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In this letter, a radio frequency identification (RFID)-based location and tracking system that uses receiver signal strength (RSS) measurements is presented. The system is intended to be deployed at industrial scenarios or warehouses involving heavy machinery and vehicles in motion such as forklifts, and/or low visibility areas, aiming at preventing accidental collisions with workers. For this purpose, RFID tags are attached to helmets of the workers. The UHF reader antennas are located in the vicinity of the area where the workers are creating a coverage area where workers close to the vehicle can be detected and tracked. To deal with the propagation effects that degrade the RSS values, a propagation model based on on-site measurements together with an accurate characterization of the radiation pattern of the RFID reader antennas has been proposed. Validation with measurements in an scenario resembling the industrial environment is presented.

SPIE Defense + Commercial Sensing (2022. Orlando), Ministerio de Ciencia, Innovación y Universidades of Spain/FEDER [RTI2018-095825-B-I00, PID2021-122697OB-I00]; Ministry of Universities of Spain; European Union - NextGenerationEU [MU-21-UP2021-030-71667225]; Gobierno del Principado de Asturias/FEDER [AYUD/2021/51706]

IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting (AP-S/URSI) (2022. Denver, USA), This work was supported in part by the Ministerio de Ciencia, Innovación y Universidades of Spain/FEDER under Project RTI2018-095825-B-I00 and META-IMAGER (PID2021-122697OB-I00), by the Ministry of Universities of Spain and European Union NextGenerationEU under project MU-21-UP2021-030-71667225, and by Gobierno del Princi-pado de Asturias/FEDER under Project AYUD/2021/51706.

This article presents a portable system for freehand antenna diagnosis and characterization based on amplitude-only data. The amplitude-only samples are acquired by moving a handheld probe, which is tracked by a motion capture system, in front of the antenna under test (AUT) aperture. The acquired measurements are processed using the phaseless sources reconstruction method to compute an equivalent current distribution on the AUT aperture. Finally, the radiation pattern of the AUT can be obtained by evaluating the corresponding radiation integrals. Unlike previous work, the use of amplitude-only data avoids the need of a phase reference, paving the way to the diagnosis and characterization of antennas under operational conditions. This fact, together with the handheld capabilities, makes the system very convenient for measurements of already deployed and onboard antennas. Moreover, these amplitude-only acquisitions also simplify the required hardware. The system has been validated through measurements in a wide frequency range from Ka-band up to 300 GHz. Despite that one cannot expect the same degree of accuracy that can be achieved under laboratory conditions (including an anechoic environment and highly accurate positioners), the system shows excellent capabilities to detect malfunctions, such as wrong amplitude/phase distributions, as well as a fair estimation of the far field. This work was supported in part by the Ministerio de Ciencia, Innovacion y Universidades of Spain/Fondo Europeo de Desarrollo Regional (FEDER) under Project RTI2018-095825-B-I00, in part by the Gobierno del Principado de Asturias under Project GRUPIN-IDI2018-000191, and in part by the Ministerio de Educacion y Formacion Profesional of Spain under Grant FPU15/06431.

A qualitative approach for microwave imaging multilayered cylindrical structures (e.g., dielectric pipes) is proposed in this paper. This approach relies on a modified circular synthetic aperture imaging technique that exploits closed-form Green’s function to account for the different propagation delays, internal reflections, and refractions. The image can then be computed by employing a matched filter expressed in terms of efficient Fourier Transforms. Consequently, a high-resolution and contactless approach for the inspection of this kind of structures is achieved. Moreover, the computational resources to render the images are negligible. Practical aspects of the technique such as sampling criteria, resolution, and limitations of the technique are discussed. The efficacy of the method is illustrated via several examples including imaging of objects and anomalies inside different types of dielectric pipes.

In this paper, a portable three-dimensional (3D) scanning system for the accurate characterization of large raw material (e.g., cereal grain, coal, etc.) stockpiles is presented. The system comprises an array of high resolution millimeter-wave radars and a cm-level accuracy positioning system to accurately characterize large stockpiles by means of a high-resolution 3D map, making it suitable for automation purposes. A control unit manages the data received by the sensors, which are sent to a computer system for processing. As a proof of concept, the entire sensor system is evaluated in a real environment for electromagnetically scan a scaled stockpile of coal, used in the industry for handling raw materials. In addition, a highly efficient processing adaptive algorithm that may reconstruct the scanned structure in real-time has been introduced, enabling continuous dynamic updating of the information. Results are compared with those from a photogrammetry-like technique, revealing an excellent agreement., This research was funded by Ministerio de Ciencia, Innovación y Universidades of Spain Government under project MILLIHAND RTI2018-095825-B-I00, by the Gobierno del Principado de Asturias and European Union (FEDER) under Project GRUPIN-IDI-2018 000191 and by the TSK company under its Open Innovation Program.

