  In this paper, we show that a multi-mode antenna (MMA) is an interesting
alternative to a conventional phased antenna array for direction-of-arrival
(DoA) estimation. By MMA we mean a single physical radiator with multiple
ports, which excite different characteristic modes. In contrast to phased
arrays, a closed-form mathematical model of the antenna response, like a
steering vector, is not straightforward to define for MMAs. Instead one has to
rely on calibration measurement or electromagnetic field (EMF) simulation data,
which is discrete. To perform DoA estimation, array interpolation technique
(AIT) and wavefield modeling (WM) are suggested as methods with inherent
interpolation capabilities, fully taking antenna nonidealities like mutual
coupling into account. We present a non-coherent DoA estimator for low-cost
receivers and show how coherent DoA estimation and joint DoA and polarization
estimation can be performed with MMAs. Utilizing these methods, we assess the
DoA estimation performance of an MMA prototype in simulations for both 2D and
3D cases. The results show that WM outperforms AIT for high SNR. Coherent
estimation is superior to non-coherent, especially in 3D, because non-coherent
suffers from estimation ambiguities. In conclusion, DoA estimation with a
single MMA is feasible and accurate.
