Number Of Frequency Samples Research Articles

Design and Analysis of a W-Band Metasurface-Based Computational Imaging System

We design and numerically analyze a coherent computational imaging system that utilizes a sparse detector array of planar, frequency-diverse, metasurface antennas designed to operate over the $W$ -band frequency range (75–110 GHz). Each of the metasurface antennas consists of a parallel plate waveguide, into which a center coaxial feed is inserted into the lower plate, launching a cylindrical guided wave. A dense array of metamaterial resonators patterned into the upper plate couples energy from the waveguide to free space radiative modes. The resonance frequency of each element, determined by its specific geometry, can be positioned anywhere within the $W$ -band. The geometry of each element is chosen to produce a resonance frequency selected randomly from the $W$ -band. Since a random subset of elements is resonant at any given frequency, the metasurface antenna forms a sequence of spatially diverse radiation patterns as a function of the excitation frequency. We analyze the metasurface aperture as an imaging system, optimizing key parameters relevant to image quality and resolution, including: aperture size; density and quality factor of the metamaterial resonators; number of detectors and their spatial distribution; bandwidth; and the number of frequency samples. A point-spread function analysis is used to compare the metasurface imager with traditional synthetic aperture radar. The singular value spectrum corresponding to the system transfer function and the mean-square-error associated with reconstructed images are both metrics used to characterize the system performance.

Parameterized Macromodeling for Efficient Analysis of Wideband Tower Grounding Structures

Field solvers are often needed to properly characterize the behavior of grounding structures in a wide frequency range. These solvers provide accurate results but at high computational cost. Design tasks, such as design optimization and sensitivity analysis, require repeated simulations for different values of system parameters (e.g., conductor geometry and soil parameters). Parameterized macromodels are suitable to speed up design tasks without compromising the accuracy and reliability of the results. These models allow describing the system behavior as a function of frequency (or time) and system parameters. In this paper, we propose a parameterized macromodeling approach for grounding structures to characterize the system impedance behavior as a function of frequency and system parameters. The method initially calculates a set of frequency-domain root macromodels for a set of system parameters combinations. An adaptive frequency sampling approach is introduced for reducing the required number of frequency samples. The model behavior at any desired intermediate parameters values can then be calculated by an enhanced interpolation scheme that uses scaling coefficients obtained via optimization. Numerical results for a tower grounding electrode example validate the proposed approach and show that it can efficiently and accurately predict the multidimensional system behavior in frequency and time domain.

Simultaneous Echo Refocused Magnetization Transfer Imaging

Purpose: To increase the efficiency of densely encoded magnetization transfer imaging of the brain, we time-multiplex multiple slices within the same readout using simultaneous echo refocusing FLASH imaging with magnetization transfer (MT) preparation (MT-SER-FLASH). Materials and Methods: Inefficiency in total scan time results from the number of frequency samples needed for sufficient quality of quantitative parameter maps for a binary spin bath model. We present a highly efficient multiplexing method, simultaneous echo refocused magnetization transfer imaging (MT-SER-FLASH) for reducing the total scan time of MT imaging by one-third. The specific absorption rate (SAR) was also reduced by reducing the number of MT-pulses per volume. Results: 2D-MT-SER-FLASH is performed in 19 minutes rather than 1 hour, acceptable for routine clinical application. The SAR could be reduced to 69% instead of more than 100% with a standard 2D or 3D-FLASH with MT-preparation. Conclusion: The net reduction of scan time and SAR enables the use of quantitative model based magnetization transfer imaging within a clinical environment.

