Showing papers on "Near and far field published in 2022"

Mixed Near-Field and Far-Field Localization and Array Calibration With Partly Calibrated Arrays

Abstract: The problem of passive localization of mixed near-field (NF) and far-field (FF) source signals in the presence of array gain-phase uncertainties is addressed. A new algorithm is aimed to use partly calibrated nonuniform linear arrays (NLAs), in which only three sensors have been fully-calibrated. Most of the existing algorithms deal with this problem by exploiting uniform linear arrays (ULAs). Moreover, they assume a simplified source-array model, in which the propagation magnitude scaling is completely neglected and the spatial phase difference is approximated by Taylor’s polynomial. As an opposite, the proposed algorithm is employed to accommodate a more general situation: the exact spatial geometries and nonuniform linear arrays. In the proposed algorithm, three cumulant matrices are firstly defined to construct two matrix pencils. Unambiguous range and angle parameter estimates of the NF sources are then obtained from the generalized eigenvalues of the two defined matrix pencils. After that, these estimates are utilized to calibrate array gain-phase errors. Finally, a spectrum-MUSIC like approach is applied to accomplish the angle estimation for the FF sources. The new algorithm is shown to be readily simple and effective and will be verified both mathematically and numerically.

Robust Far-Field Optical Image Transmission with Structured Random Light Beams

Abstract: The ability to overcome the adverse effect induced by the obstacles within the transmission link is a central challenge in long-distance optical image transmission and is significantly crucial in free-space optical communication. In this work, we introduce an efficient protocol to realize the robust far-field optical image-signal transmission by modulating the spatial-coherence structure of a partially coherent random light source. The image information encoded in the spatial-coherence structure can be stably transmitted to the far field and can resist the influence of obstructions within the communication link. This is due to the self-reconstruction property of the spatial-coherence structure embedded with the cross phase in the far field. We demonstrate experimentally that the image information can be recovered well by measuring the second-order spatial-coherence structure of the obstructed random light in the far field. Our findings open a door for robust optical signal transmission through the complex environment and may find application in optical communication through a turbulent atmosphere.

Tailoring electromagnetic responses in a coupled-grating system with combined modulation of near-field and far-field couplings

Abstract: Herein, we establish a hybrid coupling model (HCM) containing both near- and far-field couplings to describe the electromagnetic response of the coupled-grating system composed of two parallelly aligned subwavelength dielectric gratings. The HCM shows that the near-field coupling strength only contributes to the frequency splitting of two resonant modes, while the far-field one contributes to the frequency splitting and the linewidths of two resonant modes simultaneously. By changing the distance between two dielectric gratings, both the near- and far-field coupling strengths can be flexibly tuned, giving rise to rich electromagnetic responses. In addition, the formation of Fabry-Perot bound states in the continuum in coupled-grating systems can be clearly explained by the HCM. In this paper, we not only provide an all-dielectric platform for simultaneously manipulating near- and far-field couplings but also offer a viable approach to achieve reflectance/transmittance spectra with diverse shapes.

Channel Estimation for Extremely Large-Scale Massive MIMO: Far-Field, Near-Field, or Hybrid-Field?

Abstract: Extremely large-scale massive MIMO (XL-MIMO) is a promising technique for future 6G communications. However, existing far-field or near-field channel model mismatches the hybrid-field channel feature in the practical XL-MIMO system. Thus, existing far-field and near-field channel estimation schemes cannot be directly used to accurately estimate the hybrid-field XL-MIMO channel. To solve this problem, we propose an efficient hybrid-field channel estimation scheme by accurately modeling the XL-MIMO channel. Specifically, we firstly reveal the hybrid-field channel feature of the XL-MIMO channel, where different scatters may be in far-field or near-field region. Then, we propose a hybrid-field channel model to capture this feature, which contains both the far-field and near-field path components. Finally, we propose a hybrid-field channel estimation scheme, where the far-field and near-field path components are respectively estimated. Simulation results show that the proposed scheme performs better than existing schemes.

