Elektronik ve Haberleşme Mühendisliği Bölümü

Permanent URI for this communityhttps://hdl.handle.net/20.500.12416/14

Browse

Now showing 1 - 20 of 548

Citation - WoS: 18
Citation - Scopus: 22
Partially Coherent Lorentz Gaussian Beam and Its Scintillations
(Springer, 2011) Eyyuboglu, H. T.
We study the scintillation aspects of partially coherent Lorentz Gaussian (LG) beams via numerically integrating the average and average squared intensity expressions. Within the examined range of input and propagation medium parameters, the LG beams are generally found to offer less and less scintillations against the pure Gaussian beam, particularly when the Lorentzian feature of the beam is emphasized more. This lower scintillation property is exhibited for collimated coherent beams with different Lorentz widths and at on-axis and off-axis positions of the receiver plane. When focusing is introduced, at shorter propagation distances the ordering of the beams remains as described above, but at longer propagations distances a complete reversing of the beam order is observed. Raising the turbulence levels by increasing the structure constant inevitably causes rises in scintillations, while preserving the advantage of LG beams over the pure Gaussian beam. Partial coherence reduces scintillations as expected, at the same time nearly eliminating the scintillation differences between the beam types.
Citation - WoS: 4
Citation - Scopus: 5
Interaction of Plane Waves by a Half-Screen Between Two Different Media
(Springer, 2018) Umul, Yusuf Ziya
The diffraction process of plane waves by a perfectly electric conducting half-screen, residing at a planar junction between two different media, is investigated. The scattered geometrical optics waves are evaluated by subtracting the initial fields from the total geometrical optics waves. The diffracted waves are obtained from the scattered geometrical optics fields by using a relation at the transition boundaries. The diffracted waves are expressed separately for both of the media. The uniform representations of the diffracted fields are obtained by using the uniform theory of diffraction. The scattered fields are analyzed numerically.
Citation - WoS: 4
Citation - Scopus: 5
Wave Diffraction by a Soft/Hard Strip: Modified Theory of Physical Optics Solution
(Elsevier Gmbh, Urban & Fischer verlag, 2018) Umul, Yusuf Ziya
The scattering problem of waves by a strip, the faces of which are composed of soft and hard surfaces, is investigated by the method of modified theory of physical optics. The scattering integral is divided into two parts according to the single effects of the faces. The boundary conditions are integrated into the integrals and the resultant field expressions are obtained by the asymptotic evaluation methods. The scattered waves are examined numerically. (C) 2017 Elsevier GmbH. All rights reserved.
A Sequential Coding Approach for Short Length LT Codes over A WGN Channel
(2017) Abdulkhaleq, Nadhir Ibrahim; Gazi, Orhan
Luby Transform (LT) codes which can be considered as one of the first type of rate less codes are introduced to be an efficient erasure code. In such codes a fountain of encoded symbols is continuously transmitted until the successfully delivery of all the data packets. Whenever the receiver collects N noisy encoded symbols the decoding operation starts. Usually belief propagation (BP) algorithm is used to decipher the code and extract the k data symbols. In this paper, short length LT codes are generated using efficient sequential encoding approach (SEA) and performance measurements have been done over additive white Gaussian (A WGN) channel. This coding technique generates its degree in a repeated sequential manner which yields a mutual relation between adjacent codes that will be used in the decoding part. The decoding complexity of the proposed structure is similar to that of the Raptor codes. The simulation results show that the proposed approach has better performance in terms of error floor and successful decoding ratio when compared to LT codes using robust soliton distribution (RSD) and memorybased-RSD (MBRSD) even when supported by belief propagation-pattern recognition (BP-PR) technique.
Citation - WoS: 24
Citation - Scopus: 27
Partially Coherent Airy Beam and Its Propagation in Turbulent Media
(Springer Heidelberg, 2013) Sermutlu, Emre; Eyyuboglu, Halil T.
