Open-Loop Digital Predistortion Using Cartesian Feedback for Adaptive RF Power Amplifier Linearization

We present a new adaptive power amplifier (PA) linearization technique. We leverage analog Cartesian feedback (CFB) to train a Cartesian look-up table, reducing DSP and power amplifier modeling requirements to a minimum and eliminating model convergence as a design issue. Because the CFB system does not continuously operate, we overcome the bandwidth limitation traditionally associated with this technique. In addition, we exploit sample averaging to greatly relax the noise requirements of the analog feedback path. We implemented a prototype 900-MHz direct-conversion transmitter with a class-A PA. We measured a 10-dB reduction of out-of-band distortion products with no noise floor degradation for 40-MHz-bandwidth, 16-QAM test signals.