Millimeter Wave Imaging System Research Articles

Curved HSIW: an affordable performance for non-planar millimeter-wave applications

This paper introduces a Curved Hollow Substrate Integrated Waveguide (CHSIW) to address the growing demand for millimeter-wave applications, including communication, sensing, imaging, radar, and wireless power transfer systems. Leveraging advancements in integrated circuits and system-in-package (SiP) technology, the CHSIW tackles the challenges associated with hardware integration into curved structures. The design utilizes MultiJet Printing (MJP) for the M3 crystal dielectric substrate and a water laser cutter system for copper sheets, streamlining the fabrication process by eliminating complex via manufacturing steps through prefabricated through-hole vias. Operating in the frequency range of 21.7 GHz to 32 GHz, the CHSIW exhibits outstanding performance on curved surfaces, with measured average attenuation constants of 1.89 Np m−1 (16.42 dB m−1) and 1.95 Np m−1 (16.94 dB/m) for samples with radii of curvature of 166.8 mm and 125.1 mm, respectively. Beyond addressing technical challenges, the CHSIW presents a cost-effective and simplified fabrication process, positioning it as a breakthrough solution for diverse millimeter-wave applications, including future robotic and UAV communications. Furthermore, the proposed design and fabrication technique hold promise for the realization of conformal and free-form Hollow Substrate Integrated Waveguide (HSIW) devices in the future.

3D detection of flying insects from a millimeter-wave radar imaging system

Studying the movement patterns of small flying insects, such as pollinators, is a major challenge in biology. Here we introduce an original approach to track the 3D motion of flying insects using a millimeter-wave radar imaging system. 3D images are obtained from the beam scanning of a Frequency-Modulated Continuous-Wave and Single-Input Multiple-Output radar operating at 77 GHz. We derive the flight trajectory of insects and mitigate the electromagnetic clutter from the 3D radar images. To illustrate our approach, 3D flights of isolated bumblebees are radar tracked in a limited volume during 5.2 minutes with a time resolution of 60ms. The radar tracks are compared with video recordings for validation purpose. The tracking volume of an insect using a single radar is estimated at 5.3m3, and we show that the volume can be tripled when three radar imaging systems are deployed. Finally, we discuss how our new radar-based technique can be extended to track a broad diversity of small flying animals at different spatial scales and simultaneously in the lab and in the field.

Development of a High-Sensitivity Millimeter-Wave Radar Imaging System for Non-Destructive Testing.

There is an urgent need to develop non-destructive testing (NDT) methods for infrastructure facilities and residences, etc., where human lives are at stake, to prevent collapse due to aging or natural disasters such as earthquakes before they occur. In such inspections, it is desirable to develop a remote, non-contact, non-destructive inspection method that can inspect cracks as small as 0.1 mm on the surface of a structure and damage inside and on the surface of the structure that cannot be seen by the human eye with high sensitivity, while ensuring the safety of the engineers inspecting the structure. Based on this perspective, we developed a radar module (absolute gain of the transmitting antenna: 13.5 dB; absolute gain of the receiving antenna: 14.5 dB) with very high directivity and minimal loss in the signal transmission path between the radar chip and the array antenna, using our previously developed technology. A single-input, multiple-output (SIMO) synthetic aperture radar (SAR) imaging system was developed using this module. As a result of various performance evaluations using this system, we were able to demonstrate that this system has a performance that fully satisfies the abovementioned indices. First, the SNR in millimeter-wave (MM-wave) imaging was improved by 5.4 dB compared to the previously constructed imaging system using the IWR1443BOOST EVM, even though the measured distance was 2.66 times longer. As a specific example of the results of measurements on infrastructure facilities, the system successfully observed cracks as small as 0.1 mm in concrete materials hidden under glass fiber-reinforced tape and black acrylic paint. In this case, measurements were also made from a distance of about 3 m to meet the remote observation requirements, but the radar module with its high-directivity and high-gain antenna proved to be more sensitive in detecting crack structures than measurements made from a distance of 780 mm. In order to estimate the penetration length of MM waves into concrete, an experiment was conducted to measure the penetration of MM waves through a thin concrete slab with a thickness of 3.7 mm. As a result, Λexp = 6.0 mm was obtained as the attenuation distance of MM waves in the concrete slab used. In addition, transmission measurement experiments using a composite material consisting of ceramic tiles and fireproof board, which is a component of a house, and experiments using composite plywood, which is used as a general housing construction material in Japan, succeeded in making perspective observations of defects in the internal structure, etc., which are invisible to the human eye.

Millimeter-Wave Band Electro-Optical Imaging System Using Polarization CMOS Image Sensor and Amplified Optical Local Oscillator Source.

