Near-field Sources Research Articles

Theory of spoof magnetic localized surface plasmons beyond effective medium approximations

A homogeneous negative permeability sphere can support magnetic localized surface plasmons (MLSPs). Generally, negative permeability materials are metamaterial (MM) structures exhibiting very deep subwavelength spatial scales, whose effects may be detrimental in the near-field for those applications based on effective medium approximations. We suggest to overcome this fundamental limitation by demonstrating analytically that the electromagnetic spatial distribution, associated to a MLSP resonance and excited by a near-field source, can be accurately reproduced outside the sphere by substituting the negative permeability sphere with a homogeneous high-index dielectric one having the same radius. Considering that a large class of ferroelectric materials shows ultra-high dielectric constant and low-losses at low frequency (up to GHz), our spoof MLSPs theory could be a key tool for realizing high performance subwavelength magnetic photonic devices in the radiofrequency and microwave regions.

Enhancing the sensitivity of a transmissive graphene-based plasmonic biosensor

A biosensor platform based on the plasmonic resonance of graphene in the terahertz (THz) range (0.1 to 10 THz) is designed and investigated. The initial design is to create a nanofluidic channel as a sensing layer in the substrate of a biosensor grounded by metal. The sensor consists of a rectangular graphene patch over the substrate, which can be fed by either an external near-field source or an antenna. The presence of molecules in the nanosensing layer causes small changes in the channel's properties, detectable through the scattering parameters of the designed biosensor. Since biomolecules are poorly absorbed in the initial biosensor, it can be grounded by a graphene sheet that is the same size as the graphene sheet over the substrate, which results in a performance improvement of the biosensor. It is shown that, by increasing the number of graphene sheets between the ground and the patch, high absorption occurs that enhances the sensitivity of the initial surface plasmon resonance THz biosensor. With varying refractive index of the sensing layer (Ns) in the range of 1.3-1.6, the proposed biosensors are investigated and compared with the initial biosensor. It is shown that by applying a graphene sheet between the two graphene sheets in the substrate, a maximum sensitivity of 8470 nm/RIU is achieved, which is a significant improvement, and also a sensitivity improvement of 4130 nm/RIU is achieved at Ns=1.3. In the final section, it is shown that changing the substrate material from silicon (Si) to silica (SiO2) brings a significant sensitivity enhancement in the proposed biosensor with three graphene sheets, which accounts for 15410 nm/RIU in the best scenario.

Partially Matching Projection Decoding Method Evaluation Under Different Playback Conditions

The Ambisonic technique has been widely used in fields such as sound-field reproduction, sound source localization, and acoustic noise control. In this paper, a recently proposed Ambisonic decoding method, the partially matching projection method (PMPD), was described in detail and extended for more practical use with near-field sources and reverberant playback environments. Several experiments were conducted to evaluate the performance of the method. In the experiments, we compared performances of PMPD method with three other Ambisonic decoding methods, “All-round Ambisonic Decoding” (AllRAD) method, mode matching decoding (MMD) method and regularized mode matching decoding (RMMD) method. Different playback conditions were considered to investigate the generalization of the method, including unevenly and evenly distributed loudspeakers, as well as free-field and reverberant playback environments. In terms of reproduction errors near sweep-spot and energy of loudspeaker gains, PMPD method worked well for different signals and playback environments. Whereas the other three methods encontered ill-posed problems or large reproduction errors in certain conditions. This means that PMPD method achieves a balence between precise reproductioin near sweet-spot and energy of loudspeaker gains with ill-posed problems. PMPD method has a more robust performance than the other three compared methods for different signals and playback environments. And we conclude that PMPD method is effective to be used as an Ambisonic decoding method, especially for different playback environments.

Comments on “Localization of Near-Field Sources Based on Linear Prediction and Oblique Projection Operator”

In the aforementioned paper the authors investigate the localization of multiple near-field narrowband sources with a symmetric uniform linear array, and develop a new linear prediction approach based on the truncated singular value decomposition. In this comment we point out several shortcomings of the model they use for the data measured by the array. The mismatch between this highly idealized mathematical model and the true array response raises concerns about the performance of the proposed method on real arrays.

