Frequency-diverse microwave imaging using planar Mills-Cross cavity apertures

Origami Microwave Imaging Array: Metasurface Tiles on a Shape-Morphing Surface for Reconfigurable Computational Imaging

Origami is the art of paper folding that allows a single flat piece of paper to assume different 3D shapes depending on the fold patterns and the sequence of folding. Using the principles of origami along with computation imaging technique the authors demonstrate a versatile shape-morphing microwave imaging array with reconfigurable field-of-view and scene-adaptive imaging capability. Microwave/millimeter-wave based array imaging systems are expected to be the workhorse for sensory perception of future autonomous intelligent systems. The imaging capability of a planar array-based systems operating in complex scattering conditions have limited field-of-view and lack the ability to adaptively reconfigure resolution. To overcome this, here, deviations from planarity and isometry are allowed, and a shape-morphing computational imaging system is demonstrated. Implemented on a reconfigurable Waterbomb origami surface with 22 active metasurface panels that radiate near-orthogonal modes across 17-27 GHz, capability to image complex 3D objects in full details minimizing the effects of specular reflections in diffraction-limited sparse imaging with scene adaptability, reconfigurable cross-range resolution, and field-of-view is demonstrated. Such electromagnetic origami surfaces, through simultaneous surface shape-morphing ability (potentially with shape-shifting electronic materials) and electromagnetic field programmability, opens up new avenues for intelligent and robust sensing and imaging systems for a wide range of applications.

A reconfigurable intelligent surface with integrated sensing capability

Reconfigurable reflective surfaces can alter the propagation environment to improve wireless communication and power transfer. Paramount to this operation-which has attracted much attention recently-is the assumption that the reflective surface has prior knowledge of the propagation environment, for example, the direction/location of the transmitter and the intended receiver(s). To address this need, we propose a reconfigurable reflective metasurface with integrated sensing capabilities. By modifying the tunable meta-atoms constituting the metasurface, we couple small portions of the incident wave to an array of sensing waveguides. As an illustrative example, we demonstrate the ability to use the sampled incident wave to detect its angle of arrival. In addition, we propose and numerically demonstrate the possibility to reduce the required sensors, i.e., the number of radio frequency (RF) chains needed to acquire the sensed signals, by leveraging the inherent metasurface's tunable multiplexing capability. A reconfigurable reflective metasurface with integrated sensing capabilities can benefit wireless communications, wireless power transfer, RF sensing, and smart sensors.

Lowering latency and processing burden in computational imaging through dimensionality reduction of the sensing matrix

Recent demonstrations have shown that frequency-diverse computational imaging systems can greatly simplify conventional architectures developed for imaging by transferring constraints into the digital layer. Here, in order to limit the latency and processing burden involved in image reconstruction, we propose to truncate insignificant principal components of the sensing matrix that links the measurements to the scene to be imaged. In contrast to recent work using principle component analysis to synthesize scene illuminations, our generic approach is fully unsupervised and is applied directly to the sensing matrix. We impose no restrictions on the type of imageable scene, no training data is required, and no actively reconfigurable radiating apertures are employed. This paper paves the way to the constitution of a new degree of freedom in image reconstructions, allowing one to place the performance emphasis either on image quality or latency and computational burden. The application of such relaxations will be essential for widespread deployment of computational microwave and millimeter wave imagers in scenarios such as security screening. We show in this specific context that it is possible to reduce both the processing time and memory consumption with a minor impact on the quality of the reconstructed images.

Frequency-diverse multimode millimetre-wave constant-ϵr lens-loaded cavity

This paper presents a physical frequency-diverse multimode lens-loaded cavity, designed and used for the purpose of the direction of arrival (DoA) estimation in millimetre-wave frequency bands for 5G and beyond. The multi-mode mechanism is realized using an electrically-large cavity, generating spatio-temporally incoherent radiation masks leveraging the frequency-diversity principle. It has been shown for the first time that by placing a spherical constant dielectric lens (constant-ϵ_r) in front of the radiating aperture of the cavity, the spatial incoherence of the radiation modes can be enhanced. The lens-loaded cavity requires only a single lens and output port, making the hardware development much simpler and cost-effective compared to conventional DoA estimators where multiple antennas and receivers are classically required. Using the lens-loaded architecture, an increase of up to 6 dB is achieved in the peak gain of the synthesized quasi-random sampling bases from the frequency-diverse cavity. Despite the fact that the practical frequency-diverse cavity uses a limited subset of quasi-orthogonal modes below the upper bound limit of the number of theoretical modes, it is shown that the proposed lens-loaded cavity is capable of accurate DoA estimation. This is achieved thanks to the sufficient orthogonality of the leveraged modes and to the presence of the spherical constant-ϵ_r lens which increases the signal-to-noise ratio (SNR) of the received signal. Experimental results are shown to verify the proposed approach.

