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The last decades witnessed the emergence of the field of correlated disordered media, a great challenge offering a large panel of new perspectives for applications in theoretical modelling and material fabrication. The efficient design of structures with a controlled level of spatial correlation is a central challenge in this field, in a context where existing techniques generally rely on gradient descent on non-convex functions and on the use of stochastic methods to explore vast design spaces more efficiently. In this work, we propose a new generative technique based on Alan Turing's morphogenesis theory for designing correlated disordered materials. Inspired by the structuring of living organisms, this technique relies on the definition of simple local interactions guiding the self-organization of a generated medium. The decentralization of design constraints and the elimination of cost function minimization make this approach natively scalable to the design of large domains with controlled levels of disorder. As a validation, the morphogenetic generation of stealthy hyperuniform disordered structures is exploited to reproduce an experiment of isotropic electromagnetic bandgap synthesis in the microwave range using a low refractive index contrast.

In this paper, we propose a new three dimensional cylindrical imaging system based on the combination of a multiple-input multiple-output arc array in azimuth and a synthetic aperture radar on the vertical dimension for applications focused on buried threat detection. Imaging algorithms are adapted to these specific transceiver distributions in order to accelerate reconstructions in the Fourier domain. Following theoretical developments, an experimental proof of concept is proposed at 6 GHz using a circular dual positioning system specifically designed for these new studies. The validity of the proposed techniques is extended to larger cylindrical arrays using numerical simulations of complex volumetric scenes in the 5–12 GHz band. This work allows for significant gains in computation time for these specific imaging systems, which are more than three orders of magnitude shorter than those of conventional backpropagation techniques in the studied cases composed of more than 250,000 voxels.

There has been a surge of emphasis on polarimetric imaging modalities recently, in which Mueller matrix polarimetry (MMP) is at the heart of our understanding of polarization properties of an imaged sample. However, the historiography of MMP, predominantly undertaken in optics, largely lost sight of the potential of terahertz radiation on interactions with the imaged samples, especially biological ones. In addition, studies of MMP are generally concerned with intensity-only measurements requiring at least 16 polarization combinations, which dramatically add to the time cost in the process of measurements. The proposed study sets out to examine the complete 4 × 4 MMP from 0.2 to 1 THz using only orthogonal polarizations for two-dimensional (2-D) and 3-D imaging on leaf. A modified THz time domain spectrometer with self-designed linear polarizers is utilized for measurements and data acquisition. The Mueller matrix polar decomposition and Mueller matrix transform are employed to facilitate extraction of polarization characteristics of the imaged leaf. The time-of-flight technique is adopted to assist in the 3-D reconstruction of the leaf. The results demonstrate the effectiveness and feasibility of the novel measurement synthesis approach and the modified system for accurate characterization of the sample. Meanwhile, the findings indicate that MMP is superior in feature extraction as well as contrast enhancement of the sample. The insights gained from this article may be of assistance to the characterization of biological samples with MMP at higher frequencies for future research.

We study the coherence in time and space of electromagnetic fields propagated through complex media. Whether for localization, imaging or telecommunication, the development of dedicated numerical techniques is generally based on the exploitation of simplified models considering either coherent or diffuse fields. The optimization of such applications in conditions of partial coherence can therefore be particularly challenging, requiring the development of hybrid algorithms adaptable to prior knowledge on the processed fields. The objective of this work is to provide numerical techniques for decomposing an electromagnetic field into subspaces that can then be filtered according to their level of spatial and temporal coherence. In contrast to the studies carried out on space-space transfer matrices notably used for the calculation of Wigner-Smith operators, these decompositions are carried out on space-time matrices in order to facilitate the study of temporal dispersion. The theory is developed for illustrative purposes using experimental results from a leaky resonant system but seem to be applicable to any scattering and reverberating media capable of transforming localized and coherent excitations into complex and diffuse distributions. To conclude this work, the proposed technique is exploited to improve image reconstruction in a millimeter-wave computational imaging demonstration. In the studied context and from a more general perspective, we propose a technique to select the most suitable subspaces for each application operating under conditions of partial coherence, whether these correspond in the most extreme cases to ballistic paths or to diffuse fields., 12 pages, 14 figures

Deep learning offers interesting tools for radar data processing. This letter presents a signal processing method using a neural network to increase the range resolution of frequency-modulated continuous wave radar. The duration of the beat signal can be increased in the time domain for a better frequency resolution during conventional spectral estimation. This technique allows the radar bandwidth limitation to be exceeded and improves the sensing performance. A methodology is proposed as well as experimental results on real radar signal.

