Frequency-dependent magnitude pre-distortion for reducing spurious emissions in communication networks

• US 7,197,085 B1
• Filed: 05/22/2002
• Issued: 03/27/2007
• Est. Priority Date: 03/08/2002
• Status: Active Grant

• Current Assignee: CommScope Technologies, LLC (CommScope Holding Company, Inc.)
• Original Assignee: Andrew Corporation (CommScope Holding Company, Inc.)
• Inventors: Vella-Coleiro, George P.
• Primary Examiner(s): Burd; Kevin
• Assistant Examiner(s): Aghdam; Freshteh N
• Application Number:
  US10/153,446
• Time in Patent Office: 1,770 Days
• Field of Search: 375296-297
• US Class Current: 375/296
• CPC Class Codes: H03F 1/3241 using predistortion circuits H03F1/3211, H03F1/3217 take precedence
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3294 Acting on the real and imaginary components of the input signal
• CPC Class Codes: H03F 2200/102 A non-specified detector of a signal envelope being used in an amplifying circuit
• CPC Class Codes: H04L 25/03834 using pulse shaping
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

Pre-distortion, whose magnitude—and preferably phase—are frequency-dependent, is applied to an input signal in order to reduce spurious emissions resulting from subsequent amplification of the signal. In preferred embodiments, the pre-distortion technique of the present invention is implemented in combination with the (frequency-independent) magnitude and phase pre-distortion technique described in U.S. patent application Ser. No. 09/395,490 (“the '"'"'490 application”), where the frequency-dependent pre-distortion corresponds to amplifier distortion that has a magnitude that is proportional to the frequency offset from the carrier frequency and a phase shift of ±90° on either side of the carrier frequency. Since these characteristics match those of a differentiator, a thorough correction of this part of the amplifier'"'"'s distortion can be achieved using a differentiating filter. Embodiments of the present invention may be implemented in the baseband domain. Implementations may also be based on look-up tables that are adaptively updated to ensure optimal performance over time.

Claims

1. A method for reducing spurious emissions in an amplified signal, comprising the steps of:

(a) receiving an input signal; and

(b) applying pre-distortion to the input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein step (b) comprises the steps of;

(1) generating a main pre-distortion signal from the input signal;

(2) generating a secondary pre-distortion signal, whose magnitude and phase are frequency-dependent, from the input signal; and

(3) combining the main pre-distortion signal and the secondary pre-distortion signal to generate the pre-distorted signal, wherein the secondary pre-distortion signal has;

a magnitude proportional to a frequency offset from a carrier frequency of the input signal;

a phase shift of +90 degrees on one side of the carrier frequency; and

a phase shift of −

90 degrees on the other side of the carrier frequency.

2. The invention of claim 1, wherein the input signal is a baseband signal and the pre-distortion is applied in the baseband domain.

3. The invention of claim 1, wherein step (b)(1) comprises the step of applying pre-distortion, whose magnitude and phase are frequency-independent, to the input signal to generate the main pre-distortion signal.

4. The invention of claim 1, wherein the secondary pre-distortion signal is dependent on the bandwidth of the input signal.

5. The invention of claim 1, wherein the secondary pre-distortion signal is based on a temporal differentiation of a signal generated from the input signal.

6. The invention of claim 5, wherein the temporal differentiation is applied to a signal generated by complex multiplication between the input signal and a complex parameter dependent on the power of the input signal.

7. The invention of claim 5, wherein the temporal differentiation is applied to the input signal prior to complex multiplication between the resulting differentiated signal and a complex parameter dependent on the power of the input signal.

8. The invention of claim 1, wherein the frequency-dependent pre-distortion is based on data retrieved from a look-up table.

9. The invention of claim 8, wherein the look-up table is adaptively updated according to control signals generated based on the amplified signal.

10. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:

(a) a main signal processing path configured to generate a main pre-distortion signal from an input signal;

(b) a secondary signal processing path configured to generate a secondary pre-distortion signal from the input signal, wherein the magnitude and phase of the secondary pre-distortion signal are frequency-dependent; and

(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein the secondary pre-distortion signal has;

a magnitude proportional to a frequency offset from a carrier frequency of the input signal;

a phase shift of +90 degrees on one side of the carrier frequency; and

a phase shift of −

90 degrees on the other side of the carrier frequency.

