Constant gain digital predistortion controller for linearization of non-linear amplifiers
View Patent ↗The invention is related to methods and apparatus for controlling and adapting a digital predistortion linearizer for amplification of bandlimited signals using non-linear amplifiers. The control method advantageously permits the predistortion function applied by a predistortion entity to provide a relatively constant gain. This attribute is advantageous for operation within cellular radio systems, which often employ digital power control systems. However, the disclosed techniques can also be applicable to virtually any type of digital predistortion for which an input signal or reference signal to be amplified is predistorted in a manner that is complementary to the distortion induced by a non-linear amplifier. Embodiments of the invention advantageously enhance the practicality of using digital linearization and predistortion amplification techniques. Embodiments of the invention can automatically adjust the characteristics of a predistorted signal so that a deviation from overall linearity is compensated and subsequently reduced while maintaining a nearly constant gain attribute.
1. A normalizing signal pre-processor that receives a reference signal and an observed signal as inputs and generates a modified observed signal as an output, the normalizing signal pre-processor comprising:
a first multiplier circuit adapted to scale the observed signal;
a subtraction circuit adapted to generate an error signal from a difference between the modified observed signal and the reference signal; and
adaptation logic adapted to receive the reference signal, the observed signal, the modified signal, and the error signal as inputs, and to adaptively adjust the scaling by the first multiplier circuit to reduce the magnitude of the error signal.
2. The normalizing signal pre-processor as defined in claim 1 , further comprising:
a second multiplier circuit configured multiply a data stream associated with the observed signal with a signal with a frequency;
a digitally numerically controlled oscillator that provides the signal with the frequency, where the digitally numerically controlled oscillator is responsive to a control word from adaptation logic, where the control word sets the frequency of the signal; and
where the adaptation logic is configured to adaptively adjust the control word to the digitally numerically controlled oscillator to reduce a frequency offset present in the error signal.
3. The normalizing signal pre-processor as defined in claim 1 , further comprising a second multiplier circuit configured to adjust a phase of a data stream associated with the observed signal in response to a phase adjust signal from the adaptation logic, where the phase adjust signal reduces the magnitude of the error signal.
4. The normalizing signal pre-processor as defined in claim 1 , further comprising an interpolating finite impulse response filter configured to provide oversamples of a data stream associated with the observed signal in fractions of a sample period, where the interpolating finite impulse response filter is further configured to permit the adaptation logic to selectively delay the data stream through the interpolating finite impulse response filter to reduce a misalignment of time in the error signal.
5. The normalizing signal pre-processor as defined in claim 1 , further comprising:
a second multiplier circuit configured multiply a data stream associated with the reference signal with a signal with a frequency;
a digitally numerically controlled oscillator that provides the signal with the frequency, where the digitally numerically controlled oscillator is responsive to a control word from adaptation logic, where the control word sets the frequency of the signal; and
where the adaptation logic is configured to adaptively adjust the control word to the digitally numerically controlled oscillator to reduce a frequency offset present in the error signal.
6. The normalizing signal pre-processor as defined in claim 1 , further comprising a second multiplier circuit configured to adjust a phase of a data stream associated with the reference signal in response to a phase adjust signal from the adaptation logic, where the phase adjust signal reduces the magnitude of the error signal.
7. The normalizing signal pre-processor as defined in claim 1 , further comprising an interpolating finite impulse response filter configured to provide oversamples of a data stream associated with the reference signal in fractions of a sample period, where the interpolating finite impulse response filter is further configured to permit the adaptation logic to selectively delay the data stream through the interpolating finite impulse response filter to reduce a misalignment of time in the error signal.
8. The normalizing signal pre-processor as defined in claim 1 , further comprising a second subtraction circuit adapted to receive a DC adjust signal from the adaptation logic, where the adaptation logic is further configured to adaptively adjust the DC offset to reduce a DC offset in the error signal.
9. The normalizing signal pre-processor as defined in claim 1 , further comprising at least one additional signal processing circuit disposed in a signal path between the first multiplier circuit and the subtraction circuit, where the additional signal processing circuit includes at least one signal processing circuit selected from the group consisting of a frequency offset circuit, a phase adjustment circuit, a delay adjustment circuit, and a DC adjustment circuit.
10. An RF power amplifier system that embodies the normalizing signal pre-processor of claim 1 .
11. A cellular base station that embodies the normalizing signal pre-processor of claim 1 .