JP2006279775A - Distortion compensation apparatus and distortion correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To decrease miscalculation of a distortion compensation factor in a wireless communication apparatus and to dispense with adjustment of an analog section of a wireless communication section. <P>SOLUTION: A distortion compensation apparatus compensates distortion causable in a transmission signal depending on a distortion characteristic of an amplifier 21. The distortion compensation apparatus is provided with: a feedback side filter 31 that receives a feedback signal of the transmission signal and passes only a signal with a prescribed frequency band of the feedback signal; and an ACP observation evaluation section 33 that detects leakage power level at the outside of the transmission band on the basis of the feedback signal and changes the frequency band of the feedback side filter 31 in accordance with the leakage power level. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a distortion compensation device and a distortion compensation method used for a power amplifier. In particular, the present invention relates to a distortion compensation apparatus and a distortion compensation method for performing linear amplification by performing predistortion (predistortion) processing.

  In recent years, transmission systems such as CDMA (Code Division Multiple Access) have been put into practical use in the mobile communication field. A CDMA transmission amplifier is required to have high linearity for amplifying a broadband / high dynamic range signal with low distortion and high efficiency by suppressing unnecessary radio wave emission (radio wave emission outside the transmission band).

  Conventionally, as a method for compensating for nonlinear distortion of an amplifier, a negative feedback method, a feedforward method, a predistortion method, and the like are known.

  The negative feedback system is rarely used in a radio device of a base station because an oscillation phenomenon or the like is likely to occur when a signal becomes a wide band and operation instability increases.

  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 predistortion method is attracting attention as an alternative to the feedforward method because a sub-amplifier is not required, and research and development are underway. The principle of the predistortion method is shown below.

  FIG. 12 shows the amplification result when there is no distortion compensation. The amplifier has non-linear characteristics such as AM / AM characteristics (amplitude distortion characteristics) and AM / PM characteristics (phase distortion characteristics) depending on the amplitude level of the input signal. Due to this non-linear characteristic, the output waveform of the amplifier is distorted.

  The AM / AM characteristic of the amplifier is non-linear in which the output signal level is saturated when the amplitude level of the input signal exceeds a certain value. When a linear input signal is amplified by this amplifier, the antenna output signal becomes nonlinear.

  The AM / PM characteristics of the amplifier are such that when the amplitude level of the input signal exceeds a certain value, the phase of the output signal rotates. When an input signal with no phase rotation is amplified by this amplifier, the antenna output signal is rotated in phase (see FIG. 14).

  FIG. 13 shows an amplification result when a predistortion compensation unit is provided to compensate for distortion. The distortion characteristic opposite to the distortion characteristic in the amplifier is applied in advance by the predistortion unit. When this signal is input to an amplifier, distortion characteristics are canceled and linear amplification is performed.

  FIG. 15 shows the frequency spectrum with and without distortion compensation. If distortion compensation is not performed, a large amount of power leaks outside the transmission band due to distortion characteristics, but power leaking outside the transmission band can be suppressed by performing distortion compensation.

JP 2001-203539 A

  In the prior art, in order to dynamically adapt to the nonlinear characteristics of the amplifier, the amplifier output signal is fed back to calculate the distortion compensation coefficient. In addition, by detecting distortion of the amplifier output signal or detecting leakage outside the transmission band, a coefficient calculation element is generated and a table is created. However, if the frequency band of the feedback signal is not taken into account, the distortion may not be calculated correctly and the convergence efficiency may be poor. For example, in the case of aliasing (folding phenomenon) centering on an integer multiple of the Nyquist frequency at the time of digital-analog conversion and analog-digital conversion, an erroneous calculation may be caused. Under such conditions, the steps of amplifier adjustment and circuit adjustment within the range in which coefficient calculation is stably performed are required, and the production efficiency, which is the merit of the adaptive distortion compensator, is reduced.

  The present invention provides a distortion compensation apparatus and a distortion compensation method provided with a feedback-side digital filter (hereinafter also referred to as “filter”) capable of adaptive operation against power leakage outside the transmission band.

