Millimeter Wave Imaging System Research Articles

Experimental Validation on Tissue-Mimicking Phantoms of Millimeter-Wave Imaging for Breast Cancer Detection

Breast cancer is one of the leading causes of cancer death among women; to decrease the death rate for this disease, early detection plays a key role. Recently, microwave imaging systems have been proposed as an alternative to the current techniques, but they suffer from poor resolution due to the low frequencies involved. In this paper, for the first time, an innovative millimeter-wave imaging system for early-stage breast cancer detection is proposed and experimentally verified on different breast phantoms. This has the potential to achieve superior resolution for breasts with a high volumetric percentage of adipose tissue, and the merit to overcome the common misconception that millimeter-waves cannot achieve useful penetration depths for biological applications. Three phantoms were prepared according to the dielectric properties of human breast ex vivo tissues in the frequency range [0.5–50] GHz. Two cylindrical inclusions made by water and gelatin or agar, mimicking dielectric properties of neoplastic tissues, were embedded in the phantom at different depths up to 3 cm. Two double ridge waveguides, with mono-modal frequency band equal to [18–40] GHz, were used to synthetize a linear array of 24 elements in 28 positions, acquiring signals with a Vector Network Analyzer. The images were reconstructed by applying the Delay and Sum algorithm to calibrated data. The feasibility of a new imaging system with a central working frequency of about 30 GHz is experimentally demonstrated for the first time, and a target detection capability up to 3 cm within the phantom is shown. The presented results pave the way for a possible use of millimeter-waves to image non-superficial neoplasms in the breast.

Optimization of a Sparse Aperture Configuration for Millimeter-Wave Computational Imaging

We present two techniques for optimizing the position of transmitter and receiver modules on a sparse aperture for a millimeter-wave computational imaging system. The first technique uses an easily computable spatial representation of the transmitter and receiver array, called the coarray, to ideally distribute the spatial frequency components probed by the imaging setup. The second approach involves maximizing the information added by a complete measurement of the scene by the system. This approach is analogous to the system capacity maximization frequently employed in wireless communication. We show that employing aperture configurations optimized using these two techniques over commonly used standard aperture configurations results in 30% less mean-squared error when used to reconstruct a particular ensemble of 400 arbitrary 2-D objects. Finally, we discuss the similarities and differences between the two optimization strategies in terms of imaging performance and computational speed, including a case when one strategy performs better than the other.

Experimental Demonstration and Calibration of a 16-Element Active Incoherent Millimeter-Wave Imaging Array

In this article, an active incoherent millimeter-wave imaging array is presented, along with its calibration procedure. Active incoherent millimeter-wave imaging uses the transmission of incoherent signals from multiple transmitters to mimic the properties of thermal radiation, enabling interferometric image reconstruction that can be realized in a snap-shot mode, without beamsteering. Due to the use of transmitters, the sensitivity requirement on the receivers is significantly relaxed compared with passive millimeter-wave imaging systems that detect low-power thermal radiation, making it possible to use standard commercial hardware, therefore decreasing the cost considerably. No exact knowledge of the transmit illumination is needed; thus, the coordination of the transmitters is minimal, further simplifying the system implementation. In this work, a 16-element Ka-band millimeter-wave imager is built and presented using commercial components and in-house fabricated antennas, along with a calibration procedure to account for amplitude and phase variations in the hardware. Experimental 2-D snapshot image reconstructions are presented.

Millimeter-wave imaging using accelerated coherence factor based range migration algorithm

For millimeter-wave imaging systems, techniques pertaining to higher image quality seem attractive. Super-resolution (SR) approaches achieve better resolved results without extra system requirements. A computationally efficient SR imaging approach is proposed in this paper. The coherence factor (CF) is invoked and recast into the framework of range migration algorithm (RMA). Different from our previously proposed version (termed CF-RMA), CF is reformulated in the wavenumber domain, and the incoherent power of the reflectivity function is redefined and implemented with one-dimensional (1-D) inverse fast Fourier transform (IFFT) conveniently. As such, the proposed accelerated CF (ACF) is more suitable to be integrated into RMA, achieving faster implementation. As demonstrated in the simulation and experimental results, our addressed ACF based RMA (ACF-RMA) achieves spatial resolution enhancement, and sidelobes and background level suppression with respect to RMA. As indicated by the structural similarity (SSIM) index quantitatively, the image quality of ACF-RMA is on-par with that of CF-RMA. Finally, the computation load of ACF-RMA is estimated. The proposed SR imaging method is demonstrated to be an appropriate candidate for real-time systems.

