JP2007195056A - Distortion compensation device and distortion compensation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distortion compensation device and a distortion compensation method, capable of performing proper distortion compensation processing, even when the DC offset amount of an A/D converter included in a feedback system fluctuates. <P>SOLUTION: A DC offset calculating part 114 calculates the DC offset amount of A/D converters 138a and 138b, while transmitting a transmission signal. A distortion power calculating part 108 measures the distortion power of a feedback signal, a processing determining part 110 determines the need for updating distortion compensation coefficients, and a compensation coefficient calculating part 116 calculates a distortion compensation coefficients obtained by considering the DC offset amount of the A/D converters 138a and 138b to correct a predistortion characteristic, if the need for updating of the distortion compensation coefficients is determined. A switching control part 112 controls on/off of an orthogonal demodulation part 136, a DC-offset calculating part 114 or the like. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a distortion compensation device and a distortion compensation method.

  In recent years, mobile terminals using a CDMA (Code Division Multiple Access) transmission system have been put into practical use. A CDMA transmission amplifier (amplifier) is required to have a linearity for amplifying a broadband / high dynamic range signal with low distortion and a high efficiency by suppressing the emission of unnecessary radio waves. Conventionally, a negative feedback method, a feed forward method, and a predistorter method are known as methods for compensating for nonlinear distortion of an amplifier.

  The negative feedback method is rarely used in a radio device that handles a wideband signal because an oscillation phenomenon is likely to occur when the signal becomes a wideband and the operation becomes unstable.

  The feedforward method is used in many base station radios at present because it does not cause unstable operation in principle. However, the feed-forward method requires a configuration in which an error component is extracted and amplified separately by a sub-amplifier and then subtracted from the output signal of the main amplifier. There is a problem that decreases.

  On the other hand, the predistorter method is attracting attention as an alternative to the feedforward method because a sub-amplifier is not required, and research and development are underway. Hereinafter, the principle of the nonlinear distortion compensation predistorter method will be described.

  FIG. 13 is a diagram for explaining the amplification result of the transmission signal when there is no distortion compensation. In FIG. 13, I and Q signals (in-phase signal, quadrature signal) are converted into analog signals by D / A converters 500a and 500b, and the quadrature modulation unit 502 performs quadrature phase modulation on each analog signal. Or orthogonal modulation such as OFDM is performed. Then, the carrier wave generated from the carrier wave generation circuit 506 is multiplied by the up converter 504 configured by a mixer, the transmission signal is frequency-converted, amplified by the power amplifier 508, and transmitted via the antenna 510.

  In the figure, as shown on the upper side of the characteristic diagram P1 surrounded by a dotted line, the D / A converters 500a and 500b perform linear conversion with respect to the signal level, and as shown on the lower side of the characteristic diagram P1. In the D / A converters 500a and 500b, phase rotation does not occur.

  Further, as shown in a characteristic diagram P2 surrounded by a dotted line in FIG. 13, the power amplifier 508 includes an AM / AM characteristic (amplitude distortion) and an AM / PM characteristic (amplitude distortion) depending on the input amplitude as shown in the characteristic diagram P2. Non-linearity of input / output characteristics (phase distortion). Therefore, as shown in the characteristic diagram P3, nonlinearity occurs in the output waveform of the power amplifier 508.

  That is, the input / output characteristic of the power amplifier 508 is linear in a region where the input level is low, but when the input level exceeds a certain value, the AM / AM characteristic of the amplifier is saturated and becomes non-linear. Distortion occurs.

  The power amplifier 508 is in a state where the output signal has no phase rotation with respect to a signal having a low input level, but when the input signal exceeds a certain value, phase rotation occurs in the output signal. For this reason, the phase of the output signal of the power amplifier 508 rotates with respect to the input signal.

  FIG. 15 is a diagram for explaining phase distortion. In FIG. 15, a point S1 indicates a point that is actually desired to be transmitted on the phase plane, and a point S2 indicates a point after phase rotation has occurred due to phase distortion. Such phase rotation becomes a modulation error and causes deterioration of modulation / demodulation accuracy.

  FIG. 14 is a diagram for explaining an amplification result when a predistortion compensation unit is provided to compensate for distortion. 14, the same parts as those in FIG. 13 are denoted by the same reference numerals.

  As shown in FIG. 14, a predistortion compensation unit 600 is provided. In the predistortion unit 600, as shown in a characteristic diagram P5 surrounded by a dotted line, a distortion characteristic opposite to the distortion characteristic in the power amplifier 508 is given in advance to each of the I and Q signals.

  As a result, as shown in the characteristic diagram P7, when the I and Q signals are amplified by the power amplifier 508, the non-linear distortion characteristic of the power amplifier 508 is canceled and substantially linear amplification is performed.

  FIG. 16 is a diagram illustrating a frequency spectrum with and without distortion compensation. In FIG. 16, a characteristic line a shows a frequency spectrum when there is no distortion compensation, and a characteristic line b shows a frequency spectrum when there is distortion compensation. If distortion compensation is not performed, power leaks out of the band due to distortion characteristics, but it can be seen that power leaking out of band can be suppressed by performing distortion compensation.

  FIG. 17 is a block diagram illustrating an example of a configuration of a transmitter that performs general predistortion compensation. The transmitter of FIG. 17 is roughly classified into a transmission system, a feedback system, and a distortion compensation system. The transmission system includes delay units 700a and 700b, predistortion compensation unit 702, D / A converters 712a and 712b, quadrature modulation unit 714, an oscillator 724 that outputs a carrier wave, an up converter 716, and a power amplifier 718. And an antenna 722.

  The reception system includes a directional coupler 720, an oscillator 724 that outputs a carrier wave, a down converter 726, an orthogonal demodulator 728, and A / D converters 730a and 730b.

  The distortion compensation system includes a compensation coefficient calculation unit 710 that compares a transmission baseband signal (I and Q signals) with a signal fed back via a feedback system to calculate a compensation coefficient, and an input power. A distortion compensation table memory 706 in which corresponding distortion compensation coefficients are stored, and a power calculation unit 704 (the distortion compensation table 706 is addressed by the output of the power calculation unit, and the distortion compensation coefficient is table-drawn). Is done.

