JP2010166453A - Distortion compensating apparatus and radio base station - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a distortion compensating apparatus capable of preventing input signals exceeding a saturation points from being input to amplifiers. <P>SOLUTION: A DPD processing section 2 includes an inverse model estimation section 22 for estimating an inverse model to a model indicating I/O characteristics of an HPA 6, based on an input signal to the HPA 6 and an output signal from the HPA 6, a distortion compensation section 26 for correcting the input signal based on the inverse model to compensate for distortion in the I/O characteristics, a saturation point determination section 21 for detecting a saturation point H of the operation of the HPA 6, based on the I/O characteristics, and a level adjustment section 25 for adjusting the level of the input signal so that the maximum value of the input signal does not exceed the saturation point H. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a distortion compensation apparatus and a radio base station including the distortion compensation apparatus.

  In a radio base station included in a communication system using a mobile phone or the like, a high power amplifier (HPA) for amplifying and outputting a transmission signal is mounted in the transmission processing unit. Generally, HPA gives priority to amplification efficiency, and therefore has low linearity of input / output characteristics. That is, in HPA, the input / output characteristics between the input signal and the output signal exhibit nonlinear distortion characteristics. Therefore, when an input signal is amplified using an HPA having such input / output characteristics, a desired output signal may not be obtained due to the distortion. Therefore, as one of the distortion compensation methods for compensating for such distortion, a model representing the input / output characteristics of the amplifier is estimated, and an inverse model exhibiting characteristics opposite to the model is generated by digital signal processing. A method (so-called DPD: Digital Pre-Distortion) that compensates for distortion in the input / output characteristics of the amplifier by adding its inverse model to the input signal to the amplifier (digital signal before being converted into an analog signal) The following non-patent document 1 proposes. In the following Non-Patent Documents 2 and 3, HPA high-efficiency amplification techniques are proposed.

Lei Ding, "Digital predistortion of Power Amplifiers for Wireless Applications", Georgia Institute of Technology, March 2004. Donald F. Kimball, et al., "High-Efficiency Envelope-Tracking W-CDMA Base-Station Amplifier Using GaN HFETs", IEEE Transactions on Microwave Theory and Techniques, Vol.54, NO.11, November 2006. Feipeng Wang, et al., "Design of Wide-Bandwidth Envelope-Tracking Power Amplifiers for OFDM Applications", IEEE Transactions on Microwave Theory and Techniques, Vol.53, NO.4, April 2005.

  The amplifier has a saturation region in which normal amplification operation becomes difficult. However, in order to obtain high efficiency in the amplifier, it is necessary to operate the amplifier to a region close to the saturation point (starting point of the saturation region). However, the saturation point fluctuates depending on factors such as operating temperature and aging, so when the saturation point decreases due to these factors, an input signal with a signal level exceeding the saturation point is input to the amplifier. Can occur. As a result, the amplification operation of the amplifier may become abnormal, and in some cases, it may cause a failure of the amplifier.

  The present invention has been made in view of such circumstances, and obtains a distortion compensation apparatus capable of avoiding an input signal exceeding a saturation point from being input to an amplifier, and a radio base station including the distortion compensation apparatus. It is intended.

A distortion compensation apparatus according to a first aspect of the present invention includes: an estimation unit that estimates an inverse model for a model representing input / output characteristics of an amplifier based on an input signal to the amplifier and an output signal from the amplifier; By correcting the input signal based on an inverse model, a distortion compensation unit that performs distortion compensation of the input / output characteristic, a detection unit that detects a saturation point of the operation of the amplifier based on the input / output characteristic, And an adjustment unit that adjusts the level of the input signal so that the maximum value of the input signal does not exceed the saturation point.

  According to the distortion compensation apparatus according to the first aspect, the detection unit detects the saturation point of the operation of the amplifier based on the input / output characteristics of the amplifier. The adjustment unit adjusts the level of the input signal so that the maximum value of the input signal does not exceed the saturation point. Thus, by detecting the saturation point and adjusting the level of the input signal so as not to exceed the saturation point, it is possible to avoid a situation where an input signal exceeding the saturation point is input to the amplifier. .

