Adaptive digital pre-distortion system

• US 20070018722A1
• Filed: 06/28/2006
• Published: 01/25/2007
• Est. Priority Date: 07/21/2005
• Status: Active Grant

• Current Assignee: Alcatel SA (Nokia Corporation)
• Original Assignee: Alcatel SA (Nokia Corporation)
• Inventors: Jaenecke, Peter
• Granted Patent: US 7,348,844 B2
• Time in Patent Office: Days
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3258 based on polynomial terms
• CPC Class Codes: H03F 2201/3224 Predistortion being done for compensating memory effects
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion
• CPC Class Codes: H04L 27/2623 Reduction thereof by clipping
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

An adaptive Digital Pre-distortion System and a method for correcting especially power amplifier memory effects. In particular, the invention relates to an electronic circuit, for amplifying an input signal, comprising: a clipping unit for generating a signal, having a reduced peak-to-average power ratio by clipping the input signal; a pre-distorter for generating a pre-distorted signal, defined by an pre-distortion algorithm which is based on the amplifier model function; a representation unit for representing the amplifier model function; a non-linear processing unit, in particular a power amplifier, for generating a processed signal, in particular by amplifying said received pre-distorted signal; a time delay unit for compensating the processing time for the pre-distorted signal generating the delayed pre-distorted signal; and a time delay cascade for delaying the said delayed pre-distorted signal, at the integer sample clocks generating the signals.

Claims

1. An electronic circuit, for amplifying an input signal, x₀, comprising:

a clipping unit for generating a signal, x, having a reduced peak-to-average power ratio by clipping the input signal x₀′

;

a pre-distorter for generating a pre-distorted signal, ξ

, defined by an pre-distortion algorithm which is based on an amplifier model function A;

a representation unit for representing the amplifier model function, A, either in terms of coefficients of polynomials, or as look-up tables;

a non-linear processing unit, like a power amplifier, for generating a processed signal, z, like an amplified pre-distorted signal;

a time delay unit for compensating aprocessing time for the pre-distorted signal ξ

generating adelayed pre-distorted signal ξ

′

;

a time delay cascade for delaying the pre-distorted signal, ξ

′

, at integer sample clocks 1, 2, . . . , n generating delayed signals

ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n</sub>′

and an adaptation unit for learning the amplifier model function A using the processed signal, z, and the delayed signals

ξ

,ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n</sub>′

such that a measure for non-linearity L_n is minimized.

2. A method for predistortion with an electronic circuit, for amplifying an input signal, x₀, comprising:

a clipping unit for generating a signal, x, having a reduced peak-to-average power ratio by clipping the input signal x₀′

;

a pre-distorter for generating a pre-distorted signal, ξ

, defined by an pre-distortion algorithm which is based on an amplifier model function A;

a representation unit for representing the amplifier model function, A, either in terms of coefficients of polynomials, or as look-up tables;

a non-linear processing unit, like a power amplifier, for generating a processed signal, z, like an amplified pre-distorted signal;

a time delay unit for compensating aprocessing time for the pre-distorted signal ξ

generating adelayed pre-distorted signal ξ

′

;

a time delay cascade for delaying the pre-distorted signal, ξ

′

, at integer sample clocks 1, 2, . . . , n generating delayed signals

ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n′</sub>

and an adaptation unit for learning the amplifier model function A using the processed signal, z, and the delayed signals

ξ

,ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n</sub>′

such that a measure for non-linearity L_n is minimized

wherein a complex digital amplifier output signal from a feedback, path can be described as a function

