Point Source Assumption Research Articles

Intelligent Reflecting Surface for MIMO VLC: Joint Design of Surface Configuration and Transceiver Signal Processing

With the capability of reconfiguring the wireless electromagnetic environment, intelligent reflecting surface (IRS) becomes a new paradigm for designing future wireless communication systems. In this paper, we consider optical IRS for improving the performance of visible light communication (VLC) under a multiple-input and multiple-output (MIMO) setting, where the mean square error (MSE) of the IRS-aided MIMO VLC is minimized by jointly designing the IRS and transceiver signal processing. To this end, the MIMO channel gains of the IRS-aided VLC are first derived under the point source assumption, based on which the MSE minimization problem is formulated subject to the emission power constraints and the IRS configuration constraints. Next, we propose an alternating optimization algorithm, which decomposes the original problem into three subproblems, to iteratively optimize the IRS configuration, the precoding and detection matrices for minimizing the MSE. Moreover, theoretical analysis on the performance of the proposed algorithm in high and low signal-to-noise ratio (SNR) regimes is investigated, revealing that the joint optimization process can be simplified in such special cases, and the algorithm’s convergence property and computational complexity are also discussed. Finally, numerical results show that IRS-aided schemes significantly reduce the MSE as compared to their counterparts without IRS, and the proposed algorithm outperforms other baseline schemes.

Ultrasound computed tomography based on full waveform inversion with source directivity calibration

Ultrasound computed tomography based on full waveform inversion has the potential to provide high-resolution images of human tissues in a quantitative manner. A successful ultrasound computed tomography system requires the decent knowledge of acquisition array, including the spatial position and the directivity of each transducer, to meet the high-level demand of clinical applications. The conventional full waveform inversion algorithm assumes a point source with the omni-directional emission. Such assumption does not hold when the directivity of emitting transducer is not negligible. For a practical implementation, an efficient and accurate self-checking evaluation of directivity is crucial prior to the reconstruction of images. We propose to measure the directivity of each emitting transducer using the full-matrix captured data obtained with a water-immersed and target-free experiment. We introduce the weighted virtual point-source array to act as the proxy of emitting transducer during the numerical simulation. The weights of different points in the virtual array can be calculated from the observed data using the gradient-based local optimization method. Although the full waveform imaging method relies on the finite-difference solver of wave equation, such directivity estimation benefits from the introduction of analytical solver. The trick significantly reduces the numerical cost, enabling an automatic directivity self-check at boot. We verify the feasibility, efficiency, and accuracy of the virtual array method through simulated and experimental tests. For the experimental test, we also illustrate that full waveform inversion with directivity calibration can reduce the artifacts introduced by the conventional point source assumption, improving the quality of reconstructed images..

Configuring Reconfigurable Intelligent Surface for Parallel MIMO Visible Light Communications with Asymptotic Capacity Maximization

As an emerging technology, reconfigurable intelligent surface (RIS) has attracted increasing attention due to the ability to configure the wireless propagation environment. Parallel multiple-input multiple-output (MIMO) visible light communication (VLC) is suitable for some scenarios, e.g., multi-user time-division multiple access for indoor VLCs. The application of RIS in VLC is a promising trend in the future, and this paper investigates the employment of RIS in a parallel MIMO VLC system with asymptotic capacity maximization. To this end, the RIS-aided parallel MIMO VLC channel model is derived in this paper under the point source assumption, and then, as for the non-convex optimization problem, instead of searching exhaustively for the optimal solution, we propose a low-complexity near-optimal relaxing particle swarm optimization (RPSO) algorithm to optimize the RIS parameters. Meanwhile, we discover that the convex relaxation is an effective method for the optimization problems. Finally, numerical results indicate the effectiveness of the proposed design of RIS, and meanwhile, the asymptotic capacity can be improved to a great degree by adopting RIS in the parallel MIMO VLC system.