In the fields of radar imaging and detection, the use of optical components appears as an attractive solution to overcome limitations of current technologies. This paper focuses on the development of a new class of microwave-photonic radar imaging systems enabling volumetric image reconstruction. These operations are made possible using a multiple-input multiple-output antenna array architecture combined with the conception of a dedicated optical multiplexing technology to limit the complexity of the whole reception chain. Particular emphasis is placed on the development of mathematical formalisms specifically adapted to these microwave-photonic radar architectures, making it possible to model the measurements carried out and to reconstruct three-dimensional radar images. These solutions are validated by the reconstruction of complex scenes with a multiple-input multiple-output radar made of up to 16 transmitters and 16 receivers.

The frequency-modulated continuous-wave architectures are increasingly used in 3D imaging systems to reduce the constraints linked to the receptive components. For short-range applications, the phase dispersion term that appears when de-chirping these signals is generally considered negligible due to the relatively low propagation delays. However, in this paper, the frequency-modulated continuous-wave architecture is implemented into a short-range radio frequency imaging system which uses a time-division multiplexer in reception. Under these conditions, even if the scanned scene range is small, the received signals are subject to significant delays and the undesirable phase dispersion term becomes non-negligible. Its impact on the reconstructed image is studied here using simulations that consider different delay for the multiplexing and using the measurement of a scene with a multiple-input multiple-output antenna array.

An original monostatic UWB FMCW radar architecture has been developed for GPR applications. The radiation system is composed of a single antenna associated with a double switching configuration to minimize the compactness of the antenna system while ensuring a sufficient dynamic range. Moreover, an analog correction technique has been developed to linearize a low-cost free-running UWB Voltage Controlled Oscillator (VCO) and to enable operating frequency selection. A functional demonstrator has been designed and associated measurements are proposed.

This paper proposes the application of Synthetic Aperture Interferometric Imaging to the detection of electromagnetic leakage from shielded equipment and buildings. By conducting laboratory experiments, we demonstrate the efficiency of this technique for the accurate detection and location of electromagnetic leakage from a metallic cavity with faulty faradization and adjustable quality factor.

Frequency-diversity is a computational imaging technique that can offer all-electronic imaging systems by leveraging spatio-temporally incoherent radiation patterns as an enabling technology. This approach exhibits a significant contrast to conventional imaging modalities, such as synthetic aperture radars (SAR) and phased arrays in that the raster scanning requirement of a scene to be imaged (mechanical or electronic) can be broken and replaced by a quasi-random interrogation of the scene. This aspect of frequency-diverse computational imaging systems significantly simplifies the physical hardware requirements of conventional radars. Despite this advantage, the application of the frequency-diversity technique has been mostly limited to static imaging scenarios, where the position of the scene to be imaged remains fixed over the data acquisition cycle. This limitation hinders the frequency-diverse computational radars from being deployed for applications where the scene dynamics may vary over the data acquisition cycle, such as in automotive radars. In this paper, we demonstrate that by modifying the sensing matrix to account for the movement of the radar platform, frequency-diverse computational imaging radars can be successfully used in debris detection on roads. We show that operating within the frequency band of 77–81 GHz, the presented dynamic frequency-diverse radar technique can produce high fidelity point spread function (PSF) patterns eliminating the distortions caused by the motion of the radar. We also prove that the PSF patterns of the radar are in excellent agreement with theoretical diffraction limited resolution limits.

Microwave imaging systems allowing the real-time scanning of short-range objects are difficult to implement on a large scale due to their complexity and cost. In this paper we introduce a new ultra-wideband multiple-input multiple-output radar using microwave photonic components in reception. These components permit ultra-fast time division multiplexing of all receiving signals and hence their measurement with a single acquisition channel. This architecture makes possible to decrease the time of acquisition compare to architecture with a sequential reception.

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.