Fully Coherent UAV-Based Near-Field Measurement and Transformation of the S67-15 m Ground Station Antenna at the German Space Operations Center in Weilheim

Abstract: Fully coherent unmanned aerial vehicle (UAV)-based near-field measurements of the S67 parabolic ground station antenna in Weilheim, Germany at 2.06392 GHz are presented. The utilized measurement setup involves a vector network analyzer (VNA), radio frequency optical fiber connections, a laser tracking device (LT), and a specially designed light-weight dual-polarized Vivaldi antenna. The measurement data has been collected on a quasi-planar measurement surface in front of the reflector and the far-field patterns are obtained by field transformation from the irregular near-field data. Despite the complexity of this outdoor measurement setup with full exposure to varying weather conditions and the influence of the UAV on the near-field probe antenna, the far-field results show remarkable agreement with satellite based far-field measurement data.

High-efficiency generation of far-field spin-polarized wavefronts via designer surface wave metasurfaces

Abstract: Abstract Achieving a pre-designed scattering pattern from an ultra-compact platform is highly desired for on-chip integration optics, but conventional techniques suffer from the limitations of bulky size, wavelength-scale modulation and low efficiency. Here, we propose a new strategy to efficiently generate arbitrary spin-polarized scattering far-field patterns from surface-wave (SW) excitations on a designer Pancharatnam–Berry (PB) metasurface. We find that a PB meta-atom serves as a subwavelength scatter to decouple impinging SW to a spin-polarized propagating wave (PW) with tailored amplitude and phase, and thus interference among PWs generated by scatterings at different PB meta-atoms can generate a tailored far-field pattern. As a proof of concept, we design and fabricate a series of PB metasurfaces in the microwave regime and experimentally demonstrate that they can generate desired radiation patterns within a broad frequency band, including unidirectional radiation, line/point focusing, vortex beam and hologram. These findings may stimulate important applications in on-chip integrated photonics.

Codebook design and beam training for extremely large-scale RIS: Far-field or near-field?

Abstract: Reconfigurable intelligent surface (RIS) is more likely to develop into extremely large-scale RIS (XL-RIS) to efficiently boost the system capacity for future 6G communications. Beam training is an effective way to acquire channel state information (CSI) for XL-RIS. Existing beam training schemes rely on the far-field codebook. However, due to the large aperture of XL-RIS, the scatters are more likely to be in the near-field region of XL-RIS. The far-field codebook mismatches the near-field channel model. Thus, the existing far-field beam training scheme will cause severe performance loss in the XL-RIS assisted near-field communications. To solve this problem, we propose the efficient near-field beam training schemes by designing the near-field codebook to match the near-field channel model. Specifically, we firstly design the near-field codebook by considering the near-field cascaded array steering vector of XL-RIS. Then, the optimal codeword for XL-RIS is obtained by the exhausted training procedure. To reduce the beam training overhead, we further design a hierarchical near-field codebook and propose the corresponding hierarchical near-field beam training scheme, where different levels of sub-codebooks are searched in turn with reduced codebook size. Simulation results show the proposed near-field beam training schemes outperform the existing far-field beam training scheme.

Tones in the Acoustic Far Field of Jets in the Upstream Direction

Abstract: The presence of tones in the acoustic far field of isothermal round jets for angles relative to the flow direction is investigated for Mach numbers between and 2 based on data from both experiments and large-eddy simulations. For all Mach numbers, as observed in the jet near-nozzle region in previous studies, peaks are found in the far-field pressure spectra regardless of the nozzle-exit boundary-layer properties. They emerge near the cutoff frequencies of the free-stream upstream-propagating guided jet modes predicted by a vortex sheet model, sharply for but more weakly for lower Mach numbers. The variations of their characteristics with the radiation angle are shown. As the angle increases, the peaks are more prominent, and the most apparent ones are related to lower azimuthal modes. Thus, peaks associated with the axisymmetric mode strongly predominate in the upstream direction.