  The distortion compensation apparatus according to the second aspect of the present invention is characterized in that, in the distortion compensation apparatus according to the first aspect, in particular, the adjustment unit is disposed before the distortion compensation unit. is there.

  According to the distortion compensation device according to the second aspect, since the adjustment unit is arranged before the distortion compensation unit, the adjustment process of the level of the input signal by the adjustment unit is more than the distortion compensation process by the distortion compensation unit. Is also executed before. In other words, after the adjustment process is performed by the adjustment unit, the distortion compensation process is performed by the distortion compensation unit. Therefore, since the adjustment process is not performed after the distortion compensation process, it is possible to avoid that the signal adjusted to the ideal characteristic by the distortion compensation by the distortion compensation unit is deviated from the ideal characteristic by the subsequent adjustment process. As a result, adjustment processing can be performed without impairing the effect of distortion compensation.

  A radio base station according to a third aspect of the present invention includes an amplifier and the distortion compensation apparatus according to the first or second aspect.

  According to the radio base station according to the third aspect, it is possible to transmit a desired transmission signal from the radio base station by appropriately compensating the distortion of the input / output characteristics of the amplifier by the distortion compensator.

  According to the present invention, it is possible to avoid an input signal exceeding the saturation point being input to the amplifier.

It is a block diagram which shows a part of structure of the wireless base station which concerns on embodiment of this invention. It is a block diagram which shows the structural example of a DPD process part. It is a figure which shows typically an example of the AM-AM characteristic measured by the input / output characteristic measurement part. It is a figure which shows typically an example of the AM-PM characteristic measured by the input / output characteristic measurement part. It is a figure which shows the adjustment process of the input signal by a level adjustment part.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, the element which attached | subjected the same code | symbol in different drawing shall show the same or corresponding element.

  FIG. 1 is a block diagram showing a part of the configuration of radio base station 1 according to the embodiment of the present invention. As shown in the connection relationship of FIG. 1, the radio base station 1 includes a DPD (Digital Pre-Distortion) processing unit 2, a DAC (Digital-to-Analog Converter) 3, an LPF (Low Pass Filter) 4, and a frequency conversion unit 5. , HPA (High Power Amplifier) 6, coupler 7, antenna 8, frequency converter 9, LPF 10, and ADC (Analog-to-Digital Converter) 11.

  The DPD processing unit 2 outputs the signal S2 by correcting the input signal S1, which is a digital signal. The details of correction by the DPD processing unit 2 will be described later. The DAC 3 converts the signal S2 that is a digital signal into a signal S3 that is an analog signal and outputs the signal S3. The LPF 4 performs a low-pass filter process on the signal S3 and outputs a signal S4. The frequency converter 5 converts the baseband signal S4 into a high-frequency signal S5 and outputs it. The HPA 6 outputs a signal S6 by amplifying the signal S5. The signal S6 is transmitted from the antenna 8.

  A part of the signal S6 from the HPA 6 toward the antenna 8 is extracted by the coupler 7 as a signal S7. The frequency converter 9 converts the high-frequency signal S7 into a baseband signal S8 and outputs it. The LPF 10 performs low-pass filter processing on the signal S8 and outputs a signal S9. The ADC 11 converts the signal S9, which is an analog signal, into a signal S10, which is a digital signal, and outputs it. The signal S10 is input to the DPD processing unit 2.

  FIG. 2 is a block diagram illustrating a configuration example of the DPD processing unit 2. 2, the DPD processing unit 2 includes an input / output characteristic measurement unit 20, a saturation point determination unit 21, an inverse model estimation unit 22, a coefficient storage unit 23, a notification unit 24, a level adjustment unit 25, and A distortion compensation unit 26 is provided. The level adjustment unit 25 is a digital attenuator, for example, and is connected to the input of the distortion compensation unit 26.

