JP2010016422A - Distortion compensation circuit - Google Patents

Abstract

Disclosed is a distortion compensation circuit that prevents erroneous model (and inverse model) estimation.
A model estimation unit for estimating a model representing input / output characteristics of an amplifier, and distortion compensation for performing distortion compensation for canceling distortion of the input / output characteristics of the amplifier by adding an inverse model to the input signal to the input signal. In the distortion compensation circuit 1 including the unit 5, the peak detection unit 2 is provided to detect the peak value of the output signal of the amplifier 6, and the model estimation unit 3 is based on the input / output characteristics of the amplifier 6 including the peak value. To estimate the model.
[Selection] Figure 1

Description

  The present invention relates to a distortion compensation circuit having a function of compensating for nonlinear characteristics of a high-power amplifier used in, for example, a radio transceiver.

  In general, a high power amplifier (HPA) has low linearity of input / output characteristics. Therefore, when power is amplified using such an amplifier, a desired output may not be obtained due to distortion of input / output characteristics. Therefore, as a distortion compensation method for compensating such distortion, a reverse distortion characteristic opposite to the distortion characteristic of the amplifier is generated by digital signal processing for the input signal of the amplifier and added to the input of the amplifier. A technique for obtaining a desired amplifier output by performing DPD (Digital Pre-Distortion) processing has been proposed (for example, see Non-Patent Document 1).

  In addition, it has been proposed to perform distortion compensation represented by a polynomial in order to perform highly accurate DPD processing on a high-power amplifier that amplifies a broadband signal (see, for example, Patent Document 1).

Thesis by Lei Ding, "Digital predistortion of power amplifiers for wireless application", Georgia institute of Technology, March 2004 JP 2007-282066 A

  However, there is a case where the polynomial model on which the inverse distortion characteristic is based is not correctly estimated, and in this case, the result is that the nonlinear characteristic of the amplifier is promoted. FIG. 5A is a graph showing an example of input / output characteristics of the amplifier, in which the horizontal axis represents the input signal and the vertical axis represents the output signal. The dotted line shows ideal linear characteristics. This characteristic has a complicated shape and a strong non-linearity. In particular, this characteristic is complicated in a region where the input signal is large. For an amplifier having such characteristics, as shown in (b), when the model is estimated when the input signal level is small (the maximum input signal is P1), the model is estimated as shown in (c), for example. May result. This is an erroneous model estimation that is significantly different from the actual input / output characteristics of (a). Even if an inverse model which is an inverse function is obtained from such a model and added to the input signal, sufficient distortion compensation cannot be performed.

  In view of such a conventional problem, an object of the present invention is to provide a distortion compensation circuit that prevents erroneous model (and inverse model) estimation.

The present invention provides a model estimation unit that estimates a model representing input / output characteristics of an amplifier, and distortion compensation that performs distortion compensation that cancels distortion of the input / output characteristics of the amplifier by adding an inverse model to the model to the input signal. A distortion compensation circuit comprising: a peak detection unit that detects a peak value of at least one of an input signal and an output signal of the amplifier; and the model estimation unit includes the peak value of the amplifier. The model is estimated based on input / output characteristics.
In the distortion compensation circuit configured as described above, the model is estimated based on the input / output characteristics of the amplifier including the peak value detected by the peak detector, so that a necessary dynamic range is ensured and the correct model ( And inverse model) estimation can be performed.
Here, the “peak” of the input signal or the output signal refers to a signal having the maximum amplitude with a low occurrence frequency, and the peak value is a value of the maximum amplitude.

In the distortion compensation circuit, it is preferable that the model estimation unit has a buffer function for storing signals within a continuous predetermined time.
In this case, since a necessary dynamic range can be ensured including the signal before the peak value is detected, the model (and inverse model) estimation can be performed after the peak value appears.

In the distortion compensation circuit, it is preferable that the peak detection unit periodically detects a peak value.
In this case, the latest model (and inverse model) can always be estimated by periodic execution.

  According to the distortion compensation circuit of the present invention, a necessary dynamic range can be ensured and correct model (and inverse model) estimation can be performed. That is, it is possible to provide a distortion compensation circuit that prevents erroneous model (and inverse model) estimation.

  FIG. 1 is a block diagram showing a main part of a distortion compensation circuit 1 according to an embodiment of the present invention. The distortion compensation circuit 1 includes a peak detector 2 that detects a peak value of an output signal Y of a high output amplifier (HPA, hereinafter simply referred to as an amplifier) 6, and a model (specifically, an input / output characteristic of the amplifier 6). , A model estimation unit 3 that estimates the output signal Y in the form of a polynomial of the nth power series of the input signal X), a coefficient table 4 that temporarily stores the coefficients of the model, and an inverse model ( And a distortion compensation unit 5 that performs distortion compensation that cancels the distortion of the input / output characteristics of the amplifier 6 by adding the inverse function of the model to the input signal X. The peak detection unit 2, the model estimation unit 3, the coefficient table 4, and the distortion compensation unit 5 are configured by, for example, a DSP (Digital Signal Processor). Note that both the input signal X and the output signal Y are input to the model estimation unit 3.