Spatial Characterization of Electromagnetic Random Channels

Abstract: The majority of stochastic channel models rely on the electromagnetic far-field assumption, which allows to decompose the channel in terms of plane waves. The far-field assumption breaks down in future applications that push towards the electromagnetic near-field region, such as those where the use of electromagnetically large antenna arrays is envisioned. Motivated by this consideration, we show how physical principles can be used to derive a plane-wave scalar channel model that is also valid in the reactive near-field region. Precisely, we show that narrowband wave propagation through a three-dimensional scattered medium can be generally modeled as a linear and space-variant system. We first review the physics principles that lead to a closed-form deterministic plane-wave representation of the channel impulse response. This serves as a basis for deriving a stochastic representation of the channel in terms of statistically independent Gaussian random coefficients for spatially stationary random propagation environments. The very desirable property of spatial stationarity can always be retained in the radiative near-field region by excluding reactive propagation mechanisms confined in close proximity to the source. Remarkably, the provided stochastic representation is directly connected to the Fourier spectral representation of a general stationary spatial random field.

Photonics-Based Near-Field Measurement and Far-Field Characterization for 300-GHz Band Antenna Testing

Abstract: In this study, photonics-based near-field measurement and far-field characterization in a 300-GHz band are demonstrated using an electrooptic (EO) sensor with planar scanning. The field to be measured is up-converted to the optical domain (1550 nm) at the EO sensor and delivered to the measurement system with optical fiber. The typical phase drift of the system is 0.46° for the one-dimensional measurement time of 13 s, which is smaller than the standard deviation of the phase measurement of 1.2° for this time scale. The far-field patterns of a horn antenna calculated from the measured near-field distribution are compared with that measured with the direct far-field measurement system using a vector network analyzer. For the angular related parameters, the accuracy of the results obtained by our near-field measurement are comparable to that of those obtained by direct far-field measurements. The sidelobe level discrepancy (approximately 1 dB) between the results obtained based on our near-field measurement and those from the direct far-field measurements are attributed to the excess noise of the probe correction data. We believe that photonics-based near-field measurements with spherical EO probe scanning will pave the way for the characterization of high-gain antennas at the 300-GHz band.

Thermophotovoltaic Energy Conversion in Far-to-Near-Field Transition Regime

Abstract: Recent experimental studies on near-field thermophotovoltaic energy conversion have mainly focused on improving performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena induced by the interference of propagating waves, which are seldom observed in the full spectrum radiation due to the massive increase in evanescent modes. Here, we experimentally demonstrate the oscillatory nature of near-field thermophotovoltaic energy conversion in the far-to-near-field transition regime (250–2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the photovoltaic cell. Noticeably, we show that the same amount of photocurrent can be generated at different vacuum gaps of 870 and 322 nm, which is 10% larger than the far-field value.

Time of arrival as a diagnostic for far-field high explosive blast waves

Abstract: The ability to accurately determine blast loading parameters will enable more fundamental studies on the sources of blast parameter variability and their influence on the magnitude and form of the loading itself. This will ultimately lead to a better fundamental understanding of blast wave behaviour, and will result in more efficient and effective protective systems and enhanced resilience of critical infrastructure. This article presents a study on time of arrival as a diagnostic for far-field high explosive blasts, and makes use of the results from a large number of historic tests and newly performed experiments where the propagating shock front was filmed using a high-speed video (HSV) camera. A new method for optical shock tracking of far-field blast tests is developed and validated, and full-field arrival time results are compared against those determined from the historic data recorded using traditional pressure gauges. Arrival time variability is shown to be considerably lower than peak pressure and peak specific impulse, and is shown to decrease exponentially with increasing scaled distance. Further, the method presented in this article using HSV cameras to determine arrival time yields further reductions in variability. Finally, it is demonstrated that the method can be used to accurately determine far-field TNT equivalence of high explosives.

Geometry-independent antenna based on Epsilon-near-zero medium

Abstract: It is well known that electromagnetic radiation from radiating elements (e.g., antennas, apertures, etc.) shows dependence on the element's geometry shape in terms of operating frequencies. This basic principle is ubiquitous in the design of radiators in multiple applications spanning from microwave, to optics and plasmonics. The emergence of epsilon-near-zero media exceptionally allows for an infinite wavelength of electromagnetic waves, manifesting exotic spatially-static wave dynamics which is not dependent on geometry. In this work, we analyze theoretically and verify experimentally such geometry-independent features for radiation, thus presenting a novel class of radiating resonators, i.e., antennas, with an operating frequency irrelevant to the geometry shape while only determined by the host material's dispersions. Despite being translated into different shapes and topologies, the designed epsilon-near-zero antenna resonates at a same frequency, while exhibiting very different far-field radiation patterns, with beams varying from wide to narrow, or even from single to multiple. Additionally, the photonic doping technique is employed to facilitate the high-efficiency radiation. The material-determined geometry-independent radiation may lead to numerous applications in flexible design and manufacturing for wireless communications, sensing, and wavefront engineering.

