Recalibration of envelope tracking transfer function during active transmission

1. A method for adjusting a nonlinear transfer function of an envelope tracking power supply for a power amplifier, comprising:

supplying an output signal of the power amplifier to a feedback receiver to obtain a feedback signal;

determining an assumed performance of the power amplifier by evaluating the nonlinear transfer function at a first value defined by an input signal for a transmitter comprising the power amplifier;

determining an actual performance of the power amplifier by evaluating the nonlinear transfer function at a second value defined by the feedback signal,

wherein determining the actual performance comprises:

determining a total gain of the transmitter based on the feedback signal and the input signal; and

determining an actual gain of the power amplifier based on the total gain and a known gain of the transmitter without the power amplifier, wherein the actual gain is compared with a desired gain of the power amplifier for a given supply voltage for the Power amplifier; and

determining a correctional value for a parameter of the nonlinear transfer function based on a difference between the assumed performance and the actual performance.

2. The method according to claim 1 , wherein determining the assumed performance and the actual performance of the power amplifier comprises evaluating the nonlinear transfer function at a plurality of value pairs of corresponding to first and second values to obtain a plurality of differences between the assumed performance and the actual performance for a plurality of operating conditions of the envelope tracking power amplifier.

3. The method according to claim 1 , further comprising:

updating the nonlinear transfer function using the correctional value.

4. The method according to claim 1 , wherein the parameter of the nonlinear transfer function is one of:

an offset compensation for the input signal,

an offset compensation for a supply voltage for the power amplifier,

a gain compensation for the input signal,

a gain compensation for the supply voltage, and

parameters defining a shape of the nonlinear transfer function.

5. The method according to claim 1 , wherein the method is performed during an active transmission mode of the power amplifier.

6. The method according to claim 1 , further comprising:

measuring a temperature of the transmitter; and

determining a temperature-dependent overall gain of a portion of the transmitter based on the temperature, wherein the portion comprises the power amplifier;

wherein determining the actual gain of the power amplifier is based on the temperature-dependent overall gain.

7. The method according to claim 1 , further comprising:

synchronizing a variable envelope tracking gain to a variable transmitter gain before determining the actual performance of the power amplifier.

8. The method according to claim 1 , further comprising:

controlling a power of the power amplifier via a closed power-control loop concurrently with envelope tracking, wherein the feedback signal is used by both the closed power-control loop and for adjusting the nonlinear tracking function, and

correcting an overall gain of a portion of the transmitter that comprises the power amplifier by a compensation factor to compensate for a gain variation due to a change of a power amplifier supply voltage, the gain variation being caused by adjusting the nonlinear transfer function using the correctional value, wherein the compensation factor is determined based on a current feedback signal and a previous feedback signal.

9. The method according to claim 8 , wherein the compensation factor is determined as a ratio of the current feedback signal and the previous feedback signal.

10. The method according to claim 1 , further comprising:

generating the first value used for determining the assumed performance as a root-mean-square value of the input signal; and

generating the second value used for determining the actual performance as a root-mean-square value of the feedback signal.

11. The method according to claim 1 , further comprising:

generating the first value used for determining the assumed performance as a filtered value of the input signal; and

generating the second value used for determining the actual performance as a filtered value of the feedback signal.

12. The method according to claim 1 , wherein a default nonlinear transfer function is a result of a factory calibration of the power amplifier during which the nonlinear transfer function is adjusted so that a desired target gain of the power amplifier is reached for a certain load impedance at an output of the power amplifier.

13. A method for adjusting a nonlinear transfer function of an envelope tracking power supply for a power amplifier, comprising:

supplying an output signal of the power amplifier to a feedback receiver to obtain a feedback signal;

determining an assumed performance of the power amplifier by evaluating the nonlinear transfer function at a first value defined by an input signal for a transmitter comprising the power amplifier;

determining an actual performance of the power amplifier by evaluating the nonlinear transfer function at a second value defined by the feedback signal; and

determining a correctional value for a parameter of the nonlinear transfer function based on a difference between the assumed performance and the actual performance;

wherein the nonlinear transfer function models a power amplifier supply voltage as a function of a voltage of the input signal;

wherein determining the assumed performance comprises determining a first power supply voltage by evaluating the nonlinear transfer function at the voltage of the input signal as the first value;

wherein determining the actual performance comprises determining a second power supply voltage by evaluating the nonlinear transfer function at a voltage of the feedback signal as the second value; and

wherein determining the correctional value comprises evaluating a quotient of the first power supply voltage and the second power supply voltage.

14. A method for re-calibrating a nonlinear transfer function of an envelope tracking power supply for a power amplifier during active transmission, comprising:

supplying an output signal of the power amplifier to a feedback receiver to obtain a feedback signal;

determining a total gain of a transmitter comprising the power amplifier based on the feedback signal and a corresponding input signal to the transmitter;

determining an actual power amplifier gain based on the total gain and a known gain setting of the transmitter; and

determining a correctional value for a parameter of the nonlinear transfer function based on a difference between the actual power amplifier gain and a target gain for the power amplifier.

15. The method according to claim 14 ,

wherein the total gain and the actual power amplifier gain are determined for a plurality of operating conditions of the envelope tracking power amplifier to obtain a plurality of differences between corresponding actual power amplifier gain values and target gain values; and

wherein the correctional value is determined based on the plurality of differences.

16. The method according to claim 14 , further comprising:

updating the nonlinear transfer function using the correctional value.

17. The method according to claim 14 , wherein the method is performed during an active transmission mode of the power amplifier.

