Ray Launching Geometrical Optics (RL-GO) technique is known for reliable and accurate results including those in the automotive radar related field. However, the high frequency approximation can be expected to reveal inaccuracies when applied to periodical structures consisting of close-to-wavelength structure sizes. This paper investigates the limitations of RL-GO for objects or arrangements that possess dimensions below or close to the radiation wavelength lambda(g) inside the material. Here, Multilevel Fast Multipole Method (MLFMM) which is a full wave technique provides the benchmark for accuracy verification. The expected accuracy impact has been investigated in this work for periodic structures with dimensions between 1.5 lambda(g) and 0.15 lambda(g). In the scope of this work for RL-GO, accuracy condition are found and formulated.

When performing electromagnetic ray-tracing simulation for radar-environment scenarios, we often deal with a low-fidelity triangle-based surface mesh. The flat triangles composing the objects lead to wrong RCS values because they deviate significantly from the original curved surface. Therefore, in this work, we present a method to estimate the curvature of the actual curved surface enabling us to find the intersection between the rays and the surface.

Electromagnetic simulation is increasingly used in automotive industry to generate radar data in virtual scenarios instead of actual measurement campaigns. Objects in the scenario are usually converted into triangular facets before the simulation. This paper studies the effect of facetization criteria on the RCS accuracy of an object with curved surface.

In this paper we present for automotive radar applications the idea of combining RCS radial pattern with analytically calculated multi-path effect. This technique applies the multi-path phenomenon present due to ground reflection in the simulation of automotive radar targets without using a full scale ray-tracing method, which is computationally highly demanding in comparison.

This paper presents a novel local self-localization approach which formulates the registration problem as a range-Doppler response problem. The proposed registration algorithm utilizes sensor observations in range-Doppler domain in combination with a pre-built high precision map and motion sensors for velocity and yaw rate measurement. In contrast to state-of-the-art approaches which perform the registration based on a point cloud, the introduced algorithm in this work shifts the registration from detection domain down to the sensor range-Doppler response. This procedure offers several positive characteristics such as independence of targets angle estimation, registration without any necessity of further processing of range-Doppler response for extracting detections and generating a point cloud. Furthermore, the transformation of the extracted detection from range-Doppler to Cartesian domain is not required.The registration problem is constructed as an optimization-based approach. The map information is converted to the range-Doppler domain and the optimization problem searches for the best overlap between sensor observation and map information. The registration result is subsequently used for correcting the ego-pose which is initially estimated based on motion sensors. The extensive simulation results demonstrate the feasibility and precise performance of the proposed algorithm.

This paper proposes a novel registration method in range-Doppler domain which is highly robust against inaccurate initialization. In contrast to state-of-the-art approaches which perform the registration in the point cloud domain, the proposed method shifts the registration to the range-Doppler domain. This algorithm is initialized with the mechanical motion sensors measurements and combines the range-Doppler observation of a Doppler sensor with the a priori information of a high definition (HD) map to minimize the initial ego-motion estimation error. Formulating the registration problem in the range-Doppler domain allows us to benefit from the specific advantages such as angle estimation independence, saving computational costs by avoiding detections extraction and their transformation to other domains such as polar and Cartesian for point cloud generation. This method is robustly designed to cope with the noisy data of mechanical motion sensors as standard vehicle features for avoiding the usage of costly inefficient motion measurement systems. Moreover, this technique offers enough robustness to deal with Doppler sensors with a single transmit and receive antenna which leads to developing a cost-efficient registration system. As an optimization-based approach it minimizes a cost function which is formulated including the consideration of inaccurate initialization, inputted from motion-estimation systems. The evaluations using radar measurements demonstrate the plausibility and feasibility of the algorithm.

This paper covers the trade-off between the directivity of monopoles over a finite ground plane forming a uniform circular array (UCA) and the resolution capability of this UCA when utilizing the MUSIC (MUltiple SIgnal Classification) algorithm. An expression for the threshold array signal-to-noise ratio (ASNRth) with respect to uniform linear arrays (ULA) has been published before by others and is extended in this contribution to a UCA.

In this paper, sequential array processing is applied to estimate the Direction-of-Arrival (DoA). To successfully execute the DoA estimation, several signal processing stages have to be implemented. Firstly, the time-conditioned phase offset due to the sequential antenna scanning needs to be compensated. Secondly, a calibration has to be performed if it is not inherent to the DoA estimation algorithm. Additionally, a hardware demonstrator is presented that consists of a single transceiver, a SP8T (single-pole-eightfold-throw) switch and a uniform circular array (UCA) incorporating eight monopole antennas over ground. The demonstrator enables the verification of DoA estimation techniques and operates in the ISM (industrial-scientific-medical) band at 2.4 GHz with the radius of the UCA being equal to 6 cm. The presented measurement results show the feasibility of the sequential array processing concept.

