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 method of identifying a boundary between a region of realizable system gain and a region of unrealizable system gain for an RF power amplifier system with predistortion compensation, the method comprising:
retrieving a plurality of output signal levels stored in a predistortion lookup table;
computing an overall system gain based on the plurality of output signal levels and associated input signal levels;
calculating a slope of the overall system gain;
determining a point at which the slope of the overall system gain corresponds to a first predetermined amount; and
subtracting a second predetermined amount from the point to identify the boundary.
2. The method as defined in claim 1 , wherein the associated input signal levels correspond to an index to the predistortion lookup table.
3. The method as defined in claim 1 , wherein the first predetermined amount is about −45 degrees and the second predetermined amount is about 1 dB.
4. The method as defined in claim 1 , wherein determining the point at which the slope of the overall system gain corresponds to the first predetermined amount is performed in a direction from high input signal levels to low input signal levels.
5. An RF power amplifier system that embodies the method of claim 1 .
6. A cellular base station that embodies the method of claim 1 .
7. An apparatus for identifying a boundary between a region of realizable system gain and a region of unrealizable system gain for an RF power amplifier system with predistortion compensation, the apparatus comprising:
means for retrieving a plurality of output signal levels stored in a predistortion lookup table;
means for computing an overall system gain based on the plurality of output signal levels and associated input signal levels;
means for calculating a slope of the overall system gain;
means for determining a point at which the slope of the overall system gain corresponds to a first predetermined amount; and
means for subtracting a second predetermined amount from the point to identify the boundary.
8. The apparatus as defined in claim 7 , wherein the associated input signal levels correspond to an index to the predistortion lookup table.
9. The apparatus as defined in claim 7 , wherein the first predetermined amount is about −45 degrees and the second predetermined amount is about 1 dB.
10. The apparatus as defined in claim 7 , wherein the determining means is configured to operate in a direction from high input signal levels to low input signal levels.
11. The apparatus as defined in claim 7 , further comprising the RF power amplifier system.
12. The apparatus as defined in claim 7 , further comprising a cellular base station.
13. An apparatus for identifying a boundary between a region of realizable system gain and a region of unrealizable system gain for an RF power amplifier system with predistortion compensation, the apparatus comprising:
a circuit configured to retrieve a plurality of output signal levels stored in a predistortion lookup table;
a circuit configured to compute an overall system gain based on the plurality of output signal levels and associated input signal levels;
a circuit configured to calculate a slope of the overall system gain;
a circuit configured to determine a point at which the slope of the overall system gain corresponds to a first predetermined amount; and
a circuit configured to subtract a second predetermined amount from the point to identify the boundary.
14. The apparatus as defined in claim 13 , wherein the associated input signal levels correspond to an index to the predistortion lookup table.
15. The apparatus as defined in claim 13 , wherein the first predetermined amount is about −45 degrees and the second predetermined amount is about 1 dB.
16. The apparatus as defined in claim 13 , wherein determining the point at which the slope of the overall system gain corresponds to the first predetermined amount is performed in a direction from high input signal levels to low input signal levels.
17. The apparatus as defined in claim 13 , the RF power amplifier system.
18. The apparatus as defined in claim 13 , further comprising a cellular base station.