  The input / output characteristic measurement unit 20 receives the signal S2 from the distortion compensation unit 26 and the signal S10 from the ADC 11. The input / output characteristic measuring unit 20 measures the input / output characteristics of the HPA 6 based on the signals S2 and S10. FIG. 3 is a diagram schematically showing an example of AM (amplitude) -AM characteristics measured by the input / output characteristic measuring unit 20, and FIG. 4 shows AM-PM similarly measured by the input / output characteristic measuring unit 20. It is a figure which shows an example of a (phase) characteristic typically. The data S20 related to these input / output characteristics is input from the input / output characteristic measurement unit 20 to the saturation point determination unit 21.

  The saturation point determination unit 21 detects the saturation point of the operation of the HPA 6 based on the input / output characteristics given by the data S20. Regarding the AM-AM characteristic, a point where the slope of the characteristic is less than a predetermined value (for example, 0.1 dB / dB) is detected as a saturation point. In the example shown in FIG. 3, the saturation point H whose power value is P <b> 1 is detected by the saturation point determination unit 21. A region where the power value P1 is equal to or greater than the maximum power value P2 is a saturation region R0 in which normal amplification operation of the HPA 6 is difficult. That is, the saturation point H is defined as the minimum value of the saturation region R0. Regarding the AM-PM characteristic, a point where the slope of the characteristic changes from negative to positive (that is, an inflection point where the polarity of the differential coefficient reverses from negative to positive) is detected as a saturation point. In the example shown in FIG. 4, as in FIG. 3, the saturation point H whose power value is P <b> 1 is detected by the saturation point determination unit 21.

  The input / output characteristic measuring unit 20 periodically measures the input / output characteristics of the HPA 6, and the saturation point determining unit 21 periodically detects the saturation point of the HPA 6. Thereby, even when the saturation point of HPA 6 fluctuates (increases or decreases) due to operating temperature, aging deterioration, and the like, the current saturation point can be appropriately obtained. If the lower limit threshold is set for the power value at the saturation point and the power value at the saturation point detected by the saturation point determination unit 21 is equal to or lower than the lower limit threshold, the signal S24 to that effect is The information is input from the determination unit 21 to the notification unit 24, and the notification unit 24 issues an alarm for reporting the abnormality. In addition, while notifying abnormality by the alerting | reporting part 24, you may perform the process which stops the output of the transmission signal from the wireless base station 1. FIG.

The power value P1 of the saturation point H detected by the saturation point determination unit 21 is input from the saturation point determination unit 21 to the level adjustment unit 25 as data S26. The level adjustment unit 25 adjusts the level of the input signal S1 based on the data S26 so that the maximum value of the input signal S1 does not exceed the power value P1 of the saturation point H. FIG. 5 is a diagram illustrating adjustment processing of the input signal S1 by the level adjustment unit 25. As shown in FIG. 5, the level adjustment unit 25 attenuates the level of the input signal S1 as a whole to lower the average power, whereby the input signal S1 with the maximum power value P2 is input to the input with the maximum power value P1. Convert to signal S1A. 3 and 4, since the power value P1 is equal to the power value at the saturation point H, the maximum power value (power value P1) of the input signal S1A does not exceed the saturation point H. The input signal S1A adjusted by the level adjustment unit 25 is input to the distortion compensation unit 26.

  On the other hand, the inverse model estimation unit 22 receives the signal S2 from the distortion compensation unit 26 and the signal S10 from the ADC 11. In addition, data S21 related to the power value P1 of the saturation point H is input from the saturation point determination unit 21 to the inverse model estimation unit 22. Based on the data S21 and the signals S2 and S10, the inverse model estimation unit 22 estimates an inverse model for the model representing the input / output characteristics of the HPA 6 within a range where the maximum power value is the power value P1.

  Specifically, the inverse model estimation unit 22 calculates each order coefficient (that is, a coefficient set of the inverse model) for expressing the inverse model corresponding to the model in the form of a polynomial of an nth power series (n is a natural number). Ask for. Here, the inverse model is a model that exhibits characteristics opposite to the distortion characteristics of the model for compensating for nonlinear distortion in the model.