High‐Fidelity Far‐Field Microscopy at λ/8 Resolution

Abstract: The emergence of far‐field super‐resolution microscopy has rejuvenated the possibility for nanoscale imaging. Approaches to far‐field super‐resolution that utilize point scanning often depend on spatially reducing the size of the focused spot. However, the focused spot always achieves high resolution at the expense of extremely low light efficiency for the probing mainlobe and high‐intensity sidelobes, which limits the applications in nanoscale imaging and might cause misinterpretation of samples. Here, a sharp probing spot is reported with the diffraction efficiency of 3.76% at the resolution of 38% of the Airy spot size assisted by the 2D multilevel diffractive optical element (DOE) experimentally. The diffraction efficiency of DOE is improved by at least two orders of magnitude at the same resolution by breaking the limitation of circular 0–π binary structure superoscillatory lens. To eliminate the influence of the high‐intensity sidelobes, high‐fidelity images are reconstructed based on the modified deconvolution algorithm by virtue of the prior knowledge. Finally, high‐fidelity far‐field microscopy (HiFi‐FM) is constructed and experimental results show that HiFi‐FM allows the resolution of spatially complex samples better than 69 nm while acquiring high fidelity.

Acoustic far-field prediction based on near-field measurements by using several different holography algorithms.

Abstract: Near-field acoustical holography (NAH) is a useful tool for sound field reconstruction and sound source identification. In NAH, a basis model is first selected to represent the physical sound field, and then a near-field measurement is made with a microphone array. Next, the parameters in the selected model can be estimated based on the measurements by using an inverse approach, resulting in the sound field near the source being reconstructed so that the sound source location can be identified. But, in addition to being able to reconstruct the near-field of a source, the far-field can also be predicted with the identified sound source model. A significant amount of work has been performed to study the near-field reconstruction capability of different NAH algorithms, but there has been a limited number of publications in which the far-field prediction accuracy, based on the near-field measurement constructed model, has been considered. In the present experimental work, two multi-transducer loudspeakers were placed side-by-side to create a multi-component sound source, and two sets of measurements were conducted: an intensity probe scanned the sound field generated by the loudspeakers in both the near-field (0.05 m) and far-field (0.48 m) such that the true near- and far-field intensity spatial distributions and total sound power could be identified. Then, based on the near-field pressure measurements, four acoustical holography algorithms, statistically optimized near-field acoustical holography, wideband acoustical holography, l1-norm minimization, and a hybrid compressive sampling method were used to predict the near- and far-field sound intensity distributions. The near- and far-field prediction results were compared with the direct measurement, and the sound field reconstruction accuracy was studied. It was found that all of the algorithms were able to reconstruct the near-field well when the near-field measurements were used to construct the model. It was found that with the abovementioned models, far-field reconstructions could correctly predict the spatial sound field distribution, but in all of the cases, the total sound power was underestimated.

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope

Abstract: The identification of the minerals composing rocks and their dielectric characterization is essential for the utilization of microwave energy in the rock industry. This paper describes the use of a near-field scanning microwave microscope with enhanced sensitivity for non-invasive measurements of permittivity maps of rock specimens at the micrometer scale in non-contact mode. The microwave system comprises a near-field probe, an in-house single-port vectorial reflectometer, and all circuitry and software needed to make a stand-alone, portable instrument. The relationship between the resonance parameters of the near-field probe and the dielectric properties of materials was determined by a combination of classical cavity perturbation theory and an image charge model. The accuracy of this approach was validated by a comparison study with reference materials. The device was employed to determine the permittivity maps of a couple of igneous rock specimens with low-loss and high-loss minerals. The dielectric results were correlated with the minerals comprising the samples and compared with the dielectric results reported in the literature, with excellent agreements.

