Far-field Case Research Articles

Gridless three-dimensional acoustic imaging based on the concept of sonons: Reconstruction of source directivity and equivalent spatial distribution

Three-dimensional acoustic imaging with microphone arrays is a challenging task due to the numerous degrees of freedom of the physical processes at stake. Instead of meshing a region of space to identify acoustic source distributions, the aim of this work is to propose a novel gridless method to reconstruct the sound field by an equivalent set of point sources with unrestricted coordinates, named “acoustic sonons”. Their radiation is expected to reproduce the same acoustic properties as the measured acoustic field, such as its pressure level, directivity and spatial coherence by recovering phase information. It is found that the proposed concept of sonons, while being very flexible, effectively provides equivalent representations for a variety of source distributions. The simulation results show that the method can be adapted to diverse acoustic field situations, whether they are composed of elementary sources or intricate volumetric source densities. The problem is formulated within a probabilistic framework, through a hierarchical Bayesian model inferred by a dedicated Markov chain Monte Carlo algorithm. The performance of the method is evaluated on analytical radiation problems and its ability to reconstruct the directivity is tested for two far-field radiation cases.

Imaging a Moving Point Source from Multifrequency Data Measured at One and Sparse Observation Directions (Part I): Far-Field Case

Imaging a Moving Point Source from Multifrequency Data Measured at One and Sparse Observation Directions (Part I): Far-Field Case

Energy-Guided Multitarget Detection and Localization for Distributed MIMO Radar: Analysis and Solution

This article investigates the multitarget detection and localization problem for distributed multiple-input multiple-output radar. We first derive the optimal approach of this issue in the high-dimensional space according to the generalized likelihood ratio test (GLRT) and the maximum likelihood estimation. Moreover, the theoretical multitarget localization accuracy under the considered discrete time signal model is analyzed by invoking the Cram$\acute{\text{e}}$r–Rao lower bound and the sampling lower bound. To deal with the high complexity problem of the optimal solution, an energy-guided framework including two modules is proposed to detect and locate multiple targets. In the first module, the energy accumulation approach based on the GLRT is employed for multitarget detection, and a fast energy accumulation strategy in terms of the far-field case is developed to improve the computational efficiency. In the second module, an innovative iterative similarity evaluation method is designed to determine the positions of multiple targets by exploiting the relationship between the energy accumulation characteristics and the multitarget existence conditions. The effectiveness of the proposed framework is verified via extensive simulations in both far-field and near-field cases.

Channel Modeling for RIS-Assisted 6G Communications

Terahertz communication has been proposed as one of the basic key technologies of the sixth-generation wireless network (6G) due to its significant advantages, such as ultra-large bandwidth, ultra-high transmission rates, high-precision positioning, and high-resolution perception. In terahertz-enabled 6G communication systems, the intelligent reconfiguration of wireless propagation environments by deploying reconfigurable intelligent surfaces (RIS) will be an important research direction. This paper analyzes the far field and near field of RIS-assisted wireless communication and a detailed system description is presented. Subsequently, this paper presents a specific study of the channel model for an RIS-assisted 6G communication system in the far-field and near-field cases, respectively. Finally, an integrated simulation of the channel models for the far-field and near-field cases is carried out, and the performance of the RIS auxiliary link measured in terms of signal-to-noise ratio (SNR) is compared and analyzed. The results show that increasing the size of the RIS surface to improve the SNR is an effective method to enhance the coverage performance of the 6G THz communication system under the strong guarantee of the ultra-large bandwidth of THz.