In this study, we developed and demonstrated a millimeter-wave electric field imaging system using an electro-optic crystal and a highly sensitive polarization measurement technique using a polarization image sensor, which was fabricated using a 0.35-µm standard CMOS process. The polarization image sensor was equipped with differential amplifiers that amplified the difference between the 0° and 90° pixels. With the amplifier, the signal-to-noise ratio at low incident light levels was improved. Also, an optical modulator and a semiconductor optical amplifier were used to generate an optical local oscillator (LO) signal with a high modulation accuracy and sufficient optical intensity. By combining the amplified LO signal and a highly sensitive polarization imaging system, we successfully performed millimeter-wave electric field imaging with a spatial resolution of 30×60 µm at a rate of 1 FPS, corresponding to 2400 pixels/s.

Efficient single-frequency millimeter-wave three-dimensional imaging for rapid target identification

Compared to broadband systems, single-frequency millimeter-wave (SFQ-MMW) imaging systems can significantly simplify system complexity, but can only obtain two-dimensional information of the target. Moreover, the position of the reference plane must be adjusted in real time during the imaging process to obtain a high-quality focusing effects. Therefore, this paper proposes a SFQ-MMW three-dimensional imaging algorithm. In the plane wave spectrum domain, the propagation operator is used as a focusing term to accurately locate the target focus. Only single-frequency echo data is required to retrieve information and reconstruct high-quality images. This method can significantly improve the efficiency of near-field millimeter wave measurement and imaging. This work facilitates applications in rapid target identification and characterization. The performance of this method was tested through numerical simulation. Finally, validation was performed on real data in an independently developed system. The results show that the test results are in line with expectations, and the imaging efficiency has been significantly improved.

A high-resolution handheld millimeter-wave imaging system with phase error estimation and compensation

Despite the enormous potential of millimeter-wave (mmWave) imaging, the high cost of large-scale antenna arrays or stringent prerequisites of the synthetic aperture radar (SAR) principle impedes its widespread application. Here, we report a portable, affordable, and high-resolution 3D mmWave imaging system by overcoming the destructive motion error of handheld SAR imaging. This is achieved by revealing two important phenomenons: spatial asymmetry of motion errors in different directions, and local similarity of phase errors exhibited by different targets, based on which we formulate the challenging phase error estimation problem as a tractable point spread function optimization problem. Experiments demonstrate that our approach can recover high-fidelity 3D mmWave images from severely distorted signals and augment the aperture size by over 50 times. Since our system does not rely on costly massive antennas or bulky motion controllers, it can be applied for diverse applications including security inspection, autonomous driving, and medical monitoring.

Challenge for Millimeter-wave imaging system for Remote Security Inspection

Challenge for Millimeter-wave imaging system for Remote Security Inspection

Static Hand Gesture Recognition Based on Millimeter-Wave Near-Field FMCW-SAR Imaging

To address the limitations of wireless sensing in static gesture recognition and the issues of Computer Vision’s dependence on lighting conditions, we propose a method that utilizes millimeter-wave near-field SAR (Synthetic Aperture Radar) imaging for static gesture recognition. First, a millimeter-wave near-field SAR imaging system is used to scan the defined static gestures to obtain data. Then, based on the distance plane, the three-dimensional gesture is divided into multiple two-dimensional planes, constructing an imaging dataset. Finally, an HOG (Histogram of Oriented Gradients) is used to extract features from the imaging results, PCA (Principal Component Analysis) is applied for feature dimensionality reduction, and RF (Random Forest) performs classification. Experimental verification shows that the proposed method achieves an average recognition precision of 97% in unobstructed situations and 93% in obstructed situations, providing an effective means for wireless-sensing-based static gesture recognition.

Monitoring of sow postural activity from 3D millimeter-wave radar imaging

This paper presents a novel approach to monitor the postural activity of sows in farrowing pen using a millimeter-wave radar imaging system. Three-dimensional images of the scene are obtained from a 77 GHz Multiple-Input Multiple Output radar and the mechanical scanning of the radar beam. We show that the processing of radar images based on the Cell-Averaging Constant False Alarm Rate allows detecting the standing and lying postures of sows despite the electromagnetic clutter. The experimental study reported here is performed from the monitoring of 16 sows from the Large White breed for a total of 46 h. 3D positions of the sow are recorded and heatmaps of the sow locomotor activity are proposed. With few training radar data, we perform a classification of standing and laying postures with a mean balanced accuracy greater than 90%. The radar-based technique applied here in a cluttered environment inside farrowing pens may be extended to monitor pigs under other housing condition.