  A distortion compensation apparatus according to the present invention is a distortion compensation apparatus that compensates for distortion that may occur in a transmission signal in accordance with distortion characteristics of an amplifier in a wireless communication apparatus. The distortion compensation apparatus inputs a feedback signal of the transmission signal and determines a predetermined value of the feedback signal. And a leakage power evaluation unit that detects a leakage power level outside the transmission band based on the feedback signal and changes the frequency band of the filter according to the leakage power level. Prepare.

  According to the above-described configuration, the filter, particularly the feedback-side filter, can perform an adaptive operation against power leakage outside the transmission band. As a result, it is possible to reduce erroneous calculation of the distortion compensation coefficient in the wireless communication apparatus, and it is not necessary to adjust the analog part of the wireless communication unit, thereby improving the convergence efficiency of the distortion compensation coefficient.

  In particular, it is possible to configure the distortion compensation device so that the leakage power evaluation unit widens the frequency band as the leakage power level is smaller.

  Further, the distortion compensation device can be configured such that the leakage power evaluation unit scores the leakage level by comparing the power level of the transmission band of the feedback signal with the leakage power level. Here, the leakage power evaluation unit is configured to calculate a ratio between the power level of the transmission band of the feedback signal and the leakage power level, and compare the ratio with a predetermined threshold value to calculate an evaluation score. Also good. The filter may be configured to widen the frequency band as the evaluation score increases.

  A wireless communication apparatus can be configured by combining the above distortion compensation apparatus with an amplifier. The wireless communication device can be used as a wireless communication base station device or a wireless communication device.

  The distortion compensation method of the present invention is a distortion compensation method for compensating for distortion that may occur in a transmission signal in accordance with the distortion characteristics of an amplifier in a wireless communication apparatus, and receives the feedback signal of the transmission signal, and receives the feedback signal. Passing only a signal in a predetermined frequency band, detecting a leakage power level outside a transmission band based on the feedback signal, and changing the frequency band of the filter according to the leakage power level; Is provided. A program for causing an arithmetic unit to execute each step of the distortion compensation method is also included in the present invention.

  According to the present invention, it is possible to reduce the erroneous calculation of the distortion compensation coefficient in the wireless communication apparatus, reduce the adjustment process of the analog unit of the wireless communication unit, and improve the convergence efficiency of the distortion compensation coefficient.

  The present invention relates to a distortion compensation apparatus using distortion compensation by an adaptive predistortion method and a wireless communication apparatus using the distortion compensation apparatus. In the predistortion method, when noise different from distortion enters the feedback signal, the distortion compensation coefficient may be calculated erroneously. If distortion compensation is performed using an incorrect distortion compensation coefficient, the amplifier may be destroyed. The causes of such noise include 1) stationary noise of the device, and 2) noise generated by nonlinear elements other than the amplifier.

The stationary noise of the apparatus includes aliasing noise that can be caused by signal conversion between digital and analog, for example. FIG. 1 is a conceptual diagram showing a folding phenomenon that may occur when the D / A (digital / analog) conversion and the A / D (analog / digital) conversion are performed in one apparatus and the sampling frequencies of the two conversions are different. Is shown. f _sf is a sampling frequency on the D / A side (forward side), and f _sr is a sampling frequency on the A / D side (feedback side). Since the Nyquist frequency is different, aliasing indicated by a dotted line portion occurs, and aliasing noise in which the aliasing signal interferes with one signal is generated.

  In general, aliasing noise is removed by changing the sampling frequency of conversion or using an analog filter. Regardless of which method is used, it is necessary to adjust the characteristics of the analog unit, and the production efficiency, which is the merit of the adaptive system, is impaired.

  Further, as a noise factor caused by nonlinear elements other than the amplifier, there is a characteristic difference between the up converter and the down converter due to fluctuation, for example. The fluctuation mainly depends on the temperature, and when a temperature difference occurs between the up-converter and the down-converter, the feedback signal is distorted and observed. This means that a distortion that does not exist in the forward path is observed, so that a component different from the original distortion component is also observed.

  Therefore, in the distortion compensation apparatus of the present invention, a portion for observing and evaluating out-of-band leakage is provided, and filtering according to the evaluation is performed to remove noise that is not originally observed, and at the time of calculating the compensation coefficient. It is intended to obtain distortion compensation performance that prevents erroneous calculation and improves convergence efficiency.

