Wide bandwidth digital predistortion system with reduced sampling rate
A digital predistortion linearization method is provided for increasing the instantaneous or operational bandwidth for RF power amplifiers employed in wideband communication systems. Embodiments of the present invention provide a method of increasing DPD linearization bandwidth using a feedback filter integrated into existing digital platforms for multi-channel wideband wireless transmitters. An embodiment of the present invention utilizes a DPD feedback signal in conjunction with a low power band-pass filter in the DPD feedback path.
1. A wideband communications system comprising:
a digital predistorter (DPD) operable to receive an input signal, wherein the DPD is characterized by a first bandwidth;
a filter characterized by a second bandwidth coupled to the output of the DPD;
a digital-to-analog converter coupled to the output of the filter;
a modulator coupled to the output of the digital-to-analog converter;
a power amplifier coupled to the output of the modulator;
a band-pass filter characterized by a third bandwidth coupled to the output of the power amplifier;
a down-converter coupled to the output of the band-pass filter; and
an analog-to-digital converter (ADC) coupled to the output of the down-converter, wherein the ADC is characterized by a sampling rate value less than a value of the first bandwidth,
wherein the DPD comprises a coefficient estimator applying an indirect learning algorithm.
2. The wideband communications system of claim 1 , wherein the sampling rate value of the ADC is less than one-third the value of the first bandwidth.
3. The wideband communications system of claim 1 , wherein the third bandwidth is less than the first bandwidth.
4. The wideband communications system of claim 1 , wherein the third bandwidth is substantially equal to the second bandwidth.
5. The wideband communications system of claim 1 , wherein the band-pass filter comprises a low power narrowband band-pass filter.
6. The wideband communications system of claim 1 , wherein the filter comprises a narrowband digital filter.
7. The wideband communications system of claim 1 , wherein the band-pass filter comprises a radio frequency (RF) filter.
8. The wideband communications system of claim 1 , wherein the coefficient estimator comprises a polynomial function generator, a digital filter, and a least square algorithm.
9. The wideband communications system of claim 1 , further comprising a duplexer coupled to the output of the power amplifier.
10. A method of operating a communications system, the method comprising:
receiving a signal at a digital predistorter (DPD);
introducing predistortion to the signal using the DPD;
filtering the predistorted signal using a digital filter;
converting the filtered signal to an analog signal;
modulating the analog signal;
amplifying the modulated signal;
coupling a portion of the amplified signal to provide a feedback signal;
filtering the feedback signal using a band-pass filter;
downconverting the filtered feedback signal;
converting the downconverted signal to a digital signal; and
providing the digital signal to a coefficient estimator in the DPD, wherein the coefficient estimator applies an indirect learning algorithm.
11. The method of claim 10 , wherein filtering the predistorted signal is performed over a filter bandwidth less than a bandwidth of the DPD.
12. The method of claim 10 , wherein filtering the feedback signal using the band-pass filter is performed over a band-pass bandwidth less than a bandwidth of the DPD.
13. The method of claim 10 , wherein converting the downconverted signal is performed at a sampling rate less than twice the bandwidth of the DPD.
14. The method of claim 10 , wherein providing the digital signal to the coefficient estimator in the DPD comprises:
creating nonlinear components of the digital signal;
restricting a bandwidth of the nonlinear components; and
optimizing coefficients in the DPD.