Millimeter-Wave and Sub-THz Phased-Array Imaging Systems Based on Electro-Optic Up-Conversion and Optical Beamforming

Millimeter-Wave and Sub-THz Phased-Array Imaging Systems Based on Electro-Optic Up-Conversion and Optical Beamforming

Passive millimeter-wave imaging system with variable focal length optical configuration

Passive millimeter-wave imaging system with variable focal length optical configuration

Comparison of Terahertz Transceiver Array Topologies in Multiobject Imaging

We investigate the effect of side lobe level (SLL) and multipath propagation in millimeter-wave(mmWave) and terahertz (THz) imaging systems for various transceiver array topologies. This work considers practical limitations in high-resolution active imaging systems and focuses on electrically large, planar antenna array topologies. As such, we investigate the imaging performance of sparse transmitting/receiving (Tx/Rx) array topologies that enable high angular/spatial resolution imaging with minimum hardware complexity. We show that 1) properly tapering the sparse array excitation can mitigate image clutter caused by high side lobes and 2) sparse arrays are more vulnerable to multipath reflections and clutter in dense scenes with highly reflective surfaces. We present both full-wave numerical analysis and laboratory experiments (250-300 GHz) to verify and compare the imaging array performance for several exemplary scenes.

Three-Dimensional Active Incoherent Millimeter-Wave Imaging

A new 3-D active incoherent millimeter-wave (3-D AIM) imaging method is introduced in this article. By utilizing a sparse interferometric receive array and pulse modulation in the transmitted noise signals we demonstrate far-field incoherent imaging in three dimensions. Interferometric image formation is based on the spatio-temporal incoherence of the signals emitted by or scattered from the imaging scene. To support this, we use active noise transmission from multiple independent transmitters which provide incoherent illumination in space (azimuth and elevation) and time. Two-dimensional (azimuth-elevation) image formation in this manner only requires statistical knowledge of the waveforms, greatly simplifying the imaging system. In this work, we modulate the transmitted waveforms to impart range-dependent information that can be used to also obtain range, supporting imaging in three dimensions. We modulate a Gaussian pulse onto the noise waveforms and apply incoherent pulse integration to provide sufficient spatio-temporal decorrelation of the signals. We introduce a new 3-D Fourier image formation algorithm, provide imaging simulations, and include experimental image reconstructions from a 38 GHz millimeter-wave imaging system. Video-rate 3-D imagery of a moving target is presented to demonstrate the feasibility of the approach for real-time 3-D imaging applications.

Ka Band Low Channel Mutual Coupling Integrated Packaged Phased Array Receiver Front-End for Passive Millimeter-Wave Imaging

This paper presents a Ka band eight-channel integrated packaged phased array receiver front-end for a passive millimeter-wave imaging system. Since multiple receiving channels are integrated in a given package, the mutual coupling issue affecting the channel will deteriorate imaging quality. Therefore, in this study, the influence of channel mutual coupling on the system array pattern and amplitude phase error is analyzed, and the design requirements are proposed according to the results. During the design implementation, the coupling paths are discussed, and passive circuits in the path are modeled and designed to reduce the level of channel mutual coupling and spatial radiation. Finally, an accurate coupling measurement method for a multi-channel integrated phased array receiver is proposed. The receiver front-end achieves a 28~31 dB single channel gain, a 3.6 dB noise figure, less than −47 dB of channel mutual coupling. Furthermore, the array pattern of the two-dimensional 1024 channel system composed of the front end of the receiver is consistent with the simulation, and the receiver’s performance is verified by a human-body-imaging experiment. The proposed coupling analysis, design, and measurement methods are also applicable to other multi-channel integrated packaged devices.

Design and 3D printing of the Powell lens for sub-terahertz imaging.

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

A dual-band antireflection metasurface for air-ground impedance matching in ground penetrating radar

In ground-penetrating radar (GPR) detection, the impedance mismatch between air and the material under tested (MUT) creates a strong reflection at the air-ground interface. It substantially restricts the GPR’s detecting depth by reducing the electromagnetic energy fed into the ground. This paper describes a dual-band antireflection metasurface for an air-ground impedance matching layer (IML) with central frequencies of 1.0 GHz and 2.5 GHz. Its dual-band feature relies on two distinct inner and outer closed resonant ring topologies. The destructive interference theory instructs the design and optimization of the proposed IML. Through simulation and experiment, the dual-band IML can adequately eliminate reflection at the air-ground interface and enhance transmission from the air to the MUT for GPR applications. Real-world GPR experiments demonstrate that a hazy basement interface is clearer when IML is positioned at the ground’s surface. Owing to the passive and electrically thin properties of IML, the proposed approach offers a feasible impedance matching solution for GPR and through-wall radar detection. Moreover, it also has application potential in multi-band GPR and can be extended to microwave and millimeter-wave imaging systems.