Random Sampling With Iterative Recovery for Millimeter-Wave Imaging Systems

In this brief, an imaging methodology for mono-static millimeter-wave systems is introduced. When the measured object hologram is piece-wise smooth with sharp boundaries, both low-pass and high-pass components are present in a transform domain. Nevertheless, the edges are sparse in transform domain with respect to the image dimensions. It is shown that the down-sampling of the hologram with an appropriate rate determined by the sparsity level, includes the adequate information of all components for image reconstruction. Using random samples, an algorithm is proposed to recover all the sparse components, iteratively. Simulation results illustrate that a high-resolution recovery can be achieved. For instance, by using 20% of the randomly selected hologram samples, an image recovery with a PSNR of 20 dB is achieved and the object scan time is significantly reduced. The proposed methodology is applied to a practical imaging system operating at 30 GHz, where the results illustrate a successful reconstruction of the images.

An efficiently-designed wideband single-metalens with high-efficiency and wide-angle focusing for passive millimeter-wave focal plane array imaging.

Wide-angle, high-efficiency, wide-band, and ultra-compact focusing blocks are crucial for implementation and future evolution of passive millimeter-wave focal plane array imaging systems. The spherical or doublet metalens can attain high-efficiency, wide-angle field-of-view (FOV) but suffer from fabrication difficulties, complex assembly, and low compactness. Here we present an efficient single-metalens design capable of performing high-efficiency diffraction-limited wideband focusing over a wide-angle FOV. This single-metalens design can greatly mitigate the Seidel aberrations by a rational allocation of amplitude-phase of the electromagnetic waves. A proof-of-concept metalens at millimeter-wave band (33 GHz-37 GHz) confirms the validity of our design.

Accurate Near-Field Millimeter-Wave Imaging of Concave Objects—A Case Study of Dihedral Structures Under Monostatic Array Configurations

With the fast progress of high-resolution microwave and millimeter-wave (MMW) near-field imaging systems, image artifacts caused by complex high-contrast target become a nonnegligible issue for accurate surface reconstruction and recognition. Among different types of artifacts, those caused by high-order scattering within concave structures are particularly difficult to interpret and to correct. This article focuses on analyzing these unconsidered effects in conventional imaging or beamforming algorithms and chooses a typical imaging scenario with dihedral structures under near-field monostatic cylindrical imaging geometry for discussion. The mechanism of multiple reflections and the formation of artifacts are firstly investigated, and the artifacts are classified into two categories based on the number of reflection times. Then shooting and bouncing rays (SBRs) method is utilized to establish an analytical forward model including the mechanism of multiple reflections. Furthermore, a new imaging algorithm based on this forward model is proposed to obtain more accurate reconstructions. By compensating phase shifts of different multiple reflection signals with the corresponding range equations, dihedral-type objects with different opening angles can be accurately recovered. Numerical results are presented to verify the applicability of the forward model and to validate the effectiveness of the proposed imaging algorithm. In the end, experimental results confirm the practicability of the proposed method.

Millimeter-wave SFCW SAR imaging system based on in-phase signal measurement with simplified transceiver

A conventional millimeter-wave stepped frequency continuous wave (SFCW) synthetic aperture radar (SAR) imaging system generally utilizes IQ modulation technique to acquire the amplitude and phase of the waves scattered from a target object. Due to measure both in-phase signal and quadrature signal, the transceiver of the conventional system is complicate and costly, and the IQ imbalance problem makes the system difficult to calibrate. To reduce hardware complexity and enhance efficiency-cost ratio, a novel SFCW SAR imaging system only measuring in-phase signal is proposed and demonstrated. For lack of quadrature signal measurement, an algorithm based on Fourier transform is proposed to estimate the amplitude and phase. The ultimate images are obtained through an image-reconstruction algorithm, which uses the estimated amplitude and phase as input parameters. The proposed system is verified by both simulation and experiment, where the frequencies are set from 24GHz to 30GHz. The imaging results with high resolution and low noise are demonstrated. Compared to the conventional system, the image quality of the proposed system is almost identical, but the transceiver of the proposed system is greatly simplified.

