Abstract:
A distortion compensator, includes: an input terminal configured to receive a transmission signal; a processor configured to perform operations to process the transmission signal, wherein the operations includes: compensating a nonlinear distortion of an amplifier which amplifies a power of the transmission signal, by using a distortion compensation coefficient corresponding to an amplitude value of the transmission signal; calculating a difference between a power value of the transmission signal and a power value of a feedback signal from the amplifier; calculating an imaginary part of a first complex vector based on an error between the transmission signal and the feedback signal in a cartesian coordinate system; and updating the distortion compensation coefficient by using a second complex vector of which a real part is the difference, and an imaginary part is the imaginary part of the first complex vector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-040241, filed on Mar. 2, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a distortion compensator, a distortion compensation method and a radio equipment. 
     BACKGROUND 
     A radio equipment such as a base station and a user terminal in a radio communication system includes a power amplifier (hereinafter, sometimes referred to as “PA”) for amplifying a power of a transmission signal. In such radio equipment, the PA is operated near a saturation region of the PA in order to increase the power efficiency of the PA. However, when the PA is operated near the saturation region, a nonlinear distortion increases. Thus, in order to reduce an adjacent channel leakage power (ACP) by suppressing the nonlinear distortion in the PA, the radio equipment is provided with a distortion compensator that compensates the nonlinear distortion in the PA. 
     Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 2006-121408 and 2006-270638. 
     SUMMARY 
     According to one aspect of the embodiments, a distortion compensator, includes: an input terminal configured to receive a transmission signal; a processor configured to perform operations to process the transmission signal, wherein the operations includes: compensating a nonlinear distortion of an amplifier which amplifies a power of the transmission signal, by using a distortion compensation coefficient corresponding to an amplitude value of the transmission signal; calculating a difference between a power value of the transmission signal and a power value of a feedback signal from the amplifier; calculating an imaginary part of a first complex vector based on an error between the transmission signal and the feedback signal in a cartesian coordinate system; and updating the distortion compensation coefficient by using a second complex vector of which a real part is the difference, and an imaginary part is the imaginary part of the first complex vector. 
     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 
         FIGS. 1A and 1B  are an example of a relationship between an error e(t) between a transmission signal x(t) and a feedback signal y(t) and an amplitude deviation of the feedback signal y(t); 
         FIG. 2  is a block diagram illustrating an example of radio equipment according to an embodiment; 
         FIG. 3  is a view illustrating an example of a coefficient update part of the embodiment; 
         FIG. 4  is a view provided for explaining a second complex vector of the embodiment; 
         FIG. 5  is a flow chart illustrating an example of a processing operation of a distortion compensator of the embodiment; and 
         FIG. 6  is an example of simulation result. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A “pre-distortion (hereinafter, sometimes referred to as “PD”) scheme” is one of distortion compensation schemes used in a distortion compensator. A distortion compensator employing the PD scheme suppresses a distortion occurring in an output signal of a PA, by increasing the linearity of the output signal of the PA by pre-multiplying a transmission signal before being input to the PA by a distortion compensation coefficient having the reverse characteristic of a nonlinear distortion in the PA. As a result of the multiplication of the transmission signal by the distortion compensation coefficient, both a distortion of an amplitude component of the transmission signal and a distortion of a phase component of the transmission signal are compensated. Hereinafter, a signal obtained by multiplying the transmission signal by the distortion compensation coefficient may be referred to as a “pre-distortion signal (PD signal).” Therefore, the PD signal is a signal pre-distorted before being input to the PA due to the reverse characteristic of the nonlinear distortion in the PA. In addition, hereinafter, the distortion of the amplitude component of the transmission signal in the PA may be referred to as an “amplitude distortion,” and the distortion of the phase component of the transmission signal in the PA may be referred to as a “phase distortion.” 
