Near-Field Calibration of Millimeter-Wave Massive MIMO Antenna Array Using Sphere Reflectors

Millimeter-wave (mmWave) massive multiple-input–multiple-output (MIMO) array-based frequency-modulated continuous wave (FMCW) radar can deliver high-resolution angle, range, and Doppler estimation and imaging for 6G systems. However, such arrays generally suffer from severe amplitude and phase imbalances over massive antennas and must be properly calibrated. This letter develops an efficient calibration scheme based on spherical reflectors, which can readily apply to most commercial mmWave FMCW radars in regular labs. We model and analyze the impact of different error sources and accordingly convert the problem into calculating the propagation path delay between each pair of transceiving antennas. An accurate method is developed for the calculation, with low-complexity closed-form solutions derived. Simulation and over-the-air experiments with around 1.3 million transceiver pairs are provided, validating the high accuracy of the developed calibration method and its effectiveness in practical imaging applications.