Browsing by Subject "Probability density function"
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Item Open Access Accurate method for obtaining band gaps in conducting polymers using a DFT/hybrid approach(American Chemical Society, 1998) Salzner, U.; Pickup, P. G.; Poirier, R. A.; Lagowski, J. B.DFT calculations on a series of oligomers have been used to estimate band gaps, ionization potentials, electron affinities, and bandwidths for polyacetylene, polythiophene, polypyrrole, polythiazole, and a thiophene - thiazole copolymer. Using a slightly modified hybrid functional, we obtain band gaps within 0.1 eV of experimental solid-state values Calculated bond lengths and bond angles for the central ring of sexithiophene differ by less than 0.026 Å and 0.7° from those of the sexithiopnene crystal structure. IPs and EAs are overestimated by up to 0.77 eV compared to experimental bulk values. Extrapolated bandwidths agree reasonably well with bandwidths from band structure calculations.Item Open Access Adaptive filtering for non-gaussian stable processes(IEEE, 1994) Arıkan, Orhan; Çetin, A. Enis; Erzin, E.A large class of physical phenomenon observed in practice exhibit non-Gaussian behavior. In this letter, a-stable distributions, which have heavier tails than Gaussian distribution, are considered to model non-Gaussian signals. Adaptive signal processing in the presence of such a noise is a requirement of many practical problems. Since direct application of commonly used adaptation techniques fail in these applications, new algorithms for adaptive filtering for α-stable random processes are introduced.Item Open Access Average fisher information optimization for quantized measurements using additive independent noise(IEEE, 2010) Balkan, Gokce Osman; Gezici, SinanAdding noise to nonlinear systems can enhance their performance. Additive noise benefits are observed also in parameter estimation problems based on quantized observations. In this study, the purpose is to find the optimal probability density function of additive noise, which is applied to observations before quantization, in those problems. First, optimal probability density function of noise is formulated in terms of an average Fisher information maximization problem. Then, it is proven that optimal additive "noise" can be represented by a constant signal level. This result, which means that randomization of additive signal levels is not needed for average Fisher information maximization, is supported with two numerical examples. ©2010 IEEE.Item Open Access Bessel functions-based reconstruction of non-uniformly sampled diffraction fields(IEEE, 2007) Uzunov, V.; Esmer, G. Bora; Gotchev, A.; Onural, Levent; Özaktaş, Haldun M.A discrete computational model for the diffraction process is essential in forward problems related to holographic TV. The model must be as general as possible, since the shape of the displayed objects does not bear any restrictions. We derive a discrete diffraction model which suits the problem of reconstruction of diffraction fields from a set of non-uniformly distributed samples. The only restriction of the model is the wave nature of the field. The derivation takes advantage of changing the spatial and frequency coordinates to polar form and ends up with a model stated in terms of Bessel functions. The model proves to be a separable orthogonal basis. It shows rapid convergence when evaluated in the framework of the non-uniform sampling problem.Item Open Access Çokyollu ortamda çapraz belirsizlik işlevi-yön bulma tekniğinin başarım analizi(IEEE, 2008-04) Güldoǧan, Mehmet Burak; Arıkan, OrhanBu bildiride, Çapraz Belirsizlik İşlevi-Yön Bulma (CAFDF) tekniğinin çokyollu ortamlardaki sinyallerin zaman gecikmesi, Doppler kayması, geliş yönü(GY) ve genlik kestirimindeki başarımı ile yiiksek çözünürlüklü algoritmalar olan Uzay-Almaşan Genelleşmiş Beklenti-Enbüyüitme (SAGE) ve Çoklu Sinyal Sınıflandırılması(MUSIC)'in sentetik sinyaller iizerindeki başarımları kıyaslanmıştır. Algoritmalann performansları, kök Ortalama Karesel Hata (kOKH) cinsinden degişik işaret Gürültü Oranı (iGO) değerlerinde Monte Carlo denemelerine dayalı olarak sunulmuştur. Sentetik kanallarda istatiksel kıyaslama amaçlı Cramer-Rao alt sınırları eklenmiştir. Simülasyon sonuçları göstermektedir ki, orta ve düşük iGO değerlerinde CAF-DF diğer iki algoritmaya göre üstünlük sağlamaktadır.Item Open Access Density functional theory investigation of substituent effects on building blocks of conducting polymers(Elsevier, 1999) Salzner, U.Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbits method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridge dimers, HOMO-LUMO gaps correlate with π-electron densities in the carbon backbone and energy gap reduction correlate with the strength of π-π* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules.Item Open Access Does the donor-acceptor concept work for designing synthetic metals? 1. theoretical investigation of poly(3-cyano-3′-hydroxybithiophene)(American Chemical Society, 2002) Salzner, U.Homo- and copolymers of hydroxythiophene and cyanothiophene have been investigated by employing density functional theory with the aim of determining the effect of donor-acceptor substitution on the electronic structure. The band gap of the copolymer is 0.11 eV smaller than that of polythiophene. Bandwidths of valence and conduction bands are reduced by 0.22 and 0.36 eV compared to polybithiophene. Conductivity after p- and n-doping could therefore be less than that of polythiophene. All properties of the copolymer are averages between those of the homopolymers. The charge separation between hydroxy- and cyano-substituted rings is 0.12 e in the neutral state and 0.13 e and the dication. The ionization potential and electron affinity of poly(hydroxythiophene) are 1.78 and 1.63 eV smaller than those of poly(cyanothiophene). According to the donor-acceptor concept, a decrease in band gap and an increase in bandwidths compared to the homopolymers should have resulted: We rationalize the absence of band broadening with reduced interaction between fragments with very different energies in agreement with perturbation theory.Item Open Access Does the donor-acceptor concept work for designing synthetic metals? 