  In the transmitter of FIG. 17, the distortion compensation table mounted in the distortion compensation table memory 706 is created so as to compensate for the nonlinear characteristic of the power amplifier 718. In addition, the compensation coefficient calculation unit 710 calculates a distortion compensation coefficient (a control parameter for predistortion characteristics), and adaptively updates the distortion compensation coefficient stored in the distortion compensation table.

  The configuration of a transmitter that performs predistortion compensation using a table as shown in FIG. 17 is described in Patent Document 1, for example. A method for calculating a distortion compensation coefficient is described in Patent Document 2, for example.

  Furthermore, paying attention to the DC offset of the A / D converter included in the feedback system, the phase correction device that detects the DC offset amount, calculates the phase error caused by the DC offset, and corrects the phase of the transmission signal, It is described in Patent Document 3. In Patent Document 3, the phase correction considering the DC offset amount of the A / D converter included in the feedback system is performed only once at the start of transmission.

International Publication No. 01/008320 Pamphlet JP 9-69733 A JP 2002-319989 A

  In the transmitter that performs predistortion compensation shown in FIG. 17, the deterioration of the distortion compensation characteristic due to the DC offset of the A / D converter included in the feedback system is not considered.

  That is, a DC offset is generated in the A / D converter included in the feedback system. As shown in the lower characteristic diagram P10 in FIG. 17, the compensation coefficient calculation unit 710 originally needs to detect nonlinear distortion as the characteristic X1 and calculate the compensation coefficient so as to compensate for this nonlinear distortion. There is. However, when a DC offset occurs in the A / D converters 730a and 730b, the nonlinear distortion characteristic changes from the characteristic X1 to Y1, and the Y1 (the DC offset of the A / D converters 730a and 730b is superimposed and an error occurs). The compensation coefficient is calculated based on the characteristic of

  Therefore, as shown in the characteristic diagram p11 in the upper left of FIG. 17, in the predistortion compensation unit 702, predistortion like the characteristic X2 is supposed to be applied, but predistortion like Y2 is applied. Become. Then, as shown in the characteristic diagram P12 shown in the upper right of FIG. 17, the distortion is not completely canceled, and the nonlinear distortion of the transmission signal cannot be sufficiently removed.

  In the case of the prior art disclosed in Patent Document 3, although the detection of the DC offset of the feedback A / D converter is mentioned, the phase correction considering the DC offset amount of the A / D converter is mentioned. Is performed only once at the start of transmission. Therefore, since phase correction is not performed at the time of transmission, when the DC offset amount of the A / D converter varies due to changes over time (change over time), ambient temperature, etc., it is appropriate when necessary. There has been a situation in which correction cannot be performed and the accuracy of distortion compensation is reduced. In the case of the technique described in Patent Document 3, only phase correction is performed, and amplitude distortion is not corrected.

  The present invention has been made in view of the above circumstances, and a distortion compensation apparatus and distortion compensation capable of performing appropriate distortion compensation processing even when the DC offset amount of an A / D converter included in a feedback system varies. It aims to provide a method.

  The present invention firstly branches a transmission output signal of the transmission system, a transmission system including a D / A converter that converts an analog transmission digital signal, a modulation unit, and a power amplifier, and an A / D A feedback system including a converter, a predistortion processing unit that performs predistortion processing on the transmission digital signal before the D / A converter based on the transmission digital signal, and a DC offset of the A / D converter A DC offset calculation unit that calculates the amount, a distortion compensation characteristic update unit that updates a distortion compensation characteristic in the predistortion processing unit with reference to the calculated DC offset amount, and the distortion according to a state at the time of transmission A process determination unit that determines the necessity of updating the compensation characteristics, and a signal input to the A / D converter and the distortion compensation while referring to the determination result of the process determination unit. A switching control unit that switches on / off of the characteristic update unit and the DC offset calculation unit, and the switching control unit is only in a period during which the processing determination unit determines that the distortion compensation characteristic needs to be updated. When the distortion compensation characteristic update unit is allowed to be turned on and the distortion compensation characteristic update unit is turned on, the signal input to the A / D converter is turned on and the DC offset calculation unit is turned off. A distortion compensation apparatus is provided.

  With this configuration, the signal input to the A / D converter, the distortion compensation characteristic update unit, and the DC offset calculation unit are turned on / off according to the necessity of updating the distortion compensation characteristic determined based on the transmission state. Therefore, the distortion compensation characteristics can be updated efficiently and accurately in consideration of the DC offset amount during the period in which the transmission signal is transmitted from the transmission system.

  A second aspect of the present invention is the distortion compensation apparatus according to the first aspect, wherein the feedback system is provided in a preceding stage of the A / D converter and demodulates the branched transmission output signal. The switching control unit switches on / off of signal input to the A / D converter by switching on / off of the demodulating unit.

  With this configuration, it is possible to select on / off of the demodulation unit of the feedback system, so that the DC offset amount can be accurately calculated and the power consumption of the distortion compensator can be reduced by turning it off when unnecessary. Can do.

  Thirdly, the distortion compensation apparatus according to the first or second aspect described above, wherein distortion power is calculated as a state at the time of transmission based on an output signal of the A / D converter. A calculation unit; and the processing determination unit compares the distortion power value calculated by the distortion power calculation unit with a predetermined threshold value, and if the distortion is large, the distortion compensation characteristic needs to be updated. Judgment is made and it is judged that it is unnecessary when the distortion is small.

  With this configuration, it is possible to determine the necessity of updating the distortion compensation characteristics by comparing the actually measured distortion power with the threshold value and determining the magnitude of the distortion.

  Fourthly, the present invention is the distortion compensation device according to the third aspect, in which the switching control unit is in a period in which the processing determination unit determines that updating of the distortion compensation characteristic is unnecessary. The signal input to the A / D converter is turned on intermittently at a predetermined interval, and the processing determination unit is configured to output the distortion power while the signal input to the A / D converter is turned on. There is provided a distortion compensation apparatus that performs a comparison between a distortion power value measured by a calculation unit and the predetermined threshold value.