International audience; This letter describes an original approach to measure the radiation pattern of an antenna within reverberating enclosures. In such kind of enclosures creating a multipath environment, the key point is to distinguish the line-of-sight (LOS) path between the antenna under test and the measuring antenna from all the other indirect paths. This is made possible by taking advantage of the time-gating technique considering that the LOS path arrives the first on the measuring antenna. With respect to the existing methods proposed recently, the method leads to significantly simplify the measurement setup and shorten the measurement time (similar to the one obtained in anechoic chamber) as no stirring process or any movement of the antenna under test (apart of course from the rotation on itself) is required. Moreover, the accuracy of this method is presumably better in reason of the ability of the time-gating technique to discard the influence of the indirect paths. The proposed technique tested for the first time within reverberating enclosures is experimentally validated on a broadband horn antenna from 1.5 to 12 GHz. The main parameters influencing the result (the measurement bandwidth, the frequency resolution, the time-gating window width and the effect of loading the enclosure) are discussed.

We propose and demonstrate the first single channel real-time microwave photonics digital beamforming radar imaging system based on a silica photonics summator. Considering a set of microwave signals received by an antenna array, the number of receiving chains is concentrated into a single channel by achieving an analog time-division multiplexing of radio frequency modulated optical signals using well-calculated optical fiber lengths and a microwave photonics summator. This single channel output guarantees a simplified architecture and minimizes the required number of analog-to-digital converters. A fixed fiber length difference is implemented to introduce a true time delay between each channel in order to time interleave them. The image of the scenes is then retrieved by demultiplexing the received signal and applying digital beamforming techniques. The potential applications of this structure are to implement in a short-range radar imaging system.

For real-time and high-resolution radiometric imaging, the synthetic aperture interferometric radiometer (SAIR) technique seems to solve the tradeoff between imaging resolution and aperture size. Indeed, it can synthesize a large antenna array by sparsely arranging a small number of antennas to achieve high spatial resolution. Nevertheless, a conventional interferometric radiometer requires as many receivers as antennas needed. This constraint is one of the limitations of SAIR regarding the hardware cost, the system complexity, and the computation load. In this letter, a compressed acquisition technique is proposed to collect the antenna signals. In this method, the antenna’s signals are coded and combined red with a passive microwave device. From the measured waveforms at the receiver’s outputs, a decoding process is performed to estimate the received signal by each antenna. This allows the computation of the visibility function required for image reconstruction. The effectiveness of the proposed method has been tested by means of simulations analysis and has experimentally been applied to a punctual noise source detection. The results reveal that this method can be an efficient alternative for the simplification of the conventional interferometric radiometers’ architectures.

UAV-mounted GPR are a very attractive way to detect buried objects without ground contact. However, to be efficient, conventional GPR systems use a large bandwidth turned towards low frequencies for better penetration into the ground, the objective of this paper is to discuss possible solutions to reduce the system size and weight in the 0.3-1GHz frequency band by using single switched antenna. The proposed choices are supported by a complete simulation of the RF chain performed using gprMax and Advanced Design System.

This paper presents an original Ultra Wide Band (UWB) radar platform based on a coherent sub-sampling technique. The receiver of this UWB radar has a wide RF bandwidth [DC-5] GHz, and is based on the use of a Track and Hold Amplifier (THA) coupled with a Coherent Interleaving Sampling (CIS) technique. A SAR (Synthetic Aperture Radar) radar demonstration is realized and allows obtaining high dynamic images with a centimeter range resolution.

International audience

The growing need for high-performance imaging tools for terrorist threat detection and medical diagnosis has led to the development of new active architectures in the microwave and millimeter range. Notably, multiple-input multiple-output systems can meet the resolution constraints imposed by these applications by creating large, synthetic radiating apertures with a limited number of antennas used independently in transmitting and receiving signals. However, the implementation of such systems is coupled with strong constraints in the software layer, requiring the development of reconstruction techniques capable of interrogating the observed scene by optimizing both the resolution of images reconstructed in two or three dimensions and the associated computation times. In this paper, we first review the formalisms and constraints associated with each application by taking stock of efficient processing techniques based on spectral decompositions, and then, we present a new technique called the transverse spectrum deconvolution range migration algorithm allowing us to carry out reconstructions that are both faster and more accurate than with conventional Fourier domain processing techniques. This paper is particularly relevant to the development of new computational imaging tools that require, even more pronouncedly than in the case of conventional architectures, fast image computing techniques despite a very large number of radiating elements interrogating the scene to be imaged.

The ultrafast speed and the large spatial area scanning autonomy are the two main advantages of the new radar presented in this paper. Indeed, the autonomous beam scanning capability with high pulse repetition frequency (PRF) is obtained by coherent combining of electrical fields emitted from active antennas’ array integrating photoconductive switching devices. An original signal processing associated with this new radar system is presented. We show a hybrid version of a CLEAN algorithm able to overcome the depth limitation induced by high PRF with standard imaging signal processing. Simulations and measurements are performed to validate the proper operation of the algorithm.