The model estimation unit 3 includes buffers 3a and 3b for storing the input signal X and the output signal Y, respectively, within a predetermined time.
The nonlinear characteristic of the amplifier 6 is expressed by a power series polynomial of the input signal X. That is, when the output signal is Y, it can be expressed as follows. Note that a _i is a coefficient of each order.

  FIG. 2 is a flowchart showing processing in the distortion compensation circuit 1. This process is executed periodically. After the start of the process, the peak detection unit 2 determines whether or not a peak value is detected (step S1). If “No”, it waits for the peak value to appear. Since the peak value is known in advance, the appearance of the peak value can be easily detected by comparing with the threshold value. When the peak value is detected, the model estimation unit 3 that has received the detection signal reads the data, that is, the input signal and the output signal from the buffers 3a and 3b within a predetermined time with reference to that point (step S2), and performs model estimation Is performed (step S3).

For example, model estimation is performed using input / output signals within 1 μsec from 0.5 μsec before and 0.5 μsec after the reference time when the peak value appears. After model estimation, the coefficients are temporarily stored in the coefficient table 4. The distortion compensation unit 5 estimates an inverse model using these coefficients and performs distortion compensation (step S4).
“Peak” refers to a signal having a maximum amplitude with low occurrence frequency, and the peak value is a value of the maximum amplitude. “Low occurrence frequency” is, for example, about once every 1 μsec.
Usually, a necessary dynamic range (including a peak value) is included in an input / output signal of 1 μsec.

  In addition to the above, the model estimation unit 3 may obtain an inverse model, store the coefficients in the coefficient table 4, and the distortion compensation unit 5 may perform distortion compensation while referring to the coefficient table 4. Good.

  FIG. 3A is a graph showing an example of input / output characteristics (solid line) and inverse model (broken line) of the amplifier 6, wherein the horizontal axis indicates the input signal and the vertical axis indicates the output signal. The dotted line shows ideal linear characteristics. Although this characteristic has a complicated shape and strong nonlinearity, it is possible to ensure a necessary dynamic range and to estimate a correct model and an inverse model by reliably capturing a peak value. Therefore, as a result of distortion compensation by such an inverse model, input / output characteristics from which distortion is removed are obtained as shown in FIG.

  As described above, according to the distortion compensation circuit 1, the model is estimated based on the input / output characteristics of the amplifier 6 including the peak value detected by the peak detector 2, so that a necessary dynamic range is ensured. The correct model (and inverse model) can be estimated. That is, it is possible to provide a distortion compensation circuit 1 that prevents erroneous model (and inverse model) estimation.

Further, since the model estimation unit 3 includes the buffers 3a and 3b for storing signals within a continuous predetermined time, it is possible to secure a necessary dynamic range including a signal before the peak value is detected. . That is, model (and inverse model) estimation can be performed after the peak value appears.
In addition, the peak detector 2 periodically detects the peak value, so that the model estimation unit 3 can always perform the latest model (and inverse model) estimation.

  Although the output signal Y is provided to the peak detector 2 in the configuration of FIG. 1, the input signal X instead of the output signal Y can be used for peak detection. FIG. 4 is a block diagram showing an example thereof. In FIG. 4, the peak detection unit 2 detects a peak value in the input signal X, reads an input / output signal with reference to a time point when the peak value appears, and performs model (and inverse model) estimation. Others are the same as the case of the structure of FIG.

  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.

It is a block diagram which shows the principal part of the distortion compensation circuit which concerns on one Embodiment of this invention. It is a flowchart which shows the process in a distortion compensation circuit. (A) is a graph which shows an example of the input-output characteristic (solid line) and inverse model (broken line) of an amplifier, (b) is a graph which shows the input-output characteristic after distortion compensation. It is a block diagram which shows the principal part of the distortion compensation circuit of a structure different from FIG. It is a graph which shows the result of having performed distortion compensation by the conventional distortion compensation circuit with respect to an example of the input-output characteristic of an amplifier.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Distortion compensation circuit 2 Peak detection part 3 Model estimation part 3a, 3b Buffer 4 Coefficient table 5 Distortion compensation part 6 Amplifier

Claims (3)

A model estimation unit for estimating a model representing the input / output characteristics of the amplifier;
A distortion compensation circuit that performs distortion compensation to cancel distortion of input and output characteristics of the amplifier by adding an inverse model to the input signal to the input signal,
A peak detector for detecting a peak value of at least one of the input signal and the output signal of the amplifier;
The distortion compensation circuit, wherein the model estimation unit estimates the model based on input / output characteristics of the amplifier including the peak value.   The distortion compensation circuit according to claim 1, wherein the model estimation unit includes a buffer function for storing a signal within a continuous predetermined time.   The distortion compensation circuit according to claim 1, wherein the peak detection unit periodically detects the peak value.

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