Constant gain digital predistortion controller for linearization of non-linear amplifiers

A method of processing an observed signal to generate a modified observed signal in a feedback loop of an RF power amplifier, the method comprising:

• receiving a reference signal to the RF power amplifier, where the reference signal is digitally represented;
• receiving an observed signal, where the observed signal is derived from an output of the RF power amplifier and is digitally represented;
• multiplying the observed signal with a gain scaling factor to generate the modified observed signal;
• comparing the reference signal to the modified observed signal;
• adaptively adjusting the gain scaling factor in response to the comparison;
• and
• providing the modified observed signal to the feedback loop such that the modified observed signal can be used to control the feedback loop.

A method of processing an observed signal to a modified observed signal in a feedback loop of an RF power amplifier, the method comprising:

• receiving a reference signal to the RF power amplifier, where the reference signal is digitally represented;
• receiving an observed signal as a feedback signal that is derived from an output of the RF power amplifier, where the observed signal is digitally represented;
• multiplying the observed signal with a gain scaling factor;
• performing further signal procession on the multiplied observed signal, where the farther signal processing includes at least one of frequency offset correction, phase adjustment, and DC adjustment;
• comparing the reference signal to the multiplied observed signal as further signal processed by at least one of the frequency offset correction, the phase adjustment, and the DC adjustment;
• adaptively adjusting the gain scaling factor in response to the comparison such that the observed signal, as multiplied by the gain scaling factor, is normalized relative to the reference signal such that the multiplied observed signal and the reference signal are of approximately the same magnitude;
• and
• providing an at least partially processed observed signal as the modified observed signal in the feedback loop, where the at least partially processed observed signal has at least been multiplied with the gain scaling factor.

An adaptive estimator for a predistorter system, where the adaptive estimator receives at least a reference signal and an observed signal as inputs and generates a predistortion function that is implemented by a real-time predistortion engine, the adaptive estimator comprising:

• a normalizing circuit configured to receive the reference signal and the observed signal, where the normalizing circuit is configured to at least scale the observed signal to a modified observed signal such that the modified observed signal and the reference signal are of approximately the same magnitude;
• an estimator circuit adapted to compare the reference signal and the modified observed signal, where the estimator circuit is further configured to calculate a predistortion function for the real-time predistortion engine that is approximately complementary to a transfer function of a corresponding RF power amplifier based on the comparison;
• and
• a constant gain circuit configured to receive the predistortion function from the estimator circuit, where the constant gain circuit is further configured to select a region of the predistortion function, where the constant gain circuit is configured to adjust the gain of the predistortion function such that the selected region has a target gain, and where the constant gain circuit is configured to load the gain-adjusted predistortion function to the real-time predistortion engine.

A method of generating a predistortion function to be applied by a real-time predistortion engine to reduce distortion in a RF power amplifier, the method comprising:

• receiving a reference signal, where the reference signal is a digital signal that is related to an input to a real-time predistortion engine;
• receiving an observed signal, where the observed signal is a digital signal that is demodulated and downconverted from an output of the RF power amplifier;
• comparing at least a magnitude of the reference signal to the observed signal;
• normalizing the observed signal to a modified observed signal, wherein normalizing includes scaling magnitude in response to the comparison;
• comparing the modified observed signal and the reference signal;
• estimating a predistortion function for a real-time predistortion engine, where the predistortion function is estimated to provide an approximately complementary distortion to a distortion in the RF power amplifier;
• scaling the estimated predistortion function such that a selected region of the scaled predistortion function exhibits the same gain when the process is repeated;
• and
• loading the scaled predistortion function to the real-time predistortion engine.

An inner predistortion control loop for an RF power amplifier system comprising:

• a forward signal processing path starting from an input to a digital predistortion entity, the forward signal processing path further including:
• a digital predistortion entity;
• a conversion from digital to analog;
• a modulation stage;
• a baseband to RF upconversion;
• and
• an RF power amplifier;
• a feedback signal processing path starting from an actual output of the RF power amplifier, the feedback signal processing path further comprising:
• a coupling of an actual output of the RF power amplifier;
• an RF to baseband downconversion;
• a demodulation stage;
• a conversion from analog to digital;
• a gain scaling stage that normalizes the magnitude of an output of the analog to digital conversion to the magnitude of the input to the digital predistortion entity;
• an adaptation engine that is configured to communicate with the digital predistortion entity to change the predistortion behavior of the digital predistortion entity;
• and
• a constant gain stage configured to receive an output of the adaptation engine and scale the output such that the gain of a predistortion function that is applied to the input signal by the digital predistortion entity exhibits a substantially constant gain in a selected region.