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Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian; Hein, Matthias
Comparison of angle-dependent scattering of convoluted and straight microwave absorbers. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), insges. 6 S.

The increasing complexity and sensitivity of wireless communication systems enforce the requirements for test environments such as anechoic chambers. The minimum achievable level of interference between desired signal and scattered copies is essentially determined by the reflectivity of the installed absorbers, emphasizing the importance of thoroughly characterizing the scattering behavior of absorbers. In this paper, the scattering off absorbers with different geometric shapes, namely convoluted, pyramidal, wedge, and flat, is investigated using a numerical unit-cell model. To verify the simulation model, the angle-dependent reflectivity of the convoluted absorbers was measured at different angles-of-incidence between 2GHz and 18GHz. The numerical results agree well with the measured reflectivity at representative angles-of-incidence, validating the numerical model and revealing the expected increase in reflectivity for increasing beam tilts. Further, it becomes apparent that the performance of all shapes decreases similarly at oblique incidence. These results contribute to build a comprehensive database on the angle- and frequency-dependent reflectivity of absorbers, in order to develop a consistent data body, e.g., for modelling anechoic environments.
Berlt, Philipp; Altinel, Berk; Bornkessel, Christian; Hein, Matthias
Emulation of LTE link scenarios reproducibly derived from field-operational tests. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), insges. 6 S.

Virtual drive tests using the over-the-air/vehicle-in-the-loop method are becoming an essential part of testing vehicular radio systems. Different approaches ask which link scenarios and channel environments are relevant and should be tested. This paper deals with the systematic evaluation of field-operational tests and the implementation of virtual drive tests of LTE communication links focussing on the performance of the radio link close to the edges of the radio cells, which are identified as a relevant testing scenario. For this purpose, three test drives were performed on each of two test tracks. Close to cell edges the available data throughput is as much as a factor of 10 lower than the maximum available data throughput along the test track and reduced approximately by half compared to the average data throughput in the cell center. Instead of trying to recreate the entire test drive with high accuracy, this approach focusses on recreating the critical parts of a test drive in the laboratory, as these are the most likely to cause radio link failure in real operation. Therefore, the physical parameters in terms of serving signal strength and level of interfering signals were transferred to a wired virtual test, and the data rate was examined again. Despite some systematic differences between real drive test and virtual drive test, which could be clearly identified, it was possible to reproduce the behavior at the cell edge very precisely with deviations smaller than 5 %.
Schwind, Andreas; Varga, Isabella; Hofmann, Willi; Hein, Matthias
Analytical and experimental studies of ground reflections on bi-static radar signal propagation. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), insges. 6 S.

Progressing towards highly automated and connected vehicles, radar systems have evolved into reliable assistance systems for environmental perception, for a wide spectrum of traffic scenarios, and with them, accurate angle-dependent descriptions of reflectivities and scattering centers of traffic participants and road users. Depending on the electrical size of the radar object, the influence of possibly unwanted ground reflections can be significant in radar cross-section measurements. This paper presents an analytical model based on the transmitter, receiver, and single or multiple scattering center positions, that takes into account the geometric reflections at the ground floor and calculates the resulting interference. Considering also the bi-static crosstalk between the transmit and receive antennas, six different propagation paths are obtained, which differ in path delay and attenuation. Subsequent validation measurements in a semi-anechoic automotive antenna test facility confirm the analytical approach very well. Existing discrepancies between the single scattering center model and the measurements with a metal sphere could be corrected by a closer look at the position of the scattering center. Final measurements on realistic bicyclist dummies show that the model is also reliably applicable to extended radar targets.
Schwind, Andreas; Buddappagari, Sreehari; Asghar, Muhammad Ehtisham; Hein, Matthias
Virtuelle Sensorvalidierung für automatisiertes und vernetztes Fahren. - In: Automobiltechnische Zeitschrift, ISSN 2192-8800, Bd. 124 (2022), 11, S. 58-62
Schwind, Andreas; Buddappagari, Sreehari; Asghar, Muhammad Ehtisham; Hein, Matthias
Virtual sensor validation for automated and connected driving. - In: ATZ worldwide, ISSN 2192-9076, Bd. 124 (2022), 11, S. 54-57
Tayyab, Umais; Petry, Hans-Peter; Kumar, Ashish; Robbani, Md. Golam; Wack, Thomas; Hein, Matthias
Link budget and design approach of a non-terrestrial 5G automotive antenna. - In: 2022 52st European Microwave Conference, (2022), S. 864-867

5G low-earth orbiting satellites are continuously increasing attention from automotive industry for automated and connected driving. Compactness of user equipment antennas and high data rates are key performance figures for efficient satellite communication systems. Here, we present a link budget for internet-of-things applications at Ka-band frequencies (5G frequency range FR2). Anticipating a realistic high-gain satellite antenna, an uplink data rate of 4 Mbit/s can be achieved with a compact user terminal antenna with a moderate gain of 13 dBi. Along these lines, a 4×4 patch antenna array was designed for seamless embedding in the plastic part of a car body, in order to verify the link budget calculations by experiment. The radiation performance was measured under free-space conditions and with the antenna embedded in the rear spoiler wing of a modern passenger car. The array offered 11.2 dBi realized gain and 1.6 GHz of −10 dB matching bandwidth, with an uplink data rate of 2 Mbit/s, promising for many mobility applications.
Buddappagari, Sreehari; Aust, Philip; Schwind, Andreas; Hau, Florian; Hein, Matthias
Evaluation of scenario-based automotive radar testing in virtual environment using real driving data. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), S. 2379-2384

