Important requirements for automotive radar are high resolution, low hardware cost, and small size. Multiple-input, multiple-output (MIMO) radar technology has been receiving considerable attention from automotive radar manufacturers because it can achieve a high angular resolution with relatively small numbers of antennas. For that ability, it has been exploited in the current-generation automotive radar for advanced driverassistance systems (ADAS) as well as in next-generation highresolution imaging radar for autonomous driving. This article reviews MIMO radar basics, highlighting the features that make this technology a good fit for automotive radar and reviewing important theoretical results for increasing the angular resolution. The article also describes challenges arising during the application of existing MIMO radar theory to automotive radar that provide interesting problems for signal processing researchers.

MIMO Radar for Advanced Driver-Assistance Systems and Autonomous Driving: Advantages and Challenges

As the only method for all-weather, all-time and long-distance target detection and recognition, radar has been intensively studied since it was invented, and is considered as an essential sensor for future intelligent society. In the past few decades, great efforts were devoted to improving radar's functionality, precision, and response time, of which the key is to generate, control and process a wideband signal with high speed. Thanks to the broad bandwidth, flat response, low loss transmission, multidimensional multiplexing, ultrafast analog signal processing and electromagnetic interference immunity provided by modern photonics, implementation of the radar in the optical domain can achieve better performance in terms of resolution, coverage, and speed which would be difficult (if not impossible) to implement using traditional, even state-of-the-art electronics. In this tutorial, we overview the distinct features of microwave photonics and some key microwave photonic technologies that are currently known to be attractive for radars. System architectures and their performance that may interest the radar society are emphasized. Emerging technologies in this area and possible future research directions are discussed.

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Microwave Photonic Radars

In this paper, a radar demonstrator system with real-time capability operating at ${W}$ -band is presented. It operates at 90–100 GHz and provides 3-D information about the illuminated scene. The system uses frequency modulated continuous wave signals to extract range information whereupon long-range applications are aimed at. It consists of a sparse array of 22 transmitting and 22 receiving antennas and makes use of the multiple input multiple output (MIMO) principle. A back-propagation algorithm provides cross-range information. With the help of simulations and a simulated annealing algorithm, the geometry of the sparse array is optimized to meet application requirements using the available hardware. In this paper, the demonstrator system is described and the imaging theory is shortly reviewed. Measurements are presented to verify simulation results as well as 3-D imaging and long-range capability.

${W}$ -Band Time-Domain Multiplexing FMCW MIMO Radar for Far-Field 3-D Imaging

For high-resolution direction of arrival estimation, a wide aperture is necessary. This can be achieved with multiple-input multiple-output radars, which offer a wide virtual aperture. However, the requirement of orthogonal transmit signals has to be satisfied for their operation. Although time division multiplexing of the transmit elements is an orthogonality realization with low hardware effort, phase errors occur in nonstationary scenarios. This letter briefly discusses the problem of the motion-induced phase errors and describes processing steps to mitigate them without additional effort. The proposed technique is demonstrated with simulation and measurement data.

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Compensation of motion-induced phase errors in TDM MIMO radars

Terahertz (THz) links will play a major role in high data rate communication over a distance of few meters. In order to achieve this task, antenna designs with high gain and wideband characteristics will spearhead these links. In this contribution, we present different antenna designs that offer characteristics better suited to THz communication over short distances. Firstly, a single-element antenna having a dipole and reflector is designed to operate at 300 GHz, which is considered as a sub-terahertz band. That antenna achieves a wide impedance bandwidth of 38.6% from 294 GHz to 410 GHz with a gain of 5.14 dBi. Secondly, two designs based on the same dipole structure but with added directors are introduced to increase the gain while maintaining almost the same bandwidth. The gains achieved are 8.01 dBi and 9.6 dBi, respectively. Finally, an array of elements is used to achieve the highest possible gain of 13.6 dBi with good efficiency about 89% and with limited director elements for a planar compact structure to state-of-the-art literature. All the results achieved make the proposed designs viable candidates for high-speed and short-distance wireless communication systems.

Sub-THz Antenna for High-Speed Wireless Communication Systems

We present a frequency modulated continuous wave (FMCW) multiple input multiple output (MIMO) radar demonstrator system operating in the W-band at frequencies around 100 GHz. It consists of a two dimensional sparse array together with hardware for signal generation and image reconstruction that we will describe in more detail. The geometry of the sparse array was designed with the help of simulations to the aim of imaging at distances of just a few up to more than 150 meters. The FMCW principle is used to extract range information. To obtain information in both cross-range directions a back-propagation algorithm is used and further explained in this paper. Finally, we will present first measurements and explain the calibration process.

A 100 GHz FMCW MIMO radar system for 3D image reconstruction

This paper presents a novel transmitter system for applications operating in the frequency region around 300 GHz. A relatively simple packaging approach is taken, allowing for a variety of integrated or embedded applications. This paper presents a multilayered stacked patch antenna on quartz with a dielectric lens, functioning in tandem with a GaAs-based multiplier chain. When operating, an input signal between 7.8 and 8.6 GHz is multiplied 36 times to a frequency range between 280 and 310 GHz, via two in-house fabricated GaAs-based monolithic microwave integrated circuits (MMICs). The antenna is fabricated on quartz wafers via an in-house process. Both, the MMICs and the antenna, are placed on a printed circuit board. A high-density polyethylene lens encloses the system-in-package (SiP), with a footprint of 1 cm $^{2}$ . The SiP has a center frequency of 300 GHz and an absolute pattern bandwidth of 21 GHz. The measured antenna gain along the broadside is 23 dBi, which corresponds to an equivalent isotropic radiated power of 20 dBm. This is achieved with 0.3 W power consumption. The design, simulation, and the analysis are performed via an electromagnetic simulation and modeling tool (Computer Simulation Technology Microwave Studio). Both, separate measurement data for the individual parts of the SiP and complete system experimental characterization are included in this paper.

A Transmitter System-in-Package at 300 GHz With an Off-Chip Antenna and GaAs-Based MMICs

In this paper we demonstrate an antenna for 300 GHz. The antenna is processed on quartz and consists of 2 quartz layers with 2 metal layers each, which are separated by benzocyclobutene (BCB). The edge length is 2 millimeters. Design, simulation and the analysis is done in CST Microwave Studio. Measurement is done after packaging on a brass disk. This work shows the design details, simulated results and measurement data of the presented antenna.

A 300 GHz microstrip multilayered antenna on quartz substrate

This paper presents an antenna design, integrated into a printed circuit board (PCB), operating around the 100 GHz mark. A combination of a cavity-backed antenna with a dielectric lens is implemented to provide an enormous improvement in the farfield pattern and gain of the device. The PCB is manufactured via a standard multilayer fabrication process, using a glass-reinforced epoxy laminate (FR4) and a thermoset polymer (Astra MT77). The lens consists of high-density polyethylene (HDPE) material. The antenna has a relative pattern bandwidth of 25% and reaches a peak gain of 23 dBi. This work contains a brief material characterization of the utilized Astra MT77 substrate at the antenna operating bandwidth and a comparison between simulated and measured results of the antenna.

Alexander Dyck

Papers

${W}$ -Band Time-Domain Multiplexing FMCW MIMO Radar for Far-Field 3-D Imaging

A 100 GHz FMCW MIMO radar system for 3D image reconstruction

A Transmitter System-in-Package at 300 GHz With an Off-Chip Antenna and GaAs-Based MMICs

A 300 GHz microstrip multilayered antenna on quartz substrate

A Dielectric-Filled Cavity-Backed Lens-Coupled Dipole Antenna at 100 GHz