arXiv subjects

Christophe Craeye

Publications and source records attributed to Christophe Craeye.

At least 19 recordsLinked to original sources

arXiv2024

Positioning and transmission in cell-free networks: ambiguity function, and MRC/MRT array gains

Cell-free network is a new paradigm, originating from distributed MIMO, that has been investigated for a few recent years as an alternative to the celebrated cellular structure. Future networks not only consider classical data transmission but also positioning, along the lines of Integrated Communications and Sensing (ISAC). The goal of this paper is to investigate at the same time the ambiguity function which is an important metric for positioning and the understanding of its associated resolution and ambiguities, and the array gain when maximum ratio transmission (MRT) or MR combining (MRC) is implemented for data communications. In particular, the role and impact of using a waveform with non-zero bandwidth is investigated. The theoretical findings are illustrated by means of computational results.

eess.SP↗

arXiv2024

A Broadband Multipole Method for Accelerated Mutual Coupling Analysis of Large Irregular Arrays Including Rotated Antennas

We present a numerical method for the analysis of mutual coupling effects in large, dense and irregular arrays with identical antennas. Building on the Method of Moments (MoM), our technique employs a Macro Basis Function (MBF) approach for rapid direct inversion of the MoM impedance matrix. To expedite the reduced matrix filling, we propose an extension of the Steepest-Descent Multipole expansion which remains numerically stable and efficient across a wide bandwidth. This broadband multipole-based approach is well suited to quasi-planar problems and requires only the pre-computation of each MBF's complex patterns, resulting in low antenna-dependent pre-processing costs. The method also supports arrays with arbitrarily rotated antennas at low additional cost. A simulation of all embedded element patterns of irregular arrays of 256 complex log-periodic antennas completes in just 10 minutes per frequency point on a current laptop, with an additional minute per new layout.

astro-ph.IM↗

arXiv2024

Near-Field EM-Based Multistatic Radar Range Estimation

Radar targets are traditionally modelled as point target reflectors, even in the near-field region. Yet, for radar systems operating at high carrier frequencies and small distances, traditional radar propagation models do not accurately model the scatterer responses. In this paper, a novel electromagnetic-based model is thus developed for the multistatic radar detection of a rectangular plate reflector in the near-field region. This model is applied to an automotive scenario, in which a linear antenna array is spread out at the front of a vehicle, and performs a radar measurement of the distance to the back of the vehicle ahead. Based on the developed received signal model, the maximum likelihood estimator of the range is designed. By exploiting the near-field target model, this estimator is shown to provide a significant gain with respect to traditional range estimators. The impact of the system and scenario parameters, i.e. the carrier frequency, bandwidth and distance to the target, is furthermore evaluated. This analysis shows that the radar resolution in the near-field regime is improved at high carrier frequencies, while saturating to the traditional bandwidth-dependent resolution in the far-field region.

eess.SP↗

arXiv2023

Efficient Tracking of Dispersion Surfaces for Printed Structures using the Method of Moments

The dispersion surfaces of printed periodic structures in layered media are efficiently computed using a full-wave method based on the periodic Method of Moments (MoM). The geometry of the dispersion surface is estimated after mapping the determinant of the periodic MoM impedance matrix over a range of frequencies and impressed phase shifts. For lossless periodic structures in the long-wavelength regime, such as lossless metasurfaces, a tracking algorithm is proposed to represent the dispersion surface as a superposition of parameterized iso-frequency curves. The mapping process of the determinant is accelerated using a specialized interpolation technique with respect to the frequency and impressed phase shifts. The algorithm combines a fast evaluation of the rapidly varying part of the periodic impedance matrix and the interpolation of the computationally intensive but slowly varying remainder. The mapping is further accelerated through the use of Macro basis functions (MBFs). The method has been first tested on lossless metasurface-type structures and validated using the commercial software CST. The specialized technique enables a drastic reduction of the number of periodic impedance matrices that needs to be explicitly computed. In the two examples considered, only 12 matrices are required to cover any phase shift and a frequency band larger than one octave. An important advantage of the proposed method is that it does not entail any approximation, so that it can be used for lossy structure and leaky waves, as demonstrated through two additional examples.