Feasibility of collagen matrix tiles with cesium-131 brachytherapy for use in the treatment of head and neck cancer

Locoregional failure is a unique and challenging problem in head and neck cancer with controversy surrounding the use of re-irradiation in the treatment. We aimed to evaluate the dosimetry and technical parameters in utilizing a collagen matrix with embedded Cesium-131 (Cs-131) radioactive isotope seeds as it relates to dose distribution and dose to carotid artery. Cadaveric feasibility study randomizing Cs-131 strands alone or Cs-131 with collagen matrix to be placed into neck dissection defects. For the dose computation, physicists employed the TG-43 dosimetry calculation algorithm with a point source assumption to compute the dose. Carotid arteries were contoured in MIM-Symphony software and the carotid artery maximum and mean doses were calculated in accordance with TG-43 specifications. Ease of use of collagen matrix tiles on a 7-point Likert scale and mean radiation dose to the carotid artery. Ease of use score was higher in collagen matrix compared to stranded seeds with a mean score of 6.3 +/- 1.2 compared to 4.5 +/- 0.87. Time of implantation was statistically significantly, p = 0.031, lower in the collagen matrix group (M = 5.17 min, SD = 4.62) compared to stranded seeds (M = 15.83 min, SD = 3.24). Mean radiation dose to the carotid artery was 62.8 Gy +/- 9.46 in the collagen matrix group compared to 108.2 Gy +/- 55.6 in the traditional Cs-131 seeds group. We present a feasibility and concept cadaveric study using a collagen matrix with Cesium-131 demonstrating preliminary evidence to support its ease of use, decreased time to implantation, and decreased dose delivered to the carotid artery.

Joint Resource Management for Intelligent Reflecting Surface–Aided Visible Light Communications

Though visible light communication (VLC) is a significant supplement to current communication technologies, disadvantages such as the sensitivity to obstacles limit its development and commercialization. As a revolutionizing technology, intelligent reflecting surface (IRS) offers an ability to reconfigure the wireless environment dynamically and passively, which is considered beneficial to improve the performance of VLC. This paper devotes to investigating the effect of VLC IRS and putting forward a joint resource management method for an instantaneous IRS-aided VLC system. To this end, the line-of-sight (LoS) and non-LoS channel gains are first discussed under the point source assumption, after which the system model is established and the optimization problem is formulated. Then, the frozen variable algorithm and minorization-maximization algorithm are proposed to iteratively maximize the overall spectral efficiency (SE), and detailed discussions on the weak/severe interference cases and computational complexity analysis are carried out. Moreover, numerical results are provided to show the improvement of SE and the effects of the proposed algorithms, which offers beneficial insights on joint resource management of IRS-aided VLC.

Optimization on Multiuser Physical Layer Security of Intelligent Reflecting Surface-Aided VLC

This letter investigates physical layer security in intelligent reflecting surface (IRS)-aided VL communication (VLC). Under the point source assumption, we first elaborate the system model in the scenario with multiple legitimate users and one eavesdropper, where the secrecy rate maximization problem is transformed into an assignment problem by objective function approximation. Then, an iterative Kuhn-Munkres algorithm is proposed to optimize the transformed problem, and its computational complexity is in the second-order form of the numbers of IRS units and transmitters. Moreover, numerical simulations are carried out to verify the approximation performance and the VLC secrecy rate improvement by IRS.

Sum Rate Maximization for Intelligent Reflecting Surface-Aided Visible Light Communications

With the promotion of the solid-state lighting industry, visible light communication (VLC) has attracted increasing interests these years. However, the blockage problems in VLC due to the nanoscale wavelength of the visible light are severe. In this letter, a VLC system deployed with the intelligent reflecting surface (IRS) is modeled under the point source assumption, where both line-of-sight and specular non-line-of-sight paths are considered. By introducing a discrete matrix, the resource allocation process is simplified into a binary programming problem, and then we propose a low-complexity algorithm to maximize the achievable sum rate. Moreover, numerical results demonstrate that IRS can improve the rate performance and ease blockage problems of VLC.

Direct Design of Thin and High-Quality Direct-Lit LED Backlight Systems

The direct-lit LED backlight is a key component in large-area LCD displays. The existing methods which rely on point source assumption and irradiance feedback are less effective in compact designs in near field, and designing thin and high-quality back-lit LED backlights still remains a challenging issue. In this paper, we develop a direct method for designing thin and high-quality back-lit LED backlight systems. The étendue of LED light sources is considered in the numerical calculation of the lens profiles without cumbersome Monte Carlo ray tracing, which allows us to obtain compact LED lighting modules and achieve accurate control of the near-field irradiance distribution of the backlight system. Two examples are presented to demonstrate the elegance of this method in designing thin and high-quality back-lit LED backlights.

