Digital control of a linc linear power amplifier

• US 6,054,894 A
• Filed: 06/19/1998
• Issued: 04/25/2000
• Est. Priority Date: 06/19/1998
• Status: Expired due to Term

• Current Assignee: Maxlinear Asia Singapore Pte Ltd. (Maxlinear Incorporated)
• Original Assignee: Datum Telegraphic, Inc. (Microchip Technology Incorporated)
• Inventors: Wright, Andrew S., Bennett, Steven J.
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Nguyen, Khanh V.
• Application Number:
  US09/100,568
• Time in Patent Office: 676 Days
• Field of Search: 330/149, 330/124 R, 330/295, 330/136, 330/2, 330/151
• US Class Current: 330/149
• CPC Class Codes: H03F 1/0294 using vector summing of two or more constant amplitude phase-modulated signals
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion

Abstract

The inventive LINC amplifier provides substantially linear amplification from two nonlinear amplifiers by decomposing the original signal into two constant amplitude envelope, phase varying signals, which, when combined, constructively and destructively interfere to re-form the original signal. The output of the LINC amplifier, which is to be transmitted via an antenna, is an amplified form of the original signal. The inventive LINC amplifier utilizes a digital control mechanism to control and adapt a digital compensation network that directly compensates for the imperfections of the analog RF environment, including the amplifiers. The mechanism monitors the combined amplifier output and adjusts the signal components in order to precisely compensate for any differences in the characteristics of the separate signal paths which would cause the combination not to accurately represent the original signal. The mechanism also corrects the component signals using information which can be applied to the amplifiers independent of the signal to be transmitted.

Claims

1. In a LINC power amplifier, an adaptive method for compensating for an unwanted difference between analog signal paths, comprising:

(a) separating an input transmission signal into at least first and second component signals having envelopes of varying phase and substantially constant amplitude;

(b) amplifying the component signals along respective first and second analog signal paths;

(c) combining the component signals following amplification to produce an output signal;

(d) monitoring and digitally processing at least (i) the component signals prior to amplification, and (ii) the output signal, to generate compensation parameters; and

(e) modifying the first and second component signals prior to amplification thereof, based on the compensation parameters, to reduce distortion caused by said unwanted difference.

2. The method of claim 1, wherein step (d) comprises storing digital sequences of the output signal and of the component signals in a memory, and processing the sequences stored within the memory in a non-real-time mode to generate the compensation parameters.

3. The method of claim 2, wherein step (d) comprises processing the sequences at a digital processing rate which is substantially less than a sampling rate of the sequences.

4. The method of claim 1, wherein step (d) comprises implementing at least one of the following digital filtering algorithms to generate the compensation parameters:

LMS, RLS, Kalman, Extended Kalman.

5. The method of claim 1, wherein step (d) comprises storing a set of compensation parameters and a time stamp in a memory of the power amplifier for subsequent use.

6. The method of claim 1, wherein step (e) comprises digitally filtering the component signals to add at least one of propagation delay, amplitude gain and phase rotation to the component signals.

7. The method of claim 6, wherein step (e) comprises digitally filtering the component signals using finite impulse response filters.

8. The method of claim 1, wherein step (e) comprises adjusting gain settings of non-linear amplifiers that are used to amplify the first and second component signals.

9. The method of claim 1, further comprising applying training signals to the power amplifier when no input signal is present, and using the training signals in step (a) to generate compensation parameters, the method thereby enabling the compensation parameters to be updated when the power amplifier is not being used to transmit a signal.

10. The method of claim 9, wherein the step of applying training signals comprises generating first and second training signals that are in antiphase with one another so that the training signals substantially cancel each other when combined, the method thereby substantially inhibiting a transmission from the power amplifier during training.

11. The method of claim 9, wherein the step of applying training signals comprises applying a first training signal to the first analog signal path, and concurrently applying a second training signal that is orthogonal to the first training signal to the second analog signal path, the orthogonal first and second training signals allowing bulk imperfections in each analog signal path to be separately measured.

12. The method of claim 1, wherein step (b) comprises switching from a first set of compensation parameters to a second set of compensation parameters using an interpolation process.

13. The method of claim 1, further comprising the steps of:

adaptively generating a numerical model of the first and second analog signal paths, and

using the numerical model to generate an initial set of compensation parameters.

14. A power amplifier which receives a bandlimited input transmission signal and provides an output transmission signal, wherein the output transmission signal is a substantially linear amplified representation of the input transmission signal, the amplifier comprising:

a signal component separator that decomposes the input transmission signal into at least a first component signal and a second component signal, wherein each component signal has a substantially constant amplitude and a varying phase;

first and second analog signal paths along which the first and second component signals are respectively amplified;

a combiner that receives and combines the first and second component signals following amplification thereof to compose the output transmission signal;

a compensation signal processor that digitally modifies at least one of the first component signal and the second component signal prior to amplification to compensate for unwanted differences between the first and second analog signal paths, the compensation signal processor modifying the at least one component signal based on compensation parameters; and

a compensation estimator which adaptively generates the compensation parameters based on an observation of signals within the amplifier.