physics.comp-ph↗

arXiv2023

Novel Electromagnetism-Based Radar Propagation Model for 5G and Beyond

In order to evaluate the performance of radar and communication systems in future wireless networks, accurate propagation models are needed to predict efficiently the received powers at each node, and draw correct conclusions. In this paper, we present new radar propagation models based on the electromagnetism theory. The target is modelled as a flat or curved square plate to compute the scattered field and derive accurate radar cross section modellings. With a flat square plate, the model makes the link between the radar equation and the geometrical optics propagation model used in ray-tracing applications. It is then applied to popular automotive scenarios within the stochastic geometry framework to observe the impact of such modelling.

eess.SP↗

arXiv2021

Interpolation of impedance matrices for varying quasi-periodic boundary conditions in 2D periodic Method of Moments

Periodic structures can be simulated using the periodic Method of Moments. The quasi-periodicity, i.e. periodicity within a linear phase-shift, is implemented through the use of the periodic Green's function. In this paper, we propose a technique to interpolate the impedance matrix for varying phase-shifts. To improve the accuracy, the contribution of the dominant Floquet modes and a term corresponding to a linear phase-shift are first extracted. The technique is applied to planar geometries, but can be extended to non-planar configurations.

physics.comp-ph↗

arXiv2020

A beamforming approach to the self-calibration of phased arrays

In this paper, we propose a beamforming method for the calibration of the direction-independent gain of the analog chains of aperture arrays. The gain estimates are obtained by cross-correlating the output voltage of each antenna with a voltage beamformed using the other antennas of the array. When the beamforming weights are equal to the average cross-correlated power, a relation is drawn with the StEFCal algorithm. An example illustrates this approach for few point sources and a 256-element array.

astro-ph.IM↗

arXiv2020

SKA LFAA Station Design Report

This report was submitted as part of the SKA Low Frequency Aperture Array Critical Design Review describing the design of the SKA1-LOW station that took place between 2013 and 2018. The SKA1 LOW field station is inscribed in a circular area having an effective station diameter (centre to centre) of 38 meters and has 256 SKALA4 elements. This document describes the electromagnetic design of the field station. In particular it describes the layout design and the electromagnetic modelling and characteristics of the station. This document describes the effects associated with the layout and array such as mutual coupling effects, side lobe pattern and beam shape (eg. smoothness, calibration models) and presents the state of the art of our ability to measure the array performance and validate the simulation work. The current LFAA field node requirements, derived from the SKA L1 requirements, have evolved over the last years since the LFAA PDR and the System Baseline Design. The SKA1 LOW field station has been designed to meet those requirements and has therefore tracked their evolution (eg. sensitivity requirements, array diameter, etc.). The aforementioned requirements represent a very tight space with a desire for very high sensitivity over a large frequency range (7 to 1) and wide field of view (90 degrees cone around zenith) while keeping the station diameter to a minimum, so as the filling factor but at the same time allowing for sufficient space between antennas to allow for easy maintenances, amongst many others. This results in a complex design.

astro-ph.IM↗

arXiv2020

Modal characterization of thermal emitters using the Method of Moments

Electromagnetic sources relying on spontaneous emission are difficult to characterize without a proper framework due to the partial spatial coherence of the emitted fields. In this paper, we propose to characterize emitters of any shape through their natural emitting modes, i.e. a set of coherent modes that add up incoherently. The resulting framework is very intuitive since any emitter is regarded as a multimode antenna with zero correlation between modes. Moreover, for any finite emitter, the modes form a compact set that can be truncated. Each significant mode corresponds to one independent degree of freedom through which the emitter radiates power. The proposed formalism is implemented using the Method of Moments (MoM) and applied to a lossy sphere and a lossy ellipsoid. It is shown that electrically small structures can be characterized with a small number of modes, and that this number grows as the structure becomes electrically large.

physics.comp-ph↗

arXiv2020

An Ultra-wideband Battery-less Positioning System for Space Applications

An ultra-wide bandwidth (UWB) remote-powered positioning system for potential use in tracking floating objects inside space stations is presented. It makes use of battery-less tags that are powered-up and addressed through wireless power transfer in the UHF band and embed an energy efficient pulse generator in the 3-5 GHz UWB band. The system has been mounted on the ESA Mars Rover prototype to demonstrate its functionality and performance. Experimental results show the feasibility of centimeter-level localization accuracy at distances larger than 10 meters, with the capability of determining the position of multiple tags using a 2W-ERP power source in the UHF RFID frequency band.

