the storage space for storing coefficients required for distortion-compensation calculation can be reduced, and distortion compensation can be accurately executed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a block diagram illustrating a configuration of a transmission apparatus according to an embodiment of the present disclosure; and 
           [0015]      FIG. 2  is an explanatory table of an updating process for reducing a convergent time. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0016]    In the following, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. 
         [0017]      FIG. 1  is a block diagram illustrating a configuration of transmission apparatus  100  according to an embodiment of the present disclosure. Transmission apparatus  100  includes distortion-compensation section  101 , RF (Radio Frequency) modulation section  102 , antenna  103 , and feedback demodulation section  104 . 
         [0018]    Distortion-compensation section  101  compensates for signal distortion which is generated in RF modulation section  102 . To be more specific, distortion-compensation section  101  compensates for the above-mentioned distortion with use of an adaptive-digital predistortion technique. Distortion-compensation section  101  will be described in detail later. 
         [0019]    RF modulation section  102  modulates and amplifies a baseband signal in which distortion is compensated by distortion-compensation section  101 , and causes antenna  103  to radiate radio wave. Antenna  103  radiates a signal output from RF modulation section  102  in the form of a radio wave. Feedback demodulation section  104  demodulates a signal output from RF modulation section  102 , and outputs the resulting signal to distortion-compensation section  101 . 
         [0020]    Next, a configuration of distortion-compensation section  101  is described in detail. Distortion-compensation section  101  includes memoryless compensation section  200 , memory effect compensation section  201 , adder  202 , error computing section  203 , and update-processing section  204 . 
         [0021]    Memoryless compensation section  200  performs distortion compensation on a received baseband signal. Memoryless compensation section  200  includes address generation section  200   a , first storage section  200   b , and multiplier  200   c.    
         [0022]    Address generation section  200   a  computes the amplitude of a baseband signal, generates an address corresponding to the computed amplitude, and outputs the generated address to first storage section  200   b . It is to be noted that address generation section  200   a  may generate an address in accordance with the power of a baseband signal, the function of the amplitude, the function of the power and the like, instead of the amplitude of a baseband signal. 
         [0023]    First storage section  200   b  is a storage device such as a memory. First storage section  200   b  is formed as a LUT (Look Up Table) that stores compensation-coefficient candidate A, (i=1 to n) used for multiplication of a baseband signal. 
         [0024]    First storage section  200   b  outputs a compensation coefficient of the stored compensation-coefficient candidates which corresponds to the address generated by address generation section  200   a  to multiplier  200   c  and update-processing section  204 . 
         [0025]    Multiplier  200   c  multiplies a baseband signal by a compensation coefficient output by first storage section  200   b , and outputs the resulting signal to adder  202 . 
         [0026]    Memory effect compensation section  201  performs on a delay signal of a baseband signal a signal process for compensation of a memory effect. Memory effect compensation section  201  includes delayers  201   a   1  to  201   a   m , second storage section  201   b , and multipliers  201   c   1  to  201   c   m . 
         [0027]    Delayers  201   a   1  to  201   a   m  hold historical (primary, secondary, . . . , m delay) baseband signals. While the number of delayers  201   a   1  to  201   a   m  is m in the present embodiment, the number of delayers  201   a   1  to  201   a   m  is not limited as long as at least one delayer is provided. 
         [0028]    Second storage section  201   b  is a storage device such as a memory. Second storage section  201   b  is formed as an LUT (Look Up Table) that stores tap coefficient B j  (j=1 to m) used for multiplication of a delay signal of a baseband signal. Second storage section  201   b  outputs the stored tap coefficient to multipliers  201   c   1  to  201   c   m , and update-processing section  204 . 
         [0029]    Multiplier  201   c   1  to  201   c   m  multiplies a delay signal of a baseband signal by a tap coefficient output by second storage section  201   b , and outputs the resulting signal to adder  202 . 
         [0030]    Adder  202  computes a sum of a signal output by multiplier  200   c  of memoryless compensation section  200  and signals output by multipliers  201   c   1  to  201   c   m  of memory effect compensation section  201 , and outputs the resulting signal to RF modulation section  102 . 
         [0031]    Error computing section  203  computes difference e t  (=x t −y t ) between baseband signal x t  input at time t and signal y t  resulting from demodulation of an output signal in feedback demodulation section  104 , and outputs information of the difference to update-processing section  204 . The smaller the absolute value of the difference, the greater the effect of the distortion-compensation. 
         [0032]    With use of the information of difference e t  output by error computing section  203 , update-processing section  204  updates compensation coefficient A i (i=1 to n) stored in first storage section  200   b  and tap coefficient B j  (j=1 to m) stored in second storage section  201   b.    
         [0033]    To be more specific, when baseband signals at times t-j are represented by x t−j , signal y t  resulting from demodulation of an output signal in feedback demodulation section  104  is represented as 
         [0000]        y   t =( A   i   x   t   +ΣA   i   B   j   x   t−j ) f    
         [0034]    where A i  represents a compensation coefficient selected from compensation-coefficient candidates stored in first storage section  200   b  in accordance with the amplitude of baseband signal x t , and f represents an influence of distortion generated in RF modulation section  102 . 