Angular-insensitive optical filtering based on meta-GMR

In this study, the optical properties of a meta-GMR consisting of a metasurface stacked on a planar dielectric slab waveguide were theoretically investigated. Two different metasurfaces, namely chiral split-ring resonator dimer arrays with/without a rod-shaped antenna, were investigated and compared. Conventional GMR filters utilize gratings to couple the free-space electromagnetic field to the waveguide. The highly dispersive nature of grating leads to low angular tolerance. Here, the grating is replaced by metasurfaces. The metasurface unit cell can be regarded as a polarizable dipole that couples the free-space electromagnetic field to the waveguide and decouples the waveguide mode to the radiation modes. Based on the localized nature of the resonant metasurfaces, the metasurface/GMR hybrid mode exhibits a superior angular tolerance as compared with a conventional GMR filter. This study can open a new avenue to tailor the optical properties of GMR-based devices.

Lens-Loaded Coded Aperture with Increased Information Capacity for Computational Microwave Imaging

Computational imaging using coded apertures offers all-electronic operation with a substantially reduced hardware complexity for data acquisition. At the core of this technique is the single-pixel coded aperture modality, which produces spatio-temporarily varying, quasi-random bases to encode the back-scattered radar data replacing the conventional pixel-by-pixel raster scanning requirement of conventional imaging techniques. For a frequency-diverse computational imaging radar, the coded aperture is of significant importance, governing key imaging metrics such as the orthogonality of the information encoded from the scene as the frequency is swept, and hence the conditioning of the imaging problem, directly impacting the fidelity of the reconstructed images. In this paper, we present dielectric lens loading of coded apertures as an effective way to increase the information coding capacity of frequency-diverse antennas for computational imaging problems. We show that by lens loading the coded aperture for the presented imaging problem, the number of effective measurement modes can be increased by 32% while the conditioning of the imaging problem is improved by a factor of greater than two times.

Frequency-Diverse Computational Direction of Arrival Estimation Technique

We present a frequency-diverse based direction of arrival (DoA) estimation technique for millimetre-wave (mmW) 5G channel sounding. Frequency-diversity enables the creation of spatially incoherent radiation masks to encode the plane-wave signals incident on the radar aperture using a single antenna. Leveraging the frequency-diversity concept, spatial information of the plane-wave projections on the radar aperture is retrieved, resulting in high-fidelity DoA estimations by means of a simple Fourier transform operation applied to the retrieved plane-wave projection patterns. It is demonstrated that using the frequency-diversity concept, DoA estimation can be achieved through a simple frequency sweep, compressing the incoming plane-waves into a single channel through the transfer function of the radar aperture. This results in a significant simplification in the system hardware, requiring only a single antenna to achieve DoA estimation. It is also shown that the proposed technique can simultaneously detect the DoA information for multiple sources with a diffraction limited resolution.

Frequency-Diverse Bunching Metamaterial Antenna for Coincidence Imaging

A frequency-diverse bunching metamaterial antenna for coincidence imaging in the Ka band is proposed in this paper. The bunching metamaterial antenna includes a broadband circular array and a frequency-diverse bunching metalens. Firstly, in order to enhance the bunching characteristic, the broadband circular array is designed based on the 60-degree beamwidth design to generate radiation patterns from 32 GHz to 36 GHz. Then, types of metamaterial elements with different transmission phases are selected to form the frequency-diverse bunching metalens based on a random distribution design and gradient zoom coefficient design. Moreover, the bunching metamaterial antenna is constituted by loading the frequency-diverse bunching metalens to the broadband circular array, which can generate frequency-diverse bunching random radiation patterns with beamwidth less than 100 degrees from 32 GHz to 36 GHz. Furthermore, the performances of the bunching metamaterial antenna, including the reflection coefficient, the radiation efficiency, and the correlation coefficients of radiation patterns at different frequencies are evaluated. Finally, the coincidence imaging experiment is implemented using the bunching metamaterial antenna and the image of the target is reconstructed successfully. The design is verified by simulations and measurements.