The properties of partially coherent Airy beam propagating in turbulent media are investigated. Firstly the variations in the intensity profile and the location of the Airy beam peak with respect to dislocation and width parameters are examined. On the source plane, it is shown that there is a threshold value of the dislocation parameter which determines whether the beam peak will lie on the negative or positive side of the axis. The changes in source beam power and source beam size with respect to dislocation and width parameters are also examined. Then analysis is made for partially coherent Airy beam propagating in turbulence against variations in propagation distance, dislocation and width parameters, structure constant and partial coherence. It is found that the peak of the Airy beam will always shift to the right for increasing propagation distance. But this shift will become less as the dislocation and width parameters are increased. Finally it is shown that higher levels of turbulence and partial coherence will lead to more spreading and shift the beam peak more toward the on-axis position.
Citation - Scopus: 1
Information theory perspective of polar codes and polar encoding
(Springer Science and Business Media B.V., 2019) Gazi, Orhan
Polar codes are one of the recently discovered capacity achieving channel codes. What makes the polar codes different from other channel codes is that polar codes are designed mathematically and their performance are mathematically proven. © Springer Nature Singapore Pte Ltd. 2019.
Citation - WoS: 24
Citation - Scopus: 23
Scintillation Index of Flat-Topped Gaussian Laser Beam in Strongly Turbulent Medium
(Optical Soc Amer, 2011) Gercekcioglu, Hamza; Baykal, Yahya
In a strongly turbulent medium, the scintillation index of flat-topped Gaussian beams is derived and evaluated. In the formulation, unified solution of Rytov method is utilized. Our results correctly reduce to the existing strong turbulence scintillation index of the Gaussian beam, and naturally to spherical and plane wave scintillations. Another checkpoint of our result is the scintillation index of flat-topped Gaussian beams in weak turbulence. Regardless of the order of flatness, scintillations of flat-topped Gaussian beams in strong turbulence are found to be determined mainly by the small-scale effects. For large-sized beams in moderate and strongly turbulent medium, flatter beams exhibit smaller scintillations. (C) 2011 Optical Society of America
Citation - WoS: 14
Citation - Scopus: 15
Intensity Correlations of General Type Beam in Weakly Turbulent Atmosphere
(Elsevier Sci Ltd, 2011) Baykal, Yahya
In weakly turbulent atmosphere, intensity correlations at the receiver plane are formulated for a general type optical beam. Evaluating our formula at the appropriate source parameters versus the diagonal distance, intensity correlations for cos Gaussian, cosh Gaussian, annular and flat-topped Gaussian beams are obtained. As compared to Gaussian beam, intensity correlations are found smaller for cos Gaussian and larger for cosh Gaussian beams. Intensity correlations of cos Gaussian, cosh Gaussian and flat-topped Gaussian beams become larger at large source sizes. Thicker annular beams and flatter flat-topped Gaussian beams show larger intensity correlations. Without generalizing, intensity correlations tend to increase when the link length and the structure constant decrease and the wavelength increases. Our results show that the intensity correlations are not only built up using random medium effects, but also using the diffraction pattern formed at the receiver plane for the specific incidence investigated. As a check point, for all source types and medium parameters, our evaluations indicate that intensity correlations approach zero at sufficiently large diagonal distances. (C) 2011 Elsevier Ltd. All rights reserved.
Citation - WoS: 2
Citation - Scopus: 2
Diffraction by Material Half-Planes for Grazing Incidence
(Elsevier Gmbh, Urban & Fischer verlag, 2018) Umul, Yusuf Ziya
It is shown that the actual solutions of the diffraction problem of waves by material half planes do not reduce to the correct diffracted field expressions for grazing incidence. The correct expressions are obtained for the diffracted waves by resistive and conductive half screens. The case for the impedance half-plane directly emerges in terms of the diffracted fields by the previous two semi-screens. The behaviors of the geometric optics and diffraction waves are investigated numerically. (C) 2017 Elsevier GmbH. All rights reserved.
Modified Theory of Physical Optics Approach To Impedance Surfaces for Skew Incidence
(Elsevier Gmbh, 2020) Umul, Yusuf Ziya
A three dimensional modified theory of physical optics integral is constructed for the impedance surfaces, illuminated by a plane wave for skew incidence. The scalar fields are taken into account. The kernel of the physical optics integral is evaluated by using a relation with the geometric optics waves and stationary phase evaluation of the integral. The obtained surface integral is applied to the diffraction problem of waves by an impedance half-plane. The scattered fields are investigated numerically.