Escape and Accretion by Cratering Impacts: Formulation of Scaling Relations for High-speed Ejecta

Abstract Numerous small bodies inevitably lead to cratering impacts on large planetary bodies during planet formation and evolution. As a consequence of these small impacts, a fraction of the target material escapes from the gravity of the large body, and a fraction of the impactor material accretes onto the target surface, depending on the impact velocities and angles. Here, we study the mass of the high-speed ejecta that escapes from the target gravity by cratering impacts when material strength is neglected. We perform a large number of cratering impact simulations on a planar rocky target using the smoothed particle hydrodynamics method. We show that the escape mass of the target material obtained from our numerical simulations agrees with the prediction of a scaling law under a point-source assumption when v imp ≳ 12v esc, where v imp is the impact velocity and v esc is the escape velocity of the target. However, we find that the point-source scaling law overestimates the escape mass up to a factor of ∼70, depending on the impact angle, when v imp ≲ 12v esc. Using data obtained from numerical simulations, we derive a new scaling law for the escape mass of the target material for v imp ≲ 12v esc. We also derive a scaling law that predicts the accretion mass of the impactor material onto the target surface upon cratering impacts by numerically evaluating the escape mass of the impactor material. Our newly derived scaling laws are useful for predicting the escape mass of the target material and the accretion mass of the impactor material for a variety of cratering impacts that would occur on large planetary bodies during planet formation.

Double-Sided Freeform Lens for Light Collimation of Light Emitting Diodes

A double-sided freeform lens is proposed for collimating light emitted from light emitting diodes (LEDs). The surface profiles of the lens are mathematically characterized and precisely determined based on a point-source assumption and differential geometry theory. The proposed lens design method is straightforward, flexible, and effective. Moreover, the optical performance of the lens can be intuitively adjusted by tuning just a small number of design parameters. The simulation results showed that the proposed lens achieved an excellent collimating effect for a commercial ultraviolet (UV) LED. A prototype lens is fabricated in UV-grade poly(methyl methacrylate) material using a standard injection molding process. The light collimating effect of the lens/UV-LED assembly was measured experimentally and was shown to be in good agreement with the simulation results. The collimating angle at the half-energy level was equal to 1.88°. The performance of the UV-LED is thus comparable to that of conventional lithography UV light sources based on mercury arc lamps. Consequently, the proposed double freeform lens showed significant potential for photolithography applications within the industry.

Complex seismic sources in volcanic environments: Radiation modelling and moment tensor inversions

Long period (LP) signals are special seismic events observed at volcanoes, which comprise both a high frequency onset due to brittle failure and a more energetic low frequency part due to resonance in a fluid-filled conduit. They are critical for volcano monitoring since they can be used as a volcanic forecasting tool. Classic seismology assumes planar faults for seismic sources; however, there is increasing evidence that suggests different fault shapes such as dyke faults and ring faults. We consider in this study narrow dykes and conduits rather than large calderas, hence, we model these complex sources by superposing vertical single double couple (DC) sources arranged along narrow fault structures with inner upward movement. We calculate seismic radiation patterns and synthetic seismograms for a rupture along a dyke, three different partial ring ruptures and a full-ring rupture. Results show that planar faults are the most effective at radiating energy. The more the source geometry deviates from a planar fault the smaller become the amplitudes and therefore the Moment Magnitudes. For example, the amplitudes decrease to 2.4% of the planar radiation for a full-ring rupture and to 0.7% for a dyke rupture. The waveforms produced by partial ring ruptures are in accordance to what is expected in the far field, representing the derivative of the source displacement and emulating radiation of a DC with different azimuths; however, the dyke and full-ring sources produce waveforms that appear to represent the second derivative of the source displacement and negative first onset polarisations. Moment Tensor Inversions support similarities between DC ruptures and partial ring ruptures; however, they show ambiguous solutions for the other sources. This point source assumption can lead to misinterpretations of slip history on the fault and a consistent underestimation of magnitudes which has direct implications for magma ascent estimations derived from seismic amplitudes.

UV-LED radiation modeling and its applications in UV dose determination for water treatment

Abstract With the growing application of the ultraviolet light emitting diodes (UV-LED) for water treatment purposes, the significance of modeling their output for designing UV reactors becomes crucial. In this study, the radiation distributions of two UV-LEDs with different radiation profiles are modeled. These UV-LEDs represent the two most common radiation profiles of UV-LEDs – balloon shape and heart shape. Irradiance and fluence rate distribution were predicted and experimentally validated by measuring irradiance through an accurate radiometry setup and determining fluence rate by iodide-iodate actinometry. The consistency of the results among the actinometry, radiometry, and the model proves the reliability of the model. The model helps demonstrate the extend to which the irradiance distribution and average fluence rate are affected as a function of distance and how they are affected by the UV-LED’s radiation profile. For UV-LEDs with any radiation profiles, higher average fluence rate occurs at closer distances to the UV-LED; however, to study the microbiological inactivation kinetics, uniformity of the radiation distribution on the surface of the petri dish has to be taken into account. Using the validated model, common radiation modeling presumptions such as the point source assumption and symmetry assumption for radiation profile of UV-LEDs are evaluated.

