A Sub-Aperture-Based Calibration Algorithm for MIMO Antenna Arrays

In order to ensure high-resolution angle estimation results, antenna apertures of radar sensors become large relative to the wavelength. To prevent high sidelobe levels, the number of antennas is constantly increased, generally through the employment of multiple-input multiple-output operation. As systems become larger relative to the wavelength, influences of near-field effects in calibration measurements become more critical. To achieve precise calibrations despite near-field effects, calibration algorithms must be developed further. This paper proposes the deployment of sub-apertures to avoid near-field effects and to reduce the calibration effort, which is in this work related to the number of measuring points in the calibration measurement. An algorithm to create beneficial sub-apertures from a large array based on clustering is described. This allows the far-field distance to be reduced, as well as the effort required for state-of-the-art calibration methods, which depends on the aperture size. The trade-off between the benefits and error propagations as well as other limitations by the deployment of an increasing number of sub-apertures is demonstrated by simulations and measurements. Exemplary measurements show that even for large arrays in compact measuring chambers, far-field like conditions can be created. Finally, it is exemplarily demonstrated that the measurement effort is decreased by nearly 93 percent compared to a conventional calibration approach.