Uniformly Distributed Near-Field Probing Array for Enhancing the Performance of 5G Millimeter-Wave Beamforming Transmitters

This letter proposes a uniformly distributed near-field (NF) probing array (NFPA) capable of generating feedback signals suitable for monitoring and improving the performance of a 5G millimeter-wave (mm-wave) beamforming transmitter. The proposed NFPA configuration is carefully designed to ensure comparable coupling ratios between the individual RF chain outputs and the closest NF probes, across a wide frequency range. This maximizes the quality of the feedback signal available to perform array calibration and digital predistortion (DPD) when using realistic 5G modulated signals. A 16-element 27-30-GHz beamforming transmitter is designed and serves as the proof-of-concept prototype. Using a four-element NFPA, the 16 RF chains are calibrated using a 400-MHz modulated signal-achieving less than 0.27 dB and 2.8 degrees root-mean-square (RMS) magnitude and phase calibration errors, respectively. Furthermore, a DPD engine trained using the NFPA-based feedback signal improves the over-the-air transmitted signal's adjacent channel power ratio (ACPR) and error vector magnitude (EVM) from 30 dB and 8.8% to 39 dB and 2.5%, respectively. The DPD also allowed for an increase of the effective isotropic radiated power (EIRP) from 27.5 to 33 dBm while staying below the maximum EVM of 3.5%.