  With this configuration, it is possible to reduce the monitoring burden on each part and to reduce power consumption by determining whether the distortion compensation characteristics need to be updated intermittently instead of constantly. It becomes possible.

  Fifthly, the present invention provides the distortion compensation apparatus according to any one of the first to fourth aspects, further including a temperature detection unit that detects an ambient temperature as the state at the time of transmission, The determination unit monitors the temperature information detected by the temperature detection unit, determines that the distortion compensation characteristic needs to be updated, and determines that it is not necessary when the temperature fluctuation is within an allowable range. An apparatus is provided.

  With this configuration, it is possible to determine the necessity of updating the distortion compensation characteristics by determining the magnitude of the actually measured temperature fluctuation. For example, when a temperature variation exceeding an allowable range occurs and the distortion characteristic changes due to a change in the characteristic of each electronic component, it can be determined that the distortion compensation characteristic needs to be updated.

  Sixthly, the present invention provides the distortion compensation apparatus according to any one of the first to fifth aspects, wherein the switching control unit needs to update the distortion compensation characteristic by the processing determination unit. During the determination, the distortion compensation characteristic updating unit is continuously turned on, and the DC offset calculation unit is continuously turned off.

  With this configuration, when the distortion compensation characteristic needs to be updated, the distortion compensation characteristic update process is always performed, so that the calculation time for updating the distortion compensation characteristic can be shortened.

  Seventhly, the present invention provides the distortion compensation apparatus according to any one of the first to sixth aspects, wherein the predistortion processing unit calculates a distortion compensation coefficient associated with a power level of the transmission digital signal. A distortion compensation table memory to store, and a predistortion unit that performs predistortion processing on the transmission digital signal based on a distortion compensation coefficient output from the distortion compensation table memory, the distortion characteristic update unit, The distortion compensation coefficient stored in the distortion compensation table memory is updated.

  With this configuration, the distortion compensation coefficient can be updated efficiently and accurately in consideration of the DC offset amount in the predistortion processing unit having a configuration in which the distortion compensation coefficient is retrieved from the distortion compensation table memory corresponding to the transmission digital signal. Can do.

  Eighthly, according to the present invention, there is provided a radio communication apparatus equipped with the distortion compensation apparatus according to any one of the first to seventh aspects.

  With this configuration, a high-performance wireless communication apparatus capable of always performing highly accurate distortion compensation regardless of the DC offset of the A / D converter is provided.

  Ninthly, the present invention provides a mobile communication base station apparatus equipped with the distortion compensation apparatus according to any one of the first to seventh aspects.

  With this configuration, high-precision distortion compensation can be performed at all times regardless of the DC offset of the A / D converter, and linearity that amplifies a wideband and wide dynamic range signal with low distortion and emission of unwanted radio waves. It is possible to realize a high-performance mobile communication base station apparatus that can achieve both high transmission efficiency by suppressing the transmission.

  Tenth, the present invention provides a transmission system including a D / A converter that converts a transmission digital signal into an analog signal, a modulation unit, and a power amplifier, a transmission output signal of the transmission system is branched, and an A / D A distortion compensation method for a distortion compensation apparatus, comprising: a feedback system including a converter; and a predistortion processing unit that performs predistortion processing on the transmission digital signal at a preceding stage of the D / A converter based on the transmission digital signal. The step of determining the necessity of updating the distortion compensation characteristic in the predistortion processing unit according to the state at the time of transmission and the DC offset calculating unit calculating the DC offset amount of the A / D converter are calculated. The step of updating the distortion compensation characteristic of the predistortion processing unit by the distortion compensation characteristic updating unit with reference to the obtained DC offset amount. And allowing the distortion compensation characteristic updating unit to be turned on only during a period when the processing determining unit determines that the distortion compensation characteristic needs to be updated, and turning on the distortion compensation characteristic updating unit. In some cases, a distortion compensation method is provided that includes turning on a signal input to the A / D converter and turning off the DC offset calculation unit.

  By this method, the signal input to the A / D converter, the distortion compensation characteristic update unit, and the DC offset calculation unit are turned on / off according to the necessity of updating the distortion compensation characteristic determined based on the state at the time of transmission. Therefore, the distortion compensation characteristics can be updated efficiently and accurately in consideration of the DC offset amount during the period in which the transmission signal is transmitted from the transmission system.

  According to the present invention, it is possible to provide a distortion compensation apparatus and a distortion compensation method capable of performing appropriate distortion compensation processing even when the DC offset amount varies in the A / D converter included in the feedback system.

(First embodiment)
The distortion compensation apparatus according to the present embodiment (and a radio transmission apparatus equipped with the distortion compensation apparatus) detects a signal amplified by a power amplifier as a feedback signal, and uses a compensation coefficient calculation unit to calculate an update coefficient. The apparatus calculates the DC offset amount of the feedback A / D converter in real time during the period when the signal is transmitted from the transmission system, and sets the distortion compensation coefficient so as to reduce the adverse effect due to the DC offset amount. By updating, stable and highly accurate distortion compensation is realized.

  FIG. 1 is a block diagram showing the main configuration of the nonlinear distortion compensator according to the first embodiment of the present invention.

  As shown in the figure, the nonlinear distortion compensation apparatus is roughly divided into a transmission system, a feedback system, a predistortion processing unit, and a distortion compensation characteristic updating unit for updating the distortion compensation characteristic of the predistortion processing unit. .

  The transmission system includes delay units 100a and 100b for timing adjustment, a predistortion processing unit 102 that performs predistortion processing on transmission baseband signals (I and Q signals), and an analog signal obtained by performing predistortion processing on the signal. D / A converters 120a and 120b for converting to D / A, a quadrature modulation unit 122 for performing quadrature modulation on an analog signal obtained by D / A conversion, a mixer (upconverter) 124 for mixing the carrier wave from the carrier wave generation circuit 132, A power amplifier 126 and an antenna 130 are provided.

