Elektrik Elektronik Mühendisliği Bölümü Yayın Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/411

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Now showing 1 - 20 of 40

Citation - WoS: 10
Citation - Scopus: 13
Adaptive Optics Corrections of Scintillations of Hermite-Gaussian Modes in an Oceanic Medium
(Optical Soc Amer, 2020) Baykal, Yahya
Adaptive optics correction of the scintillation index is found when Hermite-Gaussian laser beams are used in oceanic turbulence. Adaptive optics filter functions are used to find how the tilt, focus, astigmatism, coma, and total correction will behave under high order mode excitation. Reduction of the oceanic scintillation under various oceanic turbulence and system parameters is examined under different high order modes. Also, the effects of the source size, wavelength, and link length on the total adaptive optics correction of Hermite-Gaussian modes in an oceanic medium are investigated for different modes. (C) 2020 Optical Society of America
Citation - WoS: 1
Citation - Scopus: 1
Antenna Synthesis by Levin's Method Using a Novel Optimization Algorithm for Knot Placement
(Applied Computational Electromagnetics Soc, 2023) Sener, Goker
- Antenna synthesis refers to determining the antenna current distribution by evaluating the inverse Fourier integral of its radiation pattern. Since this inte-gral is highly oscillatory, Levin's method can be used for the solution, providing high accuracy. In Levin's method, the integration domain is divided into equally spaced sub-intervals, and the integrals are solved by transfer-ring them into differential equations. This article uses a new optimization algorithm to determine the location of these interval points (knots) to improve the method's accuracy. Two different antenna design examples are pre-sented to validate the accuracy and efficiency of the pro-posed method for antenna synthesis applications.
Citation - WoS: 9
Citation - Scopus: 10
Aperture Averaging in Multiple-Input Single-Output Free-Space Optical Systems
(Spie-soc Photo-optical instrumentation Engineers, 2015) Baykal, Yahya; Kamacioglu, Canan; Uysal, Murat; Gokce, Muhsin C.; Kamacoʇlu, Canan
Multiple-input single-output systems are employed in free-space optical links to mitigate the degrading effects of atmospheric turbulence. We formulate the power scintillation as a function of transmitter and receiver coordinates in the presence of weak atmospheric turbulence by using the extended Huygens Fresnel principle. Then the effect of the receiver aperture averaging is quantified. To get consistent results, parameters are chosen within the range of validity of the wave structure functions. Radial array beams and a Gaussian weighting aperture function are used at the transmitter and the receiver, respectively. It is observed that the power scintillation decreases when the source size, the ring radius, the receiver aperture radius, and the number of array beamlet increase. However, increasing the number of array beamlets to more than three seems to have negligible effect on the power scintillation. It is further observed that the aperture averaging effect is stronger when radial array beams are employed instead of a single Gaussian beam. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Citation - WoS: 27
Citation - Scopus: 26
Aperture Averaging in Multiple-Input Single-Output Free-Space Optical Systems Using Partially Coherent Radial Array Beams
(Optical Soc Amer, 2016) Baykal, Yahya; Uysal, Murat; Gokce, Muhsin Caner
Multiple-input single-output (MISO) techniques are employed in free-space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. In this paper, for the MISO FSO system, a partially coherent radial array and a finite-sized receiver aperture are used at the transmitter and the receiver, respectively. Using the extended Huygens - Fresnel principle, we formulate the average power and the power correlation at the finite-sized slow detector in weak atmospheric turbulence. System performance indicators such as the power scintillation index and the aperture averaging factor are determined. Effects of the source size, ring radius, receiver aperture radius, link distance, and structure constant and the degree of source coherence are analyzed on the performance of the MISO FSO system. In the limiting cases, the numerical results are found to be the same when compared to the existing coherent and partially coherent Gaussian beam scintillation indices. (C) 2016 Optical Society of America
Citation - WoS: 29
Citation - Scopus: 35
Ber Evaluations for Multimode Beams in Underwater Turbulence
(Taylor & Francis Ltd, 2016) Arpali, Serap Altay; Baykal, Yahya; Arpali, Caglar
In underwater optical communication links, bit error rate (BER) is an important performance criterion. For this purpose, the effects of oceanic turbulence on multimode laser beam incidences are studied and compared in terms of average BER (< BER >), which is related to the scintillation index. Based on the log-normal distribution, < BER > is analysed for underwater turbulence parameters, including the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, the parameter that determines the relative strength of temperature and salinity in driving index fluctuations, the Kolmogorov microscale length and other link parameters such as link length, wavelength and laser source size. It is shown that use of multimode improves the system performance of optical wireless communication systems operating in an underwater medium. For all the investigated multimode beams, decreasing link length, source size, the relative strength of temperature and salinity in driving the index fluctuations, the rate of dissipation of the mean squared temperature and Kolmogorov microscale length improve the < BER >. Moreover, lower < BER > values are obtained for the increasing wavelength of operation and the rate of dissipation of the turbulent kinetic energy in underwater turbulence.