2. theoretical investigation of copolymers of 4-(dicyanomethylene)-4H-cyclopenta[2, 1-b: 3, 4-b′]dithiophene and 3, 4-(ethylenedioxy)thiophene(American Chemical Society, 2002) Salzner, U.; Köse, M. E.Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.Item Open Access Dynamical phase transitions in totally asymmetric simple exclusion processes with two types of particles under periodically driven boundary conditions(American Physical Society, 2016) Yeşil, A.F.; Yalabik, M.C.Driven diffusive systems have provided simple models for nonequilibrium systems with nontrivial structures. Steady-state behavior of these systems with constant boundary conditions have been studied extensively. Comparatively less work has been carried out on the responses of these systems to time-dependent parameters. We report the modifications to the probability density function of a two-particle exclusion model in response to a periodically changing perturbation to its boundary conditions. The changes in the shape of the distribution as a function of the frequency of the perturbation contains considerable structure. A dynamical phase transition in which the system response changes abruptly as a function of perturbation frequency was observed. We interpret this structure to be a consequence of the existence of a typical time scale associated with the dynamics of density shock profiles within the system.Item Open Access Economic design of EWMA control charts based on loss function(Elsevier, 2009) Serel, D. A.For monitoring the stability of a process, various control charts based on exponentially weighted moving average (EWMA) statistics have been proposed in the literature. We study the economic design of EWMA-based mean and dispersion charts when a linear, quadratic, or exponential loss function is used for computing the costs arising from poor quality. The chart parameters (sample size, sampling interval, control limits and smoothing constant) minimizing the overall cost of the control scheme are determined via computational methods. Using numerical examples, we compare the performances of the EWMA charts with Shewhart over(X, -) and S charts, and investigate the sensitivity of the chart parameters to changes in process parameters and loss functions. Numerical results imply that rather than sample size or control limits, the users need to adjust the sampling interval in response to changes in the cost of poor quality.Item Open Access Electronic and magnetic properties of zinc blende half-metal superlattices(A I P Publishing LLC, 2004) Fong, C. Y.; Qian, M. C.; Pask, J. E.; Yang, L. H.; Dag, S.Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0mB and 14.0mB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.Item Open Access Estimating distributions varying in time in a universal manner(IEEE, 2017) Gökçesu, Kaan; Manış, Eren; Kurt, Ali Emirhan; Yar, ErsinWe investigate the estimation of distributions with time-varying parameters. We introduce an algorithm that achieves the optimal negative likelihood performance against the true probability distribution. We achieve this optimum regret performance without any knowledge about the total change of the parameters of true distribution. Our results are guaranteed to hold in an individual sequence manner such that we have no assumptions on the underlying sequences. Apart from the regret bounds, through synthetic and real life experiments, we demonstrate substantial performance gains with respect to the state-of-the-art probability density estimation algorithms in the literature.Item Open Access Finite temperature studies of Te adsorption on Si(0 0 1)(Elsevier, 2002) Sen, P.; Çıracı, Salim; Batra, I. P.; Grein, C. H.; Sivananthan, S.We perform first principles density functional calculations to investigate the adsorption of Te on the Si(0 0 1) surface from low coverage up to a monolayer coverage. At low coverage, a Te atom is adsorbed on top of the Si surface dimer bond. At higher coverages, Te atoms adsorption causes the Si-Si dimer bond to break, lifting the (2 × 1) reconstruction. We find no evidence of the Te-Te dimer bond formation as a possible source of the (2 × 1) reconstruction at a monolayer coverage. Finite temperature ab initio molecular dynamics calculations show that Te covered Si(0 0 1) surfaces do not have any definitive reconstruction. Vibrations of the bridged Te atoms in the strongly anharmonic potentials prevent the reconstruction structure from attaining any permanent, two-dimensional periodic geometry. This explains why experiments attempting to find a definite model for the reconstruction reached conflicting conclusions. © 2002 Elsevier Science B.V. All rights reserved.Item Open Access Free riding in peer-to-peer networks(Institute of Electrical and Electronics Engineers, 2009) Karakaya, M.; Korpeoglu, I.; Ulusoy, ÖzgürFree riding in peer-to-peer (P2P) networks poses a serious threat to their proper operation. Here, the authors present a variety of approaches developed to overcome this problem. They introduce several unique aspects of P2P networks and discuss free riding's effects on P2P services. They categorize proposed solutions and describe each category's important features and implementation issues together with some sample solutions. They also discuss