Development and Demonstration of MIMO-SAR mmWave Imaging Testbeds

Multiple-input multiple-output (MIMO) radars and synthetic aperture radar (SAR) techniques are well researched and have been effectively combined for many imaging applications ranging from remote sensing to security. Despite numerous studies that apply MIMO concepts to SAR imaging, the design process of a MIMO-SAR system is non-trivial, especially for millimeter-wave (mmWave) imaging systems. Many issues have to be carefully addressed. Besides, compared with conventional monostatic sampling schemes or MIMO-only solutions, efficient image reconstruction methods for MIMO-SAR topologies are more complicated in short-range applications. To address these issues, we present highly-integrated and reconfigurable MIMO-SAR testbeds, along with examples of three-dimensional (3-D) image reconstruction algorithms optimized for MIMO-SAR configurations. The presented testbeds utilize commercially available wideband mmWave sensors and motorized rail platforms. Several aspects of the MIMO-SAR testbed design process, including MIMO array calibration, electrical/mechanical synchronization, system-level verification, and performance evaluation, are described. We present three versions of MIMO-SAR testbeds with different implementation costs and accuracies to provide alternatives for other researchers who want to implement their testbed framework. Several representative examples in various real-world imaging applications are presented to demonstrate the capabilities of the proposed testbeds and algorithms.

Millimeter-Wave Coherent Imaging of Moving Targets by Using Complex-Valued Self-Organizing Map and Auto-Encoder

Active millimeter-wave imaging systems using complex-valued self-organizing map (CSOM) have potentially wide applications in moving-target imaging by acquiring and classifying complex textures. However, interference in the coherent observation causes harmful effects such as amplitude fluctuation and phase distortion, resulting in the deterioration of the visualization quality achieved by the CSOM. In this article, we propose the introduction of feature refinementZ using a complex-valued auto-encoder (AE) into the feature extraction process of our imaging system to suppress these effects. We show that the complex-valued AE is extremely useful for extracting features appropriately even under such adverse influences, and improves the clustering performance. We also experimentally investigate the influence of the number of hidden-layer neurons on the AE performance to discuss the robustness of our system.

Enhancing Millimeter-Wave Computational Interferometric Imaging

Recently a progress in computational imaging has been noticed with the potential to improve the capabilities of millimeter-wave imaging systems. In this paper, an interferometric synthetic aperture systems is considered to be the conventional approach to image reconstruction that, in order to overcome its cost limitations and system complexity, both hardware and digital post-processing solutions are offered. A passive multiplexing cavity is added to encode received signals into the physical layer helping to reduce the number of measurement ports and thus the active RF chains without affecting the initial number of receiving antennas. This proposed technique leads to inverse problem solving process. A novel numerical method is then developed, based on a matrix formalism yielding a mathematical description of the computational approach. A numerical study is therefore performed to approve the feasibility of this technique for a better image reconstruction followed by an experimental study carried out at 92 GHz as a proof of concept.

Surface-roughness measurement based on scalar field correlation with active millimeter-wave imaging system.

Active millimeter-wave (MMW) imaging is of interest because it has played an important role in the area of personnel surveillance over the past decades. Through reconstructing reflectivity, potential threats can be recognized based on shape. Besides reflectivity, diverse physical characteristics should be explored so that supplementary information can be used to assist recognition. This paper presents a surface-roughness measurement method using holographic data that has already been acquired during security checks. Based on scalar diffraction theory and speckle metrology, a simple mathematical relation between the correlation of holographic fields and surface roughness has been derived. Consequently, another kind of information, that is, the surface-roughness estimate, can be used to help differentiate similarly shaped articles. Results of simulations and laboratory experiments have shown its validation and potential in application to MMW imaging systems.

