Digital pre-distortion of input signals for reducing spurious emissions in communication networks
View Patent ↗Pre-distortion, whose magnitude—and preferably phase—are frequency-dependent, is applied to an input signal in order to reduce spurious emissions resulting from subsequent amplification of the signal. In preferred embodiments, the pre-distortion technique of the present invention is implemented in combination with the (frequency-independent) magnitude and phase pre-distortion technique described in U.S. patent application Ser. No. 09/395,490 (“the '490 application”), where the frequency-dependent pre-distortion corresponds to amplifier distortion that has a magnitude that is proportional to the frequency offset from the carrier frequency and a phase shift of ±90° on either side of the carrier frequency. The frequency-dependent pre-distortion is generated by differentiating waveforms corresponding to pre-distortion parameters with respect to time and then applying the resulting differentiated pre-distortion parameters to the input signal. Depending on the implementation, the waveforms may be differentiated in either the analog or digital domain.
1. A method for reducing spurious emissions in an amplified signal by applying pre-distortion, whose magnitude is frequency-dependent, to an input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein the pre-distorted signal is generated by:
(a) generating one or more waveforms corresponding to one or more pre-distortion parameters;
(b) differentiating the one or more waveforms with respect to time to generate one or more differentiated waveforms;
(c) applying the one or more differentiated waveforms to the input signal to generate a frequency-dependent pre-distorted signal;
(d) generating a frequency-independent pre-distorted signal from the input signal; and
(e) combining the frequency-independent pre-distorted signal and the frequency-dependent pre-distorted signal to generate the pre-distorted signal.
2. The invention of claim 1 , wherein the phase of the pre-distortion is also frequency-dependent.
3. The invention of claim 1 , wherein the one or more pre-distortion parameters are retrieved from a look-up table using an index value based on a power level of the input signal.
4. The invention of claim 1 , wherein the one or more waveforms are differentiated in an analog domain.
5. The invention of claim 4 , wherein the input signal is in a non-baseband domain and the one or more differentiated waveforms are applied to the input signal in the non-baseband domain.
6. The invention of claim 1 , wherein the one or more waveforms are differentiated in a digital domain.
7. The invention of claim 6 , wherein the input signal is in a non-baseband domain and the one or more differentiated waveforms are converted to an analog domain for application to the input signal in the non-baseband domain.
8. The invention of claim 6 , wherein the input signal is in a baseband domain and the one or more differentiated waveforms are applied to the input signal in the baseband domain.
9. An apparatus for applying pre-distortion to an input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate an amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, the apparatus comprising:
(a) a main signal processing path configured to generate a main pre-distortion signal from the input signal;
(b) a secondary signal processing path configured to generate a secondary pre-distortion signal from the input signal, wherein:
the magnitude and phase of the secondary pre-distortion signal are frequency-dependent; and
the secondary pre-distortion signal is generated by:
(1) generating one or more waveforms corresponding to one or more pre-distortion parameters;
(2) differentiating the one or more waveforms with respect to time to generate one or more differentiated waveforms; and
(3) applying the one or more differentiated waveforms to the input signal to generate the pre-distorted signal; and
(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion signal to generate the pre-distorted signal.
10. The invention of claim 9 , wherein:
the input signal is a non-baseband signal;
the waveforms are differentiated in an analog domain; and
the pre-distortion is applied to the input signal in a non-baseband domain.
11. The invention of claim 10 , wherein:
the main signal processing path comprises:
(1) an envelope detector configured to generate an analog signal proportional to envelope power of the input signal;
(2) an analog-to-digital converter (ADC) configured to convert the analog signal from the envelope detector into a digital signal proportional to the envelope power of the input signal;
(3) a first look-up table configured to provide first and second pre-distortion parameters using the digital signal from the ADC as an index;
(4) a first set of digital-to-analog converters (DACs) configured to convert the first and second pre-distortion parameters into first and second analog control signals; and
(5) a first vector modulator configured to apply the first and second analog control signals to the input signal to generate the main pre-distortion signal; and
the secondary signal processing path comprises:
(1) a second look-up table configured to provide third and fourth pre-distortion parameters using the digital signal from the ADC as an index;
(2) a second set of DACs configured to convert the third and fourth pre-distortion parameters into third and fourth analog control signals;
(3) circuitry configured to differentiate the third and fourth analog control signals with respect to time; and
(4) a second vector modulator configured to apply the differentiated third and fourth analog control signals to the input signal to generate the secondary pre-distortion signal.
