Over-the-Air Calibration of mmW Imaging Radars Using Uncorrelated Continuous Wave Signals

Magnitude and phase variations of radio-frequency channels in multiple-input multiple-output imaging radars require an accurate calibration in order to reduce side-lobe levels in the radar image and thus counteract missed detections. Due to the trend towards increased angular resolution, the aperture sizes of mmWave radars are further increasing. As result, achieving far-field conditions for calibration purposes is getting tougher, since the distance for a valid physical-target based calibration rises quadratic with the aperture size. Since it is desirable to keep the calibration environment at lab size, it is beneficial to apply compact-range calibration techniques. Using physical targets for calibration in compact test ranges is barely possible. Therefore, new calibration techniques have to be developed to extract the calibration constants from the radar sensor directly. In this article, a frequency-shift-keying based calibration with vectorial coefficient scaling is presented. The coefficients are measured separately for the receive and the transmit array and synthesized in the last step to a virtual array. The performed channel measurements are completely independent from local oscillator or trigger synchronization between the radar and the calibration unit, resulting in a versatile and flexible solution. The calibration is validated in a real-world target scenario and compared to the single-target far-field calibration approach. The presented calibration shows equal side-lobe suppression in the digital beamformer compared to the conventional target-based calibration approach.