Local oscillator clock shaping for pre-distortion
Apparatus and methods for pre-distorting a radio frequency transmit signal based on local oscillator clock shaping are disclosed. In certain embodiments, one or more clock signals generated by a local oscillator and used for mixing in a transceiver are shaped to account for non-linearity of a power amplifier that amplifies the radio frequency transmit signal. Such pre-distortion can be performed in addition to or alternatively to performing digital pre-distortion on a digital representation of the radio frequency transmit signal.
1. A radio frequency communication system comprising:
a power amplifier configured to amplify a radio frequency transmit signal; and
a transceiver configured to output the radio frequency transmit signal, wherein the transceiver includes at least one mixer configured to generate the radio frequency transmit signal based on providing frequency upconversion to at least one baseband transmit signal, and a local oscillator configured to pre-distort the radio frequency transmit signal to compensate for a non-linearity of the power amplifier by providing one or more shaped clock signals to the at least one mixer.
2. The radio frequency communication system of claim 1 , wherein the local oscillator includes a numerically controlled oscillator (NCO).
3. The radio frequency communication system of claim 1 , wherein the one or more shaped clock signals include a shaped sine signal and a shaped cosine signal.
4. The radio frequency communication system of claim 1 , wherein the one or more shaped clock signals include a first shaped clock signal generated by a weighted sum of a sine signal and at least one sine harmonic signal, and a second shaped clock signal generated by a weighted sum of a cosine signal and at least one cosine harmonic signal.
5. The radio frequency communication system of claim 1 , wherein the local oscillator includes a plurality of coefficients that are adapted to provide pre-distortion based on feedback from an output of the power amplifier.
6. The radio frequency communication system of claim 5 , wherein the local oscillator receives an envelope signal indicating an envelope of the radio frequency transmit signal, wherein the coefficients are a function of the envelope signal.
7. The radio frequency communication system of claim 1 , wherein the transceiver further includes a digital transmit circuit configured to provide digital pre-distortion to the at least one baseband transmit signal.
8. The radio frequency communication system of claim 7 , wherein the digital transmit circuit is configured to provide a digital transmit phase signal to the local oscillator.
9. The radio frequency communication system of claim 8 , wherein the local oscillator is configured to generate a total phase signal based on adding the digital transmit phase signal and a local oscillator phase signal.
10. The radio frequency communication system of claim 9 , wherein the local oscillator includes an accumulator configured to generate the local oscillator phase signal based on accumulating a frequency control word.
11. The radio frequency communication system of claim 9 , wherein the local oscillator includes a look-up table configured to generate the one or more shaped clock signals based on the total phase signal.
12. The radio frequency communication system of claim 11 , wherein the digital transmit circuit is further configured to provide a transmit amplitude signal to the local oscillator, wherein the look-up table is indexed by both the total phase signal and the transmit amplitude signal.
13. The radio frequency communication system of claim 1 , wherein the local oscillator is configured to pre-distort the radio frequency transmit signal based on detecting a derivative of the at least one baseband transmit signal.
14. The radio frequency communication system of claim 13 , wherein the local oscillator is configured to pre-distort the radio frequency transmit signal to account for an asymmetric drive of the power amplifier.
15. A transceiver comprising:
a digital transmit circuit configured to generate at least one baseband transmit signal;
at least one mixer configured to generate a radio frequency transmit signal based on providing frequency upconversion to the at least one baseband transmit signal; and
a local oscillator configured to pre-distort the radio frequency transmit signal by providing one or more shaped clock signals to the at least one mixer.
16. The transceiver of claim 15 , wherein the local oscillator includes a plurality of coefficients that are adapted to provide pre-distortion based on feedback from an output of a power amplifier that amplifies the radio frequency transmit signal.
17. The transceiver of claim 16 , wherein the local oscillator receives an envelope signal indicating an envelope of the radio frequency transmit signal, wherein the coefficients are a function of the envelope signal.
18. The transceiver of claim 15 , wherein the digital transmit circuit is configured to provide a digital transmit phase signal to the local oscillator.
19. The transceiver of claim 18 , wherein the local oscillator is configured to generate a total phase signal based on adding the digital transmit phase signal and a local oscillator phase signal.
20. A method of pre-distortion in a transceiver, the method comprising:
generating at least one baseband transmit signal using a digital transmit circuit;
generating a radio frequency transmit signal based on mixing the at least one baseband transmit signal using at least one mixer; and
pre-distorting the radio frequency transmit signal by providing one or more shaped clock signals from a local oscillator to the at least one mixer.