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Discrete Fourier Transform (discrete + fourier_transform)
Selected AbstractsIntermediate-term variations in solar radius during solar cycle 23ASTRONOMISCHE NACHRICHTEN, Issue 4 2009H. Kiliç Abstract In this study, we look for the mid-term variations in the daily average data of solar radius measurements made at the Solar Astrolabe Station of TUBITAK National Observatory (TUG) during solar cycle 23 for a time interval from 2000 February 26 to 2006 November 15. Due to the weather conditions and seasonal effect dependent on the latitude, the data series has the temporal gaps. For spectral analysis of the data series, thus, we use the Date Compensated Discrete Fourier Transform (DCDFT) and the CLEANest algorithm, which are powerful methods for irregularly spaced data. The CLEANest spectra of the solar radius data exhibit several significant mid-term periodicities at 393.2, 338.9, 206.5, 195.2, 172.3 and 125.4 days which are consistent with periods detected in several solar time series by several authors during different solar cycles. The knowledge relating to the origin of solar radius variations is not yet present. To see whether these variations will repeat in next cycles and to understand how the amplitudes of such variations change with different phases of the solar cycles, we need more systematic efforts and the long-term homogeneous data. Since most of the periodicities detected in the present study are frequently seen in solar activity indicators, it is thought that the physical mechanisms driving the periodicities of solar activity may also be effective in solar radius variations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Discrete Fourier transform in arbitrary dimensions by a generalized Beevers,Lipson algorithmACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2000Martin Schneider The Beevers,Lipson procedure was developed as an economical evaluation of Fourier maps in two- and three-dimensional space. Straightforward generalization of this procedure towards a transformation in -dimensional space would lead to nested loops over the coordinates, respectively, and different computer code is required for each dimension. An algorithm is proposed based on the generalization of the Beevers,Lipson procedure towards transforms in -dimensional space that contains the dimension as a variable and that results in a single piece of computer code for arbitrary dimensions. The computational complexity is found to scale as , where N is the number of pixels in the map, and it is independent of the dimension of the transform. This procedure will find applications in the evaluation of Fourier maps of quasicrystals and other aperiodic crystals, and in the maximum-entropy method for aperiodic crystals. [source] Bridging Mucosal Vessels Associated with Rhythmically Oscillating Blood Flow in Murine ColitisTHE ANATOMICAL RECORD : ADVANCES IN INTEGRATIVE ANATOMY AND EVOLUTIONARY BIOLOGY, Issue 1 2008Aslihan Turhan Abstract Oscillatory blood flow in the microcirculation is generally considered to be the result of cardiopulmonary influences or active vasomotion. In this report, we describe rhythmically oscillating blood flow in the bridging vessels of the mouse colon that appeared to be independent of known biological control mechanisms. Corrosion casting and scanning electron microscopy of the mouse colon demonstrated highly branched bridging vessels that connected the submucosal vessels with the mucosal plexus. Because of similar morphometric characteristics (19 ± 11 ,m vs. 28 ± 16 ,m), bridging arterioles and venules were distinguished by tracking fluorescent nanoparticles through the microcirculation using intravital fluorescence videomicroscopy. In control mice, the blood flow through the bridging vessels was typically continuous and unidirectional. In contrast, two models of chemically induced inflammation (trinitrobenzenesulfonic acid and dextran sodium sulfate) were associated with a twofold reduction in flow velocity and the prominence of rhythmically oscillating blood flow. The blood oscillation was characterized by tracking the bidirectional displacement of fluorescent nanoparticles. Space,time plots and particle tracking of the oscillating segments demonstrated an oscillation frequency between 0.2 and 5.1 cycles per second. Discrete Fourier transforms demonstrated a power spectrum composed of several base frequencies. These observations suggest that inflammation-inducible changes in blood flow patterns in the murine colon resulted in both reduced blood flow velocity and rhythmic oscillations within the bridging vessels of the mouse colon. Anat Rec, 291:74,82, 2007. © 2007 Wiley-Liss, Inc. [source] A spectral method for on-line computation of the harmonics of symmetrical components in induction machinesEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 3 2005T. Assaf Abstract This paper describes a procedure for the on-line computation of the harmonics of symmetrical components for voltages, currents and impedances in working induction machines. The algorithm used to compute these components is based on the discrete Fourier transform (DFT) of complex sequences. An experimental test-bed with a 0.12,kW star-connected induction motor has been used to test the