  The feedback system mixes a carrier wave from the carrier wave generation circuit 132 with a directional coupler 128 for branching the transmission output signal and a signal branched by the directional coupler 128 (hereinafter referred to as a feedback signal). A mixer (down converter) 134, a quadrature demodulator 136 that performs quadrature demodulation processing on the signal after frequency conversion, and A / D converters 138a and 138b that convert analog signals that have passed through the quadrature demodulator 136 into digital signals With.

  The predistortion processing unit includes a power calculation unit 104, a distortion compensation table memory 106, and a predistortion compensation unit 102. The distortion compensation table and memory 106 are addressed by the output of the power calculation unit 104, and the distortion compensation coefficient is looked up in the table. The distortion compensation table mounted in the distortion compensation table memory 106 stores a distortion compensation coefficient for each input power, and the distortion compensation coefficient is created so as to compensate for the nonlinear characteristic of the power amplifier 126 in the transmission system. ing.

  The non-linear distortion compensator of FIG. 1 includes a distortion compensation characteristic updating unit 117 in order to prevent the DC offset amount of the A / D converters 138a and 138b from changing due to secular change and adversely affecting the distortion compensation characteristic. I have. In the previous stage of the distortion compensation characteristic updating unit 117, delay units 118a and 118b for performing timing adjustment on transmission baseband signals (I and Q signals) are provided.

  The distortion compensation characteristic updating unit 117 adaptively calculates a distortion compensation coefficient (a parameter for controlling the distortion compensation characteristic) stored in the distortion compensation table memory 106 and outputs it as an update coefficient. The DC offset calculation unit 114 that intermittently calculates the DC offset amount of the feedback A / D converters 138a and 138b, and the distortion power of the feedback signal are calculated based on the output signals of the A / D converters 138a and 138b. A distortion power calculation unit that performs the determination, and when the calculated distortion power value is equal to or exceeds a predetermined threshold value, a process determination unit 110 that determines that the distortion compensation coefficient needs to be updated, and The orthogonal demodulation unit 136, the DC offset calculation unit 114, and the compensation coefficient calculation unit 116 are turned on while referring to the determination result by the processing determination unit 110. And a switching control unit 112 for controlling the OFF.

  The compensation coefficient calculation unit 116 compares the transmission baseband signal and the feedback signal and adaptively calculates the distortion compensation coefficient so as to cancel the nonlinear distortion. At this time, the DC offset information from the DC offset calculation unit 114 is calculated. When calculating the distortion compensation coefficient, the information on the DC offset amount is taken into consideration.

  In the nonlinear distortion compensator of FIG. 1, whether or not the distortion compensation coefficient stored in the distortion compensation table memory needs to be updated is determined based on the distortion power of the feedback signal. That is, if the distortion power is sufficiently reduced, it can be determined that the pre-distortion is working effectively, so that it is not necessary to update the distortion compensation coefficient. In addition, in the distortion compensation apparatus of FIG. 11, if the ambient temperature does not fluctuate beyond the allowable range, there is no significant effect on the characteristics of the electronic circuit, and thus it is not necessary to update the distortion compensation coefficient.

  Specifically, the process determination unit 110 determines whether the actually measured distortion power value is equal to or greater than a threshold (or exceeds the threshold), and determines the necessity of updating the distortion compensation coefficient.

  In addition, in order to update the distortion compensation coefficient in real time during the transmission period by the transmission system, in order to obtain necessary information accurately and reduce power consumption, the quadrature demodulation unit 136, DC It is important to closely link the offset calculation unit 114 and the compensation coefficient calculation unit 116 and skillfully switch the on / off thereof. This switching is performed by the switching control unit 112.

  That is, when the DC offset amount accompanying the aging and temperature change of the A / D converters 138a and 138b is periodically measured, the feedback signal is input to the A / D converters 138a and 138b. Since the DC offset amount cannot be measured, when the DC offset amount is measured, the feedback quadrature demodulation unit 136 is turned off and the A / D conversions 138a and 138b are set to the no-input state.

  When no feedback signal is input to the A / D converters 138a and 138b, it is natural that the distortion compensation coefficient cannot be calculated. Therefore, the compensation coefficient calculation unit 116 is turned off to reduce power consumption. Note that turning off the orthogonal demodulator 136 also has an effect of reducing power consumption.

  On the contrary, when calculating distortion power or calculating a distortion compensation coefficient, a feedback signal must be input to the A / D converters 138a and 138b. Therefore, the orthogonal demodulation unit 136 is turned on. . On the other hand, since the DC offset amount cannot be accurately measured at this time, the DC offset calculation unit 114 is turned off to reduce power consumption.

  As described above, by appropriately controlling the on / off of each unit, highly accurate measurement of the DC offset amount of the A / D converters 138a and 138b and the reduction of the power consumption of the circuit can be achieved efficiently. be able to.

  Hereinafter, the functions and operations of the main components will be described more specifically. First, the basics of distortion compensation will be specifically described.

  Based on the in-phase component I signal and quadrature component Q signal of the transmission baseband signal, the power calculation unit 104 calculates transmission power. The transmission power is calculated by the following (Equation 1) calculation.

  A distortion compensation coefficient corresponding to the transmission power level calculated by the power calculation unit 104 is subtracted from the distortion compensation table mounted in the distortion compensation table memory 106 and output. The predistortion unit 102 multiplies the transmission signal by the amplitude inverse characteristic and the phase inverse characteristic of the power amplifier 126 by the passed distortion compensation coefficient. Note that the table lookup is the same even if the transmission amplitude that takes the route of the transmission power is used. In this case, an operation like (Equation 2) is performed.

  The distortion compensation table memory 106 stores distortion compensation coefficients that are inverse to the distortion characteristics of the predistortion unit. Further, the distortion compensation coefficient is adaptively updated by the update coefficients ai and bi output from the compensation coefficient calculation unit 116.

