Systems and methods of compensating a transmit signal for charge trapping effects of a power amplifier
Systems and methods for compensating a transmit signal for charge trapping effects of a power amplifier are provided. In certain embodiments, a non-linear filter is trained based on time aligning a first set of observations taken from digital transmit data prior to conversion to a radio frequency transmit signal, and a second set of observations taken from an output of a power amplifier that amplifies the radio frequency transmit signal. In certain implementations, the first set of observations and the second set of observations are obtained without decimation. Rather, decimation is provided after timing alignment. By implementing the DPD system in this manner, signal data is not lost by decimation and more accurate timing alignment between the sets of observations is achieved.
1. A radio frequency (RF) communication system comprising:
a transmitter configured to receive an input transmit signal and to output an RF transmit signal; and
a power amplifier configured to amplify the RF transmit signal,
wherein the transmitter comprises a digital pre-distortion (DPD) system configured to process the input transmit signal to pre-distort the RF transmit signal, the DPD system including a first non-linear filter along a first signal path and a second non-linear filter along a second signal path in parallel with the first signal path, wherein the DPD system is configured to train the second non-linear filter based on both a first set of observations captured from the first signal path and a second set of observations captured from the RF transmit signal.
2. The RF communication system of claim 1 , wherein the second non-linear filter compensates for charge trapping effects of the power amplifier.
3. The RF communication system of claim 1 , wherein the second non-linear filter is a Laguerre actuator.
4. The RF communication system of claim 3 , wherein the first non-linear filter is a generalized memory polynomial (GMP) actuator.
5. The RF communication system of claim 1 , wherein the first set of observations and the second set of observations are captured without any decimation.
6. The RF communication system of claim 1 , wherein the DPD system comprises a first capture buffer configured to capture the first set of observations, a second capture buffer configured to capture the second set of observations, and a time alignment block configured to time align an output of the first capture buffer and an output of the second capture buffer.
7. The RF communication system of claim 6 , wherein the DPD system is configured to update a plurality of features of the second non-linear filter based on a difference between the output of the first capture buffer and the output of the second capture buffer after time alignment.
8. The RF communication system of claim 6 , wherein the DPD system further comprises a cascade integrator comb (CIC) decimator along the second signal path, wherein the DPD system is further configured to update the plurality of features of the second non-linear filter based on an estimate of an output of the CIC decimator.
9. The RF communication system of claim 6 , wherein the DPD system further comprises a third capture buffer configured to capture a third set of observations from the second signal path.
10. The RF communication system of claim 1 , wherein the DPD system further comprises a crest factor reduction (CFR) circuit in cascade with the first non-linear filter, the first set of observations captured from an output of the CFR circuit.
11. The RF communication system of claim 1 , wherein the second non-linear filter comprises a plurality of infinite impulse response (IIR) filters.
12. The RF communication system of claim 1 , wherein a plurality of tap coefficients of the second non-linear filter are fixed, and wherein a plurality of weightings for summing a plurality of outputs of the second non-linear filter are updated based on training of the second non-linear filter.
13. A transmitter comprising:
a first non-linear filter along a first signal path configured to process an input transmit signal;
a second non-linear filter along a second signal path configured to process the input signal, wherein the first signal path and the second signal path are in parallel and operate to generate a digitally pre-distorted input transmit signal;
a digital-to-analog converter along a third signal path configured to process the digitally pre-distorted input transit signal to generate a radio frequency (RF) transmit signal; and
a training system configured to train the second non-linear filter based on both a first set of observations captured from the first signal path and a second set of observations captured from the RF transmit signal after amplification by a power amplifier.
14. The transmitter of claim 13 , wherein the first non-linear filter is a generalized memory polynomial (GMP) actuator and the second non-linear filter is a Laguerre actuator.
15. The transmitter of claim 13 , wherein the first set of observations and the second set of observations are captured without any decimation.
16. The transmitter of claim 13 , further comprising a first capture buffer configured to capture the first set of observations, a second capture buffer configured to capture the second set of observations, and a time alignment block configured to time align an output of the first capture buffer and an output of the second capture buffer.
17. The transmitter of claim 16 , wherein the DPD system is configured to update a plurality of features of the second non-linear filter based on a difference between the output of the first capture buffer and the output of the second capture buffer after time alignment.
18. The transmitter of claim 16 , wherein the DPD system further comprises a cascade integrator comb (CIC) decimator along the second signal path, wherein the DPD system is further configured to update the plurality of features of the second non-linear filter based on an estimate of an output of the CIC decimator.
19. The transmitter of claim 13 , wherein the DPD system further comprises a crest factor reduction (CFR) circuit in cascade with the first non-linear filter, the first set of observations captured from an output of the CFR circuit.
20. A method of digital pre-distortion, the method comprising:
digitally pre-distorting an input transmit signal to generate a radio frequency (RF) transmit signal using a first non-linear filter and a second non-linear filter of a digital pre-distortion system, the first non-linear filter along a first signal path and the second non-linear filter along a second signal path that is in parallel with the first signal path;
amplifying the RF transmit signal using a power amplifier; and
training the second non-linear filter based on both a first set of observations captured from the first signal path, and a second set of observations captured from the RF transmit signal after amplification by a power amplifier.