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 stabilizing a predistortion function, where at least a portion of the predistortion function is approximately complementary to a transfer function of a power amplifier, the method comprising:
receiving updates to at least some of the coefficients of the predistortion function, where a first region of the predistortion function corresponds to a region of realizable system gain, and where a second region of the predistortion function corresponds to a region of unrealizable system gain;
computing new coefficients for the second region of the predistortion function based on received coefficients of the predistortion function in the first region to provide a modified predistortion function; and
loading the modified predistortion function to a real-time predistorter.
2. The method as defined in claim 1 , further comprising:
intercepting an update originally intended for the real-time predistorter, where the update includes at least a portion of the predistortion function from adaptation logic to the real-time predistorter;
modifying a portion of a predistortion function corresponding to the second region;
loading at least the updated portion of the predistortion function in the first region to the real-time predistorter; and
loading the modified predistortion function corresponding to the second region to the real-time predistorter.
3. The method as defined in claim 2 , further comprising loading the modified predistortion function corresponding to the second region to the adaptation logic that originally generated the update.
4. The method as defined in claim 1 , wherein the new coefficients computed for the second region maintain continuity in the predistortion function between the first region and the second region.
5. The method as defined in claim 4 , wherein the new coefficients computed for the second region further provide a continuous rate of change of gain for the predistortion function between the first region and the second region.
6. The method as defined in claim 5 , wherein computing new coefficients further comprises weighting the portion of the predistortion function in the second region with a “1/x” gain curve.
7. The method as defined in claim 1 , wherein the new coefficients computed for the second region maintain continuity in the predistortion function between the first region and the second region, the new coefficients computed for the second region further provide a continuous rate of change of gain for the predistortion function between the first region and the second region, and where a transfer function defined by the new coefficients for the second region correspond to a reflected and inverted portion of a transfer function for the first region.
8. The method as defined in claim 1 , wherein the predistortion function is maintained in a multi-dimensional predistortion data structure.
9. An RF power amplifier system that embodies the method of claim 1 .
10. A cellular base station that embodies the method of claim 1 .
11. The method as defined in claim 1 , further comprising:
wherein receiving updates comprises intercepting an update for the predistortion function from adaptation logic to the real-time predistorter such that the update is not loaded into the real-time predistorter; and
wherein computing new coefficients comprises modifying a portion of the predistortion function in the update, where the modification includes changing the values of the predistortion function for input signal levels corresponding to a region of unrealizable system gain based on values for the predistortion function for input signal levels corresponding to a region of realizable system gain, wherein the change in value does not increase the gain of the predistortion function.
12. The method as defined in claim 11 , further comprising loading at least the portion of the updated predistortion function that has changed to the adaptation logic that originally generated the update.
13. An apparatus for stabilizing a predistortion function, where at least a portion of the predistortion function is approximately complementary to a transfer function of a power amplifier, the apparatus comprising:
means for receiving updates to at least some of the coefficients of the predistortion function, where a first region of the predistortion function corresponds to a region of realizable system gain, and where a second region of the predistortion function corresponds to a region of unrealizable system gain;
means for computing new coefficients for the second region of the predistortion function based on received coefficients of the predistortion function in the first region to provide a modified predistortion function; and
means for loading the modified predistortion function to a real-time predistorter.
14. The apparatus as defined in claim 13 , further comprising:
means for intercepting an update originally intended for the real-time predistorter, where the update includes at least a portion of the predistortion function from adaptation logic to the real-time predistorter;
means for modifying a portion of a predistortion function corresponding to the second region;
means for loading at least the updated portion of the predistortion function in the first region to the real-time predistorter; and
means for loading the modified predistortion function corresponding to the second region to the real-time predistorter.
15. The apparatus as defined in claim 14 , further comprising means for loading the modified predistortion function corresponding to the second region to the adaptation logic that originally generated the update.
16. The apparatus as defined in claim 13 , further comprising:
wherein the receiving means comprises means for intercepting an update for the predistortion function from adaptation logic to the real-time such that the update is not loaded into the real-time predistorter; and
wherein the computing computing means comprises modifying a portion of the predistortion function in the update, where the modification includes changing the values of the predistortion function for input signal levels corresponding to a region of unrealizable system gain based on values for the predistortion function for input signal levels corresponding to a region of realizable system gain, wherein the change in value does not increase the gain of the predistortion function.
17. The apparatus as defined in claim 13 , wherein the apparatus comprises a cellular base station.
18. An apparatus for stabilizing a predistortion function, where at least a portion of the predistortion function is approximately complementary to a transfer function of a power amplifier, the apparatus comprising:
a circuit adapted to receive updates to at least some of the coefficients of the predistortion function, where a first region of the predistortion function corresponds to a region of realizable system gain, and where a second region of the predistortion function corresponds to a region of unrealizable system gain;
a circuit adapted to compute new coefficients for the second region of the predistortion function based on received coefficients of the predistortion function in the first region to provide a modified predistortion function; and
a circuit adapted to load the modified predistortion function to a real-time predistorter.
19. The apparatus as defined in claim 18 , further comprising:
wherein the circuit adapted to receive updates is further configured to intercept an update for the predistortion function from adaptation logic to the real-time predistorter such that the update is not loaded into the real-time predistorter; and
wherein the circuit adapted to compute new coefficients is further configured to modify a portion of the predistortion function in the update, where the modification includes changing the values of the predistortion function for input signal levels corresponding to a region of unrealizable system gain based on values for the predistortion function for input signal levels corresponding to a region of realizable system gain, wherein the change in value does not increase the gain of the predistortion function.
20. The apparatus as defined in claim 18 , wherein the apparatus comprises a cellular base station.