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, the predistortion system comprising: a feedback receiver configured to receive a feedback signal representative of at least one nonlinear characteristic of the power amplifier, the power amplifier configured to amplify an input signal comprising a radio frequency (RF) modulated signal in digital form; and
a predistortion processor communicatively coupled with the feedback receiver, the predistortion processor comprising a lookup table of a predetermined size configured to receive the input signal, wherein an output of the lookup table includes a time delay correction element.
2. The predistortion system of claim 1 , wherein the input signal is a current input signal and the lookup table is configured with a memory function that enables the lookup table to store the current input signal and at least one previous input signal.
3. The predistortion system of claim 2 , wherein an addressing of the lookup table is based on a sequence of a series of previous input signals.
4. The predistortion system of claim 3 , wherein an addressing of the lookup table is implemented by a set of N-bit vector data that comprises a current input signal as well as previous N input signals.
5. The predistortion system of claim 2 , wherein each output signal of the lookup table is based on both the current input signal and the at least one previous input signal.
6. The predistortion system of claim 1 , wherein the predistortion processor is configured to generate a correction factor for correcting at least one nonlinear characteristic of the power amplifier based on at least an input of the lookup table.
7. The predistortion system of claim 6 , further comprising a processor configured to combine the RF modulated signal with a second signal corresponding to the correction factor.
8. The predistortion system of claim 7 , wherein the processor is further configured to supply the combined RF modulated signal with the feedback signal to the power amplifier to linearize the output of the power amplifier.
9. The predistortion system of claim 6 , further comprising combining logic to combine the RF modulated signal with a signal corresponding to the correction factor.
10. The predistortion system of claim 9 , wherein combining logic is performed to also supply the combined RF modulated signal and feedback signal to the power amplifier to linearize the output of the power amplifier.
11. The predistortion system of claim 1 , further comprising a serial shift register for forming an address for the lookup table.
12. The predistortion system of claim 1 , wherein the lookup table comprises at least one correction value.
13. The predistortion system of claim 12 , wherein the at least one correction value falls within a range determined by the predetermined size of the lookup table.
14. The predistortion system of claim 1 , wherein the lookup table is configured to respond to inputs derived from outputs of the lookup table.
15. The predistortion system of claim 1 , further comprising an input component comprising an analog to digital converter configured to output the input signal in digital form to the predistortion controller.