Frequency-dependent magnitude pre-distortion 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. Since these characteristics match those of a differentiator, a thorough correction of this part of the amplifier's distortion can be achieved using a differentiating filter. Embodiments of the present invention may be implemented in the baseband domain. Implementations may also be based on look-up tables that are adaptively updated to ensure optimal performance over time.
1. A method for reducing spurious emissions in an amplified signal, comprising the steps of:
(a) receiving an input signal; and
(b) applying pre-distortion to the 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 step (b) comprises the steps of:
(1) generating a main pre-distortion signal from the input signal;
(2) generating a secondary pre-distortion signal, whose magnitude and phase are frequency-dependent, from the input signal; and
(3) combining the main pre-distortion signal and the secondary pre-distortion signal to generate the pre-distorted signal, wherein the secondary pre-distortion signal has:
a magnitude proportional to a frequency offset from a carrier frequency of the input signal;
a phase shift of +90 degrees on one side of the carrier frequency; and
a phase shift of −90 degrees on the other side of the carrier frequency.
2. The invention of claim 1 , wherein the input signal is a baseband signal and the pre-distortion is applied in the baseband domain.
3. The invention of claim 1 , wherein step (b)(1) comprises the step of applying pre-distortion, whose magnitude and phase are frequency-independent, to the input signal to generate the main pre-distortion signal.
4. The invention of claim 1 , wherein the secondary pre-distortion signal is dependent on the bandwidth of the input signal.
5. The invention of claim 1 , wherein the secondary pre-distortion signal is based on a temporal differentiation of a signal generated from the input signal.
6. The invention of claim 5 , wherein the temporal differentiation is applied to a signal generated by complex multiplication between the input signal and a complex parameter dependent on the power of the input signal.
7. The invention of claim 5 , wherein the temporal differentiation is applied to the input signal prior to complex multiplication between the resulting differentiated signal and a complex parameter dependent on the power of the input signal.
8. The invention of claim 1 , wherein the frequency-dependent pre-distortion is based on data retrieved from a look-up table.
9. The invention of claim 8 , wherein the look-up table is adaptively updated according to control signals generated based on the amplified signal.
10. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:
(a) a main signal processing path configured to generate a main pre-distortion signal from an 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
(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion 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 secondary pre-distortion signal has:
a magnitude proportional to a frequency offset from a carrier frequency of the input signal;
a phase shift of +90 degrees on one side of the carrier frequency; and
a phase shift of −90 degrees on the other side of the carrier frequency.
11. The invention of claim 10 , wherein the input signal is a baseband signal and the apparatus applies the pre-distortion in the baseband domain.
12. The invention of claim 10 , wherein the main signal processing path is configured to apply frequency-independent magnitude and phase pre-distortion to the input signal to generate the main output signal.
13. The invention of claim 10 , wherein the secondary signal processing path comprises:
(1) a complex multiplication module configured to multiply the input signal by a complex parameter dependent on the power of the input signal; and
(2) a differentiating filter configured to differentiate the output of the complex multiplication module to generate the secondary pre-distortion signal.
14. The invention of claim 13 , wherein the differentiating filter comprises a cascade combination of a linear magnitude filter and a Hilbert transform filter.
15. The invention of claim 13 , wherein the differentiating filter is implemented using a single FIR or IIR filter.
16. The invention of claim 10 , wherein the secondary signal processing path comprises:
(1) a differentiating filter configured to differentiate the input signal; and
(2) a complex multiplication module configured to multiply the output of the differentiating filter by a complex parameter dependent on the power of the input signal to generate the secondary pre-distortion signal.
17. The invention of claim 10 , wherein the apparatus retrieves data for the frequency-dependent pre-distortion from a look-up table.
18. The invention of claim 17 , wherein the apparatus adaptively updates the look-up table according to control signals generated based on the amplified signal.
19. A method for reducing spurious emissions in an amplified signal, comprising the steps of:
(a) receiving an input signal; and
(b) applying pre-distortion to the 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 step (b) comprises the steps of:
(1) generating a main pre-distortion signal, whose magnitude and phase are frequency-independent, from the input signal;
(2) generating a secondary pre-distortion signal, whose magnitude and phase are frequency-dependent, from the input signal; and
(3) combining the main pre-distortion signal and the secondary pre-distortion signal to generate the pre-distorted signal, wherein the secondary pre-distortion signal is based on a temporal differentiation of a signal generated from the input signal.
20. The invention of claim 19 , wherein the temporal differentiation is applied to a signal generated by complex multiplication between the input signal and a complex parameter dependent on the power of the input signal.
21. The invention of claim 20 , wherein:
the complex multiplication is applied to (i) the input signal and (ii) parameters retrieved from a look-up table using an index value based on the envelope of the input signal; and
the signal generated by the complex multiplication is temporally differentiated to generate the secondary pre-distortion signal.
22. The invention of claim 19 , wherein the temporal differentiation is applied to the input signal prior to complex multiplication between the resulting differentiated signal and a complex parameter dependent on the power of the input signal.
23. The invention of claim 22 , wherein:
the complex multiplication is applied to (i) the result of temporally differentiating the input signal and (ii) parameters retrieved from a look-up table using an index value based on the envelope of the input signal; and
the signal generated by the complex multiplication is the secondary pre-distortion signal.
24. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:
(a) a main signal processing path configured to generate a main pre-distortion signal from an input signal, wherein the magnitude and phase of the main pre-distortion signal are frequency-independent;
(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
(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion 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 secondary signal processing path comprises:
(1) a complex multiplication module configured to multiply the input signal by a complex parameter dependent on the power of the input signal; and
(2) a differentiating filter configured to differentiate the output of the complex multiplication module to generate the secondary pre-distortion signal.
25. The invention of claim 24 , wherein the differentiating filter comprises a cascade combination of a linear magnitude filter and a Hilbert transform filter.
26. The invention of claim 24 , wherein the differentiating filter is implemented using a single FIR or IIR filter.
27. An apparatus for reducing spurious emissions in an amplified signal, wherein the apparatus comprises:
(a) a main signal processing path configured to generate a main pre-distortion signal from an input signal, wherein the magnitude and phase of the main pre-distortion signal are frequency-independent;
(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
(c) a combiner configured to combine the secondary pre-distortion signal with the main pre-distortion 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 secondary signal processing path comprises:
(1) a differentiating filter configured to differentiate the input signal; and
(2) a complex multiplication module configured to multiply the output of the differentiating filter by a complex parameter dependent on the power of the input signal to generate the secondary pre-distortion signal.