Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/260

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Citation - WoS: 33
Citation - Scopus: 34
Average Transmittance in Turbulence for Partially Coherent Sources
(Elsevier, 2004) Baykal, Y
Average intensity and power-transmittance in turbulence are formulated for a source with arbitrary degree of coherence (both spatial and temporal). Average power-transmittance is shown to reduce to the average intensity-transmittance if the receiver dimension is much less than the beam size. The average transmittance is found to attain its minimum value when the source is on the order of the Fresnel zone, irrespective of the degree of coherence of the source. We obtain the correct average intensity when the source is coherent, partially coherent and incoherent. Average transmittances due to turbulence are found for practical FSO (Free Space Optics) communication links. (C) 2003 Elsevier B.V. All rights reserved.
Citation - WoS: 31
Citation - Scopus: 31
Ber of Annular and Flat-Topped Beams in Non-Kolmogorov Weak Turbulence
(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, Yahya
The average bit error rate (BER) of multi-Gaussian beams in non-Kolmogorov weak turbulence is examined. For each specific incidence of annular and flat-topped optical beam, a power law of non-Kolmogorov spectrum is found which is defined as the worst power law at which the average BER attains the maximum value. Using these values of the worst power laws, it is observed that thinner collimated annular, larger focal length annular and flatter small sized collimated flat-topped structures have a slight advantage in obtaining smaller average BER. (C) 2012 Elsevier B.V. All rights reserved.
Citation - WoS: 17
Citation - Scopus: 15
Ber of Annular and Flat-Topped Beams in Strong Turbulence
(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, Yahya
The average bit error rate (< BER >) of annular and flat-topped beams are evaluated in strong turbulence. In this respect, our earlier results on the scintillation indices obtained by the unified Rytov method are employed and the intensity is taken to be gamma-gamma distributed. For comparison purposes, < BER > for the log-normal intensity distribution is also evaluated. It is found that for the annular beams, the ones that are thinner, possessing smaller ratio of primary to secondary beam size, and smaller focal lengths will have smaller average BER in strong turbulence. For the flat-topped beams, the ones that are flatter and possessing large source sizes have smaller average BER in strong turbulence. Large average SNR substantially reduces the average BER in weak and moderate turbulence, whereas in strong turbulence, the average BER stays at the same value no matter what the average SNR is. Comparison of the log-normal and the gamma-gamma statistics for the intensity shows that the average BER will be higher for the log-normal case when the average SNR is small and the reverse relationship holds at large average SNR. For both the gamma-gamma and the log-normal intensity distributions, < BER > obtained for the annular and the, flat-topped beams in strong turbulence is advantageous over the Gaussian beam < BER > values. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 15
Citation - Scopus: 16
Binary Phase Shift Keying-Subcarrier Intensity Modulation Performance in Weak Oceanic Turbulence
(Elsevier, 2019) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin Caner
The performance analysis of underwater wireless optical communication (UWOC) system that employs binary phase shift keying-subcarrier intensity modulation (BPSK-SIM) is investigated in weak oceanic turbulence. BPSK-SIM Gaussian beam and finite sized positive-intrinsic-negative (PIN) photodetector are employed at the transmitter and at the receiver, respectively. Bit error rate (BER) is taken as the performance indicator and in the evaluation of the BER, the required signal power and the aperture averaged scintillation index are obtained respectively by the use of the extended Huygens-Fresnel principle and the Rytov theory. Considering various noise types in underwater turbulence, BER variations are examined versus the oceanic turbulence parameters and the photodetector parameters, namely receiver aperture diameter, PIN-responsivity, noise factor, load resistor, and the electronic bandwidth. (c) 2019 Elsevier B.V. All rights reserved.
Citation - WoS: 1
Citation - Scopus: 1
Boundary Diffraction Wave Theory of Resistive Surfaces With Edge Discontinuities
(Elsevier, 2011) Umul, Yusuf Ziya
The line integral of the boundary diffraction wave theory is derived by considering the exact diffracted fields of a resistive half-plane. The line integral is generalized for arbitrary resistive surface with edge discontinuity. The method is applied to the diffraction problem of waves by a convex resistive spherical reflector and the resultant field expressions are investigated numerically. (C) 2011 Elsevier B.V. All rights reserved.