  FIG. 2 is a block diagram showing the configuration of the wireless communication device (power amplification device) 100 of the present invention. The wireless communication device 100 includes a distortion compensator 11, a transmission filter 13, a D / A converter 15, an up converter 17, an RF frequency oscillator 19, an amplifier 21, an antenna 23, an attenuator 25, An up-converter 27, an A / D converter 29, a feedback filter 31, an ACP (Adjacent Channel Power) observation evaluation unit 33, a distortion calculation unit 35, and a delay unit 37 are provided.

  In the wireless communication device 100, the distortion compensation unit 11, the feedback filter 31, the ACP observation evaluation unit (leakage power detection evaluation unit) 33, and the distortion calculation unit 35 constitute the distortion compensation device of the present invention. To do. Of course, other members such as the transmission-side filter 13 may be grasped as a distortion compensation device. The distortion compensation apparatus is an apparatus that compensates for distortion that may occur in a transmission signal by applying a distortion characteristic opposite to the distortion characteristic to the transmission signal as a predistortion according to the distortion characteristic of the amplifier in the wireless communication apparatus 100.

  The distortion compensator 11 compares the transmission signal with the feedback signal using a lookup table method using a distortion compensation coefficient corresponding to the power value of the transmission signal, a polynomial method expressing the compensation coefficient with a polynomial, or the like, and the difference The distortion compensation coefficient is calculated from In particular, the type and configuration are not limited, and existing ones can be used.

  The transmission-side filter 13 limits the band of the output signal from the distortion compensation unit 11 using FIR (Finite Impulse Response). The digital output from the transmission filter 13 is converted into an analog signal by the D / A converter 15.

  The up-converter 17 mixes this analog signal and a local oscillation signal composed of a carrier wave having a frequency as a transmission wave generated by the RF frequency oscillator 19 and performs frequency conversion to a radio frequency (transmission frequency).

  The amplifier 21 amplifies the power of the frequency conversion signal output from the up converter 17. The amplified signal is distributed into two signals by a directional coupler (or distributor) (not shown), and one signal (forward side) is emitted as a radio wave by the antenna 23.

  The signal distributed without being emitted from the antenna 23 by the directional coupler is used as a feedback signal. The level of the feedback signal is lowered by the attenuator 25 and then input to the down converter 27. The down converter 27 down-converts an input feedback signal (of radio frequency) to an intermediate frequency band or a base band. The A / D converter 29 returns the analog signal to a digital signal.

  The feedback filter 31 performs signal band limitation using FIR, similarly to the transmission filter 13. However, the feedback signal filter 31 is different in that its limited band is variable depending on the score of the ACP observation evaluation unit 33.

  The ACP observation evaluation unit 33 obtains a frequency spectrum as shown in FIG. 3 by performing Fourier transform on the feedback signal from the A / D converter 29. The process will be described later.

  The distortion calculator 35 determines the amplitude distortion compensation coefficient 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. Details of the method are introduced in JP-A-9-69733, International Publication 00/74232 and the like.

  When the delay unit 37 calculates power from the above-described input transmission signal and calculates the distortion compensation coefficient by the distortion calculation unit 35, the delay unit 37 delays the transmission signal from the transmission source by a delay time, and converts the original transmission signal and the feedback signal. The timing is adjusted.

  Next, operations of the ACP observation evaluation unit 33 and the feedback filter 31 will be described.

The ACP observation evaluation unit 33 obtains a graph indicating the power level of the feedback signal with respect to the signal frequency shown in FIG. 3, the transmission band is represented by f _bw, leakage power measurement zone is represented by f _a. Then, the average power value calculated in the transmission band _{f bw} and _{P c,} if the average power value calculated in the leakage power measurement band _{f a} was _{P a,} band leakage levels ACP _is, P a / _{P c} It is represented by

Next, it is determined and scored between which threshold value of the threshold value 1 (Th ₁ ) to the threshold value N (Th _N ) that the obtained ACP is determined in advance. For example, if there is an ACP between the threshold 1 and the threshold 2, the ACP score (ACP evaluation score) is set to 1. When there is an ACP between the threshold N-1 and the threshold N, the ACP evaluation score is set to N-1. If ACP is greater than N, the ACP evaluation score is set to N. The ACP observation evaluation unit 33 gives this ACP evaluation score to the feedback side filter 31.

