Frequency selective predistortion in a linear transmitter

A method for determining a transfer characteristic for compensating for distortion caused by a power amplifier, the method comprising:

• storing a plurality of modulation signal samples to be amplified by the power amplifier;
• storing a plurality of predistorted signal samples, wherein each modulation signal sample of said plurality of modulation signal samples has a corresponding predistorted signal sample of said plurality of predistorted signal samples;
• sampling a signal that depends on an output signal of the power amplifier to obtain a plurality of output signal samples, wherein each predistorted signal sample of said plurality of signal samples has a corresponding output signal sample of said plurality of output signal samples;
• removing the in-band distortion from said plurality of output signal samples to form a plurality of modified output signal samples;
• storing said plurality of modified output signal samples;
• determining an inverse transfer characteristic of the power amplifier for each modified output signal sample as a function of the modified output signal sample and a previous modified output signal sample, wherein each inverse transfer characteristic includes a first complex gain and a second complex gain, said first complex gain representing a gain of a level of the modified output signal sample to a level of the corresponding modulation signal sample, said second complex gain representing a gain of an average of the levels of the modified output signal sample and the previous modified output signal sample to an average of the levels of the corresponding modulation signal sample and corresponding previous modulation signal sample, whereby a plurality of inverse transfer characteristics are determined;
• quantizing said plurality inverse transfer characteristics into a plurality of bins, each bin of said plurality of bins being mapped to a corresponding address of a table;
• for each bin of said plurality of bins, averaging any inverse transfer characteristics within each bin;
• and
• storing at each address of said table a predistortion gain, each predistortion gain being a function of the inverse transfer characteristic of the bin corresponding to the predistortion gain's address.

An apparatus for determining a gain for compensating for distortion caused by a power amplifier, the apparatus comprising:

• a buffer;
• means for storing a plurality of modulation signal samples to be amplified by the power amplifier in the buffer;
• means for storing a plurality of predistorted signal samples in the buffer, wherein each modulation signal sample of said plurality of modulation signal samples has a corresponding predistorted signal sample of said plurality of predistorted signal samples;
• sampling means for sampling a signal that depends on an output signal of the power amplifier to obtain a plurality of output signal samples, wherein each predistorted signal sample of said plurality of predistorted signal samples has a corresponding output signal sample of said plurality of output signal samples, said sampling means storing said plurality of output signal samples in the buffer;
• means for removing the in-band distortion from said plurality of output signal samples to form a plurality of modified output signal samples;
• means for determining an inverse transfer characteristic of the power amplifier for each modified output signal sample as a function of the modified output signal sample and a previous modified output signal sample, wherein each inverse transfer characteristic includes a first complex gain and a second complex gain, said first complex gain representing a gain of the modified output signal sample's power to the corresponding modulation signal sample's power, second complex gain representing a gain of an average of the powers of the modified output signal sample and the previous modified output signal sample to an average of the powers of the corresponding modulation signal sample and corresponding previous modulation signal sample, whereby a plurality of inverse transfer characteristics are determined;
• a quantizer coupled to the buffer, wherein said quantizer is configured to quantize said plurality of inverse transfer characteristics inverse into a plurality of bins, each bin of said plurality of bins being mapped to a corresponding address of a table;
• an averager coupled to the buffer, wherein for each bin of said plurality of bins, said averager is configured to average any inverse transfer characteristics within each bin;
• and
• means for storing at each address of said table a predistortion gain, each predistortion gain being a function of the inverse transfer characteristic of the bin corresponding to the predistortion gain's address.