Publikationen

Sparse arrays are known to achieve an increased number of degrees-of-freedom (DOFs) for direction-of-arrival (DOA) estimation, where an augmented virtual uniform array calculated from the correlations of sub-Nyquist spatial samples is processed to retrieve the angles unambiguously. Nevertheless, the geometry of the derived virtual array is dominated by the specific physical array configurations, as well as the deviation caused by the practical unforeseen circumstances such as detection malfunction and missing data, resulting in a quite sensitive model for virtual array signal processing. In this paper, we propose a novel sparse array DOA estimation algorithm via structured correlation reconstruction, where the Nyquist spatial filling is implemented on the physical array with a compressed transformation related to its equivalent filled array to guarantee the general applicability. While the unknown correlations located in the whole rows and columns of the augmented covariance matrix lead to the fact that strong incoherence property is no longer satisfied for matrix completion, the structural information is introduced as a priori to formulate the structured correlation reconstruction problem for matrix reconstruction. As such, the reconstructed covariance matrix can be effectively processed with full utilization of the achievable DOFs from the virtual array, but with a more flexible constraint on the array configuration. The described estimation problem is theoretically analyzed by deriving the corresponding Cramér-Rao bound (CRB). Moreover, we compare the derived CRB with the performance of the virtual array interpolation-based algorithm. Simulation results demonstrate the effectiveness of the proposed algorithm in terms of DOFs, resolution, and estimation accuracy.

Antenna arrays and spatial processing techniques are among the most effective countermeasures against interference. Here, we demonstrate a new array concept consisting of spatially-distributed subarrays that are small enough to fit inside the non-metallic parts of an automobile. This will facilitate concealed installation of these devices in bumpers or side mirrors, which is a strict requirement of the industry and preferred by the customers. Using beamforming algorithms, this array was proven to be robust against jammers in the L1 band. The large distances between the individual antenna elements resulted in a non-negligible baseband delay that violated the narrowband assumption and increased with bandwidth. Hence, this paper demonstrates the influence of a jammer in the L5 band. Space-time adaptive processing that allows for compensation of the delays was introduced and analyzed. Improvements in interference mitigation capabilities were assessed and compared to those of pure spatial state-of-the-art implementation. Real-life measurement data was used to ensure realistic results.

Radio frequency exposure measurements in the surroundings of two mobile radio base stations were performed before and after their upgrade to 5G to investigate exposure changes. The measurements were carried out in an urban environment in Berlin, and a 5G Dynamic Spectrum Sharing (DSS) base station and a 5G massive MIMO base station were investigated. At the first base station, a previous UMTS system was replaced by a DSS system (LTE and 5G share common resources) with unchanged total transmission power. The maximum possible exposure at five out of six measurement points remains unchanged within the measurement uncertainty. At the second base station under investigation, a 5G massive MIMO antenna technology was additionally installed to an existing mobile radio system. Here, maximum possible exposure increases of 6 to 11 dB occur at the investigated measurement points. A parallel recording of the 5G instantaneous exposure at the massive MIMO station shows that the exposure without provoking traffic load (signalization only) and at low traffic load exploits only 5-10% of the maximum exposure in terms of field strength.

Established methods for determining the electromagnetic field exposure of mobile radio small-cell base stations to the general public are currently based either on numerical far-field computation or on field strength measurement at selected evaluation points. It means that they are not capable of providing a realistic 3-dimensional near-and far-fleld evaluation of mobile radio small-cell base stations. For this reason, a promising hybrid exposure assessment approach - which combines antenna nearfield pattern measurements and numerical computations under varying environmental conditions - was specially adapted to small cells. While the conservatively determined measurement uncertainty of ± 3 dB is comparable to already established methods such as electric field probe measurement, the hybrid assessment offers novel possibilities in terms of flexibility. By moving exposure assessment into a virtual domain, complex installation scenarios or varying antenna operation parameters can typically be investigated within a simulation environment, so that complex measurement campaigns may be substituted by hybrid assessment.

This paper presents and compares real locomotive and scaled mock-up antenna measurements to investigate the reliability of scaled model measurements and to evaluate the impact of the locomotive chassis on the performance of the installed antennas. Three different operational frequencies and two distinct mounting positions at the center and front of the locomotives were selected for comparison. Fair comparability between patterns is observed with a similarity factor above 64%. The deviations between patterns arise from different geometries and installed superstructures. However, the results reveal coherent findings, particularly the significant impact of locomotive chassis, roof geometries, and superstructures on the radiation patterns. This impact becomes more pronounced at higher frequencies. Moreover, the front position results show strong distortions in patterns compared to the center position. The results imply that when analyzing the installed locomotive antenna pattern beside the chassis, the impact of superstructures and the impact of the mounting position must be carefully considered.