Digital compensator
An approach to digital compensation uses a particular structure for a digital pre-distorter (DPD) which acts as a relatively accurate pre-inverse of a non-linear circuit (e.g., a non-linear transmit chain involving digital-to-analog converter (DAC), lowpass filter, modulator, bandpass filter, and power amplifier) while making use of a relatively small number of parameters that characterize the non-linearity and/or parameters that provide accurate linearization without requiring continual updating.
1. A method for digital predistortion of an input signal for application to a circuit exhibiting non-linear characteristics, the method comprising:
accessing a plurality of coefficient values from a stored database of coefficients;
receiving the input signal representing a desired signal for emission via the circuit;
forming a plurality of non-linear transformations of the input signal;
determining values of a plurality of measurement variables, the measurement variables including at least one of (a) an environmental variable, (b) a device signature variable, and (c) a feedback variable received from the device based on prior emission of a signal from the device;
processing the plurality of coefficient values according to the plurality of measurement variables to determine a plurality of predistortion parameters, including interpolating coefficient values according to the measurement variables to yield at least some of the predistortion parameters;
forming a predistorted signal for input to the circuit, including combining the non-linear transformations and the coefficient values to form the predistorted signal; and
providing the predistorted signal for input to the circuit.
2. The method of claim 1 , wherein the coefficient values remain fixed during operation of the circuit.
3. The method of claim 1 , wherein interpolating the coefficient values according to the measurement variables includes at least one of (a) linear interpolation, (b) piecewise linear interpolation, (c) piecewise smooth function interpolation, and (d) kernel-based interpolation, of the coefficient values.
4. The method of claim 1 , wherein interpolating the coefficient values according to the measurement variables includes applying a mathematical mapping to the measurement variables, wherein the mathematical mapping is represented by the coefficient values.
5. The method of claim 1 , wherein the determining of the values of the measurement variables, the processing of the coefficient values, and forming of the predistorted signal are repeated for successive portions of the input signal.
6. The method of claim 5 wherein the coefficient values remain fixed over the repetitions for the successive portions of the input signal.
7. The method of claim 1 , wherein forming the non-linear transformations comprises upsampling the input signal and forming the non-linear transformations based on the upsampled input signal.
8. The method of claim 1 , wherein forming the non-linear transformations comprises forming at least some of the non-linear transformations as polynomial functions of the input signal.
9. The method of claim 7 , wherein forming the non-linear transformations further comprises forming at least some other of the non-linear transformations as non-polynomial functions of the input signal.
10. The method of claim 1 , wherein forming the non-linear transformations comprises time-domain filtering at least one non-linear transformation of the input signal.
11. The method of claim 1 , wherein forming the non-linear transformations comprises includes forming at least one degree-1 product of terms, each term including (a) the input signal, (b) a delayed version of the input signal, (c) a complex conjugate of the input signal, or (d) a delayed version of the input signal, raised to a respective integer power for the term.
12. The method of claim 11 , wherein combining the non-linear transformations and the coefficient values includes computing a value of a balanced polynomial function.
13. The method of claim 1 , wherein the measurement variables include (a) the environmental variable.
14. The method of claim 13 , further comprising measuring said environmental variable, said environmental variable including a variable from a group consisting of a temperature, a supply voltage to the circuit, a transmit power, a modulation mode, a part age, a sampling rate, and a frequency band.
15. The method of claim 1 , wherein the measurement variables include (b) the device signature variable.
16. The method of claim 15 , further comprising obtained obtaining a value of the device signature variable by providing a plurality of test inputs to the device.
17. The method of claim 1 , wherein the measurement variables include (c) a feedback variable received from the device based on prior emission of the signal from the device.
18. The method of claim 17 , further comprising determining a value of the feedback variable including demodulating a radio-frequency output from the circuit in response to an input to the circuit.