Citation - WoS: 17
Citation - Scopus: 16
Coherence Length in Non-Kolmogorov Satellite Links
(Elsevier, 2013) Baykal, Yahya
Behavior of the coherence length in non-Kolmogorov satellite links is investigated. Equivalent structure constants for non-Kolmogorov spectra are employed in order to make relevant comparisons for different non-Kolmogorov power law exponents. Examining the coherence length versus the non-Kolmogorov power law exponent for different rms wind speeds, zenith angles, link lengths, structure constants and the wavelengths, the coherence length is found to decrease when the non-Kolmogorov power law exponent increases. At a fixed non-Kolmogorov power law exponent, the coherence length is found to decrease as the rms wind speed, the zenith angle or the structure constant increases and the wavelength decreases. As the link length increases, the coherence length decreases for power law exponent values smaller than that for the Kolmogorov case. However, an increase in the link length seems not to cause the coherence length to vary appreciably at power law exponent values larger than the Kolmogorov case power law exponent. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 7
Citation - Scopus: 7
Diffraction of Waves by a Perfectly Conducting Half-Plane Between Two Dielectric Media
(Elsevier, 2014) Umul, Yusuf Ziya
The scattered fields by a perfectly conducting half-plane, located at the interface of two dielectric media, are investigated. The diffracted fields are defined with novel reflection and transmission coefficients that are functions of the observation distance and angle. These coefficients reduce to the constant reflection and transmission coefficients of the geometrical optics wave. The unknown quantities in the coefficients of the diffracted waves are determined according to the boundary conditions on the half-plane and the interface of the dielectric media. The resultant field expressions are investigated numerically and the behavior of the scattered and diffracted waves are studied. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 13
Citation - Scopus: 13
The Effect of Impedance Boundary Conditions on the Potential Function of the Boundary Diffraction Wave Theory
(Elsevier, 2008) Yalcin, U.; Umul, Y. Z.
A novel potential function of the boundary diffraction wave theory is obtained for the impedance surfaces by the asymptotic reduction of the modified theory of physical integrals. The function is expressed in terms of the direction vectors of the incident and scattered rays. The application of the method is performed on the problem of diffraction of plane waves by an impedance half plane for oblique incidence. (c) 2007 Elsevier B.V. All rights reserved.
Citation - WoS: 73
Citation - Scopus: 70
Energetics and Structures of Small Clusters: Ptn, N=2-21
(Elsevier, 2003) Sebetci, A; Güvenç, ZB
The Voter and Chen version of an embedded-atom model, derived by fitting to experimental data of both diatomic molecule and bulk platinum simultaneously, has been applied to study the locally stable structures, energies and growth patterns of small platinum clusters in the size range of N = 2-21. Using molecular dynamics and thermal quenching simulations, the global minima and the other locally stable structures have been distinguished from those stationary structures that correspond to saddle points of the potential energy surface. Ten thousand independent initial configurations generated at high temperatures (about 2600 K) were used to obtain the number of isomers and the probabilities of sampling different basins of attractions, for each size of the clusters. Their energy spectra have been analyzed. Comparisons have been made with the results of previous calculations using electronic structure and empirical potential methods. Although many of the lowest energy structures correspond to icosahedral growth, a number of new structures have been identified for N = 15, 16, 17, 18, 20 and 21. It has been found that the lowest energy structures are not always the most probable isomers for each size. (C) 2002 Elsevier Science B.V. All rights reserved.
Citation - WoS: 34
Citation - Scopus: 45
Fall Detection Using Single-Tree Complex Wavelet Transform
(Elsevier, 2013) Keskin, Furkan; Toreyin, B. Ugur; Cetin, A. Enis; Yazar, Ahmet
The goal of Ambient Assisted Living (AAL) research is to improve the quality of life of the elderly and handicapped people and help them maintain an independent lifestyle with the use of sensors, signal processing and telecommunications infrastructure. Unusual human activity detection such as fall detection has important applications. In this paper, a fall detection algorithm for a low cost AAL system using vibration and passive infrared (PIR) sensors is proposed. The single-tree complex wavelet transform (ST-CWT) is used for feature extraction from vibration sensor signal. The proposed feature extraction scheme is compared to discrete Fourier transform and mel-frequency cepstrum coefficients based feature extraction methods. Vibration signal features are classified into "fall" and "ordinary activity" classes using Euclidean distance, Mahalanobis distance, and support vector machine (SVM) classifiers, and they are compared to each other. The PIR sensor is used for the detection of a moving person in a region of interest. The proposed system works in real-time on a standard personal computer. (C) 2012 Elsevier B.V. All rights reserved.