         [0035]    Update-processing section  204  performs convergence calculation with use of a widely accepted iteration method such as LMS (Least Mean Square) and RLS (Recursive Least Square Algorithm) to determine the values of compensation coefficient A, and each tap coefficient B j  (j=1 to m) such that the absolute value of difference e t  between input baseband signal x t  and signal y t  resulting from demodulation of feedback demodulation section  104 , that is, 
         [0000]        e   t   =x   t   −y   t   =x   t −( A   i   x   t   +ΣA   i   B   j   x   t−j ) f  
 
         [0036]    is a small value. 
         [0037]    Then, with use of the determined values, update-processing section  204  updates the value of compensation coefficient A i  stored in first storage section  200   b  and the value of each tap coefficient B j  (j=1 to m) stored in second storage section  201   b.    
         [0038]    As described, in the present embodiment, it suffices to store one tap coefficient for each historical baseband signal, and thus the storage space required for distortion compensation can be reduced. In addition, the signal distortion correction is performed not only by multiplying a current baseband signal by a compensation coefficient to generate a signal, but also by multiplying a historical baseband signal by a tap coefficient to generate a signal and by synthesizing the signals, and thus, highly accurate distortion compensation is achieved. 
         [0039]    Further, by performing the following updating process, the time required for convergence of compensation coefficient A i  and each tap coefficient B j  (j=1 to m) can be reduced.  FIG. 2  is an explanatory table of an updating process for reducing a convergence time. It is to be noted that  FIG. 2  shows the case where the number of compensation-coefficient candidates is 8 (n=8), and the number of tap coefficients is 2 (m=2). 
         [0040]    In this updating process, first, update-processing section  204  executes convergence calculation on compensation coefficient A i  (compensation coefficient selected from A 1  to A 8  in accordance with the amplitude of baseband signal x t ) by an iteration method for a predetermined time, and sequentially updates compensation coefficient A, (steps  0  to  4  of  FIG. 2 ). Meanwhile, the value of tap coefficients B 1  and B 2  is set to a constant value (initial value). 
         [0041]    Subsequently, update-processing section  204  executes convergence calculation on tap coefficients B 1  and B 2  by an iteration method for a predetermined time, and sequentially updates tap coefficients B 1  and B 2  (steps  5  to  9  of  FIG. 2 ). Meanwhile, the value of compensation coefficient A i  is set to a constant value (the value set at the last update at step  4  of  FIG. 2 ). 
         [0042]    Thereafter, update-processing section  204  applies an iteration method on compensation coefficient A i  and tap coefficients B 1  and B 2 , and sequentially updates compensation coefficient A i  and tap coefficients B 1  and B 2  (steps  10  to  18  of  FIG. 2 ). 
         [0043]    Through the above-mentioned processes, in comparison with the case where compensation coefficient A i  and tap coefficients B 1  and B 2  are simultaneously converged from the start, compensation coefficient A i  and tap coefficients B 1  and B 2  can be converged in a short time. 
         [0044]    In addition, when the absolute value of the convergence value of the tap coefficient becomes smaller than a predetermined threshold, update-processing section  204  determines that coefficient is 0, and excludes a compensation coefficient or a tap coefficient in which the absolute value of the convergence value is smaller than a predetermined threshold from the subjects for convergence calculation. 
         [0045]    In addition, when the absolute value of the convergence value of tap coefficient B k  becomes smaller than a predetermined threshold, the values of compensation coefficient A i  and tap coefficient B j  (j=1 to m, j≠k) are determined such that, with B k =0, the absolute value of 
         [0000]        e   t   =x   t   −y   t   =x   t −( A   i   x   t +Σ j≠k   A   i   B   j   x   t−j ) f  
 
         [0046]    is a small value. 
         [0047]      FIG. 2  shows an exemplary case where the absolute value of the convergence value of tap coefficient B 1  becomes smaller than a predetermined threshold at step  14 . Through the above-mentioned processes, each coefficient can be converged more quickly, and in this case, the circuit for the updating process of B 1  can be stopped, thus making it possible to achieve power saving. 
         [0048]    While update-processing section  204  updates compensation coefficient A i  and tap coefficient B j  (j=1 to m) with use of information of difference e t  output by error computing section  203  in the above-mentioned embodiment, it is also possible to set a part of the coefficients (for example, a compensation-coefficient candidate or a tap coefficient) or all of the coefficients to a value preliminarily obtained by an experiment and the like so as not to perform the update by update-processing section  204 . 
         [0049]    This application is entitled to and claims the benefit of Japanese Patent Application No. 2012-282752 dated Dec. 26, 2012, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0050]    The distortion-compensation apparatus and the distortion-compensation method according to the present disclosure are suitable for a distortion-compensation apparatus and a distortion-compensation method that compensate for distortion of a signal output from a predetermined circuit. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Transmission apparatus 
           101  Distortion-compensation section 
           102  RF modulation section 
           103  Antenna 
           104  Feedback demodulation section 
           200  Memoryless compensation section 
           200   a  Address generation section 
           200   b  First storage section 
           200   c  Multiplier 
           201  Memory effect compensation section 
           201   a   1  to  201   a   m  Delayer 
           201   b  Second storage section 
           201   c   1  to  201   c   m  Multiplier 
           202  Adder 
           203  Error computing section 
           204  Update-processing section