Mixed Source Sound Field Translation for Virtual Binaural Application With Perceptual Validation

Non-interactive and linear experienceslike cinema film offer high quality surround sound audio to enhance immersion, however, the perspective is usually fixed to the recording microphone position. With the rise of virtual reality, there is a demand for recording and recreating real-world experiences that allow users to move throughout the reproduction. Sound field translation achieves this by building an equivalent environment of virtual sources to recreate the recording spatially. However, the technique remains to restrict the maximum distance a user can translate away from the recording microphone's perspective due to the discrete sampling by commercial higher order microphones only being capable of recording an acoustic sweet-spot. In this paper, we propose a method for binaurally reproducing a microphone recording in a virtual application that allows the user to freely translate their body further beyond the recording position. The method incorporates a mixture of near-field and far-field sources in a sparsely expanded virtual environment to maintain a perceptually accurate reproduction. We perceptually validate the method through a Multiple Stimulus with Hidden Reference and Anchor (MUSHRA) experiment. Compared to the planewave benchmark, the proposed method offers both improved source localizability and robustness to spectral distortions at translated listening positions. A cross-examination with numerical simulations demonstrated that the sparse expansion relaxes the inherent sweet-spot constraint, leading to the improved localizability for sparse environments. Additionally, the proposed method is seen to better reproduce the intensity and binaural room impulse response spectra of near-field environments, further supporting the perceptual results.

Micro-seismic hazard assessment of Ahmedabad city, Gujarat (Western India) through near-surface characterization/soil modeling

The micro-seismic hazard estimation including quantification of the ground motion amplification has been conducted at Ahmedabad city based on near-surface characterization/soil modeling. The city has experienced substantial damage in the course of the 2001 Bhuj earthquake. A total of 20 boreholes were drilled in the city up to the depths of 40–80 m. A five-fold methodology is adopted: (1) Assessment of the seismic perspective of the area under study, (2) demarcation of the engineering bed layer (EBL) through geophysical (seismic) surveys and the soil properties, (3) soil modeling using geotechnical and the geophysical parameters, (4) assessment of the strong ground motion at EBL through simulation considering far-field earthquake scenarios and near-field earthquakes scenario and (5) surface strong-motion estimation by ground response analysis based on equivalent-linear approach. The near-surface soil models were prepared from the borehole logs, shear-wave velocity estimated from the seismic survey and the soil properties like soil classification and density. The strong motion at EBL is computed by simulating seismotectonically justified scenario earthquakes through the stochastic finite-fault source modeling technique using the region-specific input parameters. The surface-strong motion is estimated by performing ground response analysis (with SHAKE) at every borehole using EBL-strong motion and prepared soil models. The EBL was found varying from 28 to 54 m in depth in Ahmedabad city. The effect of far-field and near-field earthquake sources was considered for assessing the hazard. To compensate for the uncertainty, a total of 108 and 81 input parametric combinations for near-field earthquake scenarios, and far-field earthquake scenarios, respectively have been considered for estimating the strong motion at EBL. The peak ground acceleration (PGA) of 52–111 cm/s2 and 108 cm/s2 are estimated at EBL due to near-field earthquake scenarios and far-field earthquake scenarios, respectively. The PGA through ground response analysis at surface level is found to be varying from 101 to 279 cm/s2 for near-field earthquake scenarios and, 118–161 cm/s2 for the far-field earthquake scenarios. The spectral acceleration (SA) (at surface level) has also been calculated for damping of 5%. The average SA distribution maps for 0.2 s (1–2 story), 0.55 s (4–5 story), 1 s (high rise) and 1.25 s period (large structures) have been prepared for both types of scenario earthquakes. The strong motion amplification is computed to be in the range of 1.6–3.3 for near-field earthquake scenarios and 2.2–3.0 for near-field earthquake scenarios.