Probabilistic structural performance of RC frames with corroded smooth bars subjected to near- and far-field ground motions

Abstract: This paper investigates the seismic vulnerability of existing RC frames exposed to corrosion and subjected to near-field and far-field ground motions. A threefold approach for corrosion is adopted to illustrate the probabilistic framework and define time-dependent performance criteria for an accurate seismic fragility assessment. A bond-slip model is employed to simulate the fixed-end rotation and column-beam joints behaviour to account for the deficit in the bond strength of plain rebars. Such a model is calibrated using experimental studies from the literature and considering the effects of corrosion. An inelastic buckling model of steel bars is also incorporated in the finite element model through a hysteretic material to investigate its impact on the deformation capacity of RC members. The effects of near-field and far-field earthquakes are investigated through incremental dynamic analyses (IDA) and cloud analyses on a typical four-storey RC frame with plain bars. Results from the fragility analysis indicate that corrosion has significant effects on the seismic performance of such RC frames over time and near-field pulse-like motions are more destructive than both near-field no-pulse-like and far-field earthquakes. • Probabilistic seismic performance of aged RC structures under near- and far-fields. • Three-fold probabilistic approach for corrosion stages. • Three-linear model for bond strength deterioration of plain bars. • FEM approach for RC columns under cyclic loading including inelastic buckling. • Response surface surrogate model for shear capacity of aged RC columns.

Application of local blowing to a structured porous-coated cylinder for flow and noise control

Abstract: A comprehensive experimental study was carried out to investigate the effectiveness of a local blowing through a structured porous-coated cylinder (SPCC) as a novel technique for vortex-induced noise suppression in subcritical Reynolds number. The active flow control method was applied using a special design chamber with some holes distributed along two spanwise lines at angles 𝜃𝑏 = ±130.9○. The near- and far-field noise measurements were simultaneously carried out to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the application of the blowing to the SPCC. A highly instrumented cylinder with several pressure taps distributed around the circumference of the cylinder at midspan was used to analyze the near-field noise using remote-sensing techniques of the fluctuating pressure fields. Although quite similar acoustic spectra were demonstrated by the SPCC with and without blowing, it was found that applying the local blowing to the SPCC causes a significant reduction of the vortex shedding tone produced by the bare cylinder, for any of the non-dimensional equivalent momentum coefficient considered in this study. A substantial reduction in energy content of the surface pressure fluctuations over the whole frequency range has also been observed for the SPCC with and without blowing especially in the post-separation region. The SPCC, with or without the local blowing resulted in higher frequency contributions that were shown to be independent of the surface pressure fluctuations. The near- to far-field coherence revealed that the increased noise observed for the SPCC with and without blowing configurations is solely due to the near-field non-propagating hydrodynamic field.

Dimension and Sampling of the Near-Field and Its Intensity Over Curves

Abstract: The paper addresses the question of efficiently sampling the near field and its intensity over an arbitrary curved line. It aims at finding the minimum number of measurements and their position for discretizing the near field and its square amplitude without loss of information and follows a common approach for both the radiation operator and the correspondent lifting operator. A scalar geometry is examined for both electric and magnetic current sources. At first, the singular values of the radiation / lifting operator are evaluated by an asymptotic approach and a change of variables which lead to an operator with a widely investigated spectrum. As the number of relevant singular values is available in closed form, the dimensions of the near field and its intensity over the curve are found. Next, starting from a sampling expansion of the pertinent left singular functions, a non-redundant sampling representation of the near field and its square amplitude is provided. Some numerical results confirm the analysis. The results may be of great interest especially in antenna testing by near field data (both standard and phaseless) with an unconventional field scanning system as, for instance, an UAV mounted probe.

Morphology dependence of nanoparticle-on-mirror geometries: A quasinormal mode analysis

Abstract: Plasmonic nanoantennas are able to produce extreme enhancements by concentrating electromagnetic fields into sub-wavelength volumes. Recently, one of the most commonly used nanoantennas is the nanoparticle-on-mirror geometry, which allowed for the room temperature strong coupling of a single molecule. Very few studies offer analysis of near-field mode decompositions, and they mainly focus on spherical and/or cylindrically-faceted nanoparticle-on-mirror geometries. Perfectly spherical nanoparticles are not easy to fabricate, with recent publications revealing that a rhombicuboctahedron is a commonly occurring nanoparticle shape – due to the crystalline nature of metallic nanoparticles. In this paper, we perform a quasi-normal mode analysis for the rhombicuboctahedron-on-mirror nanoantenna and map the field distributions of each mode. We examine how the geometry of the cavity defines the near-field distribution and energies of the modes, and we show that in some cases the mode degeneracies break. This has a significant impact on the radiative emission and far-field profile of each mode, which are measured experimentally. Understanding how realistic nanoantenna geometries behave in the near-field and far-field helps us design antennas with specific properties for controlling and sensing quantum emitters in plasmonic systems.