Ray-Born inversion for fracture parameters

SUMMARY The purpose of this study is to investigate the possibility of inverting for the fracture parameters using the ray-Born approximation. The fracture parameters normal and tangential weaknesses ΔN and ΔT, derived using a low-frequency effective medium theory, are indicators of the crack density and fracture infill (liquid-filled or dry). In our implementation of ray-Born inversion, ΔN and ΔT are unknown model parameters. We treat the background medium as isotropic, and its elastic properties are assumed to be already determined using other conventional methods prior to inversion. In our current implementation, we assume that the fractures are vertical and the orientation of the symmetry axis of the effective anisotropic medium or the fracture normal is known. We formulate the inverse problem in a least-squares sense and apply the conjugate-gradient method to minimize the error function. To overcome the problem of non-uniqueness in the inversion and to ensure that the inverted model parameters are physically meaningful and unique, we added an analytical function of model parameters to the error function. We show, using a numerical example, that the fracture parameters cannot be reliably obtained from single-component seismic data (vertical or horizontal). Simultaneous inversion of both components is needed for more robust estimates of ΔN and ΔT. The inversion scheme was implemented in the frequency domain, which allowed us to experiment with the number of frequencies used in the inversion. We found that a very small number of frequency samples covering the spectrum of the source wavelet could produce reasonable estimates of ΔN and ΔT. We tested our method on a synthetic data set generated using ray-Born forward modelling. For the experiment shown in the paper, the least-squares inversion scheme produced encouraging results. We also applied our method to a synthetic data set generated using a 3-D anisotropic finite-difference code. The results indicated that our method could predict the parameter trends, but with some inaccuracies in the values of ΔN and ΔT.

Efficient power‐bus modeling based on an adaptive frequency sampling technique

AbstractThis paper presents an adaptive frequency sampling algorithm to generate a rational macromodel of a power delivery network. To this aim, the cavity model is assumed to represent the structure and accurate models of conductor and dielectric losses are incorporated. The adaptive sampling algorithm allows one to minimize the order of the macromodel and number of frequency samples used to extract the rational macromodel. The numerical results demonstrate the effectiveness of the proposed method. Copyright © 2008 John Wiley & Sons, Ltd.

Frequency-domain implementation of the transmitted-reference ultra-wideband receiver

This paper presents a mixed-signal frequency-domain implementation of the autocorrelation receiver used in the detection of ultra-wideband communication signals that are modulated with transmitted-reference signaling. The digital receiver architecture is based on samples provided by an analog-to-digital converter (ADC) in the frequency domain. Among the advantages of the new receiver are the relaxation of the conversion speed achieved by the inherent parallel architecture of the frequency-domain ADC and the flexibility and simplicity of an all-digital receiver architecture that follows the mixed-signal ADC radio front-end. The paper provides the symbol detection formulas and the corresponding system implementation and makes comparisons with the conventional time-domain implementations. Additionally, a time synchronization technique for the new receiver structure is proposed, which makes the receiver insensitive to any time offset at the pulse level. Furthermore, as a particular case of great practical interest, we consider a single bit for the ADC implementation, and we show with experimental results that the resultant frequency-domain mono-bit digital receiver provides a performance gain that increases with the number of frequency samples, when compared with its time-domain counterpart.

Broadband multicarrier communication receiver based on analog to digital conversion in the frequency domain

This paper introduces a multicarrier communication receiver for broadband applications based on analog to digital conversion (ADC) of the received signal in the frequency domain. The samples of the spectrum of the received signal are used in the digital receiver to estimate the transmitted symbols through a matched filter operation in the discrete frequency domain. The proposed receiver is aimed at the reception of high information rates in a multicarrier signal with very large bandwidth. Thus, the receiver architecture provides a solution to some of the challenging problems found in the implementation of conventional wideband multicarrier receivers based on time-domain ADC, since It efficiently parallelizes the A/D conversion reducing the sampling speed requirements. We show that the sampling rate requirements are relaxed as the number of frequency samples is increased, which introduces a trade-off between complexity and sampling rate. The new receiver possesses additional advantages, including scalability with increasing frequency samples, the possibility of optimally allocating the available number of bits for the ATD conversion across the frequency domain samples which potentially reduces the distortion introduced by the high-speed ADC, narrowband interference suppression that can be directly carried out in the frequency domain, and inherent robustness to frequency offset which makes it an attractive solution when compared with traditional multicarrier receivers. We also investigate how the proposed receiver responds to common multicarrier communication receiver problems such as phase noise and channel frequency selectivity.

Extracting PMD statistics from single emulatedfibre sample

To obtain reliable PMD statistics from frequency-swept measurements of emulated fibres, criteria are provided to relate the measurement bandwidth to the fibre mean differential group delay and the number of frequency samples.