procedure. The stability of the numerical results is verified to show the performance of the proposed method. Copyright © 2005 John Wiley & Sons, Ltd. [source] New joint frame synchronisation and carrier frequency offset estimation method for OFDM systems,EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 4 2009Zhongshan Zhang We propose a new joint frame synchronisation and carrier frequency offset estimation scheme for burst transmission mode OFDM systems. This scheme uses a central-symmetric and comb-like (CSCL) training sequence, which eases the power detection at the receiver without increasing the total training sequence power. Fine frame synchronisation as well as carrier frequency offset acquisition with a maximum acquisition range of times the sub-carrier spacing can also be performed based on the proposed CSCL training sequence, where N is the discrete Fourier transform (DFT) length and SF is an integer-valued spreading factor used to generate CSCL. The post-acquisition residual carrier frequency offset can be further estimated and corrected via a fine adjustment algorithm. In order to reduce performance loss due to the high peak-to-average power ratio (PAPR) of the CSCL training sequence, a time-domain constant-envelope (CE) training sequence is also proposed. The superior estimation accuracy of the proposed algorithm over that of the Moose algorithm and the SS (Shi and Serpedin) algorithm is proved by computer simulation. Copyright © 2008 John Wiley & Sons, Ltd. [source] Performance comparison of square root raised-cosine and lerner filters for the MDFT-TMUX filter bankEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 6 2002A. Sudana Madhu Rao This paper deals with prototype filters, namely, square root raised , cosine (SRC) and Lerner filters in the modified discrete Fourier transform (MDFT) transmultiplexer (TMUX) filter bank. The error obtained in the reconstruction of the original signal is compared for various filter orders. These filters can be employed in multicarrier modulation system- overlapped discrete multitone (overlapped DMT) or discrete wavelet multitone (DWMT) system. However, with the SRC filter, a large number of computations are required. By employing Lerner filter, we can reduce this by atleast 3- folds. We have carried out simulation studies for an 8-channel MDFT- TMUX filter bank and the results obtained are presented. [source] Applications of hybrid discrete Fourier transform moment method to the fast analysis of large rectangular dipole arrays printed on a thin grounded dielectric substrateMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 3 2002Hsi-Tseng Chou Abstract Recently a discrete Fourier transform,method of moments (DFT-MoM) scheme was developed for fast analysis of electrically large rectangular planar dipole arrays, which has been shown to be very efficient in terms of number reduction of unknown variables and computational complexity. The applications of this DFT-MoM to treat dipole arrays printed on a grounded dielectric substrate are examined in this Letter. Numerical results are presented to validate its efficiency and accuracy. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 203,207, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10417 [source] Green's function and excitation spectrum of finite latticesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2006S. Cojocaru Abstract New analytical results are obtained for the Green's function of a finite one-dimensional lattice with nearest neighbor interaction using discrete Fourier transform. The method offers an alternative to the Bethe Ansatz, but does not require any a priori assumption on the form of the wavefunction. This makes it suitable for extensions to nano-ferromagnets of higher dimensions. Solutions of the Heisenberg spin chain with periodic and open boundary conditions are considered as examples. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] On the properties of the periodogram of a stationary long-memory process over different epochs with applicationsJOURNAL OF TIME SERIES ANALYSIS, Issue 1 2010Valdério A. Reisen Primary 60G10; 60K35; Secondary 60G18 This article studies the asymptotic properties of the discrete Fourier transforms (DFT) and the periodogram of a stationary long-memory time series over different epochs. The main theoretical result is a novel bound for the covariance of the DFT ordinates evaluated on two distinct epochs, which depends explicitly on the Fourier frequencies and the gap between the epochs. This result is then applied to obtain the limiting distribution of some nonlinear functions of the periodogram over different epochs, under the additional assumption of gaussianity. We then apply this result to construct an estimator of the memory parameter based on the regression in a neighbourhood of the zero-frequency of the logarithm of the averaged periodogram, obtained by computing the empirical mean of the periodogram over adjacent epochs. It is shown that replacing the periodogram by its average has an effect similar to the frequency domain pooling to reduce the variance of the estimate. We also propose a simple procedure to test the stationarity of the memory coefficient. A limited Monte Carlo experiment is presented to support our findings. [source] | |||||||||||||