Citation - WoS: 10
Citation - Scopus: 9
Hydrogen Recombination on a Mixed Adsorption Layer at Saturation on a Metal Surface: H→(D+h)sat+ni
(Elsevier, 2003) Güvenç, ZB; Güvenç, D
Interactions of H atom beams with (D + H)-covered Ni(1 0 0) surfaces are simulated at saturation level of 0.93 monolayer using quasi-classical microcanonical trajectory method. The Ni substrate is treated as a non-rigid multilayer slab using an embedded-atom method. The model many-body potential energy surface London-Eyring-Polanyi-Sato used to characterize the interactions between H-H and H-Ni(1 0 0) systems parameterized by fitting to the results of detailed total-energy calculations based on density functional theory. Since most of the incident H atoms trap to form hot atoms, reactions between the projectile atom and adsorbates are mainly due to the hot atom process. Results of a linear behavior of the total HD and quadratic behavior of the D-2 yields with the initial D coverage, in addition, significantly low secondary reactions between the adsorbates are found to be in good agreement with the experiment. In addition distributions of the rotational states of the product molecules, molecular desorption angles, vibrational states of the product molecules, molecular formation and desorption time, total and translational energies of the product molecules are also calculated as functions of different H and D coverages on the surface. (C) 2003 Elsevier Science B.V. All rights reserved.
Citation - WoS: 3
Citation - Scopus: 5
Irradiance Fluctuations of Partially Coherent Super Lorentz Gaussian Beams
(Elsevier, 2011) Eyyuboglu, Halil T.; Gokce, Muhsin C.
By using the semi-analytic approach introduced earlier, we formulate and subsequently evaluate the irradiance fluctuations of partially coherent super Lorentz Gaussian beams for orders of 10 and 11. Within the range of examined source and propagation conditions, our calculations show that there will be less fluctuations at short propagation distances as the Lorentzian property is increased. But the reverse will be applicable, if the longer propagation distances are considered. The use of focusing will cause reductions, particularly for beams with increased Lorentzian property. (C) 2011 Elsevier B.V. All rights reserved.
The Method of the Boundary Diffraction Wave for Impedance Surfaces
(Elsevier, 2012) Umul, Yusuf Ziya
The line integral of the boundary diffraction wave theory is extended for the diffraction process of waves by the impedance surfaces with edge discontinuities. With this aim, the exact diffraction field expression of Maliuzhinets is transformed into a line integral. The method is applied to the scattering problems of waves by a spherical reflector with edge discontinuity and the diffracted fields are evaluated asymptotically. The resultant expressions of the waves are examined numerically. (C) 2011 Elsevier B.V. All rights reserved.
Citation - WoS: 87
Citation - Scopus: 95
Off-Axis Gaussian Schell-Model Beam and Partially Coherent Laser Array Beam in a Turbulent Atmosphere
(Elsevier, 2007) Cai, Yangjian; Lin, Qiang; Baykal, Yahya; Eyyuboglu, Halil T.
The propagation of an off-axis Gaussian Schell-model (GSM) beam in a turbulent atmosphere is investigated based on the extended Huygens-Fresnel integral formula. Analytical formulae for the cross-spectral density and corresponding partially coherent complex curvature tensor of an off-axis GSM beam propagating in a turbulent atmosphere are derived. Based on these formulae, the propagation properties of such kind of beam in a turbulent atmosphere are investigated in detail. Furthermore, the methods are extended to investigate the propagation properties of a partially coherent laser array beam in a turbulent atmosphere. The properties of an off-axis GSM beam and a partially coherent laser array beam in a turbulent atmosphere are closely related with the beam parameters and the structure constant of the turbulent atmosphere. (c) 2007 Elsevier B.V. All rights reserved.
Citation - WoS: 21
Citation - Scopus: 25
Paraxial Propagation of a Partially Coherent Flattened Gaussian Beam Through Apertured Abcd Optical Systems
(Elsevier, 2008) Cai, Yangjian; Lue, Xiang; Eyyuboglu, Halil T.; Baykal, Yahya
By expanding the hard aperture function into a finite sum of complex Gaussian functions, some approximate analytical formulae for the cross-spectral density of a partially coherent flattened Gaussian beam (FGB) propagating through apertured aligned and misaligned ABCD optical systems are derived based on the generalized Collins formula. The results obtained by using the approximate analytical formula are in good agreement with those obtained by using the numerical integral calculation. As a numerical example, the focusing properties (including average irradiance distribution and focal shift) of a partially coherent FGB focused by an apertured thin lens are studied, and it is found that the focusing properties of a partially coherent FGB are closely related to its initial coherence and the aperture width. Our results provide an effective and fast way for studying the paraxial propagation of a partially coherent FGB through apertured ABCD optical systems. (C) 2008 Elsevier B.V. All rights reserved.