Millimeter-Wave Single-Pixel Imaging Using Electrically-Switchable Liquid-Crystal Mask

In this study, we have proposed a millimeter-wave (MMW) single-pixel imaging (SPI) system with a liquid-crystal (LC) mask cell. The LC cell functions as an electrically switchable mask based on the change in absorption properties, which depend on the orientation of the LC. We investigated the influence of noise on the measured and estimated data (reconstructed image). The proposed system exhibited moderate robustness against random noise (that were added) compared to raster scan-based and Hadamard matrix-based SPI systems. Finally, the results of some demonstrative experiments were introduced to ensure the applicability of the constructed MMW-SPI system, and steps for improving the reconstructed image quality were discussed.

A Deep Learning Approach for Reconstruction in Millimeter-Wave Imaging Systems

In millimeter-wave (MMW) imaging, the objects of interest are oftentimes modeled as 2-D binary (black and white) shapes with white pixels representing the reflecting interior of the object. However, due to the propagation of the scattered waves, the continuous-domain binary images are convolved with a so-called point-spread function (PSF) before being digitized by means of sampling. As the 2-D PSF is both nonseparable and nonvanishing in the case of MMW imaging, exact recovery is quite complicated. In this communication, we propose a deep-learning approach for image reconstruction. We should highlight that the wave scatterings are suitably represented with complex-valued quantities, while standard deep neural networks (DNNs) accept real-valued inputs. To overcome this challenge, we separate the real and imaginary parts as if we had two imaging modalities and concatenate them to form a real-valued input with a larger size. Fortunately, the network automatically learns how to combine the mutual information between these modalities to reconstruct the final image. Among the advantages of the proposed method are improved robustness against additive noise and mismatch errors of imaging frequency and object to antenna distance; indeed, the method works well in wideband imaging scenarios over a wide range of objects to antenna distances even in the presence of high noise levels without requiring a separate calibration stage. We test the method with synthetic data simulated with software as well as real recordings in the laboratory.

A Dynamic Dual Reflector Antenna for 4-D Synthetic Aperture Radar Imaging Achieving a 0.4° Pencil Beam at 77 GHz

All existing state-of-the-art high-resolution millimeter-wave imaging systems experience a tradeoff between image acquisition time and transceiver array complexity. The proposed dual-reflector antenna breaks this tradeoff by drastically reducing the array formation time while maintaining the relative simplicity that comes with using a single transceiver element. It consists of a dual-mode horn feed, a rotating ellipsoidal subreflector, and a conic main reflector. The rotating subreflector creates a virtual phase center that rotates about an axis to produce a synthetic circular array with a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$120\lambda $ </tex-math></inline-formula> . The main reflector redirects the beams from each of these virtual phase centers to overlap and illuminate the scene over a wide field of view (FOV). Two cases are simulated, one with a 14° FOV and another with 25° FOV in both azimuth and elevation. The former case is experimentally verified and achieves a measured half-power beamwidth of 0.4° over the entire scan range. The proposed system can reduce the image acquisition time to the order of milliseconds/seconds, which makes real-time SAR imaging a practical alternative to multiple-input–multiple-output (MIMO) and phased arrays at millimeter-wave and sub-millimeter-wave frequencies.

High-resolution 96-GHz (3 mm) passive millimeter-wave imaging system and the effect of scanning resolution on images

A high-resolution 96-GHz (3 mm) passive millimeter-wave imaging system is constructed. The system has a 96-GHz radiometric receiver, which is placed on the focal point of a 50-cm diameter parabolic reflector designed and created. A specially written program for the system controls the antenna via control hardware in desired steps by a 2-axis positioner motor. A balance weight has been calculated and mounted to the rear of this parabolic reflector antenna to prevent possible damage to the rotor of the positioner motor due to the antenna’s weight. The maximum field of view of the system is 120 deg in azimuth and 90 deg in elevation. The angular scan step can be varied from 0.1 deg to 1 deg. Although the system can shoot 1190 × 890 samples (pixels) images, which is close to the HD quality (1280 × 720 pixels) in the full field of view, the actual resolution of the image is limited by the antenna’s beamwidth (BW). Some measurements were performed to investigate the effect of scanning steps on PMMW image quality for different angular scan steps. It is observed that the PMMW image scan with the smallest step gives a detailed image that is the closest to the scene’s optical image as expected. Even though the angular scan step angle is less than half of the antenna’s BW at Nyquist sampling, it is observed that the image quality still improves due to averaging effect of the received signals giving a better estimated S/N. The resolution and scan step values of our system are compared with that of some systems found in the literature and it is shown that ours has the highest value among them. This system can be used to obtain high-resolution images in remote sensing and other applications.