     For example, a distortion compensator employing the PD scheme may have a lookup table storing a plurality of distortion compensation coefficients (hereinafter, sometimes referred to as a “distortion compensation table”). The distortion compensator having the distortion compensation table reads, from the distortion compensation table, a distortion compensation coefficient corresponding to an amplitude value of a transmission signal input to the distortion compensator and multiplies the transmission signal by the read distortion compensation coefficient. The distortion compensation coefficients stored in the distortion compensation table are sequentially updated such that an error between a transmission signal as a reference signal and a signal output from the PA and fed back to a distortion compensation part (hereinafter, sometimes referred to as a “feedback signal”) becomes minimal. An error between the transmission signal and the feedback signal is represented by a complex vector coupling the transmission signal and the feedback signal in Cartesian coordinates. The method of representing an error between the transmission signal and the feedback signal by a complex vector coupling the transmission signal and the feedback signal in the Cartesian coordinates will be referred to as a “Cartesian coordinate approach.” 
       FIGS. 1A and 1B  are an example of a relationship between an error e(t) between a transmission signal x(t) and a feedback signal y(t) and an amplitude deviation of the feedback signal y(t).  FIG. 1A  represents a state where no amplitude difference is present between a transmission signal x(t 0 ) and a feedback signal y(t 0 ) at time t 0 , and only a phase difference is present between the transmission signal x(t 0 ) and the feedback signal y(t 0 ). For example, as illustrated in  FIG. 1A , the error e(t 0 ) between the transmission signal x(t 0 ) and the feedback signal y(t 0 ) is represented by a complex vector coupling a transmission signal and a feedback signal in the Cartesian coordinates. At time t 1  after the time t 0 , a distortion compensation coefficient h 0  is updated to a distortion compensation coefficient h 1  such that an error e(t 1 ) becomes smaller than the error e(t 0 ). Then, as illustrated in  FIG. 1B , the amplitude of a feedback signal y(t 1 ) becomes smaller than the amplitude of the feedback signal y(t 0 ), which results in an amplitude deviation of the feedback signal y(t). Such an amplitude deviation of the feedback signal y(t) leads to an amplitude difference between the transmission signal x(t) and the feedback signal y(t), which did not exist originally. This amplitude difference increases with the increase in the phase difference between the transmission signal x(t 0 ) and the feedback signal y(t 0 ). In addition, the above-mentioned “amplitude distortion” also increases with the increase in the amplitude difference, thereby deteriorating the distortion compensation performance. 
     As described above, there is a possibility of the deterioration of the distortion compensation performance in the Cartesian coordinate approach. 
     Thus, it may be conceivable to employ an approach of representing an error between a transmission signal and a feedback signal by the polar coordinates (hereinafter, sometimes referred to as a “polar coordinate approach”). In the polar coordinate approach, since the error between the transmission signal and the feedback signal is represented by an amplitude and a phase, the problem of the increase in the “amplitude distortion” may be is avoided and the distortion compensation performance may be improved. 
     However, the polar coordinate approach increases the circuit scale since it performs an operation of converting the Cartesian coordinates into the polar coordinates and an operation of returning the polar coordinates to the Cartesian coordinates. 
     Hereinafter, embodiments of the distortion compensator and the distortion compensation method of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited by the embodiments. In addition, elements having the same as or similar to function will be denoted by the same reference numeral, and overlapping descriptions thereof will be omitted or reduced. 
     EMBODIMENTS 
     (Exemplary Configuration of Radio Equipment) 
       FIG. 2  is a block diagram illustrating an example of radio equipment according to an embodiment. In  FIG. 2 , radio equipment  10  includes a baseband unit  11 , a distortion compensator  12 , a digital-analog converter (DAC)  13 , a quadrature modulator  14 , a carrier generator  15 , a PA  16 , a coupler  17 , and an antenna  18 . Further, the ratio equipment  10  includes a quadrature demodulator  19  and an analog-digital converter (ADC)  20 . The distortion compensator  12  includes a pre-distortion (PD) part  21 , an address generator  22 , a lookup table (LUT) storage part  23 , and a coefficient update part  24 . 