Near-Field Spatial Correlation for Extremely Large-Scale Array Communications

Extremely large-scale array (XL-array) communications correspond to systems whose antenna sizes are so large that the scatterers and/or users may no longer be located in the far-field region. By discarding the conventional far-field uniform plane wave (UPW) assumption, this letter studies the near-field spatial correlation of XL-array communications, by taking into account the more generic non-uniform spherical wave (NUSW) characteristics. It is revealed that different from the far-field channel spatial correlation which only depends on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">power angular spectrum</i> (PAS), the near-field spatial correlation depends on the scattered power distribution not just characterized by their arriving angles, but also by the scatterers’ distances, which is termed as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">power location spectrum</i> (PLS). A novel integral expression is derived for the near-field spatial correlation in terms of the scatterers’ location distribution, which includes the far-field spatial correlation as a special case. The result shows that different from the far-field case, the near-field spatial correlation no longer exhibits <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spatial stationarity</i> in general, since the correlation coefficient for each pair of antennas depends on their specific positions, rather than their relative distance only. To gain further insights, we consider a generalized one-ring model for scatterer distribution, where the ring center may be flexibly located. Numerical results are provided to show the necessity of the near-field spatial correlation modelling for XL-array communications.

Effect of hull inelasticity on whipping responses by underwater explosions

ABSTRACT This paper aims to verify why the inelastic hull responses of buckling and plastic deformation should be considered in the underwater explosion (UNDEX) simulations. A battleship was discretized into the five longitudinal segments that were composed of Timoshenko beam elements. The number of elements was determined through convergence simulations of natural frequency. The inelastic nonlinear moment-curvature curves using Smith’s method were applied to the UNDEX models. The incompressible gas bubble model was chosen for the UNDEX simulations because the compressible one produced acceleration less conservatively than the incompressible one. The UNDEX simulations were performed for the far and less far stand-off distances. The bending moment in the inelastic model was less developed than those in the elastic model for two stand-off distances. More evident Inelastic effects were observed for the reduced stand-off distance. The inelastic effect should be considered in the UNDEX analyses for even far-field UNDEX cases.

Deep neural network-based generalized sidelobe canceller for dual-channel far-field speech recognition

The traditional generalized sidelobe canceller (GSC) is a common speech enhancement front end to improve the noise robustness of automatic speech recognition (ASR) systems in the far-field cases. However, the traditional GSC is optimized based on the signal level criteria, causing it not to guarantee the optimal ASR performance. To address this issue, we propose a novel dual-channel deep neural network (DNN)-based GSC structure, called nnGSC, which is optimized by using the objective of maximizing the ASR performance. Our key idea is to make each module of the traditional GSC fully learnable and use the acoustic model to perform joint optimization with GSC. We use the coefficients of the traditional GSC to initialize nnGSC, so that both traditional signal processing knowledge and large amounts of data can be used to guide the network learning. In addition, nnGSC can automatically track the target direction-of-arrival (DOA) frame-by-frame without the need for additional localization algorithms. In the experiments, nnGSC achieves a relative character error rate (CER) improvement of 23.7% compared to the microphone observation, 13.5% compared to the oracle direction-based super-directive beamformer, 12.2% compared to the oracle direction-based traditional GSC and 5.9% compared to the oracle mask-based minimum variance distortionless response (MVDR) beamformer. Moreover, we can improve the robustness of nnGSC against array geometry mismatches by training with multi-geometry data.

Time-Domain PEEC Transient Analysis for a Wire Structure Above the Perfectly Conducting Ground With the Incident Field From a Distant Lightning Channel

This paper presents the time-domain partial element equivalent circuit (PEEC) analysis of induced transients in a wire structure during an indirect lightning strike. The full-wave PEEC formulation with an external incident field is employed. The incident field is calculated with either the Uman's or the Jefimenko's formula by using the engineering model of a lightning channel. An instability problem is, however, observed. It becomes a hurdle in the application of PEEC for evaluating induced transient currents in closed wire loops. The problem is investigated, and is primarily caused by discretization of the electric field in numerical evaluation of external voltage sources. An algorithm is provided to solve this instability problem, and is tested for both near- and far-field cases. It is validated by the finite-difference time-domain method. The issue of time step and segment length is addressed. It is also found that the Uman's formula is generally incapable of producing stable induced currents in a wire loop structure. Finally, the proposed PEEC procedure is applied to evaluate induced lightning transients in a photovoltaic panel during an indirect lightning strike.