The Physical Optics of Phase-Conjugate Mirrors: Scalar Theory
(Elsevier, 2011) Umul, Yusuf Ziya
A scalar theory of physical optics is developed for the scattered waves by finite or infinite phase-conjugate mirrors. The method is applied to the scattering problem of waves by a semi-infinite phase-conjugate mirror. The results are examined numerically and compared with the scattered waves by a conducting half-plane. The effect of the phase-conjugation to the location of the transition region is outlined. (C) 2011 Elsevier B.V. All rights reserved.
Citation - WoS: 81
Citation - Scopus: 86
Propagation of Hermite-Cosh Laser Beams in Turbulent Atmosphere
(Elsevier, 2005) Eyyuboglu, HT
The propagation characteristics of Hermite-cosh-Gaussian (HCG) beams in turbulence are studied. To this end, from a HCG source plane excitation, the average intensity of the receiver plane is developed analytically. This formulation is verified against the existing solution of HCG beams in free space. The results indicate that upon traveling in turbulent atmosphere, the HCG beam will undergo three stages of evolution. Initially, it will preserve the original profile. Then, with source parameters of right choice, this beam will turn into a sine shape if the sum of mode indices is odd, and into a cos shape if the sum of mode indices is even. Eventually however, HCG originated beam will become a pure Gassian beam. The dependence of these stages on source and propagation conditions is investigated. (C) 2004 Elsevier B.V. All rights reserved.
Citation - WoS: 42
Citation - Scopus: 49
Propagation Properties of Anomalous Hollow Beams in a Turbulent Atmosphere
(Elsevier, 2008) Cai, Yangjian; Eyyuboglu, Halil T.; Baykal, Yahya
Propagation of circular and elliptical anomalous hollow beams in a turbulent atmosphere is investigated in detail. Based on the extended Huygens-Fresnel integral, analytical formulae for the average irradiance of circular and elliptical anomalous hollow beams propagating in a turbulent atmosphere are derived. The irradiance and spreading properties of circular and elliptical anomalous hollow beams in a turbulent atmosphere and in free space are studied numerically. It is found that circular and elliptical anomalous hollow beams at short propagation distance in turbulent atmosphere have similar propagation properties to those of free space, while at long propagation distance, circular and elliptical anomalous hollow beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The conversion from an anomalous hollow beam to a circular Gaussian beam becomes quicker and the beam spot spreads more rapidly for a larger structure constant, a shorter wavelength and a smaller waist size of the initial beam. (C) 2008 Elsevier B.V. All rights reserved.
Citation - WoS: 7
Citation - Scopus: 7
Retracted: a Simple Analytical Eam Model for Some Bcc Metals (Retracted Article. See Vol. 16, Pg. 4547, 2011)
(Elsevier, 2010) Dursun, Ibrahim H.; Guvenc, Ziya B.; Kasap, E.
An analytical embedded atom method which can treat bcc transition metal iron has been developed. In this model, a new potential was presented and a modified term has been introduced to fit the negative Cauchy pressure P-C = (C-12 - C-44)/2 for Fe element. The new model was applied to calculate thermodynamic properties of binary alloys of the bcc transition metals: Fe, V and Cr. The calculated dilute-solution enthalpies and formation enthalpies of random alloys are in good agreement with the results of first principles calculations and that of the thermodynamic calculations. (C) 2009 Elsevier B.V. All rights reserved.
Citation - WoS: 15
Citation - Scopus: 17
Scintillation Analysis of Hypergeometric Gaussian Beam Via Phase Screen Method
(Elsevier, 2013) Eyyuboglu, Halil T.
We give a scintillation treatment of hypergeometric Gaussian beams via the use of random phase screens. In particular, we analyse the on-axis, point-like and aperture averaged power scintillation characteristics of this beam that cannot be undertaken easily by analytic means. Within the range of examined source and propagation parameters, our evaluations show that there will be less scintillation, with increasing hollowness at small source sizes and zero topological charge. At larger source sizes or topological charges, this is reversed and decreasing hollowness will reduce scintillation. More or less the same trend is observed for aperture averaging such that at small source sizes and zero topological charge, increased hollowness will result in lower scintillation. At larger source size and topological charges, there will be a transition from the case of smaller values of hollowness giving rise to less scintillation at smaller aperture openings to the case of larger values of hollowness giving rise to less scintillation at larger aperture openings. In general nonzero topological charges will produces more scintillations, both in on-axis and aperture averaged cases. (C) 2013 Elsevier B.V. All rights reserved.

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