     The baseband unit  11  generates a transmission signal of a baseband by subjecting input transmission data to baseband processings such as coding and modulation, and outputs the generated transmission signal to the PD part  21 , the address generator  22 , and the coefficient update part  24 . The transmission signal generated by the baseband unit  11  includes an in-phase component signal (I signal) and a quadrature component signal (Q signal). 
     The PD part  21  generates a PD signal having the reverse characteristic that cancels the distortion characteristic of the PA  16 , by multiplying the I signal and the Q signal of the transmission signal by a real part and an imaginary part of a distortion compensation coefficient output from the LUT storage part  23 , respectively, and outputs the generated PD signal to the DAC  13 . For example, the PD part  21  compensates the nonlinear distortion of the PA  16  using a distortion compensation coefficient. 
     The DAC  13  converts the PD signal corresponding to each of the I signal and the Q signal from a digital signal to an analog signal which is then output to the quadrature modulator  14 . 
     The carrier generator  15  generates a reference carrier and outputs the generated reference carrier to the quadrature modulator  14  and the quadrature demodulator  19 . 
     The quadrature modulator  14  multiplies the I signal of the PD signal by the reference carrier and multiplies the Q signal of the PD signal by a carrier obtained by phase-shifting the reference carrier by 90°. Then, the quadrature modulator  14  performs quadrature modulation and up-conversion for the PD signal by adding the two multiplication results, and outputs the quadrature modulated and up-converted PD signal to the PA  16 . 
     The PA  16  amplifies the power of the PD signal input from the quadrature modulator  14  and outputs the power-amplified PD signal to the coupler  17 . 
     The coupler  17  distributes the power-amplified PD signal to the antenna  18  and the quadrature demodulator  19 . Thus, the signal output from the PA  16  is fed back to the distortion compensator  12  via the quadrature demodulator  19  and the ADC  20 . 
     The antenna  18  transmits the power-amplified PD signal. 
     The quadrature demodulator  19  performs a down-conversion and quadrature demodulation for the signal input from the coupler  17  by multiplying the signal by each of the reference carrier generated in the carrier generator  15  and the carrier obtained by phase-shifting the reference carrier by 90°. Then, the quadrature demodulator  19  outputs a feedback signal obtained by the quadrature demodulation to the ADC  20 . The feedback signal obtained by the quadrature demodulation includes an I signal and a Q signal. 
     The ADC  20  converts the feedback signal from an analog signal into a digital signal which is then output to the coefficient update part  24 . 
     The address generator  22  obtains a power p of a transmission signal x(t) input from the baseband unit  11 , generates an address corresponding to the obtained power p, and designates the generated address for the LUT storage part  23 . 
     The LUT storage part  23  holds a distortion compensation table LUT. The LUT stores a plurality of addresses and a plurality of distortion compensation coefficients corresponding to the plurality of addresses, respectively. For example, the LUT storage part  23  uses the LUT to store the distortion compensation coefficients for cancelling the nonlinear distortion of the PA  16  in address positions corresponding to discrete powers p of the transmission signal x(t), respectively. Each of the distortion compensation coefficients stored in the LUT includes an amplitude component coefficient and a phase component coefficient. The LUT storage part  23  outputs a distortion compensation coefficient corresponding to the address designated by the address generator  22  to the PD part  21  and the coefficient update part  24 . 
     The coefficient update part  24  calculates a difference between the power of the transmission signal input from the baseband unit  11  and the power of the feedback signal input from the ADC  20  (hereinafter, sometimes simply referred as a “difference”). Then, the coefficient update part  24  calculates an “imaginary part of a first complex vector” based on an error between the transmission signal and the feedback signal in the Cartesian coordinate system. Then, the coefficient update part  24  updates the distortion compensation coefficients stored in the LUT of the LUT storage part  23  using a “second complex vector” of which a real part is the “difference” and an imaginary part is the “imaginary part of the first complex vector.” 