Design of a Compact V-Band Transceiver and Antenna for Millimeter-Wave Imaging Systems

In this paper, we introduce a synthetic aperture radar (SAR)-based millimeter-wave imaging system for high-resolution imaging of human skin, in addition to traditional nondestructive testing applications. The imaging system, which operates in the 58–64-GHz frequency range, forms the basic building block of a future millimeter-wave camera suitable for imaging burned skin and skin cancers. The system is composed of a coherent transceiver and two antennas, principally enabling a quasi-monostatic measurement configuration. The designed antenna is an antipodal Vivaldi antenna with 5.0–8.0-dB gain and half-power beamwidths of ~79.0° and ~76.0°in its $E$ – and $H$ -planes, respectively. The transceiver is designed using a complete system-on-chip transmitter and receiver capable of $I/Q$ detection. The fully designed imaging system has 47 dB of dynamic range and is phase-calibrated using a robust wideband calibration procedure based on single-frequency SAR imaging approach. Examples of images produced with this system include burned pigskin skin and a small balsa wood panel with four small embedded rubber pieces.

Millimeter-Wave Imaging With Accelerated Super-Resolution Range Migration Algorithm

Millimeter-wave imaging systems are increasingly used in human screening at checkpoints, in linear or multi-input multi-output (MIMO) array modes. Rather than pressing intrinsic system requirements to improve image quality, it seems more economical to appeal to some specific approaches, for example, super-resolution (SR) techniques. In this paper, as a variation of SR, coherence factor (CF) is incorporated in the framework of the range migration algorithm (RMA), termed CF-RMA for short. As a critical part of CF, the double integral in the wavenumber domain is approximated by a single integral in the sense of the point-matching method, resulting in a particular formulation to carry on another independent RMA program. Being computationally efficient, the proposed method is shown to be effective in suppressing sidelobes and background noise. The resolution (as gauged by the width of the main lobe of the point spread function) is enhanced as well. A structural similarity (SSIM) index is employed to quantify the image quality enhancement with respect to state-of-the-art SR approaches. In terms of computation load, the new approach is shown to be much more efficient than state-of-the-art SR approaches.

3D‐printed 4‐zone Ka‐band Fresnel lens: design, fabrication, and measurement

A planar and thin-grooved Fresnel lens is a great candidate for gain enhancement in millimetre wave communication, imaging systems, and wireless power transfer applications. The authors report the design, fabrication, and measurement of a 3D-printed Fresnel lens, using a single material. A low-profile (1.2 λ thick) 4-zone Fresnel lens with 16 annular rings is designed with a focal length of 40 mm (≃4 λ ) operating at 30 GHz. Authors' design consists of four-step heights with outer radius of 69 mm (6.9 λ ). Permittivity and loss tangent of polylactic acid are measured to be 2.79 and 0.0048 at 30 GHz, respectively. Focusing ability of the lens is studied using full-wave simulation. The lens is fabricated using a table-top commercial fused deposition modelling printer. The surface roughness, step heights, and radii of each zone are measured and verified using a 3D optical profilometer. Impact of the 3D-printed limitation on performance of the device is discussed. The gain of the fabricated prototype is measured, in conjunction with a horn antenna, in an anechoic chamber. Pattern measurement results illustrate 6.6 dB gain enhancement at broadside at 30 GHz. Gain enhancing behaviour is studied at three different focal lengths and frequencies of 29-31 GHz.

A Multi-Thresholding Method Based on Otsu’s Algorithm for the Detection of Concealed Threats in Passive Millimeter-Wave Images

Abstract In this study, an algorithm to the detection and imaging of hidden arms for passive millimeter-wave (PMMW) imaging systems is proposed. This technique is; in fact, an improved version of our previously developed auto-classification algorithm by extending it by exploiting the Otsu’s multi-level thresholding method. The detailed derivation and the brief steps of the proposed algorithm are given. The proposed algorithm is tested and validated by real PMMW images obtained by a real radiometric imaging system. Resultant measured images are obtained with the employment of signal and image processing procedures of the suggested technique. It is demonstrated by the constructed PMMW images that proposed technique successfully detects a concealed metal threat and also predicts its size by drawing the shape outline based on Otsu’s multi-level thresholding routine that was specially tailored to our auto-classification technique.

