A multimode antenna (MMA) can be 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, such as 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, an 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 noncoherent 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 the 2-D and 3-D cases. The results show that WM outperforms AIT for high signal-to-noise ratio. The coherent estimation is superior to noncoherent, especially in 3-D, because noncoherent suffers from estimation ambiguities. In conclusion, the DoA estimation with a single MMA is feasible and accurate.

Multi-mode antennas are an alternative to classical antenna arrays and, hence, a promising emerging sensor technology for a vast variety of applications in the areas of array signal processing and digital communications. An unsolved problem is to describe the radiation pattern of multi-mode antennas in the closed analytic form based on calibration measurements or on electromagnetic field (EMF) simulation data. As a solution, we investigate two modeling methods: one is based on the array interpolation technique (AIT) and the other one is based on wavefield modeling (WM). Both the methods are able to accurately interpolate quantized EMF data of a given multi-mode antenna, in our case a planar four-port antenna developed for the 6-8.5-GHz range. Since the modeling methods inherently depend on parameter sets, we investigate the influence of the parameter choice on the accuracy of both models. Furthermore, we evaluate the impact of modeling errors for coherent maximum-likelihood direction-of-arrival estimation given different model parameters. The numerical results are presented for a single polarization component. Simulations reveal that the estimation bias introduced by model errors is subjected to the chosen model parameters. Finally, we provide the optimized sets of AIT and WM parameters for the multi-mode antenna under investigation. With these parameter sets, EMF data samples can be reproduced in an interpolated form with high angular resolution.

Improving safety is of general interest in aviation. Flight transponders used in collision avoidance systems advise pilots with information of potential collisions and avoidance resolutions. Some transponders are capable of transmitting the instantaneous position of the vehicle which might be turned off. To prevent autonomous flights from collisions, the transponder signals shall be used to estimate the direction-of-arrival (DoA). Recently, multi-mode antennas have been investigated for communication and localization purposes. Multi-mode antennas provide several orthogonal ports on a single radiator surface. This leads to a reduction of space and weight, which is of interest in airborne applications. To perform direction-of-arrival estimation as well as communication, a suitable channel model is required. In this contribution, a channel model fulfilling the requirements for both multiple-input multiple-output (MIMO) communication and DoA estimation is proposed. This channel model relies on the manifold matrix representation of the UAV under consideration. The derived description of a manifold matrix for the UAV use-case is used for an optimization to achieve a close to omnidirectional radiation pattern. Therefore, different optimization functions are derived and their performance is compared. The results of the optimization using the calculated manifold matrix are verified by measurements taken in an antenna measurement chamber.

Increasing airspace safety is an important challenge, both for unmanned aerial vehicles (UAVs) as well as manned aircraft. Future developments of collision avoidance systems are supposed to utilize information from multiple sensing systems. A compact sensing system could employ a multi-mode multi-port antenna (M${ }^3$PA). Their ability to radiate multiple orthogonal patterns simultaneously makes them suitable for communication applications as well as bearing and ranging applications. Furthermore, they can be designed to flexibly originate near-omnidirectional and/or directional radiation patterns. This option of flexibility with respect to the radiation characteristic is desired for antennas integrated in collision avoidance systems. Based on the aforementioned properties, M${ }^3$PAs represent a compelling option for aircraft transponders. In this paper, direction-of-arrival (DoA) estimation using an M${ }^3$PA designed for aerial applications is put to the test. First, a DoA estimation scheme suitable to be employed with M${ }^3$PAs is introduced. Next, the validity of the proposed method is confirmed through numerical simulations. Lastly, practical experiments are conducted in an antenna measurement chamber to verify the numerical results.