Citation - WoS: 1
Citation - Scopus: 1
Speckle Contrast for Flat-Topped Field Profiles
(Elsevier Sci Ltd, 2013) Baykal, Yahya
Speckle contrast is evaluated for flat-topped field profiles where the characterization and evaluation of speckle contrast are based on mathematical formulation and theoretical parameter variation. The speckle field which is represented by the random phase shift and the random phase tilt is multiplied by the flat-topped shaped field. The increase in the source size of the flat-topped field first causes the speckle contrast to increase, then reaches saturation. Increase in the variance of the phase shift of a flat-topped field causes an appreciable increase in the speckle contrast. Depending on the diffraction characteristics of the finite sized flat-topped field distribution, upon changing the variance of the phase shift, the speckle contrast exhibits different behavior at short and long propagation distances. However, especially for small variances of phase tilts, the speckle contrast seems to monotonically increase as the propagation distance becomes larger. The effect of the order of flatness on the speckle contrast is small at short propagation distances which can be appreciable at long propagation distances. (C) 2012 Elsevier Ltd. All rights reserved.
Citation - Scopus: 1
Relativistic Wave Equation for a Quantum Particle With Potential Energy
(Elsevier Gmbh, 2019) Umul, Yusuf Ziya
A new relativistic wave equation is derived for a quantum particle which moves in a potential. The relativistic equation of energy is reconsidered with the potential energy term. The energy-momentum relation is obtained for this case. The related differential equation is derived by taking into account a wave function in terms of a plane wave. It is shown that the new equation directly reduces to the Schrodinger equation of a particle, moving in a potential in the non-relativistic limit. The wave equation is applied to the interaction of wave function with a potential barrier.
Citation - WoS: 4
Citation - Scopus: 4
The Performance Bounds of an Optical Communication System Using Irradiance Profile Modulation
(Taylor & Francis Ltd, 2017) Eyyuboglu, Halil T.
We assess the performance bounds of an optical communication system that uses irradiance profile modulation. This modulation is based on the four different orders of vortex beams. To this end, we find the turbulence induced average irradiance profiles of Gaussian, Bessel-Gaussian and modified Bessel-Gaussian vortex beams on the receiver plane. Each one is then cross correlated against the free space equivalents. Plotting the cross-correlation coefficients, it becomes possible to identify the borders of correct decision and error regions, thus, deduce the performance bounds of such a system. When measured in terms of structure constant, i.e. the turbulence strength and the propagation length being fixed to 3km, it is seen that the error region extends beyond the structure constant values of 10(-13) m(-2/3) and higher. There seem to be some variations with the beam type and the order of the vortex beam. The performance of Bessel-Gaussian vortex beam comes out to be slightly better than the rest.
Citation - WoS: 1
Propagation of a Generalized Beam in Abcd System
(Cemal Ardil-IEC Barbaros Mah Hasret Sitesi, 2005) Eyyuboglu, HT; Elektronik ve Haberleşme Mühendisliği
For a generalized Hermite sinosiodal/hyperbolic Gaussian beam passing through an ABCD system with a finite aperture, the propagation properties are derived using the Collins integral. The results are obtained in the form of intensity graphs indicating that previously demonstrated rules of reciprocity are applicable, while the existence of the aperture accelerates this transformation.
Investigation of Factors Affecting Noise Robustness in Voice Activity Detectors
(Gazi Univ, Fac Engineering Architecture, 2023) Ozaydin, Selma
In this manuscript, some voice activity detectors (VADs) in the literature were examined in terms of factors affecting their robustness under different acoustic noise conditions and in this context, the changes in detection accuracy rates according to changing noise conditions were tested. In this scope, the effect of situations such as whether the threshold value used in the decision phase in VAD methods is fixed or adaptive, the analysis window is short or long, the use of more than one feature vector together has been evaluated and analyzed comparatively. While three of the four different VAD detectors examined in this manuscript use feature vectors within the short-term analysis window while generating the decision result, one decides according to the measurement result of long-term spectral vectors. The VAD detectors in the article have been tested using the NOIZEUS noisy speech database. Thus, the performance of the analyzed VADs has been evaluated under different acoustic conditions using an extensive database that has already taken place in the literature. During the testing of the analyzed VADs, different input noise speech signals with environmental background noises between [15-0dB] such as restaurant, car, street, or station were tested. Tests were carried out using objective test measurement methods and the detection accuracy rate of each VAD method was measured. The results showed that each method gave different endurance performance in adverse environmental conditions.
