Array Calibration and Digital Predistortion Training Using Embedded Near-Field Feedback Probes and Orthogonal Coding for Enhancing the Performance of Millimeter-Wave Beamforming Arrays

Abstract

This letter proposes an active array calibration and digital predistortion (DPD) training method that relies on a series of measurement data captured using near-field (NF) probes embedded within the array to enhance the performance of millimeter-wave (mm-wave) radio frequency (RF) beamforming arrays. These measurements are obtained using phase settings that are based on orthogonal coding to enable the characterization of the linear and nonlinear errors in the array’s RF chains. The use of embedded NF probes in the proposed method makes it suitable for infield testing. Specifically, the proposed theory is formulated to allow for beamforming-phase-dependent error calibration as well as array linearization without resorting to element-wise measurement or far-field (FF)-based feedback. Furthermore, the proposed theory does not impose a flat coupling requirement between the NF probes and the array antenna elements. Experimental results are conducted on a custom-built 16-element RF beamforming array with four embedded NF probes and operated at 37.5 GHz. The measurements revealed that applying the proposed calibration method reduced the imbalance in the radiation pattern side lobes by up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2$</tex-math> </inline-formula> dB and achieved comparable performance to element-wise FF-based calibration. Furthermore, the proposed DPD training method enabled increasing the effective isotropic radiated power (EIRP) from 32 to 34.2 dBm while maintaining an error vector magnitude (EVM) below 3.5%.