Fast computation of far-field pulse-echo PSF of arbitrary arrays for large sparse 2-D ultrasound array design

Large wideband two-dimensional (2-D) arrays are essential for high-resolution three-dimensional (3-D) ultrasound imaging. Since the tremendous element number of a full sampled large 2-D array is not affordable in any practical 3-D ultrasound imaging system, it is necessary to reduce the element number through sparse 2-D array design. Sparse array design requires that both the positions and weights of the array elements should be arbitrarily alterable. Hence a proper evaluation tool that can deal with arbitrary array is integral to optimizing the array structure and apodization function. It is known that pulse-echo point spread function (PSF) has been a common tool used to evaluate the performance of wideband arrays in ultrasound imaging all along, which also plays an important role in wideband ultrasound simulations. However, so far the conventional ultrasound simulation tools can only calculate pulse-echo PSF of arbitrary wideband arrays in the time domain because of the existence of nonuniform nodes in the spatial impulse response expressions, which obstructs their application of FFT to do fast computation of the time-domain convolutions. As a result, ultra-high time consumption of pulse-echo PSF computation of a large arbitrary wideband array hampers it to be taken as the evaluation tool by any stochastic optimization methods which need massive iterations in designing large sparse 2-D arrays. This paper aims to make available the pulse-echo PSF tool in designing large sparse 2-D arrays by proposing a fast computation method of far-field pulse-echo PSFs of arbitrary wideband arrays. In the paper, fast computation of wideband spatial impulse responses of a 2-D array is first realized in frequency domain by employing the nonuniform fast Fourier transform (NUFFT), under the point source assumption in far-field. On the basis of that, fast computation of time-domain convolutions is made possible by using FFT. In addition, a short inverse FFT (IFFT) is applied in recovering the time-domain envelopes rather than the detailed waveforms of beam pulses to extract the pulse-echo PSF, which further accelerates the computation. Compared with the computation speed of the time domain method, i.e. Field II, the proposed method achieves an improvement of three orders of magnitude with comparable accuracy for a 100×100 wideband 2-D array. The proposed method makes it possible for applying stochastic optimization methods to design arbitrary large wideband 2-D arrays using pulse-echo PSF as the evaluation tool.

Robustness of the coherently distributed MUSIC algorithm to the imperfect knowledge of the spatial distribution of the sources

The MUltiple SIgnal Classification (MUSIC) estimator has been widely studied for a long time for its high resolution capabilities in the domain of the directional of arrival (DOA) estimation, with the sources assumed to be point. However, when the actual sources are spatially distributed with angular dispersion, the performance of the conventional MUSIC is degraded. This paper deals with the sensitivity of MUSIC to modeling error due to coherently distributed (CD) sources. A performance analysis of an extended MUSIC taking into account a generalized steering vector based on a CD source model (CD-MUSIC) is first studied. We establish closed-form expressions of the DOA estimation bias and mean square error due to both the model error and the effects of a finite number of snapshots. The aim of this paper is also to determine when the point source assumption is acceptable for standard MUSIC. The analytical results are validated by numerical simulations and discussed in different configurations.

Compact collimators designed with point approximation for light-emitting diodes

We present a novel freeform lens design method for application to light-emitting diode collimating illumination. The lens is designed with a point-source assumption to form compact collimators. The method is derived from a basic geometric-optics analysis and the associated construction approach. By using this method, a compact collimating lenses with an aspect ratio = 0.181 is developed. With optimization of initial parameters, a highly collimating lens for a Cree XP-E LED (chip size: 1.2 mm × 1.2 mm) with an optical efficiency of 88.5% under a beam angle of ± 1.9° is constructed by simulation. To verify the practical performance of the lens, a prototype of the collimator lens is also made. This has light distribution compatible with the simulation results.

