Digitally Assisted Feedforward Compensation of Cartesian-Feedback Power-Amplifier Systems

This brief describes digitally assisted feedforward compensation to extend the bandwidth and to enhance the in-band linearity of Cartesian-feedback (CFB) power-amplifier (PA) systems. The proposed technique combines the strength of the CFB, which is chiefly relaxed PA modeling requirements, with the increased speed of digital predistortion (DPD). CFB is used initially to train efficiently a lookup table for a predistorted feedforward path. Then, both the CFB and the predistorted feedforward path run simultaneously during transmission. Viewed as a bandwidth-enhancement technique for analog CFB, the feedforward-compensated system shows a linearization bandwidth improvement of almost six times over the unaided feedback system. Viewed as a means of improving the in-band linearity of CFB, the proposed system realizes up to 10 dB of reduction in distortion products over CFB or DPD alone and does so without compromising the stability of the feedback system.