Source Localization of EM Waves in the Near‐Field of Spherical Antenna Array in the Presence of Unknown Mutual Coupling

To obtain an antenna array with isotropic radiation, spherical antenna array (SAA) is the right array configuration. The challenges of locating signals transmitted within the proximity of antenna array have been investigated considerably in the literature. However, near‐field (NF) source localization of signals has hitherto not been investigated effectively using SAA in the presence of mutual coupling (MC). MC is another critical problem in antenna arrays. This paper presents an NF range and direction‐of‐arrival (DoA) estimation technique via the direction‐independent and signal invariant spherical harmonics (SH) characteristics in the presence of mutual coupling. The energy of electromagnetic (EM) signal on the surface of SAA is captured successfully using a proposed pressure interpolation approach. The DoA estimation within the NF region is then calculated via the distribution of pressure. The direction‐independent and signal invariant characteristics, which are SH features, are obtained using the DoA estimates in the NF region. We equally proposed a learning scheme that uses the source activity detection and convolutional neural network (CNN) to estimate the range of the NF source via the direction‐independent and signal invariant features. Considering the MC problem and using the DoA estimates, an accurate spectrum peak in the multipath situation in conjunction with MC and a sharper spectrum peak from a unique MC structure and smoothing algorithms are obtained. For ground truth performance evaluation of the SH features within the context of NF localization, a numerical experiment is conducted and measured data were used for analysis to incorporate the MC and consequently computed the root mean square error (RMSE) of the source range and NF DoA estimate. The results obtained from numerical experiments and measured data indicate the validity and effectiveness of the proposed approach. In addition, these results are motivating enough for the deployment of the proposed method in practical applications.

Multichannel Blind Source Separation Based on Evanescent-Region-Aware Non-Negative Tensor Factorization in Spherical Harmonic Domain

There is growing interest in new audio formats in the context of virtual reality (VR), and higher-order ambisonics (HOA) is preferred for VR systems to transmit recorded scenes owing to its transmission efficiency and its flexibility to work with different loudspeaker setups. However, the conversion between another well-known format, i.e., object format, and the HOA format is not fully addressed in the literature. To address this issue, blind source separation in a spherical harmonic (SH) domain can be considered as the best way to extract objects in terms of efficiency, i.e., decoding HOA signals for separation can be omitted. A few authors attempted to extract objects from encoded HOA signals directly by using multichannel non-negative matrix factorization (MNMF), but these approaches either assume only far-field sources or do not take array characteristics into account, which make these methods difficult to use for VR in practical situations where singers or speakers often perform close to microphones. Furthermore, MNMF generally requires a huge computational cost, although dimensional reduction to the SH domain is performed. In this work, we also model near-field sources by estimating the model parameters of non-negative tensor factorization (NTF) in the SH domain assuming that microphone signals can be obtained with a rigid spherical array. We propose a masking scheme to exclude noisy evanescent regions in the SH domain from the NTF cost function. Evaluations show that our method outperforms existing methods devised for the HOA format and that our masking approach is effective in improving the separation quality.

Semidefinite relaxation method for unified near-Field and far-Field localization by AOA

This paper investigates the unified near-field and far-field localization using angle-of-arrival measurements, where the knowledge that the source is in the near-field or far-field is not required. We propose a semidefinite relaxation method to solve this problem by using the modified polar representation of the source. By approximating the measurement model, we formulate the localization problem as a weighted least squares problem, which is a non-convex constrained optimization problem. This problem is then relaxed to a convex semidefinite program (SDP) by applying semidefinite relaxation. The relaxed SDP problem is tightened by adding a set of second-order cone constraints to reach a tighter mixed semidefinite and second-order cone program. The proposed method provides an accurate position estimate for the near-field source, and yields a precise direction of arrival estimate for the far-field source. To reduce the estimation bias introduced by the approximation of the measurement model, we further derive the analytical expression of the bias and subtract it from the solution to obtain an approximately unbiased estimate. Both simulations and theoretical analysis show that the proposed method reaches the Cramer-Rao lower bound when the noise is not large under Gaussian noise, and has a comparable bias with the maximum likelihood estimator.