  The transmission delay units 100a and 100b are for adjusting the timing of the delay time when calculating the power from the transmission signal and drawing the distortion compensation coefficient in the table. The delay units 118a and 118b in front of the compensation coefficient calculation unit 116 are for delaying the transmission signal by the delay time generated in the analog unit and matching the timing with the feedback signal.

  The D / A converters 120a and 120b convert the predistorted I and Q signals into analog signals, respectively. The I and Q signals are input to the quadrature modulation unit 122. The quadrature modulation unit 122 performs quadrature modulation by multiplying the input analog I and Q signals by a reference carrier wave and a signal that is phase shifted by 90 °, and adding the multiplication results.

  The up-converter 124 mixes the quadrature modulation signal and the local oscillation signal and converts the frequency to a radio frequency. The power amplifier 126 amplifies the frequency conversion signal output from the up-converter 126 and radiates it from the antenna 130 into the air.

  The modulation signal output from the power amplifier 126 is distributed by the directional coupler 128 and used as a feedback signal. The feedback signal is used to calculate a distortion compensation coefficient. The feedback signal is frequency-converted by the down converter 134.

  The quadrature demodulator 136 extracts the in-phase component IF and the quadrature component QF by performing quadrature demodulation, and the A / D converters 138a and 138b return the analog signals to digital signals.

  FIGS. 2A and 2B are diagrams showing the state of the A / D conversion signal when the A / D converter has no DC offset and when there is no DC offset.

  As shown in the figure, the A / D conversion signal has a waveform as shown in FIG. 2A with the center being zero when the A / D converters 138a and 138b have no DC offset, and the DC offset If there is, a waveform including the DC offset amount α is obtained as shown in FIG.

  If the DC offset amount α can be obtained with high accuracy, the distortion compensation table excluding the influence of the DC offset amount α can be created by the compensation coefficient calculation unit 116 at the next stage, and the distortion compensation coefficient can be calculated with high accuracy. it can. The DC offset amount α varies depending on the aging (or temperature) of the characteristics of the A / D converters 138a and 138b. In order to follow this variation, it is necessary to appropriately calculate the DC offset amount α.

  The compensation coefficient calculation unit 116 determines amplitude distortion and phase distortion compensation coefficients so that the error between the transmission signal and the feedback signal is minimized. As a method for calculating the distortion compensation coefficient, there are a least square method (LMS), an exponentially weighted sequential least square method (RLS), and the like.

  The DC offset calculation unit 114 calculates the DC offset amount by performing section averaging of the output signals of the A / D converters 138a and 138b. If the values of the feedback signal after A / D conversion are I-FD and Q-FD, the DC offset amount α_I of the D / A converter on the I signal side and the DC offset amount α_Q of the A / D converter on the Q signal side Is as follows: N is the number of I-FD and Q-FD samplings. t is a sampling time, and I-FD (t) and Q-FD (t) indicate values after A / D conversion at the sampling time t.

  The compensation coefficient calculation unit calculates the distortion compensation coefficient after correcting the α_I and α_Q. In order to obtain the DC offset amount α with high accuracy by the DC offset calculation unit 114, the switching control unit 112 performs on / off control of the orthogonal demodulation unit 136.

  When the quadrature demodulating unit 136 is on, the quadrature demodulated signals are input to the A / D converters 138a and 138b. When the quadrature demodulating unit 136 is off, the A / D converters 138a and 138b are not input.

  Even when quadrature demodulated signals are input to the A / D converters 138a and 138b, it is not impossible to determine the DC offset amount α. However, this is because the input signals have the same occurrence probability of positive and negative. Without it, accurate values cannot be calculated. Further, even if the positive and negative occurrence probabilities are the same rate, a certain degree of section average length is required to calculate an accurate value.

  When the quadrature demodulator 136 is off, the output I_FD (or Q_FD) of the A / D converters 138a and 138b is only the component of the DC offset portion as shown in FIG. Calculation is possible. FIG. 3 is a waveform diagram showing the state of the calculated DC offset when the feedback A / D converter is not input.

  The switching control unit 112 controls on / off of the quadrature demodulation unit 136 so that the quadrature demodulated signal is input to the A / D converters 138a and 138b, and the A / D converters 138a and 138b are not input. Create a section. When the quadrature demodulator 136 is turned on and a feedback signal is input to the A / D converters 138a and 138b, the compensation coefficient calculator 116 calculates a distortion compensation coefficient, and the quadrature demodulator 136 is turned off and the A / D converter 136 is turned off. When the D converters 138a and 138b are in the no-input state, the DC offset calculation unit 114 calculates the DC offset amount α.

  FIG. 4 is a timing diagram showing the on / off control timings of the orthogonal demodulation unit, the compensation coefficient calculation unit, and the DC offset calculation unit according to the on / off control signal output from the switching control unit.

  As shown in the figure, during the period (t2 to t3, t4 to t5, t6 to t7) in which the DC offset calculation unit 114 is on, the quadrature demodulation unit 136 is off and the compensation coefficient calculation unit 116 is off. . On the other hand, during the period (t1 to t2, t3 to t4, t5 to t6) in which the DC offset calculation unit 114 is off, the quadrature demodulation unit 136 is on and the compensation coefficient calculation unit 116 is on.

  In other words, when the distortion power is small, sufficient performance is obtained with the distortion compensation coefficient currently used, so there is no need to update the distortion compensation coefficient (in other words, the power of the quadrature demodulation unit is turned off) It may be anyway). Therefore, the distortion power is measured based on the feedback signal of the transmission signal. When the distortion is large, both the calculation of the distortion compensation coefficient and the calculation of the DC offset amount are performed. When the distortion is small, only the calculation of the DC offset amount is performed. To do.

  The digital I_FD signal and Q_FD signal after A / D conversion are input to a distortion power calculation unit, and distortion power is calculated.

  FIGS. 5A and 5B are diagrams for explaining the distortion power calculation section. FIG. 5A shows a case where the transmission carrier is one carrier. FIG. 5B shows a case where the transmission carrier is two carriers. FIG.