Radiometric imaging is a passive technique that measures the radiation emitted by a hot object and finds applications in many fields. In particular, the detection of buried threats requires the joint optimization of the spatial resolution, field of view and acquisition time of imaging systems that impose strong constraints on the redundancy of active systems. In this paper, it is proposed to explore the implementation of a computational interferometric imaging solution in the millimeter band, making it possible to limit the number of signals to be measured by a multiplexing operation carried out passively and in the physical layer with the use of electrically large cavities. This approach is studied within the framework of a numerical study then of a proof of experimental concept in the W band.

International audience; Abstract:This paper introduces a Passive Millimeter-Wave Imager based on a physical layer compressive sensing approach. Previous works demonstrated the effectiveness of an original interferometric imager architecture operating in the S-Band. The system associates a passive microwave coder to an interferometric array leading to a drastically reduced number of the receiver chains compared to conventional architectures. Considering the preliminary results, this concept is an attractive prospect for a simplified architecture of passive screeners dedicated to security applications.

International audience; In this article, a matching pursuit algorithm is developed to improve the performance of a passive multiplexing technique based on compressed sensing. This deconvolution technique is applied to RADAR imaging in the microwave range, starting from previous studies based on a compact coding device and L 1-norm regularizations. This study demonstrates that in this context, the quality of the reconstructed RADAR images can be improved using an algorithm close to Hogbom's Clean, and based on a dictionary built with Tikhonov pseudo-inversions. The theoretical principle of this new algorithm is developed, followed by a parameters study. Finally, an experimental validation is presented to demonstrate the efficiency of this iterative algorithm.

Surface acoustic wave (SAW) filters have been developed to code the information received by an antenna array and sum them into a single signal. In post-processing, these waveforms are retrieved exploiting the orthogonal propagation in each SAW filter. Thus, a proof a concept of beamforming and imaging using SAW filters is presented.

Multiple Input Multiple Output (MIMO) Radar has many advantages compared with conventional Radars including improved target parameters estimation, improved angular resolution while keeping a small number of antennas. Nevertheless, these advantages can be obtained when probing simultaneously the scattering matrix with orthogonal transmit signals. Recently, a passive orthogonal waveform generation technique was introduced using a 1 × M ports Mode-Mixing microwave cavity whose transfer functions are uncorrelated. This approach makes MIMO Radar system able to satisfy transmit beamforming constraint with a cost-efficient architecture. In this paper, more conceptual clarifications are brought and the orthogonality metrics are assessed. Furthermore, the proposed method is experimentally compared with a conventional MIMO Radar based on frequency hopping waveforms.

This work is focused on the conception of a new kind of mm-wave radiometer based on a computational imaging technique. The aim is the design of a radiating aperture associated with a dispersive component able to passively multiplex the received signals, allowing for a massive simplification of the active part in comparison with conventional solutions. This approach also requires the implementation of fast algorithms for retrieving the coded signals and reconstruct images of the target, desirably at the speed of several frames per second.

International audience; —Based on the synthetic interferometric imaging technique (SAIR), a new microwave radiometer architecture is proposed in this paper to detect thermal noise sources with a passive coded measurement approach so called CSAIR. In this new system, a passive microwave device is used to intrinsically code and multiplex the antenna signals. This allows the reduction of the number of RF chains while keeping the same antenna array configuration needed in a conventional interferometric radiometers. The system principle is described and the signal processing required for the target brightness temperature rendering is also discussed. Simulation and measurement results show the effectiveness and the potential of the proposed system.

In a recent paper, a novel technique that allows for the first time to perform actual antenna radiation pattern measurements in reverberation chamber (RC) has been presented. Even though its results were in good agreement with the ones obtained in anechoic chamber (AC), it was necessary to repeat stationary measurements locating the antenna under test (AUT) at N different fixed positions along its line-of-sight to a fixed probe antenna per angle under study, thus multiplying by N the measurement time when compared to measuring in AC. On the contrary, in this contribution, an improvement of this technique is presented, obtaining an antenna radiation pattern by performing only one measurement per angle under study, taking advantage of real-time Doppler effect. Moreover, the antenna gain is obtained from only one of these measurements, without the need to know the antenna efficiency, by using the Friis transmission equation. In order to demonstrate the validity of this improved technique, the radiation pattern for both E- and H- planes and the gain of a horn antenna have been measured in an RC, being the results in good agreement with the ones obtained in AC and the values provided by the antenna manufacturer, respectively.