open issues, including common attacks and security considerations. © 2009 IEEE.Item Open Access Input sequence estimation and blind channel identification in HF communication(IEEE, 2000) Khames, Mariam; Miled, B. H.; Arıkan, OrhanA new algorithm is proposed for reliable communication over HF tropospheric links in the presence of rapid channel variations. In the proposed approach, using fractionally space channel outputs, sequential estimation of channel characteristics and input sequence is performed by utilizing subspace tracking and Kalman filtering. Simulation based comparisons with the existing algorithms show that the proposed approaches significantly improve the performance of the communication system and enable us to utilize HF communication in bad conditions.Item Open Access Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra(2008) Salzner, U.The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory. The credibility of the method was established by comparing the results with a variety of theoretical levels and with experiment. Effects due to counterions (Cl3-) and solvent (CH 2Cl2) were examined. It was found that TDDFT employing hybrid functionals is accurate enough to distinguish the absorbing species. The findings offer an explanation for the experimentally observed difference in UV-spectra of medium-sized and long oligomers upon doping. As chain lengths of the oligomers increase and energy levels get closer, configuration interaction leads to additional absorption peaks in the high energy sub-band region (at around 1.5-2.5 eV). Thus, long oligomers do not behave differently from medium-sized ones upon doping, only their spectra are different. At low doping levels radical cations (polarons) are produced. At higher doping levels, dications that harbor weakly interacting polaron pairs are formed. Bipolarons are predicted only on very short chains or at high doping levels. There is no bipolaron binding energy and disproportionation of monocations into dications and neutral species is energetically unfavorable. © 2008 American Chemical Society.Item Open Access Large deviations of probability rank(IEEE, 2000) Arıkan, ErdalConsider a pair of random variables (X,Y) with distribution P. The probability rank function is defined so that G(x|y) = 1 for the most probable outcome x conditional on Y = y, G(x|y) = 2 for the second most probable outcome, and so on, resolving ties between elements with equal probabilities arbitrarily. The function G was considered in [1] in the context of finding the unknown outcome of a random experience by asking question of the form 'Is the outcome equal to x?' sequentially until the actual outcome is determined. The primary focus in [1], and the subsequent works [2], [3], was to find tight bounds on the moments E[G(X|Y)θ]. The present work is closely related to these works but focuses more directly on the large deviations properties of the probability rank function.Item Open Access Lattice dynamics and elastic properties of lanthanum monopnictides(2008) Gökoǧlu G.; Erkişi, A.In this study, first principles calculation results of the second order elastic constants and lattice dynamics of two lanthanum monopnictides, LaN and LaBi, which crystallize in rock-salt structure (B1 phase), are presented. Calculations were based on plane wave basis sets and pseudopotential methods in the framework of Density Functional Theory (DFT) with generalized gradient approximation. Elastic constants are calculated by tetragonal and orthorhombic distortions on cubic structure. Phonon dispersion spectra was constructed in the linear response approach of the Density Functional Perturbation Theory (DFPT). The complete phonon softening with negative frequencies and large elastic anisotropy were observed for LaN single crystal as a sign of the structural instability. The phonon dispersion curve for LaBi is typical for lanthanum monopnictides and does not show any anomalous physical property. The calculated structural quantities for both LaN and LaBi systems agree well with the available experimental and theoretical data. © 2008 Elsevier Ltd. All rights reserved.Item Open Access Mean-shift tracking of moving objects using multi-dimensional histograms(Society of Photo-Optical Instrumentation Engineers (SPIE), 2004-04) Cüce, Halil I.; Çetin, A. EnisIn this paper, a moving object tracking algorithm for infrared image sequences is presented. The tracking algorithm is based on the mean-shift tracking method which is based on comparing the histograms of moving objects in consecutive image frames. In video obtained after visible light, the color histogram of the object is used for tracking. In forward looking infrared image sequences, the histogram is constructed not only from the pixel values but also from a highpass filtered version of the original image. The reason behind the use of highpass filter outputs in histogram construction is to capture structural nature of the moving object. Simulation examples are presented.Item Open Access Multiperson tracking with a network of ultrawideband radar sensors based on gaussian mixture PHD filters(Institute of Electrical and Electronics Engineers Inc., 2015) Gulmezoglu, B.; Guldogan, M. B.; Gezici, SinanIn this paper, we investigate the use of Gaussian mixture probability hypothesis density filters for multiple person tracking using ultrawideband (UWB) radar sensors in an indoor environment. An experimental setup consisting of a network of UWB radar sensors and a computer is designed, and a new detection algorithm is proposed. The results of this experimental proof-of-concept study show that it is possible to accurately track multiple targets using a UWB radar sensor network in indoor environments based on the proposed approach. © 2014 IEEE.