Circular array design based on Bayesian Cramer–Rao bound

In this paper, we consider circular array design in the presence of a far-field or a near-field signal source. The location of the source is introduced to our optimization problem by its probability density function (PDF) as a priori information. We consider Bayesian Cramer–Rao bound as the cost function to be minimized to specify the best locations of the sensors on a circular boundary. In the far-field case, a closed form solution is derived for an arbitrary PDF of the bearing. In the near-field scenario, we divide the design problem into three categories: known source bearing, known source range, and the general case. We present some examples to exhibit the process of array design by the proposed method. Finally, we show the array optimized by the proposed method outperforms arrays with other configurations in source localization.

A note on near-field site amplification effects of ground motion from a radially inhomogeneous valley

To improve the understanding of the near-field soil and topographic amplification effects, an analytical solution by the authors for the scattering of plane SH waves by a radially inhomogeneous semi-cylindrical valley is extended to the case of a line source of cylindrical SH waves. Upon confirmation of its accuracy with past exact solutions for a homogeneous and an inhomogeneous semi-cylindrical valley under far-field plane SH waves, the extended solution is used to calculate the ground motion amplification factors for both the homogeneous and inhomogeneous valleys subjected to near-field waves. A comprehensive parametric study is conducted with respect to the location of the wave source, the dimensionless frequency of the incident waves, and the inhomogeneity degree of the covering soil layer. It is found that more amplifications and reductions of ground motions will occur within a certain range in and around the valley as the sources are located further. Consistent with the far-field case, it is confirmed that an increase of the degree of inhomogeneity of the covering soil layer generally amplifies the ground motions significantly.

Shared secret key generation from signal fading in a turbulent optical wireless channel using common-transverse-spatial-mode coupling

Common randomness arising from turbulence-induced signal fading in reciprocal optical wireless channels is a beneficial resource that can be used to generate secret keys shared by two legitimate parties. The concept of optical wireless channels using common-transverse-spatial-mode coupling (CTSMC) that can maintain perfect fading reciprocity in atmospheric turbulence is first developed in a general manner. Subsequently, by performing Monte Carlo simulations, the Johnson SB probability distribution is demonstrated to be appropriate for statistical description of turbulence-induced signal fading in an optical wireless channel constructed by use of two identical CTSMC transceivers, and the nature of correlation between signal fadings detected by two contiguous reception spatial modes is further quantitatively characterized, revealing that rapid spatial decorrelation between signal fadings observed by a legitimate party and an eavesdropper holds for scenarios of practical interest. Finally, the information theoretic capacity for generating secret keys from CTSMC-based optical wireless channels is theoretically formulated and quantitatively examined under different conditions, manifesting that the turbulence strength and average electrical signal-to-noise ratio have a noticeable combined impact on the secret key capacity, especially in the far-field case.

Talbot carpet at the transverse plane produced in the diffraction of plane wave from amplitude radial gratings.

We experimentally demonstrate and theoretically predict a new and unprecedented optical carpet that includes all the geometric shadow and far-field and near-field diffraction patterns at the transverse plane in the diffraction from a radial grating illuminated by a plane wavefront. The main feature of using radial grating is the continuous change of the spatial period along the radial direction. Therefore, the geometric shadow, and the near-field and far-field diffraction regimes are mixed at various propagation distances, and the traditional definitions for the different diffraction regimes would not apply here. We show that for a given propagation distance, at a certain radial distance the shadow regime changes to the near-field regime and at another certain radial distance, the diffraction pattern changes from a near-field to a far-field case.

Uniform refraction in negative refractive index materials.

