The Fundamentals of Radar with Applications to Autonomous Vehicles

Radar systems can be extremely useful for applications in autonomous vehicles. This paper seeks to show how radar systems function and how they can apply to improve autonomous vehicles. First, the basics of radar systems are presented to introduce the basic terminology involved with radar. Then, the topic of phased arrays is presented because of their application to autonomous vehicles. The topic of digital signal processing is also discussed because of its importance for all modern radar systems. Finally, examples of radar systems based on the presented knowledge are discussed to illustrate the effectiveness of radar systems in autonomous vehicles

Backscatter Transponder Based on Frequency Selective Surface for FMCW Radar Applications

This paper describes an actively-controlled frequency selective surface (FSS) to implement a backscatter transponder. The FSS is composed by dipoles loaded with switching PIN diodes. The transponder exploits the change in the radar cross section (RCS) of the FSS with the bias of the diodes to modulate the backscattered response of the tag to the FMCW radar. The basic operation theory of the system is explained here. An experimental setup based on a commercial X-band FMCW radar working as a reader is proposed to measure the transponders. The transponder response can be distinguished from the interference of non-modulated clutter, modulating the transponder’s RCS. Some FSS with different number of dipoles are studied, as a proof of concept. Experimental results at several distances are provided

Design of Digital FMCW Chirp Synthesizer PLLs Using Continuous-Time Delta-Sigma Time-to-Digital Converters

Radar applications for driver assistance systems and autonomous vehicles have spurred the development of frequency-modulated continuous-wave (FMCW) radar. Continuous signal transmission and high operation frequencies in the K- and W-bands enable radar systems with low power consumption and small form factors. The radar performance depends on high-quality signal sources for chirp generation to ensure accurate and reliable target detection, requiring chirp synthesizers that offer fast frequency settling and low phase noise. Fractional-N phase locked loops (PLLs) are an effective tool for synthesis of FMCW waveform profiles, and advances in CMOS technology have enabled high-performance single-chip CMOS synthesizers for FMCW radar. Design approaches for FMCW chirp synthesizer PLLs need to address the conflicting requirements of fast settling and low close-in phase noise. While integrated PLLs can be implemented as analog or digital PLLs, analog PLLs still dominate for high frequencies. Digital PLLs offer greater programmability and area efficiency than their analog counterparts, but rely on high-resolution time-to-digital converters (TDCs) for low close-in phase noise. Performance limitations of conventional TDCs remain a roadblock for achieving low phase noise with high-frequency digital PLLs. This shortcoming of digital PLLs becomes even more pronounced with wide loop bandwidths as required for FMCW radar. To address this problem, this work presents digital FMCW chirp synthesizer PLLs using continuous-time delta-sigma TDCs. After a discussion of the requirements for PLL-based FMCW chirp synthesizers, this dissertation focuses on digital fractional-N PLL designs based on noise-shaping TDCs that leverage state-of-the-art delta-sigma modulator techniques to achieve low close-in phase noise in wide-bandwidth digital PLLs. First, an analysis of the PLL bandwidth and chirp linearity studies the design requirements for chirp synthesizer PLLs. Based on a model of a complete radar system, the analysis examines the impact of the PLL bandwidth on the radar performance. The modeling approach allows for a straightforward study of the radar accuracy and reliability as functions of the chirp parameters and the PLL configuration. Next, an 18-to-22GHz chirp synthesizer PLL that produces a 25-segment chirp for a 240GHz FMCW radar application is described. This synthesizer design adapts an existing third-order noise-shaping TDC design. A 65nm CMOS prototype achieves a measured close-in phase noise of -88dBc/Hz at 100kHz offset for wide PLL bandwidths and consumes 39.6mW. The prototype drives a radar testbed to demonstrate the effectiveness of the synthesizer design in a complete radar system. Finally, a second-order noise-shaping TDC based on a fourth-order bandpass delta-sigma modulator is introduced. This bandpass delta-sigma TDC leverages the high resolution of a bandpass delta-sigma modulator by sampling a sinusoidal PLL reference and applies digital down-conversion to achieve low TDC noise in the frequency band of interest. Based on the bandpass delta-sigma TDC, a 38GHz digital FMCW chirp synthesizer PLL is designed. The feedback divider applies phase interpolation with a phase rotation scheme to ensure the effectiveness of the low TDC noise. A prototype PLL, fabricated in 40nm CMOS, achieves a measured close-in phase noise of -85dBc/Hz at 100kHz offset for wide loop bandwidths >1MHz and consumes 68mW. It effectively generates fast (500MHz/55us) and precise (824kHz rms frequency error) triangular chirps for FMCW radar. The bandpass delta-sigma TDC achieves a measured integrated rms noise of 325fs in a 1MHz bandwidth.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147732/1/dweyer_1.pdfDescription of dweyer_1.pdf : Restricted to UM users only

Design and Implementation of a FPGA and DSP Based MIMO Radar Imaging System

The work presented in this paper is aimed at the implementation of a real-time multiple-input multiple-output (MIMO) imaging radar used for area surveillance. In this radar, the equivalent virtual array method and time-division technique are applied to make 16 virtual elements synthesized from the MIMO antenna array. The chirp signal generater is based on a combination of direct digital synthesizer (DDS) and phase locked loop (PLL). A signal conditioning circuit is used to deal with the coupling effect within the array. The signal processing platform is based on an efficient field programmable gates array (FPGA) and digital signal processor (DSP) pipeline where a robust beamforming imaging algorithm is running on. The radar system was evaluated through a real field experiment. Imaging capability and real-time performance shown in the results demonstrate the practical feasibility of the implementation

Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar

We show experimentally that a high-resolution imaging radar operating at 576–605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4–25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources

Frequency-modulated continuous-wave LiDAR compressive depth-mapping

We present an inexpensive architecture for converting a frequency-modulated continuous-wave LiDAR system into a compressive-sensing based depth-mapping camera. Instead of raster scanning to obtain depth-maps, compressive sensing is used to significantly reduce the number of measurements. Ideally, our approach requires two difference detectors. % but can operate with only one at the cost of doubling the number of measurments. Due to the large flux entering the detectors, the signal amplification from heterodyne detection, and the effects of background subtraction from compressive sensing, the system can obtain higher signal-to-noise ratios over detector-array based schemes while scanning a scene faster than is possible through raster-scanning. %Moreover, we show how a single total-variation minimization and two fast least-squares minimizations, instead of a single complex nonlinear minimization, can efficiently recover high-resolution depth-maps with minimal computational overhead. Moreover, by efficiently storing only $2m$ data points from $m<n$ measurements of an $n$ pixel scene, we can easily extract depths by solving only two linear equations with efficient convex-optimization methods

Precise control of broadband frequency chirps using optoelectronic feedback

We demonstrate the generation of wideband frequency sweeps using a semiconductor laser in an optoelectronic feedback loop. The rate and shape of the optical frequency sweep is locked to and determined by the frequency of a reference electronic signal, leading to an agile, high coherence swept-frequency source for laser ranging and 3-D imaging applications. Using a reference signal of constant frequency, a transformlimited linear sweep of 100 GHz in 1 ms is achieved, and real-time ranging with a spatial resolution of 1.5 mm is demonstrated. Further, arbitrary frequency sweeps can be achieved by tuning the frequency of the input electronic signal. Broadband quadratic and exponential optical frequency sweeps are demonstrated using this technique