Response of concrete buildings with steel shear walls to near- and far-field earthquakes

Abstract: The research reported in this paper was aimed at studying the effect of higher modes and degrees of freedom on the estimation of displacement, velocity, acceleration and shear forces in reinforced concrete buildings with steel plate shear walls during earthquakes. Modelled building storey displacements were found to be conservatively distributed during far-field earthquakes in low- and mid-rise buildings with elastic behaviour as well as in high-rise buildings with linear and non-linear behaviour. In the case of low- and mid-rise structures, it was found acceptable to apply the base shear induced by the far-field earthquake rather than that of the near-field earthquake. However, for high-rise buildings, it was found that non-conservative results were obtained if far-field base shear was used instead of near-field. On the other hand, the patterns of velocity and acceleration distribution were affected by the higher modes.

On the Study of Reconfigurable Intelligent Surfaces in the Near-Field Region

Abstract: As reconfigurable intelligent surface (RIS) is expected to be implemented with a large aperture in terms of wavelengths, using a RIS to perform near-field rather than conventional far-field communications might gain some benefits. The electric fields in the near-field region from the near-field focusing and conventional far-field RISs are simulated and compared to assess and validate the benefits. Two metrics of benefit distance and near-field gain are defined to characterize the electric fields in the near-field region. The cumulative distribution function (CDF) related to the near-field gain is calculated to describe the benefits in the area of interest where the total scan patterns from the near-field focusing and conventional far-field RISs are plotted. For demonstration, the benefit distance, near-field gain, and CDFs are presented and calculated for near-field focusing RISs with assigned focal distances of 80 and 300 mm. It is concluded that the benefit distance is a critical factor affecting the CDF directly. The effects of the size of a RIS and phase quantization of elements on the benefit distance, near-field gain, and CDF are also discussed. Moreover, the maximum focal distance of a near-field focusing RIS with a certain size is derived and specified. Finally, the electric fields of three prototypes (e.g., conventional far-field RIS, near-field focusing RISs with assigned focal distances of 500 and 1500 mm) at 26 GHz are measured and compared, where the benefit distances and near-field gains are experimentally observed.

Symmetric flipped nested array for mixed near‐field and far‐field non‐circular source localisation

Abstract: As it is known, existing mixed near-field and far-field non-circular source localisation rely on the uniform linear array (ULA). Compared with the ULA, the sparse array can obtain more consecutive lags and a larger physical array aperture. So a novel symmetric flipped nested array (SFNA) is proposed for mixed near-field and far-field non-circular source localisation. At the same time, according to the special geometry of the array, the non-circular phase information can be removed and the existing algorithm can be used to solve the problem of sparse array estimating the mixed near-field and far-field non-circular signals. First, by exchanging the positions of the subarrays in the traditional nested array and then flipping them, a new SFNA is formed; second, the authors present the maximum consecutive lags and composition method of SFNA. For example, 4MN +4M −3 consecutive lags can be obtained by using a 2M + 2N −1 array element; third, in the estimation of mixed near-field and far-field non-circular sources, a special cumulant is used to eliminate the range of the mixed near-field and far-field non-circular sources and a one-dimensional (1-D) spectral search is used to obtain all the directions of arrival, and the range can be obtained by defining the range spectrum; finally, numerical simulation results demonstrate the superiority of the proposed array.

Optimization of the Far-field Pattern in an Apodized Optical Phased Array by Refractive Index Liquids

Abstract: We investigate the far-field pattern broadening of an apodized optical phased array when changing the waveguide cladding indices. Results indicate the beam broadening can be optimized and circumvented by tuning liquid claddings.