eess.SP↗

arXiv2019

Numerically stable eigenmode extraction in 3D periodic metamaterials

A numerical method is presented to compute the eigenmodes supported by three dimensional (3D) metamaterials using the Method of Moments (MoM). The method relies on interstitial equivalent currents between layers. First, a parabolic formulation is presented. Then, we present an iterative technique that can be used to linearize the problem. In this way, all the eigenmodes characterized by their transmission coefficients and equivalent interstitial currents can be found using a simple eigenvalue decomposition of a matrix. The accuracy that can be achieved is only limited by the quality of simulation, and we demonstrate that the error introduced when linearizing the problem decreases doubly exponentially with respect to the time devoted to the iterative process. We also draw a mathematical link and distinguish the proposed method from other transfer-matrix based methods available in the literature.

physics.comp-ph↗

arXiv2019

All-analytical evaluation of the singular integrals involved in the Method of Moments

Surface Integral Equation (SIE) methods routinely require the integration of the singular Green's function or its gradient over Basis Functions (BF) and Testing Functions (TF). Many techniques have been described in the literature for the fast and accurate computation of these integrals for TF that is located close to the BF. In this paper, we propose an all-analytical formula for the singular part of the integral for both the Electric and Magnetic Field Integral Equations (EFIE and MFIE). The method works for any flat polygonal BF and TF of any order, and proves to be competitive with existing techniques.

physics.comp-ph↗

arXiv2019

The radioscience LaRa instrument onboard ExoMars 2020 to investigate the rotation and interior of Mars

LaRa (Lander Radioscience) is an experiment on the ExoMars 2020 mission that uses the Doppler shift on the radio link due to the motion of the ExoMars platform tied to the surface of Mars with respect to the Earth ground stations (e.g. the deep space network stations of NASA), in order to precisely measure the relative velocity of the lander on Mars with respect to the Earth. The LaRa measurements shall improve the understanding of the structure and processes in the deep interior of Mars by obtaining the rotation and orientation of Mars with a better precision compared to the previous missions. In this paper, we provide the analysis done until now for the best realization of these objectives. We explain the geophysical observation that will be reached with LaRa (Length-of-day variations, precession, nutation, and possibly polar motion). We develop the experiment set up, which includes the ground stations on Earth (so-called ground segment). We describe the instrument, i.e. the transponder and its three antennas. We further detail the link budget and the expected noise level that will be reached. Finally, we detail the expected results, which encompasses the explanation of how we shall determine Mars' orientation parameters, and the way we shall deduce Mars' interior structure and Mars' atmosphere from them. Lastly, we explain briefly how we will be able to determine the Surface platform position.

astro-ph.EP↗

arXiv2019

Geometrical MoM formulation for eigenmode analysis

The resonant frequencies of a structure and the associated field distributions are generally determined by solving a non-linear eigenvalue problem. Using frequency-domain solvers, the response of the structure needs to be evaluated at many different frequencies in order to solve the non-linear problem. Moreover, these frequencies may be complex, the inverse of the imaginary part physically corresponding to the e-folding time of the energy of the mode. In this paper, we propose to use the so-called "Geometrical Method of Moments" (GMoM) to accelerate the computation of the resonant frequencies of a structure using the Method of Moments. First, purely geometrical reaction integrals are precomputed, which do not depend on the frequency nor material parameters. Then, by summing these terms with proper weights, the impedance matrix can be obtained for any complex frequency. This method easily accommodates for dispersive materials provided that the permittivity and permeability of the material can be extrapolated to complex frequencies.

physics.comp-ph↗

arXiv2018

Identification of the absorption processes in periodic plasmonic structures using Energy Absorption Interferometry