We study the problem of constructing an optical surface separating two homogeneous, isotropic media, one of which has a negative refractive index. In doing so, we develop a vector form of Snell's law, which is used to study surfaces possessing a certain uniform refraction property, in both the near- and far-field cases. In the near-field problem, unlike the case when both materials have positive refractive indices, we show that the resulting surfaces can be neither convex nor concave.

Multiple 3D Far-Field/Near-Field Moving Target Localization Using Wideband Echo Chirp Signals

In this paper, we propose a technique for active detection and estimation of the range, radial velocity and direction-of-arrival (DOA) of multiple moving targets in the three-dimensional space for both far-field and near-field cases using wideband chirp signals. The proposed technique is an extension of the method recently proposed by Leong in [“Multiple target localization using wideband echo chirp signals,” IEEE Trans. Signal Process., vol. 61, no. 16, pp. 4077-4089, Aug. 2013] for the localization of multiple far-field targets in the two-dimensional space. In our approach, the received signals at a sensor array are first decomposed into the spherical harmonic components and a modal preprocessing step is then carried out on the received wideband signals so that they can be further processed using narrowband techniques for parameter estimation. The application of fractional Fourier transform on the modal preprocessed signals allows the proposed method to identify closely spaced targets. This is demonstrated via computer simulations where the proposed technique achieved low root-mean-square errors in the parameters estimated in a scenario with closely spaced targets.

A Broadband Model of the Characteristic Currents for Rectangular Plates

This paper presents an approximate model of the characteristic currents for rectangular plates. This model provides a broadband description of the characteristic current distributions, which could be used for the far-field or near-field cases. For narrow rectangular plates, a modified cosine function and sine function are adapted for the odd and even modes, respectively. As the narrow case is fundamentally a 1-D problem, the presented solution could be scaled for arbitrary combinations of plate length and operating frequency. For wide rectangular plates, each characteristic current is decomposed into four parts along the plate edges, which decay exponentially toward the opposite edges. Variance of the characteristic currents with plate dimension is observed and briefly discussed. The frequency-dependent but axis-independent mode coefficients are utilized and numerically evaluated to verify the proposed model. The modal solutions of two rectangular plates that are illuminated by a normally incident planar wave are given and compared with the full-wave solutions. The modal solutions composed of the lowest four terms provide an explicit description of the surface currents with reasonable accuracy. Axis dependence of the mode coefficients is briefly discussed, and the first-order expression contributes an improved accuracy when the mode number is larger than 1.

Real Time High Accuracy 3-D PHAT-Based Sound Source Localization Using a Simple 4-Microphone Arrangement

This paper investigates wideband sound source localization in outdoor cases. In such cases, time difference of arrival (TDOA)-based methods are commonly used for 2-D and 3-D wideband sound source localization. These methods have lower accuracy in comparison with direction of arrival-based approaches. However, they feature fewer microphones and less computation time. High accuracy sound source localization using these methods needs highly accurate time delay measurement, and therefore, high frequency signal sampling rates. Moreover, the need to use numerical analysis methods for local calculations (solving nonlinear equations of closed-form methods) will increase computation time while the calculations may still not converge. Also, a good initial guess close to the true solution is needed to avoid local minima. In this paper, a simple, fast (real time) and accurate pure geometric phase transform-based exact location calculation approach for 3-D localization of wideband sound sources in outdoor far-field and low degree reverberation cases, using only four microphones, is proposed. Based on the proposed method, a simple arrangement of microphones is implemented. Experimental results show that the proposed method has more accuracy and less computation time simultaneously, in comparison with previous closed-form hyperbolic intersection and other TDOA-based state-of-the-art location calculation methods, overcoming their major weaknesses. Also, as the nonlinear closed-form equations are linearized, no initial guess is required. It features less than 0.2° error for angle of arrival, less than 5% error for 3-D location finding, and computation times as low as 250 ms for the localization of a typical wideband sound source such as a flying object (helicopter).

