A new direction-of-arrival (DOA) and power estimation method of unknown number of source signals is proposed. The direction and power of coherent and/or non-coherent signals are estimated by controlling the roots of the array polynomial on the unit circle. The genetic algorithm is used to find the phases of the array polynomial roots that minimize the array output power. The pseudo-spectrum is obtained by phase rotation of the estimated roots, and the real power spectrum is derived from the pseudo-spectrum and the array factor. The results indicate that the direction of arrivals, power of the signals, and number of source signals are estimated from the real power spectrum.
To improve absorbing properties of phase-modulated surface (PMS), the multi-active-layer PMS composed of multiple active frequency-selective surface (AFSS) layers and one background plane is theoretically studied using time-modulation theory in this paper. The optimization of PMS's switching scheme using differential evolution (DE) algorithm is also proposed for minimizing scattering echo energy at the incident frequency. We provide analytical formulation for the scattering problem and obtain the angular scattering pattern of PMS after optimization. Simulation results indicate that the optimized switching scheme is beneficial for reducing the spatial coverage of scattering echo at incident frequency. This coverage can be further confined by the increasing number of active layers in PMS. Furthermore, it is shown that floor effect appears when the number of active layers reaches a certain value, which limits the PMS structure conversely.
A large aperture quasi-optical dielectric lens antenna for passive imaging at W-band frequency is proposed. The lens is designed to obtain best resolution at a designate distance of 3.5 m from it. The lens has biconvex aspheric surface to achieve low aberration. The initial parameters of the optical path are obtained with Gaussian beam method, and then the optical simulator ZEMAX is applied to optimize the shape of the lens which improves design efficiency greatly. A hybrid numerical method is used to analyze near field distribution of the lens, and the final design of the lens is evaluated and determined by the results. The method is the combining of ANSOFT HFSS software, ray tracing method and integration algorithm based on Huygens' Principle. It is feasible and efficient for the analysis of various lens antennas, such as large aperture lens antennas which are difficult to be simulated by commercial electromagnetic simulation software. The lens is fabricated with HDPE. Experimental results show that its 3 dB beam size is 29 mm at distance of 3.5 m, which is in good agreement with theoretical calculation. The measured patterns on the image plane show that the lens has 0.3 dB decrease of field intensity in field view of 690 mm. Imaging result shows that the lens is a good candidate for focal plane imaging.
The rigorous modeling and analysis of surface waves at the boundary of two metamaterials are presented. The nature of the phenomenon of the surface-plasmon-polaritons and the influence of various parameters on it are investigated. We have analyzed the properties of structures incorporating nanostructured metamaterials. Surface-plasmon-polaritons at the interface of such metamaterials are studied. We demonstrate the ways to control the properties of the surface waves. Each metamaterial comprises alternating metal and dielectric layers. We analyze the dependence of the dispersion characteristics on the materials employed in metal-dielectric compound. The consistency of the dispersion diagrams and effective permittivity is studied. The Drude model is introduced in the metal dispersion in order to take into account the effects of the structure on dielectric properties.
Clifford's Geometric Algebra provides an elegant formulation of Maxwell's equations in the spacetime setting. Its clear geometric interpretation is used to derive a goal function, whose minimization results in Hodge-optimized material matrices being diagonal or diagonal-dominant. Effectively it is an optimization of the primal/dual mesh pair of a finite difference based discretization scheme taking into account the material properties. As a research example a standing wave in 2D cavity filled with an anisotropic material is investigated. Convergence of the scheme for various choices of mesh pairs is discussed. The limitations of the method in the 3D case are presented.
The degree of polarization (DoP) can be utilized as a detection statistic in the polarimetric radar to achieve target detection performance improvement. In this paper, a polarimetric radar model is established, which includes reflections from both target and clutter at first. Then, probability density functions (PDFs) of the estimated DoP are expressed in closed form, which is derived from joint eigenvalue distributions of complex noncentral Wishart matrices. The detector is developed and evaluated theoretically on the basis of the statistical properties of the DoP. Finally, a comparison between the new DoP detector and single-polarization detector is presented against real data. The performance improvement is demonstrated by the comparison results.
Multispectral imaging is an important tool for understanding composite materials in many disciplines. Spectral unmixing enables the determination of individual fluorophore distributions. Due to the dispersive nature of biomaterials the observed spectra of fluorescent dyes is unknown. Spectral unmixing can be accomplished for unknown endmember spectra using minimum volume simplex analysis (MVSA). Compressive sampling (CS) is a method to reduce the computational cost of operating on sparse data sets and can be performed efficiently using NESTA based on Nesterov's algorithm. Here we demonstrate that NESTA and MVSA can be combined with a denoising threshold to create a compressive sampling and multispectral unmixing (CSMIU) method that enables efficient bioimaging and unmixing with high levels of accuracy (spectral angle distances (SADs) < 0.05). This CSMIU method may potentially enable broadband and in vivo bioimaging modalities.