Radiofrequency electromagnetic fields from mobile communication: Description of modeled dose in brain regions and the body in European children and adolescents

BackgroundLittle is known about radiofrequency electromagnetic fields (RF) from mobile technology and resulting dose in young people. We describe modeled integrated RF dose in European children and adolescents combining own mobile device use and surrounding sources. MethodsUsing an integrated RF model, we estimated the daily RF dose in the brain (whole-brain, cerebellum, frontal lobe, midbrain, occipital lobe, parietal lobe, temporal lobes) and the whole-body in 8358 children (ages 8–12) and adolescents (ages 14–18) from the Netherlands, Spain, and Switzerland during 2012–2016. The integrated model estimated RF dose from near-field sources (digital enhanced communication technology (DECT) phone, mobile phone, tablet, and laptop) and far-field sources (mobile phone base stations via 3D-radiowave modeling or RF measurements). ResultsAdolescents were more frequent mobile phone users and experienced higher modeled RF doses in the whole-brain (median 330.4 mJ/kg/day) compared to children (median 81.8 mJ/kg/day). Children spent more time using tablets or laptops compared to adolescents, resulting in higher RF doses in the whole-body (median whole-body dose of 81.8 mJ/kg/day) compared to adolescents (41.9 mJ/kg/day). Among brain regions, temporal lobes received the highest RF dose (medians of 274.9 and 1786.5 mJ/kg/day in children and adolescents, respectively) followed by the frontal lobe. In most children and adolescents, calling on 2G networks was the main contributor to RF dose in the whole-brain (medians of 31.1 and 273.7 mJ/kg/day, respectively). ConclusionThis first large study of RF dose to the brain and body of children and adolescents shows that mobile phone calls on 2G networks are the main determinants of brain dose, especially in temporal and frontal lobes, whereas whole-body doses were mostly determined by tablet and laptop use. The modeling of RF doses provides valuable input to epidemiological research and to potential risk management regarding RF exposure in young people.

Near-Field Localization for Non-circular Sources in the Presence of Sensor Phase Uncertainties

We address the problem of locating multiple near-field non-circular sources in the presence of sensor phase errors. Such phase error may arise in MIMO applications where perfect phase synchronization is not achieved. Using source non-circularity and based on the virtual ESPRIT idea, we define three cumulant matrices to form two matrix pencils, whose generalized eigenvalues are dependent only on the amplitude but not on the phase of the near-field propagation geometry. We introduce a linear operator to estimate the generalized eigenvalues of the defined matrix pencils, and then apply our recently proposed SPatial Amplitude Ratio based Estimator (SPARE) approach to estimate angle and range parameters of the sources. We define the presented algorithm as Rotational SPARE (R-SPARE) since it obtains the spatial amplitudes from the rotational structures embedded in the defined matrices. R-SPARE shares all advantages offered by the SPARE. We finally examine the performance of R-SPARE via numerical simulations.

Bearing and range estimation with an exact source‐sensor spatial model

In sensor array processing literature, near-field bearing and range estimation algorithms generally use spherical wavefront to model only array sensors’ phase response (sometimes with Fresnel approximation) and assume equality in amplitude response. Ignoring the range dependent amplitudes, though facilitating the algorithmic development, will cause systematic estimation errors due to model mismatch. By taking the spherical wavefront amplitude into account, a new bearing and range estimation algorithm for locating multiple near-field sinusoid sources is presented. With the estimation of the near-field sensor array's response vector, closed-form formulas for bearing and range estimates are derived from its magnitude, or phase, or both. The problem of estimation ambiguity is discussed as well. Cramér-Rao bound is also derived to serve as a benchmark for performance study.