11. The invention of claim 10, wherein the input signal is a baseband signal and the apparatus applies the pre-distortion in the baseband domain.

12. The invention of claim 10, wherein the main signal processing path is configured to apply frequency-independent magnitude and phase pre-distortion to the input signal to generate the main output signal.

13. The invention of claim 10, wherein the secondary signal processing path comprises:

(1) a complex multiplication module configured to multiply the input signal by a complex parameter dependent on the power of the input signal; and

(2) a differentiating filter configured to differentiate the output of the complex multiplication module to generate the secondary pre-distortion signal.

14. The invention of claim 13, wherein the differentiating filter comprises a cascade combination of a linear magnitude filter and a Hilbert transform filter.

15. The invention of claim 13, wherein the differentiating filter is implemented using a single FIR or IIR filter.

16. The invention of claim 10, wherein the secondary signal processing path comprises:

(1) a differentiating filter configured to differentiate the input signal; and

(2) a complex multiplication module configured to multiply the output of the differentiating filter by a complex parameter dependent on the power of the input signal to generate the secondary pre-distortion signal.

17. The invention of claim 10, wherein the apparatus retrieves data for the frequency-dependent pre-distortion from a look-up table.

18. The invention of claim 17, wherein the apparatus adaptively updates the look-up table according to control signals generated based on the amplified signal.

19. A method for reducing spurious emissions in an amplified signal, comprising the steps of:

(a) receiving an input signal; and

(b) applying pre-distortion to the input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein step (b) comprises the steps of;

(1) generating a main pre-distortion signal, whose magnitude and phase are frequency-independent, from the input signal;

(2) generating a secondary pre-distortion signal, whose magnitude and phase are frequency-dependent, from the input signal; and

(3) combining the main pre-distortion signal and the secondary pre-distortion signal to generate the pre-distorted signal, wherein the secondary pre-distortion signal is based on a temporal differentiation of a signal generated from the input signal.

20. The invention of claim 19, wherein the temporal differentiation is applied to a signal generated by complex multiplication between the input signal and a complex parameter dependent on the power of the input signal.

21. The invention of claim 20, wherein:

the complex multiplication is applied to (i) the input signal and (ii) parameters retrieved from a look-up table using an index value based on the envelope of the input signal; and

the signal generated by the complex multiplication is temporally differentiated to generate the secondary pre-distortion signal.

22. The invention of claim 19, wherein the temporal differentiation is applied to the input signal prior to complex multiplication between the resulting differentiated signal and a complex parameter dependent on the power of the input signal.

23. The invention of claim 22, wherein:

the complex multiplication is applied to (i) the result of temporally differentiating the input signal and (ii) parameters retrieved from a look-up table using an index value based on the envelope of the input signal; and

the signal generated by the complex multiplication is the secondary pre-distortion signal.

24. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:

(a) a main signal processing path configured to generate a main pre-distortion signal from an input signal, wherein the magnitude and phase of the main pre-distortion signal are frequency-independent;

(b) a secondary signal processing path configured to generate a secondary pre-distortion signal from the input signal, wherein the magnitude and phase of the secondary pre-distortion signal are frequency-dependent; and

(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein the secondary signal processing path comprises;

(1) a complex multiplication module configured to multiply the input signal by a complex parameter dependent on the power of the input signal; and

(2) a differentiating filter configured to differentiate the output of the complex multiplication module to generate the secondary pre-distortion signal.

25. The invention of claim 24, wherein the differentiating filter comprises a cascade combination of a linear magnitude filter and a Hilbert transform filter.

26. The invention of claim 24, wherein the differentiating filter is implemented using a single FIR or IIR filter.

27. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:

(a) a main signal processing path configured to generate a main pre-distortion signal from an input signal, wherein the magnitude and phase of the main pre-distortion signal are frequency-independent;

(b) a secondary signal processing path configured to generate a secondary pre-distortion signal from the input signal, wherein the magnitude and phase of the secondary pre-distortion signal are frequency-dependent; and

(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein the secondary signal processing path comprises;

(1) a differentiating filter configured to differentiate the input signal; and

(2) a complex multiplication module configured to multiply the output of the differentiating filter by a complex parameter dependent on the power of the input signal to generate the secondary pre-distortion signal.