Power dissipation in electromagnetic absorbers is a quadratic function of the incident fields. To characterize an absorber, one needs to deal with the coupling that may occur between different excitations. Energy Absorption Interferometry (EAI) is a technique that highlights the independent degrees of freedom through which a structure can absorb energy: the natural absorption modes of the structure. The coupling between these modes vanishes. In this paper, we use the EAI formalism to analyse different kinds of plasmonic periodic absorbers while rigorously accounting for the coupling: resonant golden patches on a grounded dielectric slab, parallel free-standing silver wires and a silver slab of finite thickness. The EAI formalism is used to identify the physical processes that mediate absorption in the near and far field. First, we demonstrate that the angular absorption, which is classically used to characterize periodic absorbers in the far field and which neglects the coupling between different plane waves, is only valid under stringent conditions (subwavelength periodicity, far field excitation and negligible coupling between the two possible polarizations). Using EAI, we show how the dominant absorption channels can be identified through the signature of the absorption modes of the structure, while rigorously accounting for the coupling. We then exploit these channels to improve absorption. We show that long-range processes can be exploited to enhance the spatial selectivity, while short-range processes can be exploited to improve absorptivity over wide angles of incidence. Lastly, we show that simply adding scatterers with the proper periodicity on top of the absorber, the absorption can be increased by more than one order of magnitude.

physics.optics↗

arXiv2017

Main Beam Modeling for Large Irregular Arrays: The SKA1-LOW telescope case

Large radio telescopes in the 21st century such as the Low-Frequency Array (LOFAR) or the Murchison Widefield Array (MWA) make use of phased aperture arrays of antennas to achieve superb survey speeds. The Square Kilometer Array low frequency instrument (SKA1-LOW) will consist of a collection of non-regular phased array systems. The prediction of the main beam of these arrays using a few coefficients is crucial for the calibration of the telescope. An effective approach to model the main beam and first few sidelobes for large non-regular arrays is presented. The approach exploits Zernike polynomials to represent the array pattern. Starting from the current defined on an equivalence plane located just above the array, the pattern is expressed as a sum of Fourier transforms of Zernike functions of different orders. The coefficients for Zernike polynomials are derived by two different means: least-squares and analytical approaches. The analysis shows that both approaches provide a similar performance for representing the main beam and first few sidelobes. Moreover, numerical results for different array configurations are provided, which demonstrate the performance of the proposed method, also for arrays with shapes far from circular.

astro-ph.IM↗

arXiv2017

Fast and Accurate Simulation Technique for Large Irregular Arrays

A fast full-wave simulation technique is presented for the analysis of large irregular planar arrays of identical 3-D metallic antennas. The solution method relies on the Macro Basis Functions (MBF) approach and an interpolatory technique to compute the interactions between MBFs. The Harmonic-polynomial (HARP) model is established for the near-field interactions in a modified system of coordinates. For extremely large arrays made of complex antennas, two approaches assuming a limited radius of influence for mutual coupling are considered: one is based on a sparse-matrix LU decomposition and the other one on a tessellation of the array in the form of overlapping sub-arrays. The computation of all embedded element patterns is sped up with the help of the non-uniform FFT algorithm. Extensive validations are shown for arrays of log-periodic antennas envisaged for the low-frequency SKA (Square Kilometer Array) radio-telescope. The analysis of SKA stations with such a large number of elements has not been treated yet in the literature. Validations include comparison with results obtained with commercial software and with experiments. The proposed method is particularly well suited to array synthesis, in which several orders of magnitude can be saved in terms of computation time.

astro-ph.IM↗

arXiv2016

Characterization of Power Absorption Response of Periodic 3D Structures to Partially Coherent Fields

In many applications of absorbing structures it is important to understand their spatial response to incident fields, for example in thermal solar panels, bolometric imaging and controlling radiative heat transfer. In practice, the illuminating field often originates from thermal sources and is only spatially partially coherent when reaching the absorbing device. In this paper, we present a method to fully characterize the way a structure can absorb such partially coherent fields. The method is presented for any 3D material and accounts for the partial coherence and partial polarization of the incident light. This characterization can be achieved numerically using simulation results or experimentally using the Energy Absorption Interferometry (EAI) that has been described previously in the literature. The absorbing structure is characterized through a set of absorbing functions, onto which any partially coherent field can be projected. This set is compact for any structure of finite extent and the absorbing function discrete for periodic structures.

physics.optics↗