Power amplifier time-delay invariant predistortion methods and apparatus
An embodiment of the invention is a time-delay invariant predistortion approach to linearize power amplifiers in wireless RF transmitters. The predistortion architecture is based on the stored-compensation or memory-compensation principle by using a combined time-delay addressing method, and therefore, the architecture has an intrinsic, self-calibrating time-delay compensation function. The predistortion architecture only uses a lookup table to conduct both the correction of non-linear responses of a power amplifier and the compensation of any time-delay effects presented in the same system. Due to the time-delay invariant characteristic, the predistortion design has a wider dynamic range processing advantage for wireless RF signals, and therefore can be implemented in multi-carrier and multi-channel wireless systems.
1. A predistortion system for linearizing the output of a power amplifier comprising:
a first receiver configured to receive a first signal representative of a radio frequency (RF) modulated signal;
a second receiver configured to receive a feedback signal representative of at least one nonlinear characteristic of a power amplifier;
a predistortion controller communicatively coupled with both the first receiver and the second receiver, the predistortion controller comprising at least one lookup table of a predetermined size; and
wherein the at least one lookup table is configured to respond to:
a first input derived from a difference between the square of a quadrature component of the first signal and the square of a quadrature component of the feedback signal, and
a second input derived from a difference between the square of an in-phase component of the first signal and the square of an in-phase component of the feedback signal.
2. The predistortion system of claim 1 , further comprising:
a reference path communicatively coupled with the first receiver and the predistortion controller;
a feedback path communicatively coupled with the second receiver and the predistortion controller;
wherein the at least one lookup table is configured to store at least one time-delay value that compensates for a time delay between the reference path and the feedback path.
3. The predistortion system of claim 2 wherein the at least one lookup table is further configured to adaptively update at least one correction value in the lookup table by combining a non-linear correction value of the power amplifier with the at least one time-delay value.
4. The predistortion system of claim 1 wherein the predistortion controller is configured to generate a correction factor for correcting at least one nonlinear characteristic of the power amplifier based on at least the first input or the second input.
5. The predistortion system of claim 1 , further comprising combining logic to:
combine the RF modulated signal with a signal corresponding to the correction factor; and
supply the combined RF modulated signal and second signal to the power amplifier to linearize the output of the power amplifier.
6. The predistortion system of claim 1 , further comprising a serial shift register for forming an address for the lookup table.
7. The predistortion system of claim 1 wherein the lookup table is addressed by parallel signals.
8. The predistortion system of claim 1 wherein the feedback signal is an analog signal.