Estimation of directions of arrival of multiple distributed sources for nested array

We consider the problem of estimating the nominal directions of arrival (DOA) of multiple distributed sources using nested array. In practical applications, angular spread of distributed source arises due to the multipath propagation and local scattering around the vicinity of a source. DOA estimation methods based on point-source assumption are known to severely degrade in the presence of the angular spread. To solve this problem, we develop a distributed source model based approach for a nested array. Based on the concept of difference co-array, redundancy averaging technique is applied to the sample covariance matrix. By exploiting the annihilating property of the redundancy averaged terms, an efficient method based on annihilating filter (AF) is proposed. This method can be applied to the case of more sources compared with the actual physical sensors without a priori knowledge of the spreading parameters. Whereas the sample covariance matrix is corrupted by either noise or estimation error due to finite snapshots, so an AF method with structured low rank approximation (SLRA) is further proposed, which results in a better estimate. Finally, numerical simulations demonstrate the effectiveness of the proposed methods.

Reprint of : Dynamics of a quantum wave emitted by a decaying and evanescent point source

We put forward a model that describes a decaying and evanescent point source of non-interacting quantum waves in 1D. This point-source assumption allows for a simple description that captures the essential aspects of the dynamics of a wave traveling through a classically forbidden region without the need to specify the details of the inner region. The dynamics of the resulting wave is examined and several characteristic times are identified. One of them generalizes the tunneling time-scale introduced by Büttiker and Landauer and it characterizes the arrival of the maximum of the wave function. Diffraction in time and deviations from exponential decay are also studied. Here we show that there exists an optimal injection frequency and detection point for the observation of these two quantum phenomena.

Improved Multi-Bernoulli Filter for Extended Stealth Targets Tracking Based on Sub-Random Matrices

In this paper, we address a radar detection and tracking of multi-stealth targets. Multi-Bernoulli filter is a provable optimal Bayes approach of multi-target filtering and shows excellent performance over the other filters. The original multi-Bernoulli filters rely on the assumption that the interest targets are point source objects. Normally, in realistic scenarios, the point source assumption is not suitable for estimating the extent stealth targets (ESTs) scenarios. Recently, the random matrices approach has been proposed for ellipsoidal extended object tracking by additional state variables. In multiple EST scenarios, although the extension ellipsoid is efficient, it may not be accurate enough because of lacking useful information, such as size, shape, and orientation. To this point, we introduce a non-ellipsoidal EST composed of multiple ellipsoidal sub-objects, and each one is represented by a random matrix. Based on such models, a multi-Bernoulli filter is used to estimate kinematic states and extensions of sub-objects for each EST. However, in multiple EST scenarios, the detection profile is prior unknown due to fluctuation of EST parameters. The parameter, such as detection probability, is of critical importance in this extended filter. To tackle these problems, a beta Gaussian inverse Wishart implementation is proposed to estimate the EST extent augment with unknown detection probability and kinematic state. The results show that the proposed filter has more accurate cardinality estimation and smaller optimal sub-pattern assignment errors than the recently extended probability hypothesis density (PHD) and cardinalized PHD filters.

Dynamics of a quantum wave emitted by a decaying and evanescent point source

We put forward a model that describes a decaying and evanescent point source of non-interacting quantum waves in 1D. This point-source assumption allows for a simple description that captures the essential aspects of the dynamics of a wave traveling through a classically forbidden region without the need to specify the details of the inner region. The dynamics of the resulting wave is examined and several characteristic times are identified. One of them generalizes the tunneling time-scale introduced by Büttiker and Landauer and it characterizes the arrival of the maximum of the wave function. Diffraction in time and deviations from exponential decay are also studied. Here we show that there exists an optimal injection frequency and detection point for the observation of these two quantum phenomena.

The effect of mesh division of a volume source on calculated radiation flux distribution

A sensitivity analysis of a point kernel code was performed to investigate the effect of mesh division of a volume source on the radiation flux at points of interest. The QAD-CGGP, known as a representative point kernel code, was employed for a series of calculations and these calculation results compared with the reference data obtained from the MCNP5-1.60 code. The spherical volume source widely used in radiation shielding was also considered in this work and the mesh division along the radius was performed in two ways (regular and irregular). In addition, an approximate equation was defined to correct the significant error that occurs as an outcome of the point source assumption. As a result, in the case of a regular mesh division, a minimum mesh size of 1 cm is required to produce accurate results in comparison to the MCNP ones, while in the other instance, a half-level mesh division is sufficient to obtain the same result from the standpoint of the level of accuracy. In addition, by introducing the approximate equation presented in this paper, a significant error resulting from the point source assumption is exponentially reduced from a maximum of ~30% to a maximum of ~11%. Therefore, it is to be expected that the appropriate level of mesh division is required so as to increase the accuracy of the calculation using a point kernel method.