Congress & Conference Contributions

Accurate characterization of locomotive antennas is key to safe and robust railway signaling and control communication. With the introduction of new technologies and the foreseeable migration from the GSM-R standard towards FRM CS, new wireless applications and specifications arise, and suitable antenna solutions need to be developed and tested. Moreover, the rooftops of modern locomotives present a dense and harsh environment; therefore, potential antenna mounting spaces should be carefully evaluated to avoid undesirable degradations of the antenna radiation patterns. Due to the electrically large and complex structure of locomotives, full-scale testing is challenging to perform, especially under laboratory conditions. Antenna measurements with geometrically scaled models present a powerful alternative to address this issue. In this paper, we present and discuss antenna measurement results of a scaled locomotive mockup. The mockup incorporates two different cabin geometries, one with a step-like rooftop contour, and one with a smooth slightly tilted geometry. First, the optimum scaling factor was identified and validated through numerical simulations. Afterwards, antenna measurements with a scaled locomotive mockup were carried out in our automotive antenna measurement facility VISTA. The measured results were compared with the numerical simulations, where a good correlation above 80 % was found. Secondly, the impact of the rooftop geometries, and superstructures on the roof has been investigated for a range of operational frequencies between 700 and 2600 MHz. The results reveal that the parasitic impact of the antenna environment becomes more pronounced at higher frequencies.

One benefit of cooperative automated and connected driving lies in the fusion of multiple mobile wireless sensor and data transmission nodes, covering complementary technologies like radar, cellular and ad-hoc communications, and alike. Current developments indicate enormous potential to increase the environmental awareness through joint communication and radar sensing. In this respect, future channel models require knowledge of bi-static reflectivities of road users over a range of illumination and observation angles, both in the nearfield and in the far-field. To establish reference data and model such angle-dependent RCS variations, this paper deals with realistic pedal and wheel rotations of a bicycle based on electromagnetic simulations. In the simulation setup, idealized far-field conditions with plane-wave illumination and observation were assumed, while the angles covered the entire azimuth with 201 variations of the pedal and wheel positions. The fluctuation of the RCS is analyzed and discussed in terms of its probability density and cumulative distribution functions. Depending on the angular constellation, the range of the fluctuation varied between 1 dB and 14 dB, while the specular reflection and forward-scattering showed almost no fluctuation.

Deploying antenna arrays with an asymptotically large aperture will be central to achieving the theoretical gains of massive MIMO in beyond-5G systems. Such extra-large MIMO (XL-MIMO) systems experience propagation conditions which are not typically observed in conventional massive MIMO systems, such as spatial non-stationarities and near-field propagation. Moreover, standard precoding schemes, such as zero-forcing (ZF), may not apply to XL-MIMO transmissions due to the prohibitive complexity associated with such a large-scale scenario. We propose two novel precoding schemes that aim at reducing the complexity without losing much performance. The proposed schemes leverage a plane-wave approximation and user grouping to obtain a low-complexity approximation of the ZF precoder. Our simulation results show that the proposed schemes offer a possibility for a performance and complexity trade-off compared to the benchmark schemes.

This document outlines the measurement methodology currently employed to evaluate wireless technology on vehicles. Near Field Over the Air measurement results are provided and the Figure of Merit developed to assess the performance is reviewed. Simulation results of a more complex measurement scenario are also presented.

Ultrasound Computed Tomography (UCT) is challenging due to phenomena such as strong refraction, multiple scattering, and mode conversion. In NDT, large speed of sound contrasts lead to strong artifacts if such phenomena are not modeled correctly; however, enhanced models are computationally expensive. In this work, a two-step framework for Compressed UCT based on the integral approach to the solution of the Helmholtz equation is presented. It comprises a physically motivated forward step and an imaging step that solves a suitable inverse problem. Multiple scattering is accounted for through the use of Neumann series. Convergence problems of Neumann series in high contrast settings are addressed via Padé approximants. Compressed sensing is employed to reduce the computational complexity of the reconstruction procedure by reducing data volumes directly at the measurement step, avoiding redundancy in the data and allowing the ability to steer the admissible computational effort at the expense of reconstruction quality. The proposed method is shown to yield high quality reconstructions under heavy subsampling in the frequency and spatial domains.