Real-time Calibration for Digital Beamforming in 5G Systems With Experiments on Testbed

Beamforming is a core physical layer technique of the New Radio (NR) air interface, which is the standard for 5G specified by 3GPP. Beamforming can improve system coverage and capacity by focusing transmitted energy toward a certain region or user equipment rather than radiating energy in many directions. However it requires accurate control of relative phases between transmission chains in order to form the desirable beams. In practice, due to unavoidable properties of the hardware such as the unequal lengths of RF transmission lines, the imperfect synchronization between all the active chips, those relative phases can not be guaranteed unless those components are properly calibrated. In this paper, we propose a real-time calibration method for digital beamforming in 5G systems with validation using measurement on hardware prototype. By using a built-in calibration line which is connected to all transceivers in the circuit of the radio unit, reference signal for calibration can be transmitted and captured in real time. The captured data then be processed using the proposed calibration algorithm to extract the relative phase errors between transceivers, then they can be compensated to suppress those errors to certain threshold in order to form the desirable beams. Measurements on hardware prototype show that using the proposed calibration procedure, the radiation patterns are in good agreement with simulation for different beam angles.