Computational polarimetric microwave imaging

We propose a polarimetric microwave imaging technique that exploits recent advances in computational imaging. We utilize a frequency-diverse cavity-backed metasurface, allowing us to demonstrate high-resolution polarimetric imaging using a single transceiver and frequency sweep over the operational microwave bandwidth. The frequency-diverse metasurface imager greatly simplifies the system architecture compared with active arrays and other conventional microwave imaging approaches. We further develop the theoretical framework for computational polarimetric imaging and validate the approach experimentally using a multi-modal leaky cavity. The scalar approximation for the interaction between the radiated waves and the target- often applied in microwave computational imaging schemes-is thus extended to retrieve the susceptibility tensors, and hence provides additional information about the targets. Computational polarimetry has relevance for existing systems in the field that extract polarimetric imagery, and particular for ground observation. A growing number of short-range microwave imaging applications can also notably benefit from computational polarimetry, particularly for imaging objects that are difficult to reconstruct when assuming scalar estimations.

Millimeter-wave spotlight imager using dynamic holographic metasurface antennas

Computational imaging systems leverage generalized measurements to produce high-fidelity images, enabling novel and often lower cost hardware platforms at the expense of increased processing. However, obtaining full resolution images across a large field-of-view (FOV) can lead to slow reconstruction times, limiting system performance where faster frame rates are desired. In many imaging scenarios, the highest resolution is needed only in smaller subdomains of interest within a scene, suggesting an aperture supporting multiple modalities of image capture with different resolutions can provide a path to system optimization. We explore this concept in the context of millimeter-wave imaging, presenting the design and simulation of a single frequency (75 GHz), multistatic, holographic spotlight aperture integrated into a K-band (17.5-26.5 GHz), frequency-diverse imager. The spotlight aperture - synthesized using an array of dynamically tuned, holographic, metasurface antennas - illuminates a constrained region-of-interest (ROI) identified from a low-resolution image, extracting a high-fidelity image of the constrained-ROI with a minimum number of measurement modes. The designs of both the static, frequency-diverse sub-aperture and the integrated dynamic spotlight aperture are evaluated using simulation techniques developed for large-scale synthetic apertures.

Cavity-backed metasurface antennas and their application to frequency diversity imaging

Frequency diversity antennas with spatially structured radiation patterns reduce the reliance on actively switched elements for beamforming which become increasingly expensive and impractical as frequency increases. As the quality factor Q of a frequency diverse antenna increases, the antenna samples more spatial structure as the number of unique radiated coded spatial patterns correspondingly increases. Antennas that combine hollow cavities and metamaterial apertures achieve both large fractional bandwidth, in excess of 40%, and a high Q of 1600, so that each antenna radiates over 640 unique coded patterns. As compared to switched active antennas, such a passive antenna replaces the 50 antennas and switches that would produce at most (50/2)²=625 unique patterns. Furthermore, the engineered metamaterial apertures enable a radiation efficiency exceeding 60% to be achieved in a single desired polarization. The theory of cavity-backed metasurface antennas is explained, and frequency diverse imaging is demonstrated with a pair of these antennas.

Large Metasurface Aperture for Millimeter Wave Computational Imaging at the Human-Scale

We demonstrate a low-profile holographic imaging system at millimeter wavelengths based on an aperture composed of frequency-diverse metasurfaces. Utilizing measurements of spatially-diverse field patterns, diffraction-limited images of human-sized subjects are reconstructed. The system is driven by a single microwave source swept over a band of frequencies (17.5–26.5 GHz) and switched between a collection of transmit and receive metasurface panels. High fidelity image reconstruction requires a precise model for each field pattern generated by the aperture, as well as the manner in which the field scatters from objects in the scene. This constraint makes scaling of computational imaging systems inherently challenging for electrically large, coherent apertures. To meet the demanding requirements, we introduce computational methods and calibration approaches that enable rapid and accurate imaging performance.