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 controlling gain in an adaptive predistorter for an RF power amplifier system, the method comprising:
identifying a region to be used as an anchored gain region within a predistorter response;
selecting an overall target gain for the RF power amplifier system;
scaling at least a portion of the predistorter response such that the gain of the predistorter response in the region identified as the anchored gain region corresponds to the overall target gain; and
storing the scaled predistorter response in a predistorter engine.
2. The method as defined in claim 1 , wherein the portion of the predistorter response that is scaled corresponds to the portion of the predistorter response in the region of realizable system gain.
3. The method as defined in claim 1 , wherein the overall target gain is in a range of about 2 dB to about 6 dB less than a boundary between a region of realizable system gain and a region of unrealizable system gain.
4. The method as defined in claim 1 , wherein the overall target gain is about 3 dB less than a boundary between a region of realizable system gain and a region of unrealizable system gain.
5. The method as defined in claim 1 , wherein the overall target gain is selectable during operation within a predetermined range.
6. The method as defined in claim 1 , including identifying a boundary between a region of realizable system gain and a region of unrealizable system gain, the method further 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.
7. The method as defined in claim 1 , including stabilizing a predistortion response that is stored in a lookup table, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the method further comprising:
identifying a portion of the lookup table that corresponds to a region of unrealizable system gain;
scaling entries from the identified portion of the lookup table such that the overall gain of the predistortion response in the identified portion of the lookup table is reduced; and
storing the scaled entries back into the lookup table.
8. The method as defined in claim 1 , including stabilizing a predistortion response, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the method further comprising:
receiving updates to at least some of the coefficients of the predistortion response, where a first region of the predistortion response corresponds to a region of realizable system gain, and where a second region of the predistortion response corresponds to a region of unrealizable system gain;
computing new coefficients for the second region of the predistortion response based on received coefficients of the predistortion response in the first region to provide a modified predistortion response; and
loading the modified predistortion response to a real-time predistorter.
9. The method as defined in claim 1 , including stabilizing a predistortion function, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the method further comprising:
intercepting an update for a predistortion response from adaptation logic to a real-time predistortion engine such that the update is not loaded into the real-time predistortion engine;
modifying a portion of the predistortion response in the update, where the modification includes changing the values of the predistortion response for input signal levels corresponding to a region of unrealizable system gain based on values for the predistortion response 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 response; and
loading the updated predistortion response, as modified, to the real-time predistorter.
10. An RF power amplifier system that embodies the method of claim 1 .
11. A cellular base station that embodies the method of claim 1 .
12. A gain controller for an adaptive predistorter for an RF power amplifier system, the gain controller comprising:
a scanning circuit configured to identify a region to be used as an anchored gain region within a predistorter response;
a gain select module configured to select an overall target gain for the RF power amplifier system;
a scaling circuit configured to scale at least a portion of the predistorter response such that the gain of the predistorter response in the region identified as the anchored gain region corresponds to the overall target gain; and
a data transfer circuit configured to store the scaled predistorter response in a predistorter engine.
13. The gain controller as defined in claim 12 , wherein the scaling circuit is configured such that the portion of the predistorter response that is scaled corresponds to the portion of the predistorter response in the region of realizable system gain.
14. The gain controller as defined in claim 12 , further comprising a boundary identification circuit that identifies a boundary between a region of realizable system gain and a region of unrealizable system gain, wherein the boundary identification circuit further comprises:
a module adapted to retrieve a plurality of output signal levels stored in a predistortion lookup table;
a gain computation circuit to compute an overall system gain based on the plurality of output signal levels and associated input signal levels;
a module adapted to calculate a slope of the overall system gain;
a module adapted to determine a point at which the slope of the overall system gain corresponds to a first predetermined amount; and
an arithmetic circuit adapted to subtract a second predetermined amount from the point to identify the boundary.
15. The gain controller as defined in claim 12 , further comprising a modification circuit that stabilizes a predistortion response that is stored in a lookup table, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the modification circuit further comprising:
a module configured to identify a portion of the lookup table that corresponds to a region of unrealizable system gain;
a scaling circuit configured to scale entries from the identified portion of the lookup table such that the overall gain of the predistortion response in the identified portion of the lookup table is reduced; and
an interface circuit configured to store the scaled entries back into the lookup table.
16. The gain controller as defined in claim 12 , a modification circuit adapted to stabilize a predistortion response, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the modification circuit further comprising:
a module configured to receive updates to at least some of the coefficients of the predistortion response, where a first region of the predistortion response corresponds to a region of realizable system gain, and where a second region of the predistortion response corresponds to a region of unrealizable system gain;
an updating circuit configured to compute new coefficients for the second region of the predistortion response based on received coefficients of the predistortion response in the first region to provide a modified predistortion response; and
an interface circuit configured to load the modified predistortion response to a real-time predistorter.
17. The gain controller as defined in claim 12 , further comprising a modification circuit adapted to stabilize a predistortion function, where at least a portion of the predistortion response is approximately complementary to a transfer function of a power amplifier of the RF power amplifier system, the modification circuit further comprising:
a first interface circuit adapted to intercept an update for a predistortion response from adaptation logic to a real-time predistortion engine such that the update is not loaded into the real-time predistortion engine;
an overdrive circuit configured to modify a portion of the predistortion response in the update, where the modification includes changing the values of the predistortion response for input signal levels corresponding to a region of unrealizable system gain based on values for the predistortion response 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 response; and
a second interface circuit adapted to load the updated predistortion response, as modified, to the real-time predistorter.
18. An RF power amplifier system that embodies the gain controller of claim 12 .
19. A cellular base station that embodies the gain controller of claim 12 .