High reliability and low electromagnetic interference (EMI) are two important factors for many industrial applications such as air based electric transport system charger (AETSC). Therefore, it is essential to introduce high reliability and low EMI power converters. This paper presents a new high reliability and low current ripple DC-DC converter. For the proposed converter, a spectrum analysis approach for suppressing the EMI using chaotic sinusoidal pulse width modulation is provided. In addition, the proposed converter has radio frequency (RF) EMI lower than 100 kHz. However, for higher than 100 kHz, EMI issue of the proposed converter has unsuitable situation.
In this paper, two new hybrid excited vernier machines with surface and interior V-shaped PM arrays are proposed. By integrating the vernier structure and field excitation windings together, the proposed machines not only retain the merit of high torque of permanent magnet vernier, but also offer flexible flux adjustment to enable a wide speed range with the introduction of field windings. Different from existing hybrid excited vernier machines having magnets on the rotor, the proposed machines is designed with all excitation sources on the stator. Therefore, temperature rise of magnets of the proposed machines is much easier to control, which in turn reduce the risk of irreversible demagnetization of magnets and enhance the reliability. The electromagnetic performances of the two proposed machines are comprehensively analyzed and quantitatively compared by using the time-stepping finite-element method, verifying the theoretical analysis.
In low grazing angle scenario, target detection performance is seriously deteriorated due to multipath effect. This paper deals with moving target detection in low grazing angle with orthogonal frequency division multiplexing (OFDM) multi-input multi-output (MIMO) radar. We show that the detection performance can be improved through utilizing the multipath effect. Realistic physical and statistical effects such as refraction of the lower atmosphere and the Earth's curvature are incorporated into the multipath propagation model. Then, we derive a generalized likelihood ratio test (GLRT) detector by taking advantage of the frequency diversity of OFDM and MIMO configuration. Based on the fact that the target responses resonate at different frequencies and statistical characteristics of the test, we propose an algorithm which adaptively allocates the transmitted energy to improve the detection performance. The effectiveness of the GLRT detector as well as the adaptive design method is demonstrated via numerical examples.
As for the lack of the contribution by decision fusion in pose estimation and the demand for the combination of the feature fusion and the decision fusion in SAR ATR, in this paper, with the help of pose estimation, a new multi-look SAR ATR method is proposed in order to improve the performance, which is based on two-level decision fusion of neural network and sparse representation. The first-level decision fusion is acted for the combination of the pose estimation result by neural network and sparse representation. Based on the constraint of pose, these two models are exerted for the multi-look SAR ATR, and the second-level decision fusion is used to achieve the final recognition result. Several experiments based on MSTAR are conducted, and experimental results show that our method can achieve an acceptable result.
Within the framework of the finite-difference time-domain (FDTD) and the weighted Laguerre polynomials (WLPs), we derive an effective update equation of the electromagnetic in the dispersive media by introducing the factorization-splitting (FS) schemes and auxiliary differential equation (ADE). As two examples, we employ a 2-D parallel plate waveguide loaded with two dispersive medium columns and a thin grapheme sheet to calculate the plane wave propagation by using the FS-ADE-WLP-FDTD method. Compared with the ADE-FDTD and the ADE-WLP-FDTD methods, the results from our proposed method show its accuracy and efficiency for dispersive media simulation.
In traditional over-the-horizon radar (OTHR), multipath propagation due to the multi-layer ionospheric structure always deteriorates the detection performance. The properties of multiple-input multiple-output (MIMO) radar technique, which transmits wide beams with low gain at the transmitter and achieves receiver beam-forming to obtain narrow beams with high gain, make it an ideal choice for OTHR to detect target through multi-layer ionosphere and suppress strong clutter. This paper investigates the assumption of a two-layer ionospheric model and proposes a two-step Max-Min algorithm based on the mutual information theory to optimize MIMO-OTHR waveform so as to suppress clutter, interference and noise. The first step is to maximize the mutual information between the echo and target response from the same direction of arrival (DOA) in order to reduce the impact of noise. The second step is to minimize the mutual information between the echoes from different DOAs, in order to suppress the clutter and interference by reducing the correlation of the echoes from the different DOAs. Numerical experiments validate that this algorithm can improve range resolution and detection probability significantly. Experiment results also demonstrate that the previously harmful multipath propagation can be utilized to enhance the detection performance in MIMO-OTHR.