12. The invention of claim 11 , wherein:
the main signal processing path further comprises:
(6) a first delay element configured to delay the input signal prior to input to the first vector modulator; and
(7) a second delay element configured to delay the main pre-distortion signal prior to input to the combiner; and
the secondary signal processing path further comprises:
(5) a third delay element configured to delay the input signal prior to input to the second vector modulator.
13. The invention of claim 9 , wherein:
the input signal is a non-baseband signal;
the waveforms are differentiated in a digital domain; and
the pre-distortion is applied to the input signal in a non-baseband domain.
14. The invention of claim 13 , wherein:
the main signal processing path comprises:
(1) an envelope detector configured to generate an analog signal proportional to envelope power of the input signal;
(2) an ADC configured to convert the analog signal from the envelope detector into a digital signal proportional to the envelope power of the input signal; and
(3) a first look-up table configured to provide first and second pre-distortion parameters using the digital signal from the ADC as an index;
the secondary signal processing path comprises:
(1) a second look-up table configured to provide third and fourth pre-distortion parameters using the digital signal from the ADC as an index; and
(2) circuitry configured to differentiate the third and fourth pre-distortion parameters with respect to time; and
the combiner comprises:
(1) a set of adders configured to combine the first and second pre-distortion parameters with the differentiated third and fourth pre-distortion parameters to generate first and second combined pre-distortion parameters;
(2) a set of DACs configured to convert the first and second combined pre-distortion parameters into first and second analog control signals; and
(3) a vector modulator configured to apply the first and second analog control signals to the input signal to generate the pre-distorted signal.
15. The invention of claim 14 , wherein:
the main signal processing path further comprises:
(4) a first delay element configured to delay the digital signal from the ADC prior to input to the first look-up table; and
the apparatus further comprises a second delay element configured to delay the input signal prior to input to the vector modulator.
16. The invention of claim 14 , wherein the combiner is configured to convert the first and second combined pre-distortion parameters from rectangular coordinates into a coordinate system corresponding to the vector modulator.
17. The invention of claim 9 , wherein:
the input signal is a baseband signal;
the waveforms are differentiated in a digital domain; and
the pre-distortion is applied to the input signal in a baseband domain.
18. The invention of claim 17 , wherein:
the main signal processing path comprises:
(1) an index generator configured to generate index values proportional to envelope power of the input signal; and
(2) a first look-up table configured to provide first and second pre-distortion parameters using the index values;
the secondary signal processing path comprises:
(1) a second look-up table configured to provide third and fourth pre-distortion parameters using the index values; and
(2) circuitry configured to differentiate the third and fourth pre-distortion parameters with respect to time; and
the combiner comprises:
(1) a set of adders configured to combine the first and second pre-distortion parameters with the differentiated third and fourth pre-distortion parameters to generate first and second combined pre-distortion parameters; and
(2) a complex multiplier configured to apply complex multiplication between the first and second combined pre-distortion parameters and the input signal to generate the pre-distorted signal.
19. The invention of claim 18 , wherein:
the main signal processing path further comprises:
(3) a first delay element configured to delay the index values prior to input to the first look-up table; and
the apparatus further comprises a second delay element configured to delay the input signal prior to input to the complex multiplier.
20. A method for reducing spurious emissions in an amplified signal by applying pre-distortion, whose magnitude is frequency-dependent, to an input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate the amplified signal, the pre-distortion reduces spurious emissions in the amplified signal, wherein the pre-distorted signal is generated by:
(a) generating one or more waveforms corresponding to one or more pre-distortion parameters;
(b) differentiating the one or more waveforms with respect to time to generate one or more differentiated waveforms; and
(c) applying the one or more differentiated waveforms to the input signal to generate the pre-distorted signal, wherein the input signal is in a non-baseband domain and the one or more differentiated waveforms are applied to the input signal in the non-baseband domain.
21. The invention of claim 20 , wherein the one or more waveforms are differentiated in an analog domain.
22. The invention of claim 20 , wherein the one or more waveforms are differentiated in a digital domain.