Safety assurance of intended functionality through rigorous testing is a key to large-scale homologation and deployment of automated driving. It is therefore imperative to transfer real world tests into efficient and quantifiable virtual testing procedures and environments without compromising reliability. In earlier work, we presented a fully operational over-the-air vehicle-in-the-loop test system for automotive millimeter-wave radar, where we generated a virtual electromagnetic environment with physically realistic radar target echoes. We evaluated the performance of the implemented test system with an exemplary scenario parameterised with analytically pre-defined vehicle manoeuvres. In this work, we significantly proceed with the performance evaluation through re-simulation of scenarios based on real driving data and traffic manoeuvres. We have measured a standard Euro-NCAP scenario, namely, the Car-to-Pedestrian Longitudinal Adult on a proving ground and re-simulated the ground truth parameters in the test bed. We compare the consistency of the test results at several data abstraction levels using parameter trajectories. Additionally, we introduce and evaluate quality metrics such as difference and root mean square error. For a driving scenario approximately 20 seconds long, we achieved promisingly low root mean square errors in range, azimuth and RCS of 0.3 m, 0.5˚ and 2 dB, respectively.
Schilling, Lisa-Marie; Bornkessel, Christian; Hein, Matthias
Human RF electromagnetic exposure to V2X-communication. - In: Advances in radio science, ISSN 1684-9973, Bd. 19 (2022), S. 233-239

In the era of automated and connected driving, more and more cars will be equipped with wireless transmission technologies such as mobile communications 4G (LTE) and 5G, WiFi, Bluetooth, and V2X. For the technical implementation of V2X-communications, different standards like cellular-V2X from the cooperation 3rd Generation Partnership Project and ITS-G5, based on the WiFi standard 802.11p from the Institute of Electrical and Electronics Engineers, are under consideration. The electromagnetic environment of cars and the corresponding exposure of the general public to wireless emission will be significantly influenced by new radio technologies. Under all circumstances, it must be ensured that the exposure of the electromagnetic fields inside a car does not cause any harmful effects on humans. In order to quantitatively assess the resulting exposure, the generated exposure must be correctly recorded and evaluated according to their specific time-frequency spectra. This paper describes a new measurement procedure suitable for the V2X-standard ITS-G5 together with various exposure measurements performed in different cars with WiFi, Bluetooth and ITS-G5. In comparison of all wireless standards studied here, the V2X-service generated the highest electric field strengths inside a car, when a transmitting di-patch antenna was mounted on the windscreen inside the driver's cabin. The maximum fraction of the corresponding ICNIRP reference level amounted to 15.1 %. We conclude that the total exposure of wireless on-board automotive devices will be dominated by ITS-G5, if the transmitting antenna is located inside the passenger cabin. As V2X-communications will increasingly penetrate road traffic, the resulting exposure should be carefully monitored, in order not to exceed the corresponding reference levels for general public.
Kumar, Ashish; Petry, Hans-Peter; Tayyab, Umais; Hein, Matthias
Link budget considerations for automotive 5G LEO satellite-based communications. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), S. 106-107

Satellite communication receives strongly increasing attention for automated and connected driving. Compact size, low power consumption, and high data rates are key performance parameters for such systems. Based on realistic assumptions on user equipment terminal and satellite antenna for low-earth orbiting satellites using large space structures, we present relevant link budget considerations. The high-gain satellite antenna opens the potential to employ compact user equipment antennas of moderate gain and without beam tracking. We study selected C-, Ku-, and Ka-frequency bands for 5G satellite communications based upon usual key performance indicators. Our findings indicate that C-band and Ka-band offer a suitable spectrum, with an uplink data rate of 7 Mbit/s at Ka-band, which is suitable for IoT and broadband mobile access applications.
Hofmann, Willi; Schwind, Andreas; Bornkessel, Christian; Hein, Matthias
Angle-dependent reflectivity of microwave absorbers at oblique wave incidence. - In: 2021 51st European Microwave Conference, (2022), S. 233-236

For frequencies in the GHz-range, anechoic chambers are usually evaluated using ray-tracing techniques to locate disturbing reflections off the chamber walls. Most approaches reduce this wave-absorber interaction to a specular reflection, although the absorbers may extend over several wavelengths in size and display a rough surface. In order to develop more realistic ray-tracing models, the reflection characteristics of absorbers must be evaluated based on physical wave phenomena. In this paper, a measurement method is proposed which extends the established NRL-arch to measure the angle-dependent reflectivity for non-specular cases. First measurement results of commercial pyramidal absorbers in the frequency range between 1GHz and 10GHz indicate that the assumption of specular reflections is not justified, as power is reflected over a wide angular range with approximately the same intensity. This effect is, to our knowledge, currently not implemented in ray-tracing methods. These results contribute to a better understanding of the properties of RF absorbers to improve the efficiency of their use.