Systems and methods for spurious emission cancellation
View Patent ↗In accordance with some embodiments of the present disclosure, a method may include digitally pre-distorting a digital baseband signal with an opposite phase of a C-IM3 distortion term such that the pre-distortion and C-IM3 distortion cancel each other out in a transmitter. The method may also include digitally conditioning the pre-distorted digital signal in order to provide a flat amplitude response of a composite filter comprising a baseband filter and a digital half-band filter of the transmitter, and to provide a linear phase response for the composite filter.
1. A transmitter comprising:
a digital-to-analog converter configured to convert a digital baseband signal into an analog baseband signal;
a baseband filter configured to filter the analog baseband signal;
an upconverter configured to modulate the analog baseband signal to provide a radio-frequency signal;
a variable gain amplifier configured to amplify the radio-frequency signal;
a digital pre-distortion module configured to pre-distort the digital baseband signal with the opposite phase of a C-IM3 distortion term such that the pre-distortion and C-IM3 distortion cancel each other out; and
a digital pre-emphasis filter configured to condition the pre-distorted digital signal in order to provide a flat amplitude response of a composite filter comprising the baseband filter and a digital half-band filter, and to provide a linear phase response for the composite filter.
2. A transmitter according to claim 1 , wherein the response of the digital pre-distortion module is based on one or more pre-distortion terms, each pre-distortion term including a third-order polynomial, wherein variables of the polynomial comprise frequency response components of the digital baseband signal and a coefficient of a third-order term of the polynomial comprises a pre-distortion coefficient.
3. A transmitter according to claim 2 , wherein the pre-distortion coefficient is determined at power on or reset of the transmitter based on a search algorithm.
4. A transmitter according to claim 3 , wherein:
the pre-distortion coefficient is, at power on or reset, swept across a range of potential values;
amplitude of C-IM3 distortion is measured for each of the values; and
the value providing the lowest amplitude C-IM3 distortion is selected as the pre-distortion coefficient for operation of the pre-distortion module.
5. A transmitter according to claim 4 , wherein the value of the pre-distortion coefficient may be modified during operation in response to changes in operating characteristics of the transmitter.
6. A transmitter according to claim 5 , wherein the operating characteristics may comprise at least one of a change in a gain of the variable gain amplifier; a change in a digital gain of the transmit path; and a change in temperature of one of more components of the transmitter.
7. A transmitter according to claim 2 , wherein the pre-distortion coefficient is determined by lab validation of the transmitter and is stored in a memory of the transmitter.
8. A transmitter according to claim 2 , wherein the pre-distortion coefficient is determined by hardware testing of the transmitter based on analysis of the C-IM3 distortion term and stored in a memory of the transmitter, further wherein the pre-distortion coefficient may be loaded from the memory during power on or reset of the transmitter.
9. A transmitter according to claim 1 , wherein the pre-emphasis filter is selected from a plurality of possible pre-emphasis filters such that the selected pre-emphasis filter provides a desired 3-dB frequency for the composite filter.
10. A transmitter according to claim 7 , wherein characteristics of the pre-emphasis filter may be modified during operation in response to changes in operating characteristics of the transmitter.
11. A wireless communication element, comprising:
a receive path configured to receive wireless communications signal and convert the wireless communications signal into digital signals; and
a transmit path configured to digital signals into a wireless communication signals, the transmit path comprising;
a digital-to-analog converter configured to convert a digital baseband signal into an analog baseband signal;
a baseband filter configured to filter the analog baseband signal;
an upconverter configured to modulate the analog baseband signal to provide a radio-frequency signal;
a variable gain amplifier configured to amplify the radio-frequency signal;
a digital pre-distortion module configured to pre-distort the digital baseband signal with the opposite phase of a C-IM3 distortion term such that the pre-distortion and C-IM3 distortion cancel each other out; and
a digital pre-emphasis filter configured to condition the pre-distorted digital signal in order to provide a flat amplitude response of a composite filter comprising the baseband filter and a digital half-band filter, and to provide a linear phase response for the composite filter.
12. An element according to claim 11 , wherein the response of the digital pre-distortion module is based on one or more pre-distortion terms, each pre-distortion term including a third-order polynomial, wherein variables of the polynomial comprise frequency response components of the digital baseband signal and a coefficient of a third-order term of the polynomial comprises a pre-distortion coefficient.
13. An element according to claim 12 , wherein the pre-distortion coefficient is determined at power on or reset of the element based on a search algorithm.
14. An element according to claim 13 , wherein:
the pre-distortion coefficient is, at power on or reset, swept across a range of potential values;
amplitude of C-IM3 distortion is measured for each of the values; and
the value providing the lowest amplitude C-IM3 distortion is selected as the pre-distortion coefficient for operation of the pre-distortion module.
15. An element according to claim 14 , wherein the value of the pre-distortion coefficient may be modified during operation in response to changes in operating characteristics of the element.
16. An element according to claim 15 , wherein the operating characteristics may comprise at least one of a change in a gain of the variable gain amplifier; a change in a digital gain of the transmit path; and a change in temperature of one of more components of the element.
17. A element according to claim 12 , wherein the pre-distortion coefficient is determined by lab validation of the transmitter and is stored in a memory of the transmitter.
18. A element according to claim 12 , wherein the pre-distortion coefficient is determined by hardware testing of the element based on analysis of the C-IM3 distortion term and stored in a memory of the element, further wherein the pre-distortion coefficient may be loaded from the memory during power on or reset of the element.
19. An element according to claim 11 , wherein the pre-emphasis filter is selected from a plurality of possible pre-emphasis filters such that the selected pre-emphasis filter has a desired 3-dB frequency for the composite filter.
20. An element according to claim 19 , wherein characteristics of the pre-emphasis filter may be modified during operation in response to changes in operating characteristics of the element.
21. A method comprising:
digitally pre-distorting a digital baseband signal with an opposite phase of a C-IM3 distortion term such that the pre-distortion and C-IM3 distortion cancel each other out in a transmitter; and
digitally conditioning the pre-distorted digital signal in order to provide a flat amplitude response of a composite filter comprising a baseband filter and a digital half-band filter of the transmitter, and to provide a linear phase response for the composite filter.
22. A method according to claim 21 , wherein the response of the digital pre-distortion module is based on one or more pre-distortion terms, each pre-distortion term including a third-order polynomial, wherein variables of the polynomial comprise frequency response components of the digital baseband signal and a coefficient of a third-order term of the polynomial comprises a pre-distortion coefficient, the method further comprising:
at power on or reset, sweeping the pre-distortion coefficient across a range of potential values;
measuring an amplitude of C-IM3 distortion for each of the values; and
selecting the value providing the lowest amplitude C-IM3 distortion as the pre-distortion coefficient for digitally pre-distorting the digital baseband signal.
23. A method according to claim 22 , further comprising modifying the value of the pre-distortion coefficient during operation in response to changes in operating characteristics of the transmitter.
24. A method according to claim 21 , further comprising selecting the pre-emphasis filter from a plurality of possible pre-emphasis filters such that the selected pre-emphasis filter provides a desired 3-dB frequency for the composite filter.