Suitability of Compact Antenna Arrays for Direction-of-Arrival Estimation

The suitability of compact antenna arrays for the direction-of-arrival (DoA) estimation of radio emissions is investigated. Some applications may set constraints with respect to the antenna array physical size. If we increase the number of radiating elements in a given array size to maximize the degrees of freedom (DoF), the spacing between neighboring elements reduces to less than half of the free-space wavelength. Consequently, the electromagnetic coupling between the antenna elements becomes stronger. The result is a distortion of the radiation patterns, and mismatch at the antenna ports. This will have serious effects on the performance of any DoA estimator. Whereas the radiation pattern distortion, which is known from calibration measurements, can be considered by the DoA estimator, the power mismatch causes a loss of received signal power, and therefore, the SNR is reduced. This can only be mitigated by an analog decoupling and matching network (DMN), applied directly at the antenna array outputs, before digitalization. The DMN together with the antennas will exhibit mutually orthogonal radiation patterns, referred to as eigenpatterns. The output ports are, as a consequence, ideally "decoupled," and the power spread of the eigenmodes will reflect the available DoF. We investigate the influence of the DMN design to the DoA performance in terms of the Cramer-Rao lower bound (CRLB). We show that both decoupling and matching will have a positive impact on the average CRLB. Moreover, for the application case of DoA estimation using small flying unmanned aerial vehicle platforms, we describe the performance of a specific circular array with L-quad antenna elements, and demonstrate the need for dual-polarimetric design and calibration.