The combination of OFDM with joint pre-processing in adaptive multi-antenna systems offers both an ease of equalization in frequency-selective channels and keeping the signal processing at the mobile stations simple. In addition, the spatial dimension can be efficiently exploited to ensure high system throughput. With the utilization of higher-order modulation the performance of the system is highly sensitive to multiple access interference and nonorthogonal subchannels due to hardware impairments or insufficient adaptation to the current channel conditions. A further source of error in TDD systems are the non-reciprocal transceivers inhibiting the baseband-to-baseband channel reciprocity required for accurate channel state acquisition based on the uplink channel estimate. In this paper, measurement results of a low-cost hardware-based calibration are presented and the drawbacks are discussed leading to the utilization of a recently introduced relative calibration. The latter is applied to an OFDM system and achieves or at least approximates the baseband-to-baseband reciprocity. Thus, it enables the link adaptation using the uplink channel state information. Furthermore, preliminary hardware implementations of the relative calibration running on a real-time system show accurate results.

In this paper, a framework is established to quantify the influence of hardware imperfections realizing a Direction-of-Arrival (DoA) estimation system using a circular antenna array. The latter exploits one transceiver and therefore samples the different antennas sequentially. To accomplish this, a Single-Pole-Multifold-Throw (SPMT) switch is needed to connect antennas and transceiver. Describing the different components by their respective scattering parameter matrices, the aforementioned framework is assembled. It can be demonstrated that hardware imperfections such as finite isolation and finite matching perturb the original pattern of every single antenna element within the array. Furthermore, a calibration algorithm is successfully applied to compensate for the pattern perturbation introduced by the hardware imperfections.

In this paper, a hardware-based calibration scheme at the base station is used to mitigate the impact of the non-reciprocal transceivers in a time division duplex (TDD) multiple-input-multiple-output orthogonal-frequency-division-multiplexing (MIMOOFDM) system. The calibration setup consists of two single-pole-double-throw (SPDT) switches to bypass the calibration and the data signals. Additionally, an attenuator is needed to avoid overdriving the receive chain of the transceivers. Simulation results show that this calibration scheme results in lower bit error rate (BER) values of the communication system. These results are underlined by measurement results exploiting the low-cost and simple-technology calibration solution.

In adaptive time division duplex (TDD) broadcast multi-antenna orthogonal frequency division multiplexing (OFDM) systems, non-reciprocal transceiver chains at the base station (BS) cause multi-user interference. This is due to the inappropriate spatial filter design at the BS based on the reverse link estimate. Hence, BS transceiver calibration is required. Provided that an estimate of the forward link channel is available at the BS, e.g., in a calibration phase, the transceiver parameters can be estimated by solving a total least squares (TLS) problem. In addition, if mutual coupling between the antennas exists the number of unknown front-end parameters to be estimated increases. Consequently, large matrices need to be decomposed via singular value decomposition (SVD) to attain a calibrated system. To deal with these large matrices a conjugate gradient (CG) method for solving the TLS problem iteratively is proposed in this paper. Simulation results show that the calibration based on the CG method achieves almost the same performance compared to the TLS solution but with significantly reduced complexity.

The performance of multiple-input multiple-output (MIMO) systems is affected by the spatial correlation properties, which depend on the array configuration and the channel characteristics. The configuration can be influenced by manipulating the radiation patterns of the antennas, the antenna spacing and the array geometry. To decrease the spatial correlation effects, e.g., the antenna spacing can be increased, which is usually undesirable in mobile devices. Here, the utilization of polarization diversity techniques comes into consideration. This paper deals with the application and comparison of different polarized transmit/receive array setups in indoor environments using a multiple antenna demonstrator in uncoded adaptive MIMO-OFDM scenarios. Measurement results indicate a decreased correlation of the spatial subchannels, a higher MIMO capacity for cross-polarized antenna arrays and lower bit error rates with bit and power loading algorithms.

The influence of mutual coupling of antenna elements is usually not considered when dealing with MIMO (Multiple Input Multiple Output) communication systems. This is justifiable if mobile subscribers feature only one antenna and base station antenna elements are separated by a distance of several wavelengths. However, regarding pico-cell base stations that consist of multiple antenna elements, the coupling has to be taken into account due to the reduced antenna element spacing. In this paper, the influence of antenna coupling on adaptive MIMOOFDM (Orthogonal Frequency Division Multiplex) systems is investigated in terms of the attainable signal-to-interferenceplus-noise ratio (SINR). The mutual coupling of the antenna elements is calculated for different antenna models based on the "infinitesimally thin" lambda/2 dipole and the Hertzian dipole. Additionally, a coupling model based on the isotropic radiator is considered. The different results for the SINR stemming from the antenna models when exploiting linear pre-equalization are presented. It is shown that using an isotropic radiator based model is not sufficient for describing the effect of mutual coupling for lambda/2 dipoles.

In this paper, the impact of nonreciprocal transceivers is demonstrated by measurement results for different adaptive transmission strategies in a time division duplex MIMO-OFDM system. These strategies rely on the channel state information (CSI) at the transmitter, thus requiring either appropriate feedback or reciprocity of both communication links. The measurements were performed for point-to-point communication applying a bit and power loading algorithm. Furthermore, a point-tomultipoint scenario with zero forcing pre-equalization is considered. For both scenarios, equalization at the receiver is mandatory for imperfect CSI at the transmitter. To quantify the influence of the nonreciprocal transceivers on data transmission, the bit error rate determined by using the CSI obtained via the reverse link is compared with the bit error rate achieved by using perfectly fed back CSI of the forward link.