As shown in FIG. 4, the feedback filter 31 changes the pass frequency band of the filter according to the received ACP evaluation score. That is, the FIR coefficient is set. In the example of FIG. 4, the filter change characteristic is such that the lower the ACP evaluation score, the lower the cutoff frequency (the pass frequency band becomes narrower), and the higher the ACP evaluation score, the higher the cutoff frequency (the wider the pass frequency band). Is set. Here, P _a / P _c is calculated in units of decibels.

  FIG. 5 shows a flowchart of ACP observation evaluation and filter setting processing by the ACP observation evaluation unit 33 and the feedback side filter 31.

  First, when the ACP observation evaluation unit 33 receives a feedback signal from the A / D converter 29, the ACP observation evaluation unit 33 uses the Fourier transform (discrete Fourier transform, fast Fourier transform, etc.) on the signal to obtain the frequency spectrum shown in FIG. , ACP is calculated (step S11).

  Then, the ACP observation evaluation unit 33 compares the above-mentioned ACP with thresholds 1 to N prepared in advance and scores them (calculation of ACP evaluation score; step S12).

  Based on the above ACP evaluation score, the filter coefficient of the pass frequency band is set in the feedback filter 31 as shown in FIG. 4 (step S13).

  Finally, the feedback filter 31 performs a feedback signal filtering process (step S14).

  FIG. 6 shows the relationship between the observed power level and the filter characteristics when the ACP evaluation score is calculated as N (maximum value) in the above embodiment. The power level outside the transmission band is low, and the pass frequency bandwidth of the filter is large. FIG. 7 shows the relationship between the observed power level and the filter characteristics when the ACP evaluation score is calculated as 2. The power level outside the transmission band is higher than in the case of FIG. 6, and the pass frequency bandwidth of the filter is narrower than in the case of FIG. Further, FIG. 8 shows the relationship between the observed power level and the filter characteristics when the ACP evaluation score is calculated as 1. The power level outside the transmission band is even higher than in the case of FIG. 7, and the pass frequency bandwidth of the filter is even narrower than in the case of FIG.

  As described above, the present embodiment employs a configuration in which the level of leakage power outside the transmission band of the feedback signal is observed and the filter characteristic on the feedback side is changed according to the level. Under this configuration, particularly when the level outside the transmission band is large due to noise other than the distortion component, the noise can be removed and excluded from consideration when calculating the distortion compensation coefficient. Even when a distortion component is included in the component outside the transmission band, there is no problem because compensation is performed step by step from the distortion component in the filter band. Accordingly, it is possible to reduce the erroneous calculation of the distortion compensation coefficient and improve the coefficient convergence efficiency.

FIG. 9 shows a modification in which the leakage power measurement band is changed to be observed in N sections in the ACP observation. In this example, the leakage power measurement band f _a is divided into N sections of f _a1 , f _a2 ,..., F _aN , and average powers P _a1 , P _{a2 ,.}

In the above-described embodiment, threshold values 1 to N are prepared. In this example, only one threshold value Th is prepared. And the largest section number _kmax ( _kmax = 1,2, ..., N) is _{calculated |} required among the sections where _Pak / _Pc exceeded Th. In this example, the following formula can be considered as a formula for calculating the evaluation score in step S12 in FIG. 5, but there is no particular limitation. The evaluation score is determined by the calculation formula: (N−k _max +1).

  FIG. 10 shows the relationship between the observed power level and the filter characteristics when the ACP evaluation score is calculated as N-1 near the maximum value in the above-described modification. The power level outside the transmission band is low, and the pass frequency bandwidth of the filter is large. FIG. 11 shows the relationship between the observed power level and the filter characteristics when the ACP evaluation score is calculated as 1. The power level outside the transmission band is higher than in the case of FIG. 10, and the pass frequency bandwidth of the filter is narrower than in the case of FIG.

  The wireless communication device and the power amplifying device of the above-described embodiment can be used as a wireless communication base station device. The radio communication base station apparatus is installed in, for example, a mobile phone base station. The wireless communication device can also be used as a wireless mobile communication device. An example of the wireless mobile communication device is a portable wireless device represented by a mobile phone.