$\begin{array}{c}A<br><br>\left({\mathrm{\xi <br><br>}}_n,{\mathrm{\xi <br><br>}}_{n-1},{\mathrm{\xi <br><br>}}_{n-2},\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>},{\mathrm{\xi <br><br>}}_{n-K}\right)=<br><br>=A_1<br><br>\left({\mathrm{\xi <br><br>}}_n\right)·<br><br>A_2<br><br>\left({\mathrm{\xi <br><br>}}_{n-1}\right)<br><br>\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>A_{K_1}<br><br>\left({\mathrm{\xi <br><br>}}_{n-K_1}\right)++\\ <br><br>B_1<br><br>\left({\mathrm{\xi <br><br>}}_{n-1}\right)<br><br>{\mathrm{\xi <br><br>}}_{n-1}+B_2<br><br>\left({\mathrm{\xi <br><br>}}_{n-2}\right)<br><br>{\mathrm{\xi <br><br>}}_{n-2}+\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>}+\\ <br><br>B_{K_2}<br><br>\left({\mathrm{\xi <br><br>}}_{n-K_2}\right)<br><br>{\mathrm{\xi <br><br>}}_{n-K_2},\end{array}$

having a multiplicative and additive terms, where

K=max(K₁,K₂), K₁=0,1, . . . , K₂=1,2, . . .

3. The method for predistortion according to claim 2 comprising to learning method for determining adaptively the functions A₁, A₂, . . . according learning parameter set b=(β

₁,β

₂, . . . ), where β

₁ defines the function A₁, . . . for which an error characteristic E_n, i.e. the derivation between real distortion and the model function has the limit

E_n→

0

and the measure for the non-linearity L_n has the limit

L_n→

0

4. The method for predistortion according to claim 3, characterized in that for each given clipped sample

x_n

a constant

C(x_n;

ξ

<sub>n−

1</sub>,ξ

<sub>n−

2</sub>, . . . ,ξ

<sub>n−

k</sub>)

is calculated according to equation

${\mathrm{\xi <br><br>}}_n=\frac{{\mathrm{\gamma <br><br>}}_{\mathrm{fb}}-\underset{k_2=1}{\overset{K_2}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>B_{k_2}<br><br>\left({\mathrm{\xi <br><br>}}_{n-k_2}\right)<br><br>{\mathrm{\xi <br><br>}}_{n-k_2}}{A_1<br><br>\left({\mathrm{\xi <br><br>}}_n\right)·<br><br>\underset{k_1=1}{\overset{K_1}{\prod <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>A_{k_1+1}<br><br>\left({\mathrm{\xi <br><br>}}_{n-k_1}\right)}$

, where

y_fb

is a constant feedback gain.

5. Method for predistortion according to claim 3 characterized in that for each given clipped sample

x_n

a predistorted sample

ξ

_n

is calculated iteratively according to equation

${\mathrm{\xi <br><br>}}_n^{\left(r\right)}=\frac{C<br><br>\left(C_n;<br><br>{\mathrm{\xi <br><br>}}_{n-1},\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>},{\mathrm{\xi <br><br>}}_{n-k}\right)}{A_1<br><br>\left({\mathrm{\xi <br><br>}}_n^{\left(r-1\right)}\right)},\text{\hspace{1em}<br><br>}<br><br>r=1,2,\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>};<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\xi <br><br>}}_n^{\left(0\right)}=x_n$

6. A wireless communication system comprising a power amplifier, characterized by further comprising an electronic circuit, for amplifying an input signal, x₀, comprising:

a clipping unit for generating a signal, x, having a reduced peak-to-average power ratio by clipping the input signal x₀′

;

a pre-distorter for generating a pre-distorted signal, ξ

, defined by an pre-distortion algorithm which is based on an amplifier model function A;

a representation unit for representing the amplifier model function, A, either in terms of coefficients of polynomials, or as look-up tables;

a non-linear processing unit, like a power amplifier, for generating a processed signal, z, like an amplified pre-distorted signal;

a time delay unit for compensating aprocessing time for the pre-distorted signal ξ

generating adelayed pre-distorted signal ξ

′

;

a time delay cascade for delaying the pre-distorted signal, ξ

′

, at integer sample clocks 1, 2, . . . , n generating delayed signals

ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n</sub>′

and an adaptation unit for learning the amplifier model function A using the processed signal, z, and the delayed signals

ξ

,ξ

<sub>−

1</sub>′

,ξ

<sub>−

2</sub>′

, . . . ,ξ

<sub>−

n</sub>′

such that a measure for non-linearity L_n is minimized.