  As can be seen from FIG. 5A, for one carrier, distortion power for y [Hz] is calculated excluding the transmission band x [Hz]. As can be seen from FIG. 5B, in the case of two carriers, distortion power for y [Hz] is calculated excluding the transmission band 2 · x [Hz]. Similarly, for n carriers, distortion power for y [Hz] is calculated excluding the transmission band n · x [Hz]. (X and y can be set arbitrarily)

  FIGS. 6A and 6B are diagrams for explaining a method of calculating distortion power, and FIG. 6A is a diagram for explaining a method of calculating power by extracting only a distortion band by a bandpass filter. FIG. 5B is a diagram for explaining a method of calculating the power of the corresponding band after obtaining the frequency spectrum by FFT (Fast Fourier Transform).

  In FIG. 5A, only the distortion band is extracted by the band pass filters (BPF) 140 a and 140 b, and the distortion power is directly calculated by the distortion power system calculation unit 142. In FIG. 5B, the frequency spectrum is obtained by a complex fast Fourier transformer 144 that performs complex FFT (fast Fourier transform), and the spectrum distortion power calculation unit 146 calculates the power of the corresponding band.

  Further, the distortion power is measured periodically at a certain timing. The processing determination unit 110 in FIG. 1 compares the measured distortion power with a threshold value, and determines that the distortion characteristic has deteriorated when the distortion power exceeds the threshold value (or exceeds the threshold value), and the switching control unit. 112 is notified. The switching control unit 112 starts calculating the distortion compensation coefficient. The distortion compensation coefficient is updated, and when the process determining unit 110 determines that the distortion power is equal to or lower than the threshold value, the switching control unit 112 is notified, thereby stopping the calculation of the distortion compensation coefficient. .

  FIG. 7 is a timing diagram showing an example of the periodic measurement timing of distortion power and on / off timings of the orthogonal demodulation unit, the compensation coefficient calculation unit, and the DC offset calculation unit.

  In FIG. 7, t1 to t7 are distortion power observation (actual measurement) timings. Observation (actual measurement) is started at time t1. Degradation of distortion characteristics is observed at t3. From t3 to t6, the distortion compensation coefficient is updated. During this correction period, it is necessary to perform both the DC offset amount calculation operation and the distortion compensation coefficient calculation process, so that the compensation coefficient calculation unit 116 and the DC offset calculation unit 114 are alternately turned on. At t6, the update of the distortion compensation coefficient ends.

  In the period during which only the observation is performed (monitoring mode period: t1 to t3), it is not necessary to calculate the distortion compensation coefficient, and it is sufficient if the distortion power measurement can be performed. Minimize the on period of (ie, make the off period as long as possible). In the distortion compensation coefficient update period (t3 to t6), since it is necessary to calculate the distortion compensation coefficient, the period during which the demodulation unit is turned on is lengthened.

  FIG. 8 is a timing diagram showing another example of distortion power periodic measurement timing and on / off timings of the quadrature demodulation unit, the compensation coefficient calculation unit, and the DC offset calculation unit.

  As shown in FIG. 7, in principle, the compensation coefficient calculation unit 116 and the DC offset calculation unit 114 are alternately turned on. However, the average value of the DC offset amounts detected before the correction is started. In addition, since the distortion compensation coefficient can be updated with high accuracy even using the value of the DC offset amount measured immediately before, the DC offset calculation operation during the correction period may be completely stopped. Therefore, in the case of FIG. 8, the DC offset calculation operation is completely stopped in the correction period (t3 to t6).

  FIG. 9 is a block diagram illustrating configurations of the distortion compensation table memory and the predistortion unit. Using the transmission power value P output from the power calculation unit 104 as an address variable, the distortion compensation table memory 106 is addressed, distortion compensation coefficients ai and bi are output from the table, and the compensation coefficients are supplied to the predistortion unit 102. .

  The predistortion compensation unit 102 includes a multiplier and an adder, and calculates a distortion-compensated phase signal (I ′, Q ′) by the following calculation (Equation 4).

  1 determines the amplitude distortion and phase distortion compensation coefficients so that the error between the transmission signal and the feedback signal is minimized. Here, the embodiment will be described based on the least square method. A signal to be transmitted (a signal without distortion) is represented by the following (formula 5).

  The feedback signal is represented by the following (formula 6).

  At this time, considering the DC offset amount, Z (P) becomes Z ′ (P) as shown in (Expression 7) below.

  Here, when an error between a signal to be transmitted and a signal considering the DC offset amount of the feedback signal is e (P), the error is expressed by the following (Equation 8). Here, P is the power of the transmission signal.

  The compensation coefficient calculation unit 116 determines the distortion compensation coefficients ai and bi so that the error e (P) is minimized (see Equation 9 below).

  Note that hn (P) is a distortion compensation coefficient that is newly updated this time, and hn−1 (P) is the value of the previously calculated compensation coefficient stored in the memory. u (P) is an output value obtained by amplifying the input when no compensation coefficient is applied (distortion compensation is not performed and the distortion compensation coefficient is 1), and is expressed as (Equation 10) (* is a conjugate complex number). To express). Y (P) is an output signal that is distorted by the predistortion unit (Formula 11). Μ is an LMS step size parameter, and an appropriate value must be set in order for h (P) to converge.

  The distortion compensation coefficient at a certain input power P is h (P) = ai + j · bi, and the real part and imaginary part of h (P) are ai and bi, respectively. This value is written into the distortion compensation table memory. Ai and bi are expressed by Equation 12. Note that real (h (p)) and imag (h (p)) represent the real part and the imaginary part of h (p), respectively.

  This calculation is performed at the timing when the distortion compensation coefficient calculation is turned on, and the distortion compensation table memory is updated.

(Second Embodiment)
FIG. 10 is a block diagram showing a configuration of the nonlinear distortion compensator according to the second embodiment of the present invention (an example in which an analog switch is used for setting the A / D converter in a no-input state). 10, the same parts as those in FIG. 1 are denoted by common reference numerals.