International audience; —There has been growing interest in the use of microwave synthetic aperture radiometers since their could solve the trade-off between the image resolution and the antennas aperture size. Compared to classical techniques, SAIR (Synthetic Aperture Interferometric Radiometer) can reach a large field of view (FOV) and high imaging rate. Nevertheless, the architecture of a conventional radiometer still bulky and costly. In this paper, a passive combining technique is proposed to reduce the hardware complexity as well as the dimensional requirements of the receiving antennas. Experimental results are presented to highlight the effectiveness of the proposed technique.

It is well known that MIMO radar waveforms should be orthogonal to probe simultaneously the MIMO channel matrix H. This method is not easily implementable since it requires as much transmitter chains as antennas. Herein, a compressive technique is introduced to measure the channel information avoiding multiple waveforms generation. This technique is thus based on the generation of a single waveform to probe the channel by the mean of a passive chaotic component connected to the transmit array. Mathematical formulations are presented considering a device with uncorrelated transfer functions allowing the transmission of orthogonal waveforms. Simulation and experimental results are presented to highlight the advantages of such a technique.

Recent works have demonstrated the feasibility of microwave imaging using compressive techniques, exempting the use of active delay lines, phase shifters, or moving parts to achieve beamforming. With this method, waves are coded in a passive way by a compressive device to reduce the complexity of the transmitter and/or receiver chains of the telecommunication and radar systems requiring beamsteering. Such a technique is based on the exploitation of the frequency diversity, implying that a reduction of the compressive device's volume imposes a diminution of the number of driven antennas. In this paper, the improvement brought by simultaneous excitations of the compressive device is presented. Adapting a new mathematical formulation, it is shown that M inputs can send independent waveforms allowing the beamsteering of an N-elements antenna array, while maintaining N > M.

This paper presents a compressive technique that simplifies the architecture of multiple-input multiple-output (MIMO) radar imaging systems. By means of a passive device connected to an MIMO array of antennae, a novel approach is introduced to extract the interaction between antennae from a compressed signal measured between two ports. This technique relies on a multiplexing principle that exploits the frequency diversity and it is thus particularly suitable for ultrawideband imaging systems. This paper presents the theoretical principle underlying this compressive technique, defining the key parameters affecting the information retrieval from a measured frequency signal and the prior characterization of the compressive device’s transfer functions. Simulations are performed to demonstrate the potential of the technique for MIMO ultrawideband imaging systems and its compatibility with fast range migration algorithms based on fast Fourier transforms. Finally, an experimental validation is performed with a two-dimensional radiating aperture connected to a compressive device, allowing for the three-dimensional near-field imaging of different targets.

International audience; A compact multiband antenna system using a dielectric resonator antenna (DRA) is presented in this paper. Designed to be integrated in a tablet, it is not only heavily miniaturized (λ0/15×λ0/38×λ0/94 at 800 MHz), but also able to cover three frequency bands for different wireless applications (DVB-H, WiFi and WiMAX). Since a reconfigurable radiation pattern can result in an improved quality and reliability of wireless links, two DRAs have been integrated to implement this feature on the three frequency bands. These improvements are demonstrated by the presentation of the correlation coefficient and the effective diversity gain, both measured in a reverberation chamber. The experimental measurements are in very good agreement with the simulated ones. Good overall performances are obtained, and the requirements of all three applications are perfectly met. Moreover, the antenna operative frequencies are independent of the ground plane dimensions, making this system a very good candidate for all kinds of mobile devices.

Electromagnetic reverberation chambers can be used for multiple-input multiple-output (MIMO) systems testing.Currently, the tests focus on parameters such as correlation, diversity gain, efficiency, etc., by using a vector network analyzer. In contrast with these passive tests, a novel way of MIMO systems characterization is described in this article. It consists on evaluating bit error rate (BER) levels of a MIMO WiMAX-OFDM system according to the Signal-to-Noise Ratio (SNR) by establishing an active link between the transmitter and the receiver. The measurement process is set up in a reverberation chamber, where multipath frequency selective channels are emulated. The obtained results are compared to a reference case with single-input single-output (SISO) to evaluate the real improvements made by the studied system. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2347–2352, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25465