This paper presents a freehand imaging system, which takes full-advantage of a compact multiple-input multipleoutput (MIMO) radar to provide real-time results by means of freehand movements. The system is based on a commercial radar-on-chip, equipped with four receivers and two transmitters, and an optical tracking system (i.e., motion capture system) for retrieving its position and attitude. The reduced size of the radar enables its handheld operation, making possible to create a synthetic aperture by moving the scanner over the area of interest. Due to the new degrees of freedom with respect to previous implementations based on quasi-monostatic topologies, a novel calibration strategy for the system is required in order to mitigate positioning errors. After calibration, simulations and measurements are accomplished revealing that the novel freehand multistatic setup significantly outperforms the monostatic setup in terms of scanning speed and image quality. Multimedia material is provided to illustrate this performance., This work has been supported by the Ministerio de Educación y Formación Profesional of Spain under the FPU grant FPU15/06431, by the Ministerio de Ciencia, Innovación y Universidades under project RTI2018-095825-B-I00 and by the Principado de Asturias/FEDER under project IDI/2018/000191.

In this paper, a synthetic aperture radar (SAR) approach, for imaging internal structures of generally lossy layered dielectric cylindrical objects, is presented. This method which properly accounts for different transmission and refraction paths at each boundary between different layers produces a properly focused image of embedded targets. This approach is also capable of addressing imaging needs for asymmetrical multilayered cylindrical bodies. Consequently, this approach overcomes the limitation associated with the conventional methodology, in which free-space propagation is assumed. The calculation method of angular sampling criterion for circular and cylindrical SARs (i.e., circumferential and in height) is also presented. Electromagnetic simulations are performed on a three-layer cylindrical object, symmetrical and asymmetrical, with embedded targets to validate the approach. In addition, representative measurements are conducted at X-band (8.2–12.4 GHz), demonstrating the effectiveness of the approach for practical nondestructive evaluation applications.

In this letter, an algorithm for synthetic aperture radar (SAR) imaging of electrically small targets embedded in multilayered cylindrical geometries (e.g., pipes) using non-uniform measurement points is presented. In contrast to previous approaches, this algorithm is able to efficiently handle non-uniform points without exhibiting relevant side lobes in the image. For these purposes, the approach exploits a flexible time-reversal (TR) algorithm enhanced by compressed sensing (CS). The theoretical performance of the algorithm is studied in terms of the point spread function (PSF) and several images are presented using synthetic full-wave data from CST Microwave Studio. In addition, the approach is empirically validated by performing microwave imaging of a PVC pipe. The results demonstrated that the TR-CS algorithm provides an effective focusing technique for dense non-uniform measurement points, as well as for sparse non-uniform measurement points., This work was supported in part by the Ministerio de Ciencia, Innovacinó y Universidades of Spain/FEDER under Project RTI2018-095825-B-I00, in part by the Gobierno del Principado de Asturias under Project GRUPINIDI-2018-000191, and in part by FPU under Grant FPU15/06431

This article presents an agile system for antenna diagnosis. The system is operated by means of freehand movements of the probe antenna avoiding the need of bulky positioners and thus providing portability to the system. For that purpose, the probe antenna position, which is required by the antenna diagnosis algorithm, is retrieved by means of an affordable and quickly deployable motion capture system. The probe position and the acquired field are used to characterize the antenna under test by computing an equivalent currents distribution on the antenna aperture and its far-field pattern in real-time. To achieve that, an ad hoc workflow is designed including a method for spatially balancing the acquired field samples. Although the system is not intended to provide an accuracy comparable to the one achieved by anechoic chamber facilities, it opens a new horizon of possibilities for in situ agile characterization of antennas enabling the detection of faulty elements or radiation pattern deviations. Results at Ku- and Ka-bands, supported by attached multimedia material, are presented to illustrate the capabilities of the system., This work was supported in part by the Ministerio de Ciencia, Innovación y Universidades of Spain/FEDER, under Project RTI2018-095825-B-I00, in part by the Gobierno del Principado de Asturias under Project GRUPIN-IDI-2018-000191, and in part by the Ministerio de Educación y Formación Profesional of Spain under the FPU Grant FPU15/06431

In this paper, a method to estimate the attitude of an object using uniform arrays of radio frequency Identification (RFID) tags is presented. This technique is based on using the multiple signal classification algorithm with the phase measurements of the signals backscattered by the RFID tags of the array. Two array topologies have been studied (uniform linear arrays and uniform circular arrays). The performance of the system has been assessed by means of simulations and measurements in a controlled environment and in an indoor scenario. Results show that, after calibration, the attitude can be estimated with an error of a few degrees for a large range of orientations as long as the signals sent by the RFID reader do not imping the tags at grazing angles.