Analytical vectorial structure of Bessel–Gauss beam in the near field

Based on the vectorial angular spectrum representation of electromagnetic beams starting from Maxwell’s equations and the paraxial approximation, analytical expressions for the TE and TM terms of a linearly polarized Bessel–Gauss beam diffracted in the near field are derived. Effects of radial components, beam orders and styles of orbital angular momentum (OAM) distributions on the internal vectorial structures of the beam are separately analyzed by numerical examples. It is shown that the near-field distributions of the TE and TM terms of the beam are totally different from those studied for the far-field case. Within the near-field region, the propagating electric field components for TE and TM terms would no longer remain orthogonal to each other because of the emergence of z-component terms except for the incidence of zero-order Bessel-Gauss beam.

A geometric approach to multi-view compressive imaging

In this paper, we consider multi-view imaging problems in which an ensemble of cameras collect images describing a common scene. To simplify the acquisition and encoding of these images, we study the effectiveness of non-collaborative compressive sensing encoding schemes wherein each sensor directly and independently compresses its image using randomized measurements. After these measurements and also perhaps the camera positions are transmitted to a central node, the key to an accurate reconstruction is to fully exploit the joint correlation among the signal ensemble. To capture such correlations, we propose a geometric modeling framework in which the image ensemble is treated as a sampling of points from a low-dimensional in the ambient signal space. Building on results that guarantee stable embeddings of manifolds under random measurements, we propose a manifold algorithm for recovering the ensemble that can operate even without knowledge of the camera positions. We divide our discussion into two scenarios, the near-field and far-field cases, and describe how the lifting algorithm could be applied to these scenarios. At the end of this paper, we present an in-depth case study of a far-field imaging scenario, where the aim is to reconstruct an ensemble of satellite images taken from different positions with limited but overlapping fields of view. In this case study, we demonstrate the impressive power of random measurements to capture single- and multi-image structure without explicitly searching for it, as the randomized measurement encoding in conjunction with the proposed lifting algorithm can even outperform image-by-image transform coding.

Enhanced scattering of acoustic waves at interfaces

We propose a general method to realize a total scattering of an incident acoustic wave at interfaces between different media while allowing the flow of air, fluids and/or particles. This originates from the enlargement of the equivalent acoustic scattering cross section of an embedded object coated with acoustic metamaterials, which causes the coated object to behave as a scatterer bigger than its physical size. We theoretically design a model circular cylindrical object coated with such metamaterials whose properties are determined according to two different, but identical, methods. The desired function is confirmed for both far-field and near-field cases with full wave simulations based on the finite element method. This work reveals a promising way to achieve noise shielding and naval camouflage.

Conditional and Unconditional Cramér–Rao Bounds for Near-Field Source Localization

Near-field source localization problem by a passive antenna array makes the assumption that the time-varying sources are located near the antenna. In this context, the far-field assumption (i.e., planar wavefront) is, of course, no longer valid and one has to consider a more complicated model parameterized by the bearing (as in the far-field case) and by the distance, named range, between the source and a reference coordinate system. One can find a plethora of estimation schemes in the literature, but their ultimate performance in terms of mean square error (MSE) have not been fully investigated. To characterize these performance, the Cramer-Rao bound (CRB) is a popular mathematical tool in signal processing. The main cause for this is that the MSE of several high-resolution direction of arrival algorithms are known to achieve the CRB under quite general/weak conditions. In this correspondence, we derive and analyze the so-called conditional and unconditional CRBs for a single time-varying near-field source. In each case, we obtain non-matrix closed-form expressions. Our approach has two advantages: i) due to the fact that one has to inverse the Fisher information matrix, the computational cost for a large number of snapshots (in the case of the conditional CRB) and/or for a large number of sensors (in the case of the unconditional CRB), of a matrix-based CRB can be high while our approach is low and ii) some useful information can be deduced from the behavior of the bound. In particular, an explicit relationship between the conditional and the unconditional CRBs is provided and one shows that closer is the source from the array and/or higher is the signal carrier frequency, better is the range estimation.