Multi-mode antennas have recently been studied for communication as well as localization purposes. In this work, the capabilities provided by a single planar multi-mode radiator as a steerable multi-port antenna are explored. As an original contribution, the radiation characteristics of individual groups of modes of the single radiator are combined to optimize beamforming performance. Three possible codebook realizations resulting from the optimization of gain, element factor, and, as a new criterion, gain by element factor are studied and compared. Numerical results show that the gain can be enhanced by about 3 dB at some angles.

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.

The anticipated trends in mobility, either autonomous driving or urban air mobility (UAM), require wireless services to achieve mandatory reliability and safety levels. The number of wireless systems already in-use, especially airborne, occupy regulated spectra. In order to mitigate the impact of a large number of systems mounted to airborne platforms, we propose to combine enabling techniques. These include the joint waveform design, multi-mode multi-port antennas (M ³ PAs), and appropriate beamforming techniques for joint communication, sensing and localization (JCSL). Our study finds that employing DFT-spread OFDM results in a more consistent radar performance compared to conventional OFDM, highlighting a novel application of this waveform design in JCSL systems. M³PAs are explored as a candidate system for performing JCSL using a single antenna radiator. It is shown how the orthogonality properties of M ³ PAs are beneficial for avoiding crosstalk in JCSL. We therefore review mature and actively employed techniques and discuss the applicability of M ³ PAs and novel waveform designs for JCSL. This study aims to conduct a thorough analysis of current systems in operation. Furthermore, design concepts that facilitate the implementation of the JCSL concept in UAM and airborne applications are introduced and discussed.

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.

Joint communication and sensing (JCAS) is of special interest due to densification in various contexts like wireless networks, airspace, and transportation networks. In this paper, a JCAS approach using a single multi-mode multi-port antenna (M3PA) element is proposed. Towards this goal, circular beamforming is applied in order to separate communication and sensing channels, using the unique capabilities of M3PA s. The sensing possibilities using only a single M3PA element are evaluated assuming compressive sensing and MIMO radar signal processing including the polarization domain. Simulation results for an unmanned aerial vehicle scenario indicate a good separation of the communication link and sensing system at the cost of a slightly increased angular error.

Flight transponder signaling standards must be continuously improved in light of increasing requirements for the safe integration of an increasing number of aircraft. Since the introduction of the airborne collision avoidance system (ACAS), transponders are not only a major part of the air surveillance systems of flight controls, but also communicating between each other. The purpose of this communication is the aforementioned collision avoidance. The most common current system is the traffic collision avoidance system (TCAS) II. Towards the integration of unmanned aerial vehicles, the ACAS Xu standard is under investigation. However, interrogations between different airborne transmitters help to improve the mutual information exchange between the vehicles. Hybrid beamforming with multi-mode multi-port antennas (M ³ PAs) is proposed to reduce the effects of interference from other users as well as ambiguities in the directional domain. The performance of different hybrid beamforming variants is compared and an example pattern measured in an antenna measurement chamber is shown.