Feed-forward correction loop with adaptive predistortion injection for linearization of RF power amplifier

• US 5,760,646 A
• Filed: 07/23/1996
• Issued: 06/02/1998
• Est. Priority Date: 03/29/1996
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Spectrian Corp. (REMEC, Inc.)
• Inventors: Belcher, Donald K., Wohl, Michael A., Bagwell, Kent E.
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US08/685,222
• Time in Patent Office: 679 Days
• Field of Search: 330/149, 330/151, 375/297, 455/63
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3229 using a loop for error extraction and another loop for error subtraction
• 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 2200/102 A non-specified detector of a signal envelope being used in an amplifying circuit
• CPC Class Codes: H03F 2200/438 Separate feedback of amplitude and phase signals being present
• CPC Class Codes: H03F 2201/3212 Using a control circuit to adjust amplitude and phase of a signal in a signal path
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion

Abstract

A linearization scheme for an RF power amplifier combines an adaptive predistortion modulator with a feedforward error correction loop, which cancels noise imparted by predistortion modulation to the amplified signal, and minimizes distortion in the RF amplifier'"'"'s output to a level that allows the use of a low cost auxiliary RF error amplifier in the feed-forward loop. The predistortion correction mechanism produces a predistortion signal based upon the input signal and is adaptively adjusted by an error signal extracted from the output of the a main RF power amplifier. The input signal is supplied to a work function generator unit and to a subtraction unit, which is also coupled to receive a fractional portion of the amplifier output signal and outputs the RF error component. The RF error component is coupled to a predistortion function generator, which is driven by the work function generator unit. The predistortion modulator uses the output of the predistortion function generator to predistort the input signal by a compensation characteristic equal and opposite to the distortion expected at the output of the main RF amplifier. When subjected to the transfer function of the RF amplifier, the predistortion signal injected into the input signal path will effectively cancel the amplifier'"'"'s anticipated distortion behavior. The predistortion is made adaptive by tracking the error signal and coupling this error signal to the predistortion function generator.

Claims

1. A method for correcting for distortion in an RF power amplifier comprising the steps of:

(a) monitoring an input signal supplied to and an amplified output signal from by said RF power amplifier, and extracting therefrom a distortion signal that varies with a rate of variation of the envelope of said amplified output signal;

(b) monitoring the instantaneous amplitude of said input signal to said amplifier, and generating at least one work function that varies with a variation of the instantaneous amplitude of said input signal;

c) generating a predistortion function in accordance with said distortion signal extracted in step (a) and said at least one work function of the instantaneous amplitude of said input signal as generated in step (b), so that said predistortion function varies with a rate of variation of the envelope of said amplified output signal;

(d) adaptively predistorting said input signal to be amplified by said RF amplifier at a rate of variation of the envelope of said amplified output signal, by predistorting said input signal by said predistortion function generated in step (c); and

(e) subjecting said amplified output signal from said RF power amplifier to a feed-forward correction loop.

2. A method according to claim 1 wherein step (e) comprises combining said distortion signal extracted in step (a) with said amplified output signal from said RF power amplifier, so as to effectively cancel the distortion signal of said amplified output signal by said RF power amplifier.

3. A method according to claim 2, wherein step (e) comprises amplifying said distortion signal further extracted in step (a) by means of an auxiliary RF error amplifier having an operational power range less than that of said RF power amplifier.

4. A method according to claim 3, wherein said feed-forward correction loop includes a subtraction unit coupled to the output of said RF amplifier and to the output of said auxiliary RF error amplifier.

5. A method according to claim 1, wherein said predistortion function has a distortion compensation characteristic that is generally complementary to distortion expected at the output of said RF amplifier.

6. A method according to claim 5, wherein step (b) comprises generating a plurality of respectively different work functions of said monitored input signal, and step (c) comprises controllably adjusting said plurality of respectively different work functions in accordance with said distortion signal extracted in step (a), and generating said predistortion function in accordance with a combination of controllably adjusted respectively different work functions.

7. A method according to claim 6, wherein step (c) further comprises generating a plurality of weights in accordance with a distortion signal minimization mechanism that is operative to reduce said distortion signal, and adjusting said plurality of respectively different work functions in accordance with said weights.

8. A method according to claim 7, wherein step (a) further includes applying said input signal and said output signal to a coherent receiver to extract said distortion signal.

9. A method according to claim 1, wherein said predistortion function is controllably weighted by respective amplitude and phase component-associated weighting coefficients generated in accordance with said distortion signal in said RF amplifier'"'"'s output signal.

10. A signal processing system for correcting for distortion in an RF power amplifier comprising:

a work function generator coupled to receive an input signal to be amplified by said RF power amplifier and being operative to generate at least one work function that varies with a variation of the instantaneous amplitude of said input signal;

a distortion signal extraction unit which is coupled to monitor said input signal and an amplified output signal from by said RF power amplifier, and to extract therefrom a distortion signal that varies with a rate of variation of the envelope of said amplified output signal;

a predistortion function generator, coupled to said work function generator and to said distortion signal extraction unit, and being operative to generate a predistortion function in accordance with said distortion signal and said at least one work function of the instantaneous amplitude of said input signal, so that said predistortion function varies with a rate of variation of the envelope of said amplified output signal;

a predistortion modulator coupled in the path of an input signal to be amplified by said RF power amplifier, and being operative to adaptively predistort said input signal at a rate of variation of the envelope of said amplified output signal, by predistorting said input signal by said predistortion control function; and

a feed-forward correction loop connected in circuit with an output of said RF power amplifier.

11. A signal processing system according to claim 10, wherein said feed-forward correction loop includes an RF error amplifier which amplifies said distortion signal extracted by said distortion signal extraction unit, and a signal combining unit which is coupled to subtract said distortion signal from said amplified output signal from said RF power amplifier, so as to effectively cancel the distortion signal in said amplified output signal from said RF power amplifier.

12. A signal processing system according to claim 11, wherein said RF error amplifier has an operational power range less than that of said RF power amplifier.

13. A signal processing system according to claim 10, wherein said predistortion function has a distortion compensation characteristic that is generally complementary to distortion expected at the output of said RF amplifier.

14. A signal processing system according to claim 10, wherein said predistortion modulator is operative to adaptively adjust respective amplitude and phase components of said input signal to be amplified by said RF power amplifier.

15. A signal processing system according to claim 10 wherein said work function generator is operative to generate a plurality of respectively different work functions of said input signal, said predistortion function generator is operative to controllably adjust said plurality of respectively different work functions in accordance with said distortion signal and to generate said predistortion function in accordance with a combination of controllably adjusted respectively different work functions.

16. A signal processing system according to claim 15, wherein said predistortion function generator is operative to generate a plurality of weights in accordance with a distortion signal minimization mechanism that reduces said distortion signal, and to adjust said plurality of respectively different work functions in accordance with said weights.

17. A signal processing system according to claim 10, wherein said distortion signal extraction unit includes a coherent receiver.

18. A signal processing system according to claim 10, wherein said distortion signal extraction unit is operative to measure energy in a prescribed spectral portion of the output signal produced by said RF amplifier, so as to derive a frequency domain measure of distortion contained in the output signal of said RF amplifier.

19. A signal processing system according to claim 10, wherein said distortion signal extraction unit includes a bandpass filter which has a frequency pass band exclusive of a desired frequency content of said output signal is and is operative to provide a measure of energy contained in said pass band indicative of distortion in the output signal amplified by said RF amplifier.