     For example, as illustrated in  FIG. 3 , the coefficient update part  24  includes a power calculator  31 , a power calculator  32 , a subtractor  33 , a conjugate complex signal output part (Conj)  34 , and a complex multiplier  35 . Further, as illustrated in  FIG. 3 , the coefficient update part  24  includes subtractors  36  and  37 , a complex multiplier  38 , multipliers  39  and  40 , and adders  41  and  42 .  FIG. 3  is a view illustrating an example of the coefficient update part of the present embodiment. 
     The power calculator  31  calculates a power value |x(t)| 2  (=x re (t) 2 +x im (t) 2 ) of the transmission signal x(t) input from the baseband unit  11  and outputs the calculated power value |x(t)| 2  to the subtractor  33 . Here, x re (t) represents a real part of the transmission signal x(t) and corresponds to the I signal of the transmission signal x(t). In addition, x im (t) represents an imaginary part of the transmission signal x(t) and corresponds to the Q signal of the transmission signal x(t). 
     The power calculator  32  calculates a power value |y(t)| 2  (=y re (t) 2 +y im (t) 2 ) of the feedback signal y(t) input from the baseband unit  11  and outputs the calculated power value |y(t)| 2  to the subtractor  33 . Here, y re (t) represents a real part of the feedback signal y(t) and corresponds to the I signal of the feedback signal y(t). In addition, y im (t) represents an imaginary part of the feedback signal y(t) and corresponds to the Q signal of the feedback signal y(t). 
     The subtractor  33  calculates a difference (|x(t)| 2 −|y(t)| 2 ) between the power value |x(t)| 2  input from the power calculator  31  and the power value |y(t)| 2  input from the power calculator  32  and outputs the calculated difference to the multiplier  39 . 
     The conjugate complex output part  34  outputs a conjugate complex signal y*(t) of the feedback signal y(t) to the complex multiplier  35 . 
     The complex multiplier  35  performs a complex multiplication of a distortion compensation coefficient h n-1 (p) and the conjugate complex signal y*(t) to obtain a complex multiplication result rot(t) (=h n-1 (p)y*(t)). 
     The subtractors  36  and  37  calculate an error e(t) between the transmission signal x(t) and the feedback signal y(t) in the Cartesian coordinate system and output the calculated error e(t) to the complex multiplier  38 . That is, the subtractor  36  outputs a difference (x re (t)−y re (t)) between the I signal x re (t) of the transmission signal x(t) and the I signal y re (t) of the feedback signal y(t) as a real part of the error e(t). The subtractor  37  outputs a difference (x im (t)−y im (t)) between the Q signal x im (t) of the transmission signal x(t) and the Q signal y im (t) of the feedback signal y(t) as an imaginary part of the error e(t). 
     The complex multiplier  38  calculates only an imaginary part of a complex multiplication result e(t)rot(t) of the error e(t) output from the subtractors  36  and  37  and the complex multiplication result rot(t) obtained by the complex multiplier  35 , and outputs the calculated imaginary part to the multiplier  40 . The complex multiplication result e(t)rot(t) is an example of the “first complex vector,” and the imaginary part of the complex multiplication result e(t)rot(t) is an example of the “imaginary part of the first complex vector.” The imaginary part of the complex multiplication result e(t)rot(t) is expressed by (x re (t)−y re (t))rot im (t)+(x im (t)−y im (t))rot re (t). Here, rot re (t) is a real part of the complex multiplication result rot(t), and rot im (t) is an imaginary part of the complex multiplication result rot(t). 
     Here, the difference (|x(t)| 2 −|y(t)| 2 ) calculated by the subtractor  33  and the imaginary part of the complex multiplication result e(t)rot(t) calculated by the complex multiplier  38  form the “second complex vector” δh(t).  FIG. 4  is a view provided for explaining the second complex vector of the present embodiment. As illustrated in  FIG. 4 , the “second complex vector” δh(t) is a complex vector of which a real part δh re (t) is the difference (|x(t)| 2 −|y(t)| 2 ) and an imaginary part δh im (t) is the imaginary part of the complex multiplication result e(t)rot(t). The real part δh re (t) of the “second complex vector” δh(t) is irrelevant to a phase difference between the transmission signal x(t) and the feedback signal y(t). For this reason, even when the phase difference between the transmission signal x(t) and the feedback signal y(t) increases, the increase of the “amplitude distortion” in the Cartesian coordinate approach as described above with respect to  FIG. 1  may be avoided. As a result, a lowering of the distortion compensation accuracy may be reduced. 
