A new adjustment method for the frequency-dependent IMD compensator of the digital predistortion linearizer

A new adjustment method is proposed that controls the frequency characteristics compensator of a digital predistortion linearizer (DPDL) when modulated signals are input for digital mobile communications. The digital predistortion linearizer is equipped with an odd-order distortion generator, a vector regulator and a frequency characteristics compensator in the distortion generation paths. In order to compensate the frequency-dependent intermodulation distortion (IMD) components, the proposed method divides them into narrow bandwidth segments whose frequency characteristics can be approximated as flat. The proposed method ensures that the frequency characteristics compensator has uniform amplitude and phase in each segment and adjusts them repeatedly until the IMD components are suppressed in all segments. Experiments show that the DPDL with the proposed method can compensate frequency-dependent IMD. Adjacent channel leakage power ratios at ±5 MHz offset frequency are suppressed to under -45 dB at the output back-off point of 6.6 dB from the saturation output power for a root roll-off QPSK signal.