The aim of this paper is to introduce the use of sparse multistatic arrays to reduce the number of elements in novel portable and muti-view millimiter-wave scanners. Thus, the complexity of these novel scanners is significantly reduced. Furthermore, sparsity is expected to enable embedding a conventional optical camera sensor inside the aperture of the scanner allowing a reduction of the scanner size. This camera sensor is used to build a complementary 3-D optical model as well as to estimate the millimeter-wave scanner position so that the imaging merging techniques can be applied. The approach is validated for concealed weapon detection by means of simulation as well as measurements, in which the scanner aperture is emulated by raster scan, showing a performance comparable to the case of dense arrays.

In this chapter focuses on two specific techniques, both derived from optics. In the first technique, an interferometry between the unknown radiated field and a known radiated field is used. As it will be illustrated, if certain conditions are met, then it is possible to directly retrieve the radiated field with a negligible error by means of simple operations in the spatial spectral domain, also known as k-space.

The aim of this paper is to present a freehand scanning system with a compact mm-wave radar. In order to achieve high-resolution images, the system exploits the free movements of the radar to create a synthetic aperture. However, in contrast to conventional synthetic aperture radar (SAR), in which canonical acquisition surfaces (e.g., planes or cylinders) are used, the system allows for a given tolerance compatible with real hand-made trajectories. Moreover, different techniques are studied to compensate for the impact of irregular sampling to reduce the artifacts in the image. As a result, real-time scanning can be readily performed even by inexperienced users. The scanning system, comprising a commercial motion capture system and an mm-wave module, can be easily deployed and calibrated. Several results involving different objects are shown to illustrate the performance of the system.

IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting (2019. Atlanta, Georgia, USA), This work has been supported by the Ministerio de Educación y Formación Profesional of Spain under the FPU grant FPU15/06431, by the Ministerio de Ciencia, Innovación y Universidades under project RTI2018-095825-BI00 and by the Principado de Asturias/FEDER under project IDI/2018/000191.

This letter presents a cost-effective technique for through-the-wall imaging of objects beyond a wall. The approach relies on an amplitude-only multimonostatic radar that operates as a synthetic aperture radar. In contrast to conventional approaches, the system employs recent broadband techniques for phase retrieval. Thus, the complexity of the scanner is reduced, whereas it preserves the capacities of a conventional broadband system to retrieve the 3-D profile of objects. Moreover, the system is compatible with state-of-the art techniques that require full (i.e., amplitude and phase) acquisitions. Results at different frequency bands are shown to illustrate how the system can provide accurate estimation of the profile of metallic objects behind building materials such as plywood, plasterboard, or hollow bricks and mortar.

Ministerio de Educacion y Formacion Profesional of Spain under the FPU grant [FPU15/06431]; Ministerio de Ciencia, Innovacion y Universidades [RTI2018-095825-BI00]; Principado de Asturias/FEDEREuropean Union (EU) [IDI/2018/000191], This work has been partly supported by the Ministerio de Educación y Formación Profesional of Spain under the FPU grant FPU15/06431, by the Ministerio de Ciencia, Innovación y Universidades under project RTI2018-095825-BI00, and by the Principado de Asturias/FEDER under project IDI/2018/000191.

This contribution shows the application of multiview processing technique to a recently-developed real-time 3D microwave camera. The camera, which operates based on efficient synthetic aperture radar (SAR) imaging techniques, provides a good trade-off between fast image production and image quality. These cameras, as it happens with standard optical cameras, provide a good image of a volume in front of their apertures, but they cannot provide information about the complete object under test. Furthermore, it is usually convenient to consider multiple view angles as the best quality image is achieved when the area to be image is parallel to the aperture (as the specular reflection is captured). In this paper, we illustrate how recently-developed techniques, inspired from computer vision approaches, can be used to merge multiple views to obtain a more complete image of an object.