A one-step algorithm for mixed far-field and near-field sources localization

In this paper, a one-step method is proposed for the classification and estimation of the parameters in the scenario of mixed near-field and far-field stationary sources. By constructing several temporal-spatial cumulant matrices, utilizing the stationary property of sources, and introducing a mechanism for controlling the rank, a parameters estimation matrix is formed. This matrix contains electric angles and signals information. By introducing a method based on the rotational invariant technique and using the estimation signal parameter, the direction-of-arrival of all sources' signals and the range of the near-field sources (NFSs) are estimated. Also, a reasonable classification of the signals types is provided. Unlike conventional methods, the proposed method does not require any spectral search stage, so it reduces the computational complexity to a great extent. The proposed algorithm prevents the array aperture loss, has no need to know the number of NFSs or field-field sources, and does not require any extra stage to pair the parameters. Mathematical analysis and simulation results validate the good performance of parameters estimation and sources classification in the proposed method.

DOA and Range Estimation Using a Uniform Linear Antenna Array Without A Priori Knowledge of the Source Number

A new efficient root-propagator method based on a uniform linear antenna array is proposed to estimate the directions of arrival (DOAs) and ranges of near-field sources without a priori knowledge of the source number. A non-Hermitian matrix is first designed, which allows the applications of rooting methods. Then, by constructing two propagators, the parameter estimation for near-field sources can be achieved through rooting methods in a decoupled way without eigenvalue decomposition (EVD) and spectrum searches. Finally, the importance of a priori source number to source localization problems is studied, and especially, an improvement is proposed to get rid of this dependence. Numerical examples are carried out to show the effectiveness and validate the theoretical analysis of the proposed root-propagator method. The results show that the proposed method provides more superior performance for near-field source localization than other existing similar methods. Even in the case without a priori knowledge of the source number, the performance of the proposed method is still satisfactory.

Localization for mixed far-field and near-field sources utilizing improved symmetric nested array

Nested arrays exhibit higher spatial resolution (SR) and enhanced degrees-of-freedom (DOFs) with fewer sensors, and they have been utilized for direction-of-arrival (DOA) estimation of both far-field (FF) and near-field (NF) sources. In this study, an improved symmetric nested array configuration with a given number of sensors was developed, which achieved increased consecutive and unique lags and thus resolved more targets than the actual number of inherent array sensors. In particular, the analytical expressions of the number of consecutive lags, the number of unique lags, and the virtual array aperture were derived for quantitative evaluation and comparison, as well as the corresponding array composition parameters of the optimal array geometry were obtained. In the mixed sources localization scheme, a special cumulant matrix was constructed to eliminate the range parameter in the NF steer vectors by exploiting the symmetric feature. Both subspace and sparse reconstruction techniques were exploited in order to directly obtain the DOAs of both FF and NF sources. With the estimated DOAs and the covariance matrix of the array output, the NF sources could be identified, and the corresponding range parameters could also be obtained by a one-dimension (1-D) spectrum search scheme. Numerous simulation results demonstrated that the proposed array showed a remarkable performance in terms of estimation accuracy, SR capacity, and numerous DOFs compared to state-of-the-art symmetric nested arrays.

A comparison between off and on-body control surfaces for the FW-H equation: Application to a non-compact landing gear wheel

This work is a contribution to the ongoing debate about the role of quadrupoles in low Mach number flows, studied through the use of solid and permeable surface Ffowcs-Williams & Hawkings (FW-H) integrals for landing gear numerical noise predictions. It rests upon the key idea that the dominance of surface sources over volume sources can only be guaranteed when the compact-source condition is met. We propose here to express this property as a condition on the smallness of the product between the Mach and Strouhal numbers, or formally as MSt < 1. We consider a canonical isolated wheel, that basically consists in a shallow circular cavity inscribed in a coin-like cylinder, thus presenting few different length scales, as compared to a full landing gear assembly. Zonal Detached Eddy Simulations are performed on several gradually refined grids to assess the grid convergence of the numerical result. In particular, important computational effort is put into the accurate resolution of acoustic waves up to Strouhal numbers such that MSt > 1. Overall, determining the real significance of quadrupoles was challenging as numerical errors or misleading effects such as near-field terms, surface discretization, or source domain truncation biased the initial permeable surface results. Even when these sources of bias are removed, non-negligible differences are found between the solid and permeable noise spectra at non-compact St values, while both formulations are equivalent at lower Strouhal numbers. The analysis of these differences is extended to the determination of their impact on frequency-domain noise maps obtained with the DAMAS algorithm fed with far-field signals computed with both FW-H approaches. A tentative interpretation of the wheel noise sources is finally proposed, highlighting the dominant role of the scattering of aerodynamic sources by the cavity downstream edge at low Strouhal numbers, while wake sources dominate at higher Strouhal numbers.