     Referring back to  FIG. 3 , the multiplier  39  multiplies a step size parameter μ and the difference (|x(t)| 2 −|y(t)| 2 ) (for example, the real part δh re (t) of the “second complex vector” δh(t)) calculated by the subtractor  33 . The multiplier  40  multiplies the step size parameter μ and the imaginary part of the complex multiplication result e(t)rot(t) (for example, the imaginary part δh im (t) of the “second complex vector” δh(t)) calculated by the complex multiplier  38 . 
     The adder  41  adds an output μ(|x(t)| 2 −|y(t)| 2 ) of the multiplier  39  and a real part of the distortion compensation coefficient h n-1 (p) to calculate a real part of a new distortion compensation coefficient h n (p). The adder  42  adds an output μ{(x re (t)−y re (t))rot im (t)+(x im (t)−y im (t))rot re (t)} of the multiplier  40  and an imaginary part of the distortion compensation coefficient h n-1 (p) to calculate an imaginary part of the new distortion compensation coefficient h n (p). Thus, the new distortion compensation coefficient h n (p) is calculated. The coefficient update part  24  updates the distortion compensation coefficient stored in the LUT of the LUT storage part  23  to the new distortion compensation coefficient h n (p). 
     With the above-described configuration, the coefficient update part  24  performs the following calculation process.
 
 h   n ( p )= h   n-1 ( p )+μδ h ( t )
 
δ h ( t )=δ h   re ( t )+ jδh   im ( t )
 
δ h   re ( t )=| x ( t )| 2   −|y ( t )| 2  
 
= x   re ( t ) 2   +x   im ( t ) 2 −( y   re ( t ) 2   +y   im ( t ) 2 )
 
δ h   im ( t )=( x   re ( t )− y   re ( t )) rot   im ( t )+( x   im ( t )− y   im ( t )) rot   re ( t )
 
 e ( t )= x ( t )− y ( t )
 
= x   re ( t )− y   re ( t )+ j ( x   im ( t )− y   im ( t ))
 
 rot ( t )= h   n-1 ( p ) y *( t )
 
 p=|x ( t )| 2  
 
     Here, each of x, y, f, h, u, and e is a complex number, and * is a conjugate complex number. The coefficient update part  24  repeats this calculation process to update the distortion compensation coefficient h n (p) such that the magnitude of the “second complex vector” δh(t) becomes minimal. 
     (Exemplary Operation of Distortion Compensator) 
     An example of the processing operation of the distortion compensator  12  included in the above-configured radio equipment  10  will be described.  FIG. 5  is a flow chart illustrating an exemplary processing operation of the distortion compensator of the present embodiment. 
     As illustrated in  FIG. 5 , the coefficient update part  24  of the distortion compensator  12  acquires a transmission signal input from the baseband unit  11  and a feedback signal input from the ADC  20  (Operation S 101 ). 
     The coefficient update part  24  calculates a “difference” between a power of the transmission signal and a power of the feedback signal (Operation S 102 ). 
     The coefficient update part  24  calculates an “imaginary part of a first complex vector” based on an error between the transmission signal and the feedback signal in the Cartesian coordinate system (Operation S 103 ). 
     The coefficient update part  24  updates a distortion compensation coefficient stored in the LUT of the LUT storage part  23 , by using a “second complex vector” of which a real part is the “difference” and an imaginary part is the “imaginary part of the first complex vector” (Operation S 104 ). Then, the PD part  21  generates a PD signal having the reverse characteristic that cancels the distortion characteristic of the PA  16 , by multiplying an I signal and a Q signal of the transmission signal by a real part and an imaginary part of the distortion compensation coefficient updated by the coefficient update part  24 , respectively, and outputs the generated PD signal to the DAC  13 . 