A scalar calibration method for broadband off-axis holography phaseless antenna characterization and diagnostics is presented in this communication. With the proposed method, the need of vector calibration, inherent to off-axis holography setups, is bypassed by means of an intermediate step in which a well-known antenna, whose phase can be analytically modeled, is used to retrieve the phase of the reference antenna by applying the off-axis holography method twice. The method is especially suited for near-field characterization of high-gain millimeter- and submillimeter-wave antennas in which phase acquisition is a difficult task, requiring complex and expensive instrumentation and steady conditions. The phase of the field is retrieved at each point of the acquisition surface for a set of frequencies, and, consequently, the technique can be applied independently of the acquisition geometry. Validation examples are presented in the W-band.

The aim of this paper is to present two different schemes to retrieve the phase of the field scattered by a UHF RFID tag without requiring a physical connection between the transmitter and the receiver systems. Thus, the phase angle can be comfortably measured in a multistatic, easy to deploy RFID sensor network for different purposes such as location and imaging. The accuracy of both schemes is validated against the one achieved by resorting to a physical connection between the transmitter and the receiver, demonstrating an excellent accuracy. Finally, a localization scheme based on this acquired information is presented to illustrate the use of the proposed phase retrieval approaches.

This paper introduces a hybrid method that combines two 3-D imaging methods of very different nature but complementary. On the one hand, a depth camera, also known as RGB-D camera, is used in order to obtain a 3-D scan of the target under test. On the other hand, a synthetic aperture radar at millimeter-wave frequency band is used to exploit the penetration capabilities of these waves, allowing electromagnetic imaging. As a result, the system can provide two complementary and aligned 3-D models. In contrast to the previous work, in which the position estimation and the external 3-D model relied on conventional optical cameras, this setup can construct the model even in the case of flat textures as it does not resort to color information at any stage. Several examples, in which the millimeter-wave scanner is validated by raster scan, are presented for people screening revealing the validity of the approach.

A real-time imaging system based on a frequency scanning antenna for conveyor belt setups is presented in this paper. The frequency scanning antenna together with an inexpensive parabolic reflector operates at the W band enabling the detection of details with dimensions in the order of 2 mm. In addition, a low level of sidelobes is achieved by optimizing unequal dividers to window the power distribution for sidelobe reduction. Furthermore, the quality of the images is enhanced by the radiation pattern properties. The performance of the system is validated by showing simulation as well as experimental results obtained in real time, proving the feasibility of these kinds of frequency scanning antennas for cost-effective imaging applications. This work was supported by the Spanish Ministry of Economy and Competitiveness Project Nos. TEC2016-76997-C3-1-R, TEC 2013-47753-C3-1-R, RTC-2014-2110-2, and postdoctoral fellowship FPDI-2013-16278.

This paper presents the first demonstrator of a portable, multiview, high-resolution, 3-D, and real-time microwave imaging system. The system is based on a recently developed real-time 3-D microwave camera, which performs quasi-monostatic acquisitions, equipped with an optical depth camera, providing target surface profile information. In addition, the entire system can be arbitrarily moved along the target performing microwave and depth camera synchronized acquisitions from different views with a twofold purpose, namely: 1) enabling a coverage area much larger than that possible with a static imaging system and b) allowing for incorporation of several tilt angles (or views) to enhance capturing specular reflection imaging data to improve the overall image quality. At each scanning position, the imaging data from the microwave camera are processed to build a local 3-D microwave image. The information is then merged, using recently proposed techniques for multiview synthetic aperture imaging, to compose the global image. The synchronized optical camera depth acquisitions enable tracking the entire imager movements so that the position and attitude are known. Moreover, the data acquired by the depth camera are also use to build a complementary 3-D outer surface profile model of the target, producing a combined and realistic image of the internal and external geometries of the target. Finally, the performance of the combined system is evaluated using several examples related to hidden contraband covered by clothing (i.e., people screening).

Summary Support vector regression is proposed to obtain the coupling matrix that accounts for the real behavior of an antenna array in the near-field multifocusing framework. The obtained model can then be easily included in this synthesis method in order to obtain the feeding weights which satisfy the radiation pattern requirements under real conditions. An example of the performance of support vector regression using information from method of moments is also provided to demonstrate the performance and accuracy of the technique. Copyright © 2015 John Wiley & Sons, Ltd.