Improved underwater acoustic imaging with non‐uniform spatial resampling RL deconvolution

Underwater acoustic imaging (UAI) can be utilised to observe the spatial distribution of a near-field sound source. The image quality depends on the resolution and sidelobe level of conventional beamforming. The linear array-based UAI can be considered as deconvolution of a two-dimensional point spread function shift-variant model. The performance of UAI can be improved via innovative deconvolution algorithms. In this study, a non-uniform spatial resampling Richardson–Lucy (RL) fast algorithm is designed in which the amount of samples is determined by the power of the UAI output. This allows for a significant decrease in the number of samples compared to the traditional RL algorithm with similar positioning accuracy. Computer simulations and sea trials are performed to validate the effectiveness and feasibility of the proposed method.

A Geometric Calibration Method of Hydrophone Array Based on Maximum Likelihood Estimation with Sources in Near Field

Considering the requirement of the near-field calibration under strong underwater multipath condition, a high-precision geometric calibration method based on maximum likelihood estimation is proposed. It can be used as both auxiliary-calibration and self-calibration. According to the near-field geometry error model, the objective function of nonlinear optimization problem is constructed by using the unconditional maximum likelihood estimator. The influence of multipath on geometric calibration is studied. The strong reflections are considered as the coherent sources, and the compensation strategy for auxiliary-calibration is realized. The optimization method (differential evolution, DE) is used to solve the geometry errors and sources’ position. The method in this paper is compared with the eigenvector method. The simulation results show that the method in this paper is more accurate than the eigenvector method especially under high signal-to-noise ratio (SNR) and multipath environment. Experiment results further verify the effectiveness.

An efficient solver for the generalized normal modes of non-uniform open optical resonators

Modal expansion is an attractive technique for solving electromagnetic scattering problems. With the one set of resonator modes, calculated once and for all, any configuration of near-field or far-field sources can be obtained almost instantaneously. Traditionally applied to closed systems, a simple and rigorous generalization of modal expansion to open systems using eigenpermittivity states is also available. These open modes are suitable for typical nanophotonic systems, for example. However, the numerical generation of modes is usually the most difficult and time-consuming step of modal expansion techniques. Here, we demonstrate efficient and reliable mode generation, expanding the target modes into the modes of a simpler open system that are known. Such a re-expansion technique is implemented for resonators with non-uniform permittivity profiles, demonstrating its rapid convergence. Key to the method's success is the inclusion of a set of longitudinal basis modes.

Subspace and sparse reconstruction based near-field sources localization in uniform linear array

This work studies the near-field localization problem using a symmetry uniform linear array (ULA). To decouple the range and direction of arrival (DOA), by exploiting the symmetry property of the array, two spatial correlation sequences are constructed, where each sequence only corresponds to one parameter, i.e., DOA or range. After decoupling, an attractive property is that the resulting sequences still share the similar ULA spatial structure. To perform DOA estimation, two approaches have been developed. The first one is based on the power of R (POR) method, which obtains the noise subspace without the eigendecomposition and prior information of the number of sources. The second one is developed using atomic norm minimization, which eliminates the off-grid issue. For range estimation, since the constructed sequence that corresponds to the range parameter shares the same spatial structure with the DOA sequence, the developed approaches are readily applied to obtain the range estimates. The proposed approach is also studied under one-bit measurement to show its robustness. The numerical studies including simulation and real-world data demonstrate the performance of the proposed method.