       FIG. 6  is an example of simulation result.  FIG. 6  represents a simulation result for an example of a temporal variation of the “amplitude distortion” in a PA when a phase difference is present between the transmission signal and the feedback signal. In  FIG. 6 , a curve  51  indicates a temporal variation of the “amplitude distortion” in a PA when a distortion compensator employing a polar coordinate approach is used. A curve  52  indicates a temporal variation of the “amplitude distortion” in the PA  16  when the distortion compensator  12  of the present embodiment is used. A curve  53  indicates a temporal variation of the “amplitude distortion” in a PA when a distortion compensator employing the Cartesian coordinate approach is used. 
     As represented in  FIG. 6 , in the distortion compensator employing the Cartesian coordinate approach, time from the initiation of the simulation until the “amplitude distortion” in the PA converged at 0 was about 3,000 ms. This time did not meet the pre-allowed specification. 
     In contrast, in the distortion compensator  12  of the present embodiment, the time from the initiation of the simulation until the “amplitude distortion” in the PA  16  converged at 0 was about 2,000 ms. This time did meet the pre-allowed specification. For example, in the distortion compensator  12  of the present embodiment, the distortion compensation performance has been improved, as compared to the distortion compensator employing the Cartesian coordinate approach. 
     According to the above-described embodiment, in the distortion compensator  12 , the coefficient update part  24  calculates the “difference” between the power of the transmission signal and the power of the feedback signal. Then, the coefficient update part  24  calculates the “imaginary part of the first complex vector” based on the error between the transmission signal and the feedback signal in the Cartesian coordinate system. Then, the coefficient update part  24  updates the distortion compensation coefficient stored in the LUT of the LUT storage part  23 , by using the “second complex vector” of which a real part is the “difference” and an imaginary part is the “imaginary part of the first complex vector.” 
     With the configuration of the distortion compensator  12 , the distortion compensation coefficient may be updated by using the “second complex vector” of which the real part is irrelevant to the phase difference between the transmission signal and the feedback signal. Therefore, even when the phase difference between the transmission signal and the feedback signal increases, since the distortion compensation coefficient may approach to a proper value, the increase of the “amplitude distortion” in the PA  16  may be avoided. In addition, since the distortion compensator  12  does not perform the operation of converting the Cartesian coordinates into the polar coordinates and the operation of returning the polar coordinates to the Cartesian coordinates, an amount the computation may be reduced, as compared to the distortion compensator employing the “polar coordinate system.” As a result, according to the present embodiment, the distortion compensation performance may be improved while suppressing the increase of the circuit scale. 
     In addition, in the distortion compensator  12 , the coefficient update part  24  updates the distortion compensation coefficient stored in the LUT of the LUT storage part  23  such that the magnitude of the “second complex vector” becomes minimal. 
     With the configuration of the distortion compensator  12 , even when the phase difference between the transmitting signal and the feedback signal increases, since the distortion compensation coefficient may quickly approach to a proper value, the increase of the “amplitude distortion” in the PA  16  may be avoided more stably. 
     OTHER EMBODIMENTS 
     The distortion compensator  12  is implemented with hardware such as a field programmable gate array (FPGA), a large scale integrated (LSI) circuit, or a processor. The baseband unit  11  is also implemented with hardware such as a FPGA, an LSI, or a processor. An example of the processor may be a central processing unit (CPU), a digital signal processor (DSP) or the like. In addition, the DAC  13 , the quadrature modulator  14 , the carrier generator  15 , the PA  16 , the coupler  17 , the antenna  18 , the quadrature demodulator  19 , and the ADC  20  are implemented with hardware such as radio communication modules. In addition, the radio equipment  10  and the distortion compensator  12  may include a memory. For example, the memory stores a table in which distortion compensation coefficients are stored. 
     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 an illustrating 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.