18. The method according to claim 14 , further comprising:

measuring a temperature of the transmitter; and

determining a temperature-dependent overall gain of a portion of the transmitter based on the temperature, wherein the portion comprises the power amplifier;

wherein determining the actual gain of the power amplifier is based on the temperature-dependent overall gain.

19. The method according to claim 14 , further comprising:

synchronizing a variable envelope tracking gain to a variable transmitter gain before determining the total gain of the transmitter.

20. The method according to claim 14 , further comprising:

controlling a power of the power amplifier by using a closed power-control loop concurrently with envelope tracking, wherein the feedback signal is used by both the closed power-control loop and for re-calibrating the nonlinear tracking function, and

correcting an overall gain of a portion of the transmitter that comprises the power amplifier by a compensation factor to compensate for a gain variation due to a change of a power amplifier supply voltage, the gain variation being caused by adjusting the nonlinear transfer function using the correctional value, wherein the compensation factor is determined based on a current feedback signal and a previous feedback signal.

21. The method according to claim 14 , further comprising:

generating a root-mean-square value of the input signal to be used when determining the total gain; and

generating a root-mean-square value of the feedback signal to be used when determining the total gain.

22. The method according to claim 14 , wherein a default nonlinear transfer function is a result of a factory calibration of the power amplifier during which the nonlinear transfer function is adjusted so that a desired target gain of the power amplifier is reached for a certain load impedance at an output of the power amplifier.

23. An envelope tracking system, comprising:

a transmitter comprising an envelope tracking power amplifier;

a supply voltage provider configured to provide a variable supply voltage for the envelope tracking power amplifier based on an input signal of the transmitter and a nonlinear transfer function of the variable supply voltage based on the input signal;

a feedback receiver configured to receive an output signal of the envelope tracking power amplifier and provide a corresponding feedback signal; and

a transfer function adjuster configured to receive the input signal and the feedback signal, and determine a correctional value for the nonlinear transfer function based on a difference between an assumed performance of the envelope tracking power amplifier and an actual performance of the envelope tracking amplifier, the assumed performance being determined based on the input signal and the nonlinear transfer function and the actual performance being determined based on the feedback signal

wherein the nonlinear transfer function models a power amplifier supply voltage as a function of a voltage of the input signal;

wherein determining the assumed performance comprises determining a first power supply voltage by evaluating the nonlinear transfer function at the voltage of the input signal;

wherein determining the actual performance comprises determining a second power supply voltage by evaluating the nonlinear transfer function at a voltage of the feedback signal; and

wherein determining the correctional value comprises evaluating a quotient of the first power supply voltage and the second power supply voltage.

24. The envelope tracking system according to claim 23 , wherein the transfer function adjuster is further configured to evaluate the nonlinear transfer function at a plurality of value pairs of corresponding first and second values to obtain a plurality of differences between the assumed performance and the actual performance for a plurality of operating conditions of the envelope tracking power amplifier.

25. The envelope tracking system according to claim 23 , wherein the transfer function adjuster is further configured to update the nonlinear transfer function using the correctional value.

26. The envelope tracking system according to claim 23 , wherein the transfer function adjuster is further configured to receive a temperature measurement relative to a temperature of the transmitter and determine a temperature-dependent overall gain of a portion of the transmitter based on the temperature measurement, wherein the portion comprises the envelope tracking power amplifier, and wherein the determination of the actual gain of the envelope tracking power amplifier is based on the temperature-dependent overall gain.

27. The envelope tracking system according to claim 23 , further comprising:

a closed power-control loop configured to control an output power of the envelope tracking power amplifier, wherein the feedback signal is used by both the closed power-control loop and the transfer function adjuster.

28. The envelope tracking system according to claim 27 , further comprising:

an overall gain corrector configured to correct an overall gain of a portion of the transmitter that comprises the power amplifier by a compensation factor to compensate for a gain variation due to a change of the power amplifier supply voltage, the gain variation being caused by adjusting the nonlinear transfer function using the correctional value, wherein the compensation factor is determined based on a current feedback signal and a previous feedback signal.

29. A mobile communication device, comprising:

an RF circuit configured to provide or receive RF signals; and

an antenna port coupled to the RF circuit;

wherein the RF circuit comprises an envelope tracking system, the envelope tracking system comprising:

a transmitter comprising an envelope tracking power amplifier;

a supply voltage provider configured to provide a variable supply voltage for the envelope tracking power amplifier based on an input signal of the transmitter and a nonlinear transfer function that is a function of the input signal;

a feedback receiver configured to receive an output signal of the envelope tracking power amplifier and provide a corresponding feedback signal; and

a transfer function adjuster configured to determine a total gain of the transmitter based on the input signal and the feedback signal, derive an actual power amplifier gain based on the total gain and a known gain setting of the transmitter, and determine a correctional value for the nonlinear transfer function based on a difference between the actual power amplifier gain and a target gain for the power amplifier.

30. An envelope tracking system, comprising:

a transmitter comprising an envelope tracking power amplifier;

a lookup table having stored at least one nonlinear transfer function of a variable supply voltage based on an input signal to the transmitter, wherein the lookup table is configured to provide in response to an input signal value a variable supply voltage value;

a DC-DC converter configured to provide the variable supply voltage for the envelope tracking power amplifier based on the variable supply voltage value provided by the lookup table;

a feedback receiver configured to receive an output signal of the envelope tracking power amplifier and provide a corresponding feedback signal; and

a transfer function adjuster configured to determine a total gain of the transmitter based on the input signal and the feedback signal, to derive an actual power amplifier gain based on the total gain and a known gain setting of the transmitter, and determine a correctional value for a parameter of the nonlinear transfer function based on a difference between the actual power amplifier gain and a target gain for the power amplifier.