  The distortion compensation method of the present invention can be executed by a program that causes an arithmetic processing device such as a CPU or DSP incorporated inside or outside the wireless communication device to execute a predetermined procedure. This program is stored in the form of a memory or other recording medium provided inside or outside the distortion compensation apparatus.

  Although various embodiments of the present invention have been described above, the present invention is not limited to the matters shown in the above-described embodiments, and those skilled in the art can make modifications and applications based on the description and well-known techniques. This is also the scope of the present invention, and is included in the scope of seeking protection.

  According to the present invention, a distortion compensation device and a distortion compensation method are provided that reduce erroneous calculation of a distortion compensation coefficient in a wireless communication apparatus, eliminate the need for adjustment of an analog part of the wireless communication unit, and improve convergence efficiency of the distortion compensation coefficient. Is done.

The figure explaining the folding phenomenon produced by the difference of the sampling frequency of D / A conversion and A / D conversion. The block diagram which shows the structure of the radio | wireless communication apparatus (power amplifier) of this invention. The graph which shows the power level of the feedback signal with respect to signal frequency. The graph which shows the frequency characteristic of a filter. The flowchart of an ACP observation evaluation and a filter setting process. The graph which shows an observation signal spectrum and filter characteristic in case an ACP evaluation score is N (maximum). The graph which shows an observation signal spectrum in case ACP evaluation score is 2, and a filter characteristic. The graph which shows an observation signal spectrum in case an ACP evaluation score is 1, and a filter characteristic. The graph which shows the modification which divides a leakage electric power measurement zone | band into N area. The graph which shows an observation signal spectrum and filter characteristic in case a ACP evaluation score is N-1 (near maximum) in a modification. The graph which shows an observation signal spectrum and filter characteristic in case an ACP evaluation score is 1 in a modification. The figure which shows the amplification result when there is no predistortion compensation. The figure which shows the amplification result at the time of providing a front distortion compensation part and carrying out distortion compensation. The figure which shows the concept that the phase of an antenna output signal rotates. The figure which shows the frequency spectrum when not carrying out distortion compensation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Distortion compensation part 13 Transmission side filter 15 D / A converter 17 Up converter 19 RF frequency oscillator 21 Amplifier 23 Antenna 25 Attenuator 27 Up converter 29 A / D converter 31 Feedback side filter 33 ACP observation evaluation part 35 Distortion calculation part 37 Delay Device 100 Wireless Communication Device

Claims (10)

A distortion compensation device that compensates for distortion that may occur in a transmission signal in accordance with distortion characteristics of an amplifier in a wireless communication device,
A filter that inputs a feedback signal of a transmission signal and passes only a signal of a predetermined frequency band of the feedback signal;
A distortion compensation device comprising: a leakage power evaluation unit that detects a leakage power level outside a transmission band based on the feedback signal and changes the frequency band of the filter according to the leakage power level. The distortion compensation apparatus according to claim 1,
The leakage power evaluation unit is a distortion compensation device that expands the frequency band as the leakage power level is smaller. The distortion compensation apparatus according to claim 1,
The leakage power evaluation unit is a distortion compensation device that scores a leakage level by comparing a power level of a transmission band of the feedback signal with the leakage power level. The distortion compensation apparatus according to claim 3,
The leakage power evaluation unit is a distortion compensation device that calculates a ratio between a power level of a transmission band of the feedback signal and the leakage power level, and compares the ratio with a predetermined threshold to calculate an evaluation score. The distortion compensation apparatus according to claim 4, wherein
The filter is a distortion compensation device that expands the frequency band as the evaluation score increases.   A wireless communication device comprising the distortion compensation device according to any one of claims 1 to 5 and an amplifier.   A radio communication base station apparatus comprising the radio communication apparatus according to claim 6.   A wireless mobile communication device comprising the wireless communication device according to claim 6. A distortion compensation method for compensating for distortion that may occur in a transmission signal in accordance with distortion characteristics of an amplifier in a wireless communication device,
Inputting a feedback signal of a transmission signal and passing only a signal of a predetermined frequency band of the feedback signal; and
Detecting a leakage power level outside a transmission band based on the feedback signal;
Changing the frequency band of the filter in accordance with the leakage power level.   The program for making a computer perform each step of the distortion compensation method of Claim 9.

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