  Data S23 related to the coefficient set of the inverse model is input from the inverse model estimation unit 22 to the coefficient storage unit 23 and stored in the coefficient storage unit 23. In other words, the inverse model is stored in the coefficient storage unit 23. The distortion compensation unit 26 receives the input signal S1A from the level adjustment unit 25 and the data S23 related to the coefficient set from the coefficient storage unit 23. The distortion compensator 26 corrects the input signal S1A based on a coefficient set (inverse model) given by the data S23. As a result, the signal S <b> 2 in which appropriate distortion compensation is performed on the input signal S <b> 1 </ b> A is output from the distortion compensation unit 26.

  In the above description, the lower limit value of the saturation region in which normal amplification operation by the HPA 6 is difficult is set as the saturation point. However, the saturation point can be set from another viewpoint. For example, when the HPA 6 is operated with high efficiency, when the power value of the input signal increases and exceeds a predetermined level, the parasitic capacitance and the mutual conductance in the device rapidly change, and as a result, the AM− In the PM characteristic, an inflection point where the slope of the characteristic changes from negative to positive similarly to the point H in FIG. By setting this inflection point as a saturation point, modeling can be performed only in a region below the inflection point. When modeling is performed including a region above the inflection point, the shape of the AM-PM characteristic is complicated, and thus a higher-order function is required in the inverse model. On the other hand, modeling is limited to the area below the inflection point, so that the shape of the AM-PM characteristic is simplified, so that the order of the inverse model can be reduced. As a result, it is possible to reduce the memory capacity and the processing time.

  As described above, according to the DPD processing unit 2 (distortion compensation device) according to the present embodiment, the saturation point determination unit (detection unit) 21 determines the operation of the HPA 6 based on the input / output characteristics (data S20) of the HPA 6. A saturation point H is detected. Then, the level adjustment unit 25 adjusts the level of the input signal S1 so that the maximum value of the input signal S1 does not exceed the saturation point H. In this way, by detecting the saturation point H and adjusting the level of the input signal S1 so as not to exceed the saturation point H, a situation where the input signal S1 exceeding the saturation point H is input to the HPA 6 is avoided. It becomes possible.

Further, according to the DPD processing unit 2 according to the present embodiment, the level adjusting unit 25 is arranged before the distortion compensating unit 26, so that the level adjusting unit 25 can adjust the level of the input signal S1. This is executed before the distortion compensation processing by the distortion compensator 26. In other words, after the adjustment process is performed by the level adjustment unit 25, the distortion compensation process is performed by the distortion compensation unit 26. Therefore, since the adjustment process is not performed after the distortion compensation process, it is possible to avoid that the signal S2 adjusted to the ideal characteristic by the distortion compensation by the distortion compensation unit 26 is deviated from the ideal characteristic by the subsequent adjustment process. As a result, adjustment processing can be performed without impairing the effect of distortion compensation.

  Further, according to the radio base station 1 according to the present embodiment, a desired transmission signal is transmitted from the radio base station 1 by appropriately compensating for distortion of the input / output characteristics of the HPA 6 by the DPD processing unit 2. Is possible.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined not by the above-mentioned meaning but by the scope of claims for patent, and is intended to include all modifications within the scope and meaning equivalent to the scope of claims for patent.

1 Wireless base station 2 DPD processor 6 HPA
20 Input / output characteristic measurement unit 21 Saturation point determination unit 22 Inverse model estimation unit 23 Coefficient storage unit 25 Level adjustment unit 26 Distortion compensation unit H Saturation point

Claims (3)

An estimation unit for estimating an inverse model for a model representing input / output characteristics of the amplifier based on an input signal to the amplifier and an output signal from the amplifier;
A distortion compensator that performs distortion compensation of the input / output characteristics by correcting the input signal based on the inverse model;
A detection unit for detecting a saturation point of operation of the amplifier based on the input / output characteristics;
A distortion compensation apparatus comprising: an adjustment unit that adjusts a level of the input signal so that a maximum value of the input signal does not exceed the saturation point.   The distortion compensation apparatus according to claim 1, wherein the adjustment unit is disposed before the distortion compensation unit. An amplifier;
A radio base station comprising the distortion compensation apparatus according to claim 1.