This paper presents a broadband and phaseless synthetic aperture radar (SAR) with efficient sampling. The design relies on a novel phaseless monostatic element comprising a transmitter and a receiver. This element, together with a computationally inexpensive algorithm, can retrieve the phase of the monostatic scattered field at all the working band except for two small safety margins at the lower and upper frequencies. Furthermore, the phase retrieval works independently of the transmitter/receiver position. Consequently, conventional approaches to reduce the number of monostatic acquisition points can be employed. Thus, the proposed strategy is suitable to implement either arrays that benefit from a reduced number of low-complexity elements or raster scan systems that benefit not only from the cost reduction of the scanning components but also from a remarkable speed-up due to the reduced number of acquisition points. Moreover, in contrast to other off-axis schemes, the proposed system does not require neither mechanical nor electrical phase shifting and, therefore, it can be directly adapted to a large number of frequency bands. The performance of the system is validated by simulation and measurement examples in the millimeter-wave band.

Off-axis holography techniques, which are commonly used to perform phase retrieval from amplitude-only data, require a full-characterization of the reference signal in terms of amplitude and phase. In this letter, it is demonstrated that the recently proposed version of the off-axis holography, working with a broadband synthetic aperture radar, can bypass the need for this calibration. The strategy is based on employing a calibration object to adjust the system. Furthermore, certain properties of the reflectivity calculation are exploited to correct potential misalignment errors. The performance of the approach is demonstrated by reconstructing the profile of several objects at ${K}_{a}$ -band.

The aim of this contribution is the study of a multi-view electromagnetic scanner. In this system, the final image is dynamically built from several partial images taken from arbitrary views. Thus, the system opens the possibility to produce the surface profiling of an entire target without suffering shadow effects due to occlusions.

The aim of this paper is to present a 3D positioning system based on phase measurements of the signals backscattered by the UHF RFID tags of a uniform circular array (UCA). Apart from the location estimations, the system is also able to provide the 2D attitude (azimuth and elevation) of the UCA. This complementary information can be very useful in some applications such as in loading operations at warehouses. The accuracy of the system is validated through simulations using different number of transmitters and several Signal to Noise Ratio (SNR) values.

An analysis of different strategies for an efficient implementation of multiview millimeter-wave imaging systems is presented. The goal is to show the impact on Synthetic Aperture Radar (SAR) image quality of different strategies to combine multiple SAR images obtained with a portable mm-wave scanner in order to obtain a full image of the area-under-scan. Simulation results for a 15 cm linear array working in the 75–79 GHz frequency band are presented.

In this paper, we propose a ball grid array (BGA) module with an integrated 3-D-printed plastic lens antenna for application in a dedicated 130 GHz OOK transceiver that targets the area of 5G backhaul/fronthaul systems. The main design goal was the full integration of a small footprint antenna with an energy-efficient transceiver. The antenna system must be compact and cost effective while delivering an approximately 30 dBi gain in the working band, defined as 120 to 140 GHz. Accordingly, a 2×2 array of aperture-coupled patch antennas was designed in the 7×7×0.362 mm3 BGA module as the feed antenna of the lens. This achieved a 7.8 dBi realized gain, broadside polarization purity above 20 dB, and over 55% total efficiency from 110 to 140 GHz (20% bandwidth). A plastic elliptical lens 40 mm in diameter and 42.3 mm in height was placed on top of the BGA module. The antenna achieved a return loss better than ?10 dB and a 28 dBi realized gain from 114 to 140 GHz. Finally, active measurements demonstrated a >12 Gbps Tx/Rx link at 5 m with bit error rate (BER) < 10?6 at 1.6 pJ/b/m. These results pave the way for future cost-effective, energy-efficient, high-data rate backhaul/fronthaul systems for 5G communications. info:eu-repo/semantics/acceptedVersion

A phaseless setup for a submillimetre-wave frequency scanning-based radar system is presented. The phase retrieval setup is implemented by modifying a previous implementation at microwave frequencies. In the new setup, the number of involved components (e.g. directional couplers, power combiners etc.) is reduced by including a time-domain reference signal by means of direct radiation, yielding a low-cost submillimetre-wave radar system that avoids the need for phase shifters. Several measurement examples are shown to illustrate the performance of the system.

An antenna diagnostics method based on off-axis holography is presented in this letter. In this novel phaseless antenna measurement setup, the reference wave is synthesized by means of a mechanical phase shifting, and therefore the use of a phase shifter is avoided, yielding an accurate low-cost antenna diagnostics system. The technique is validated by means of simulation as well as measurement. Accurate results prove the system capability for antenna diagnostics tasks, This work was supported in part by theEuropean Union under COST Action IC1102 (VISTA); the European UnionSeventh Framework Programme (FP7/2007-2013) under Grant Agreement No.600849 (INSIDDE)