  The basic configuration of the nonlinear distortion compensator of this embodiment is the same as that shown in FIG. However, in this embodiment, analog switches 150a and 150b are provided in order to put the A / D converters 138a and 138b in the no-input state (terminating circuits 152a and 152b are connected to one end of each switch) The on / off of the switch is controlled by an on / off control signal output from the switching control unit 112.

(Third embodiment)
FIG. 11 is a block diagram showing a configuration of a nonlinear distortion compensator according to the third embodiment of the present invention (configuration for measuring the ambient temperature and determining the necessity of updating the distortion compensation coefficient). In FIG. 11, the same parts as those in FIG.

  The basic configuration of the nonlinear distortion compensator of this embodiment is the same as that shown in FIG. However, in the nonlinear distortion compensator of FIG. 11, the ambient temperature is actually measured by the temperature sensor 160, the temperature information is acquired by the temperature measurement unit 162, and the temperature information is given to the processing determination unit 110. The process determining unit 110 determines the necessity of updating the distortion compensation coefficient depending on whether or not the ambient temperature has fluctuated beyond the allowable range.

  That is, when a sudden temperature change occurs, the characteristics of the electronic circuit change, and the DC offset amounts of the A / D converters 138a and 138b are expected to change rapidly. Therefore, the necessity of updating the distortion compensation coefficient is determined by monitoring a sudden change in the ambient temperature. For example, when a temperature variation exceeding an allowable range occurs and the distortion characteristic changes due to a change in the characteristic of each electronic component, it can be determined that the distortion compensation characteristic needs to be updated.

  Specifically, when the ambient temperature is periodically measured, the processing determination unit 110 when the current measurement temperature is higher than the previous measurement temperature by a predetermined temperature (x degrees). Determines that a temperature fluctuation exceeding the allowable range has occurred. As a result, the update of the distortion compensation coefficient is started.

  FIG. 12 is a timing diagram showing periodic measurement timings of the ambient temperature and on / off timings of the orthogonal demodulation unit, the compensation coefficient calculation unit, and the DC offset calculation unit.

  In FIG. 12, t1, t2, t3, t5, and t6 are the actual measurement timings of the ambient temperature. At t3, the distortion compensation coefficient is updated, and the correction ends at t4.

  The end timing of the distortion compensation coefficient update is, for example, a timing at which the compensation coefficient calculation unit 116 updates the distortion compensation coefficient a predetermined number of times (here, y is a natural number of 2 or more). This is because when the distortion compensation coefficient is updated y times, it can be determined that an appropriate distortion compensation characteristic considering the DC offset amount of the A / D converters 138a and 138b is realized. In the update period from t3 to t4, the calculation of the distortion compensation coefficient and the calculation of the DC offset are alternately performed as in the above-described embodiment.

  As described above, according to the embodiment of the present invention, the DC offset of the feedback system is detected in real time in the period in which the transmission signal is transmitted from the transmission system, the DC offset amount is referred to, and the DC By updating the distortion compensation characteristics of the predistortion processing unit so as to reduce the influence of the offset, a DC offset occurs in the A / D converter due to aging (change over time) and a sudden change in ambient temperature. Even in such a case, the adverse effect of the DC offset can be reduced, and highly accurate nonlinear distortion compensation (predistortion processing) can always be stably performed.

  In addition, according to the embodiment of the present invention, it is possible to achieve both highly accurate distortion compensation coefficient updating and circuit power consumption reduction by skillfully switching on / off of each unit.

  In addition, since the wireless communication apparatus includes the distortion compensation apparatus according to the embodiment of the present invention, high-performance wireless that can always perform highly accurate distortion compensation regardless of the DC offset amount of the A / D converter. A communication device can be provided.

  In addition, since the mobile communication base station apparatus includes the distortion compensation apparatus according to the embodiment of the present invention, the distortion compensation apparatus can always perform highly accurate distortion compensation regardless of the DC offset amount of the A / D converter. A high-performance mobile communication base station device that can achieve both linearity for amplifying signals with a wide dynamic range with low distortion and high transmission efficiency by suppressing the emission of unnecessary radio waves. Can be realized.

  In the first to third embodiments, the predistortion processing unit stores the power level of the transmission digital signal and the distortion compensation coefficient in association with each other, and calculates the distortion compensation coefficient according to the power level of the transmission digital signal. A case has been described in which a distortion compensation table memory that outputs to a previous distortion compensation unit is provided and the compensation coefficient calculation unit updates the distortion compensation coefficient in the distortion compensation table memory as a method for updating distortion compensation characteristics in the predistortion processing unit. However, the present invention is not limited to this configuration, and other configurations can be applied. For example, the predistortion processing unit includes a calculation unit that calculates a distortion compensation coefficient from the power level of the transmission digital signal using a polynomial instead of the distortion compensation table memory, and the distortion compensation characteristic update unit includes a distortion compensation coefficient calculation unit. Instead of the above, a polynomial update unit that updates the distortion compensation characteristic by adjusting the coefficient of the polynomial used in the calculation unit may be provided.

  The distortion compensation apparatus and the distortion compensation method of the present invention have an effect that an appropriate distortion compensation process can be performed even when the DC offset amount of the A / D converter included in the feedback system fluctuates. This is useful for a communication radio base station apparatus or the like.

The block diagram which shows the main structures of the nonlinear distortion compensation apparatus which concerns on the 1st Embodiment of this invention. The figure which shows the mode of an A / D conversion signal Waveform diagram showing the state of the calculated DC offset when the feedback A / D converter is not input Timing chart showing timing of on / off control by switching control unit Diagram for explaining the distortion power calculation section The figure for demonstrating the method of calculating distortion electric power Timing chart showing an example of periodic measurement timing of distortion power and on / off timing of a quadrature demodulator, a compensation coefficient calculator, and a DC offset calculator Timing chart showing other examples of periodic measurement timing of distortion power and on / off timing of quadrature demodulation unit, compensation coefficient calculation unit, and DC offset calculation unit Block diagram showing configurations of distortion compensation table memory and predistortion unit The block diagram which shows the main structures of the nonlinear distortion compensation apparatus which concerns on the 2nd Embodiment of this invention. The block diagram which shows the main structures of the nonlinear distortion compensation apparatus which concerns on the 3rd Embodiment of this invention. Timing chart showing periodic measurement timing of ambient temperature and on / off timing of quadrature demodulation unit, compensation coefficient calculation unit, and DC offset calculation unit The figure for demonstrating the amplification result of the transmission signal when there is no distortion compensation The figure for explaining the amplification result when the predistortion part is provided and the distortion is compensated Diagram for explaining phase distortion Diagram showing frequency spectrum with and without distortion compensation Block diagram showing an example of the configuration of a transmitter that performs general predistortion compensation

