Future driver assistance and autonomous driving systems require high-resolution 4D imaging radars that provide detailed and robust information about the vehicle's surroundings, even in poor weather or lighting conditions. In this work, a novel high-resolution radar system with 1728 virtual channels is presented, exceeding the state-of-the-art channel count for automotive radar sensors by a factor of 9. To realize the system, a new mixed feedthrough and distribution network topology is employed for the distribution of the ramp oscillator signal. A multilayer printed circuit board is designed and fabricated with all components assembled on the back side, while the radio frequency signal distribution is on a buried layer and only the antennas are on the front side. The array is optimized to enable both multipleinput multiple-output operation and transmit beamforming. A sparse array with both transmit and receive antennas close to the transceivers is realized to form a 2D array with a large unambiguous region of 130 • × 75 • with a maximal sidelobe level of −15 dB. The array features a 3 dB beamwidth of 0.78 • × 3.6 • in azimuth and elevation, respectively. Radar measurements in an anechoic chamber show that even the individual peaks of the absorber in the chamber can be detected and separated in the range-angle cut of the 4D radar image. The performance is validated by measurements of a parking lot, where cars, a pedestrian, a fence, and a street lamp can be detected, separated, and estimated correctly in size and position.INDEX TERMS Advanced driver assistance systems (ADAS), automotive radar, chirp sequence modulation, direction-of-arrival (DoA) estimation, frequency modulated continuous wave (FMCW), imaging radar, local oscillator (LO) feedthrough, mm-wave, multiple-input multiple-output (MIMO), time delay correction.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

A novel vertical transition through a multilayer printed circuit board is presented. The transition is designed to fulfill the cost and manufacturing constraints of next generation automotive radar sensors. This is achieved both by employing FR4 as material in the transition core and the fabrication in a standard high density interconnect process. The modified design of a quasi-coaxial structure solves the limitations determined by the constraints. Although FR4 has high dielectric losses, a vertical transition through multiple FR4 layers is realized with a low insertion loss of less than 2.1 dB within the automotive radar frequency range of 76 GHz to 81 GHz. A return loss better than 10 dB is achieved up to a frequency of 83.2 GHz. The reliability and repeatability of the transition is shown by measurements of several transitions from different production cycles.

A novel synthesis method for holographic multi-feed antennas is presented to combine all sub-holograms into an angle-dependent shared holographic aperture. In order to find the global error minimum between the shared holographic aperture and all ideal sub-holograms, a non-pixel-based genetic optimization is used. For a more accurately implementation of the analytical impedance tensor an eigenvector approach taking all tensor components into account is introduced. The optimized holographic antenna has four feeds for an exemplary integration into a 2D-monopulse radar system and is realized on a fused silica wafer due to its lower loss at millimeter-wave frequencies compared to Teflon-based materials. The antenna prototype provides a measured gain of 23 dBi, a polarization purity of 26 dB and a side-lobe level of 20 dB for feed 1, 2 and 4 is reached across 76 GHz−81 GHz. The S-parameters are measured from 60 GHz to 90 GHz, and a reflection coefficient of −14 dB and an inter-port coupling <−38 dB are achieved. The measured 2D-monopulse patterns provide a FOV of 10°, the sum beam has a gain of 26 dBi leading to an aperture efficiency of 45 % and the differential beams show a null depth of 30 dB.INDEX TERMS holographic antenna, metasurface antenna, multi-feed antenna, multi-beam antenna, surface wave, leaky wave, monopulse radar, glass technology.

In radar systems with local oscillator feedthrough topology, range shifts occur due to the time delays between the radar transceivers. These shifts be corrected efficiently in postprocessing, except for the short-range spectrum, as disturbing peaks, caused by range shifts of the transmit-to-receive leakage, prevent the use of such system in short-range applications like automated parking. In this contribution, the capabilities of complex sampling, compared to real sampling, are analyzed both in theory and by measurements, with respect to the effects in systems with feedthrough topology. The imaging performance at very short ranges is investigated with an exemplary scenario, representing a typical automated parking situation. The measurements show that with complex sampling, the image peaks vanish and the noise level is reduced by 2.8 dB. In this way, radar images suitable for automated parking and other short range applications can be obtained.