Citation - WoS: 6
Citation - Scopus: 6
Applications of Cognitive Radio Networks: Recent Advances and Future Directions
(Sage Publications inc, 2016) Pan, Miao; Joshi, Gyanendra Prasad; Gazi, Orhan; He, Jianhua; Coupechoux, Marceau; Kim, Sung W.
Citation - WoS: 8
Citation - Scopus: 8
Dynamics of the D2+ni(100) Collision System: Analysis of the Reactive and Inelastic Channels
(Wiley-blackwell, 2001) Böyükata, M; Güvenç, ZB; Jackson, B; Jellinek, J
The reactive and scattering channels of the D(2)(v, j) + Ni(100) collision system are studied using quasiclassical molecular dynamics simulations. The interaction between the D(2) and the atoms of the surface is modeled by a LEPS (London-Eyring-Polani-Sato) potential energy function. The molecule is aimed at three different impact sites (atop, bridge, and center) of a rigid Ni(100) surface along the normal direction with various collision energies less than or equal to1.0 eV. Dissociative chemisorption probabilities are computed for different rotational states of the molecule. Probability distributions of the final rovibrational states of the ground-state Dp molecule scattered from those impact sites are also computed as a function of the collision energy. Higher collision energy results in excitation of higher rotational and/or vibrational states of the scattered molecule. At collision energies below 0.1 eV an indirect dissociation mechanism (through molecular adsorption) dominates the reaction. (C) 2001 John Wiley & Sons, Inc.
Citation - WoS: 7
Citation - Scopus: 8
The Theory of the Boundary Diffraction Wave for Wedge Diffraction
(Taylor & Francis Ltd, 2008) Umul, Yusuf Z.
A new potential function, line integration which gives the edge diffracted fields, is constructed for wedge diffraction by using the method of modified theory of physical optics. The surface integrals are transformed into line integrals by the technique of asymptotic reduction. As an application of the novel potential function, the diffracted field is obtained for the geometry of a wedge for arbitrary incidence of plane waves.
Citation - WoS: 67
Influence of Turbulence on the Effective Radius of Curvature of Radial Gaussian Array Beams
(Optical Soc Amer, 2010) Ji, Xiaoling; Eyyuboglu, Halil T.; Baykal, Yahya
The analytical formula for the effective radius of curvature of radial Gaussian array beams propagating through atmospheric turbulence is derived, where coherent and incoherent beam combinations are considered. The influence of turbulence on the effective radius of curvature of radial Gaussian array beams is studied by using numerical calculation examples. (C) 2010 Optical Society of America
Citation - WoS: 5
Citation - Scopus: 5
Performance of M-Ary Pulse Position Modulation for Aeronautical Uplink Communications in an Atmospheric Turbulent Medium
(Optical Soc Amer, 2019) Baykal, Yahya; Gokce, Muhsin Caner; Ata, Yalcin
This paper discusses the bit-error-rate (BER) performance of an aeronautical uplink optical wireless communication system (OWCS) when a Gaussian beam is employed and the M-ary pulse position modulation technique is used in an atmospheric turbulent medium. Weak turbulence conditions and log-normal distribution are utilized. The Gaussian beam is assumed to propagate on a slant path, the transmitter being ground-based, and the airborne receiver is on-axis positioned. Variations of BER are obtained against the variations in the link length, Gaussian beam source size, zenith angle, wind speed, wavelength, modulation order, data bit rate, equivalent load resistor, avalanche photodetector gain, and detector quantum efficiency. It is observed that the performance of the aeronautical uplink OWCS is affected from atmospheric turbulence significantly. (C) 2019 Optical Society of America

Browse

Browsing Elektronik ve Haberleşme Mühendisliği Bölümü by Title