15. The power amplifier of claim 14, further comprising a memory which stores sequences of samples of observed signals within the amplifier, and wherein the compensation estimator processes the samples stored within the memory in a non-real-time mode to generate updates to the compensation parameters.

16. The power amplifier of claim 14, further comprising a signal generator which generates training signals to stimulate the first and second analog signal paths when no input transmission signal is present, the training signals enabling the compensation estimator to generate updates to the compensation parameters between transmissions.

17. The power amplifier of claim 16, wherein the signal generator generates first and second training signals which are substantially in antiphase so that the training signals substantially cancel each other when combined.

18. The power amplifier of claim 14, further comprising a circuit which recovers energy associated with signals applied to the first and second analog signal paths.

19. The power amplifier of claim 14, wherein the first and second analog signal paths comprise respective non-linear amplifiers which amplify the first and second component signals.

20. The power amplifier of claim 19, wherein the compensation signal processor further compensates for the unwanted differences by adjusting gain settings of the non-linear amplifiers.

21. The power amplifier of claim 14, wherein the first and second analog signal paths comprise respective Class A, AB, B, C, D, E, F or S amplifiers which amplify the first and second component signals.

22. The power amplifier of claim 14, wherein the first and second analog signal paths further comprise respective up-converters which convert the first and second component signals to RF frequencies, and wherein the compensation signal processor compensates for distortions caused by the up-converters.

23. The power amplifier of claim 14, wherein the first and second analog signal paths further comprise analog quadrature (IQ) modulators, and the compensation signal processor further comprises first and second IQ modulator correction circuits.

24. The power amplifier of claim 14, wherein the compensation signal processor is capable of introducing propagation delays, amplitude gains and phase rotations to the first and second component signals.

25. The power amplifier of claim 14, wherein the compensation signal processor comprises a first multi-tap finite impulse response (FIR) filter which digitally filters the first component signal, and comprises a second multi-tap FIR filter which digitally filters the second component signal, the first and second FIR filters operating based on the compensation parameters.

26. The power amplifier of claim 14, wherein the compensation estimator generates the compensation parameters based on an observation of at least (i) the first and second component signals prior to amplification, and (ii) the output transmission signal.

27. The power amplifier of claim 14, wherein the compensation estimator implements at least one of the following digital filtering algorithms to generate compensation parameters:

LMS, RLS, Kalman, Extended Kalman.

28. The power amplifier of claim 14, wherein the compensation estimator generates initial values of the compensation parameters using a numerical model of at least the first and second analog signal paths.

29. A power amplifier which receives an input transmission signal and provides an output transmission signal, wherein the output transmission signal is a substantially linear amplified representation of the input transmission signal, the amplifier comprising:

means for separating the input transmission signal into a plurality of components signals, wherein each component signal has an amplitude envelope which varies in phase and is substantially constant in amplitude;

means for adaptively modifying the component signals to generate compensated component signals, the means for adaptively modifying compensating for unwanted differences in analog signal paths along which the compensated component signals are amplified;

means for amplifying the compensated component signals to generate amplified, compensated component signals; and

means for combining the amplified, compensated component signals to generate the output transmission signal.

30. The power amplifier of claim 29, further comprising means for capturing and storing digital sequences of at least the output transmission signal and of the component signals, wherein the means for adaptively modifying processes the digital sequences to generate updates to compensation parameters that are used to modify the component signals.

31. The power amplifier of claim 30, wherein the means for adaptively modifying processes the stored sequences at a digital processing rate which is substantially less than a sampling rate of the observation signals.

32. The power amplifier of claim 29, further comprising a signal generation means which generates training signals to stimulate the analog signal paths when no input transmission signal is present, the training signals used by the means for adaptively modifying to make adaptive adjustments between transmissions.

33. The power amplifier of claim 29, further comprising a means for recovering energy associated with the component signals.

34. The power amplifier of claim 29, wherein the means for amplifying comprises non-linear amplifiers which amplify the component signals.

35. The power amplifier of claim 34, wherein the means for adaptively modifying comprises means for adaptively adjusting gain settings of the non-linear amplifiers.

36. The power amplifier of claim 29, wherein the means for amplifying comprises amplifiers which fall within at least one of the following classes:

A, AB, B, C, D, E, F and S.

37. The power amplifier of claim 29, wherein the means for adaptively modifying is capable of independently modifying each of the plurality of component signals.

38. The power amplifier of claim 29, wherein the means for adaptively modifying is capable of introducing propagation delays, amplitude gains and phase rotations to the component signals.

39. The power amplifier of claim 29, wherein the means for adaptively modifying implements at least one of the following digital filtering algorithms to generate compensation parameters:

LMS, RLS, Kalman, Extended Kalman.

40. The power amplifier of claim 29, wherein the means for adaptively modifying comprises a first multi-tap finite impulse response (FIR) filter which digitally filters a first component signal, and comprises a second multi-tap FIR filter which digitally filters a second component signal.

41. The power amplifier of claim 29, wherein the analog signal paths further comprise analog quadrature (IQ) modulators, and the means for adaptively modifying comprises IQ modulator correction circuits.