Digital Predistortion of Millimeter-Wave Phased Array Transmitter with Over-The-Air Calibrated Simplified Conductive Feedback Architecture
Digital predistortion (DPD) of a phased array requires that multiple transmit paths must be measured by a feedback (FB) receiver (Rx). This paper proposes a simplified FB architecture to be used for phased array DPD. A single FB line collects the waveform samples from the parallel paths to the FB Rx. The gain and phase of the common FB line is obtained by comparing the FB outputs to over-the-air (OTA) measurements. The overall DPD training is done by collecting the PA outputs with the common FB line, post-equalizing them to model the main lobe waveform and combining them to create the object used for array DPD. The DPD performance was verified by OTA measurements with 5GNR waveform and 28 GHz 8-path phased array transmitter. The DPD trained through the local FB line with the proposed calibration method achieved < -45dB ACPR which was close to OTA DPD performance.