I. Introduction

Radar sensors with ultrahigh range resolution, accuracy, and sensitivity have great potential in various noncontact sensing applications [1], [2], [3], [4]. In this article, range accuracy is referred to the minimum static displacement that can be detected by radar, and range sensitivity is referred to the minimum vibrating displacement that can be detected by radar, while range resolution is referred to the minimum range difference that can be distinguished by radar when detecting multiple objects. Human vital signs such as respiration, heart, and epilepsy induce mm-level displacement at very low frequencies around 1 Hz [5], [6], [7], [8], [9]. Material thickness and flatness measurement and detection in high-precision industrial manufacturing require -level accuracy. Detection of -level mechanical vibration of building structures in smart cities, machines, and instruments in labs, factories, and other specialty environments also call for high-sensitivity sensors [1], [7], [10], [11]. Utilizing dielectric lenses, beamforming, and phased-array techniques, millimeter-wave (mmWave) and terahertz (THz) radars can be designed as high-resolution imagers for security monitoring and human gesture detection [12], [13]. In general, radars with carrier signals at higher frequencies such as mmWave and THz band have better range accuracy and sensitivity, since the same displacement induces larger phase change on the shorter wavelengths.