Explanation of symbols

100a, 100b, 700a, 700b Delay devices 102, 600, 702 Predistortion compensation unit 104, 704 Power calculation unit 106, 706 Distortion compensation table memory 108 Distortion power calculation unit 110 Processing judgment unit 112 Switching control unit 114 DC offset calculation unit 116 , 710 Compensation coefficient calculation unit 117 Distortion compensation characteristic update unit 118a, 118b, 708a, 708b Delay device 120a, 120b, 500a, 500b, 712a, 712b D / A converter 122, 502, 714 Quadrature modulation unit 124, 504, 716 Up converter 126, 508, 718 Power amplifier 128, 720 Directional coupler 130, 510, 722 Antenna 132, 506, 724 Carrier wave generation circuit 134, 726 Down converter 136, 728 Quadrature demodulator 138a , 138b, 730a, 730b A / D converters 140a, 140b Band pass filter 142 Distortion power calculation unit 144 Complex fast Fourier transformer 146 Distortion power calculation unit 150a, 150b Analog switch 152a, 152b Termination circuit 160 Temperature sensor 162 Temperature measurement unit

Claims (10)

A transmission system including a D / A converter that converts a transmission digital signal into analog, a modulator, and a power amplifier;
A branching of the transmission output signal of the transmission system, and a feedback system including an A / D converter;
A predistortion processing unit that performs predistortion processing on the transmission digital signal at a preceding stage of the D / A converter based on the transmission digital signal;
A DC offset calculator for calculating a DC offset amount of the A / D converter;
A distortion compensation characteristic updating unit that updates the distortion compensation characteristic in the predistortion processing unit with reference to the calculated DC offset amount;
A process determining unit that determines the necessity of updating the distortion compensation characteristic according to a state at the time of transmission;
With reference to the determination result of the processing determination unit, a signal input to the A / D converter, the distortion compensation characteristic update unit, and a switching control unit that switches on / off the DC offset calculation unit,
The switching control unit permits the distortion compensation characteristic update unit to be in an on state only during a period when the process determination unit determines that the distortion compensation characteristic needs to be updated. A distortion compensation apparatus that turns on a signal input to the A / D converter and turns off the DC offset calculation unit when the A / D converter is turned on. The distortion compensation device according to claim 1,
The feedback system includes a demodulator that is provided in a preceding stage of the A / D converter and demodulates the branched transmission output signal.
The distortion control device, wherein the switching control unit switches on / off of signal input to the A / D converter by switching on / off of the demodulating unit. The distortion compensation device according to claim 1 or 2,
A distortion power calculator that calculates distortion power as the transmission state based on the output signal of the A / D converter;
The processing determination unit compares the distortion power value calculated by the distortion power calculation unit with a predetermined threshold value, determines that the distortion compensation characteristic needs to be updated when the distortion is large, and the distortion is A distortion compensator that determines that it is unnecessary when it is small. The distortion compensation apparatus according to claim 3, wherein
The switching control unit intermittently turns on the signal input to the A / D converter at a predetermined interval during a period in which the processing determination unit determines that the distortion compensation characteristics need not be updated. ,
The process determination unit compares the distortion power value measured by the distortion power calculation unit with the predetermined threshold value while the signal input to the A / D converter is on. Distortion compensation device. The distortion compensation device according to any one of claims 1 to 4, wherein
As a state at the time of transmission, further comprising a temperature detection unit for detecting the ambient temperature,
The process determination unit monitors temperature information detected by the temperature detection unit, determines that the distortion compensation characteristic needs to be updated when a temperature variation exceeding an allowable range occurs, and the temperature variation is allowed. A distortion compensator that determines that it is unnecessary when it is within the range. The distortion compensation device according to any one of claims 1 to 5,
The switching control unit continuously turns on the distortion compensation characteristic update unit and continuously turns on the DC offset calculation unit while the process determination unit determines that the distortion compensation characteristic needs to be updated. Distortion compensation device to turn off. The distortion compensation apparatus according to any one of claims 1 to 6,
The predistortion processing unit includes a distortion compensation table memory storing a distortion compensation coefficient associated with a power level of the transmission digital signal, and the transmission digital based on the distortion compensation coefficient output from the distortion compensation table memory. A predistortion unit that performs predistortion processing on the signal,
The distortion characteristic updating unit updates a distortion compensation coefficient stored in the distortion compensation table memory.   A wireless communication apparatus equipped with the distortion compensation apparatus according to claim 1.   A base station apparatus for mobile communication equipped with the distortion compensation apparatus according to claim 1. A transmission system including a D / A converter that converts a transmission digital signal into analog, a modulation unit, and a power amplifier; a transmission system that branches the transmission output signal of the transmission system; and a feedback system that includes an A / D converter; A distortion compensation method for a distortion compensator, comprising: a predistortion processing unit that performs predistortion processing on the transmission digital signal before the D / A converter based on the transmission digital signal,
Determining the necessity of updating distortion compensation characteristics in the predistortion processing unit according to the state at the time of transmission;
A step of updating the distortion compensation characteristic in the predistortion processing unit by a distortion compensation characteristic updating unit with reference to the DC offset amount calculated by a DC offset calculation unit that calculates a DC offset amount of the A / D converter. When,
Allowing the distortion compensation characteristic update unit to be in an on state only during a period when the process determination unit determines that the distortion compensation characteristic needs to be updated, and when turning on the distortion compensation characteristic update unit, Turning on the signal input to the A / D converter and turning off the DC offset calculation unit.

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