The microwave scattering characteristics of a metamaterial (MTM) sphere and an MTM coated conducting sphere is compared to that of its DPS (Real (ε) > 0, Real (μ) > 0) counterpart in the presence of an infinite conducting plane using the multipole expansion method and is presented in this article. The DPS medium may be an artificial dielectric or natural dielectric. The differential scattering cross sections and the differential backscattering cross sections of the different types of spheres are presented for a circularly polarized (left or right) beam incident normally on the sphere. The results presented may be useful for maritime applications.
The measurement of far-field radiation patterns is time consuming and expensive. Therefore, a novel technique that reduces the samples required to measure radiation patterns is proposed where random far-field samples are measured to reconstruct two-dimensional (2D) or three-dimensional (3D) far-field radiation patterns. The proposed technique uses a compressive sensing algorithm to reconstruct radiation patterns. The discrete Fourier transform (DFT) or the discrete cosine transform (DCT) are used as the sparsity transforms. The algorithm was evaluated by using 3 antennas modeled with the High-Frequency Structural Simulator (HFSS) --- a half-wave dipole, a Vivaldi, and a pyramidal horn. The root mean square error (RMSE) and the number of measurements required to reconstruct the antenna pattern were used to evaluate the performance of the algorithm. An empirical test case was performed that validates the use of compressive sensing in 2D and 3D radiation pattern reconstruction. Numerical simulations and empirical tests verify that the compressive sensing algorithm can be used to reconstruct radiation patterns, reducing the time and number of measurements required for good antenna pattern measurements.
We investigate optical bistability (OB) and optical multistability (OM) behaviors in a triply driven five-level atomic system. It is shown that the system has bistable behavior and can be controlled by intensity of applied fields. We find that OB switches to OM via interference induced among the Rabi-split resonance. We consider a superposed one-dimension standing wave, generated by two optical fields, and it is demonstrated that the OB and OM behaviors depend on the position of localized atoms as well as the relative phase of applied fields.
The light propagation through a one-dimensional symmetrical photonic structure, determined by the symmetric Silver mean Ag4 distribution embedded between two Bragg structures Bg27 (Bg27/Ag4/Bg27), is studied using the transfer matrix method (TMM). The focus lies on the investigation of the influence of symmetry of the structure as well as the dependence of transmission on the frequency, angle of incidence of the light striking the structure and symmetrical deformation of the structure. The deformation was introduced by applying a power law, so that the coordinates y of the deformed object were determined through the coordinates x of the non-deformed structure in accordance with the following rule: y = x1+k. Here, k is the degree of the law. A comparison will be made with a symmetrical periodic structure having the same number of layers. All results will be discussed in relation with the k values. Indeed, in the case of low k values close to zero a monochromatic filter was obtained, and in the case of relatively high values, an omnidirectional mirror is obtained.
A detailed study on the performance of square loop High impedance Surface (HIS) on lossy dielectric with its Artificial Magnetic Conductor (AMC) Property changing to narrow band absorber and then to Perfect Electric Conductor (PEC) depending on the loss in the dielectric is presented in this paper. An equivalent circuit modelling is used to theoretically explain how this transition is happening. This observed narrow band absorption (0.08 GHz) on the thin (0.016λ) lossy dielectric is scalable to different operating frequencies by varying the dimension of the geometry. The simulation studies on the effect of different geometrical, dielectric and incident wave parameters on the absorption property of this lossy HIS are also dealt with in this paper. Experimental investigation is in good agreement with simulated result and equivalent circuit modelling.
This paper describes a method of measurement of miniaturized antenna gain in HF band based on a parallel plate cell. Compared to a free space outdoor approach this method offers two advantages: the use of a well defined environment and time efficiency. For the same external dimensions, it also has an advantage compared to TEM/GTEM cells designs in terms of useful antenna under test (AUT) space. This space is of a major importance in HF band since even miniature antennas can have considerable proportions. The proposed structure is composed of a parallel plate cell, whose construction is simple and not expensive. It offers a precision measurement with an error not exceeding 2.3 dB with respect to calibrated antenna gain and simulation results.
A maximum a posteriori (MAP) approach, based on the Bayesian criterion, is proposed to overcome the low cross-range resolution problem in forward-looking imaging. We adapt scanning radar system to record received data and exploit deconvolution method to enhance the real-aperture resolution because the received echo is the convolution of target scattering coefficient and antenna pattern. The Generalized Gaussian distribution is considered as the prior information of target scattering coefficient in MAP approach for the reason that it could express different target scattering coefficient properties with the control of statistic parameter. This constraint term makes the proposed algorithm useful in different applications. On the other hand, the reconstruction problem can also be viewed as the lp-norm (0 < p ≤ 2) regularization. Simulation results show the robustness of the proposed algorithm against additive noise compared with other superresolution methods.
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