Robust, Sensitive, and Inexpensive 2D Focal Plane Array Upconverting MMW Imaging Into the Visible

A new concept of millimeter-wave (MMW) imaging system is demonstrated in this letter. The new concept is based on glow discharge detector (GDD) focal plane array (FPA) and a basic CMOS camera. The GDD elements emit optical light as a function of the incident MMW radiation intensity. By synchronizing the modulation of the MMW source with the exposure time of the CMOS camera in a way that every two consecutive frames were captured with the influence of the MMW radiation and without it, we were able to observe the effect of the MMW radiation alone on the GDD FPA without the light deriving from the GDD DC bias.

Near-Field MIMO-SAR Millimeter-Wave Imaging With Sparsely Sampled Aperture Data

The primary challenge of a cost-effective and low-complexity near-field millimeter-wave (mmWave) imaging system is to achieve high resolution with a few antenna elements as possible. Multiple-input multiple-output (MIMO) radar using simultaneous operation of spatially diverse transmit and receive antennas is a good candidate to increase the number of available degrees of freedom. On the other hand, higher integration complexity of extremely dense transceiver electronics limits the use of MIMO only solutions within a relatively large imaging aperture. Hybrid concepts combining synthetic aperture radar (SAR) techniques and sparse MIMO arrays present a good compromise to achieve short data acquisition time and low complexity. However, compared with conventional monostatic sampling schemes, image reconstruction methods for MIMO-SAR are more complicated. In this paper, we propose a high-resolution mmWave imaging system combining 2-D MIMO arrays with SAR, along with a novel Fourier-based image reconstruction algorithm using sparsely sampled aperture data. The proposed algorithm is verified by both simulation and processing real data collected with our mmWave imager prototype utilizing commercially available 77-GHz MIMO radar sensors. The experimental results confirm that our complete solution presents a strong potential in high-resolution imaging with a significantly reduced number of antenna elements.

Microwave Imaging Using Noise Signals

A new method of active incoherent microwave imaging is presented that uses noise signals and spatial frequency sampling. Building on interferometric spatial frequency sampling arrays developed in radio astronomy, active incoherent microwave imaging utilizes the transmission of noise signals to implement the first active imaging method that samples in the spatial frequency domain. In comparison to passive microwave imaging systems, active incoherent imaging requires receivers with far less sensitivity, and thus less overall cost. We present the theory behind the imaging technique and show experimental results from a 5.85-GHz system imaging in one and two dimensions. For 1-D images, the transmitter consisted of two noise generators, while the receive array was a synthesized linear array with elements placed in $1\lambda $ increments with the widest spacing of $15\lambda $ . For 2-D images, the transmitter consisted of three noise generators, while the receive array was a synthesized T-array with elements placed in $0.5\lambda $ increments. We demonstrate the reconstruction of 1-D and 2-D scenes consisting of spherical reflecting targets, using only 25 MHz of signal bandwidth and 10 $\mu \text{s}$ of integration time, both of which are an order of magnitude less than passive microwave and millimeter-wave imaging systems.

A 7.52-dB Noise Figure 128.75–132.25-GHz Super-Regenerative Receiver With 0.615-fW/<inline-formula> <tex-math notation="LaTeX">$\sqrt{\mathrm{Hz}}$ </tex-math> </inline-formula> NEP by Coupled Oscillator Networks for Portable Imaging System in 65-nm CMOS

This paper presents a low noise figure (NF) and high gain super-regenerative receiver (SRR) with coupled networks for portable millimeter-wave (mm-wave) imaging systems. Compared with the previous SRR designs, this paper presents a novel SRR structure comprising of two oscillators coupled by a serial resonator, which consists of the inductors and capacitors. Due to the coupled oscillators, the gain of the SRR is significantly improved and the NF of the SRR structure is reduced by 3 dB simultaneously. These benefits lead to a high-sensitivity SRR at mm-wave frequency. The circuit is realized in 65-nm CMOS process with a core area of 0.06 mm2. Measured results show that the receiver can detect −84-dBm minimum input power with 541-MHz integration bandwidth (BW). It has a noise-equivalent-power of 0.615 fW/(Hz), a NF of 7.52 dB, a BW of 3.5 GHz, and a power consumption of 8.1 mW. With frequency calibration procedure, these performances show the proposed chip meets portable imaging system requirement.