Citation - WoS: 4
Citation - Scopus: 6
Case Study on Thermal Optimization of Oil Immersed Transformer Used in Solar Power Plant Based on Genetic Algorithm and Computational Fluid Dynamics
(Vinca inst Nuclear Sci, 2023) Iskender, Ires; Yukselen, Emir
Transformers are one of the most capital investments in the solar power generation. Their safe and stable operations in the electrical networks are important. The main failure factor of transformers is the high temperature generated by the losses during operation, which increases the probability of insulation damage that significantly affects the useful life of transformer. Considering the importance of oil temperature and its effects on the life of the transformer, a numerical method is developed in this paper to optimize the cooling system of the transformer. In this regard, genetic algorithm is used as an optimization method to minimize the total cost of the cooling system while maintaining the required thermal conditions of the transformer. A comprehensive parametric study is carried out among the effective cooling geometry parameters using 3-D electromagnetic and thermal models of the photovoltaic transformer to evaluate and analyze the temperature distribution. The accuracy and feasibility of the proposed method is established by comparing the numerical results with those obtained from the experimental test. The results of the proposed method are found to be in a good agreement with the experimental and simulation results.
Citation - WoS: 10
Citation - Scopus: 10
Cross-Beam Scintillations in Underwater Medium
(Spie-soc Photo-optical instrumentation Engineers, 2016) Baykal, Yahya
The fluctuation in the intensity, which is quantified by the scintillation index, is evaluated for cross beams when such beams propagate in an underwater medium experiencing turbulence. The variations in the scintillation index are investigated against the changes in the size of the cross beams, the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean-squared temperature, and the rate of dissipation of kinetic energy per unit mass of fluid. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)
Design of a Novel Converter Between Li-Ion Battery and Supercapacitor To Feed Synthetic Aperture Radar Loads for Satellite Applications
(int Journal Renewable Energy Research, 2020) Tulay, Gencer; İskender, İres; Iskender, Ires; Elektrik-Elektronik Mühendisliği
For satellite applications the active duty life of a satellite depends mainly on the battery lifespan. The charging or discharging current has an important role in useful life of batteries. For this purpose, the converters used in power supplies including batteries should be accurately designed such that the large current of batteries can be prevented. The power converter proposed in this paper is used to transfer energy from battery to supercapacitor when SAR (Synthetic Aperture Radar) load is active. Accordingly, the lifespan of battery and hence the lifespan of satellite will be extended. Parallel to preventing the large discharging current of batteries the proposed converter has another important property such that it is able to vary the input voltage without having any stability problems. The operating performance of the converter that is composed of Weinberg and Buck converters is analyzed using PSpice software and the results of simulations are verified through experiments. The results show that how well the converter operates satisfying the conditions required for satellite applications.
Citation - WoS: 1
Citation - Scopus: 1
Editorial: Optical Wave Propagation and Communication in Turbulent Media
(Frontiers Media Sa, 2023) Baykal, Yahya
Citation - WoS: 28
Citation - Scopus: 30
Effect of Anisotropy on Intensity Fluctuations in Oceanic Turbulence
(Taylor & Francis Ltd, 2018) Baykal, Yahya
For an optical spherical wave propagating in an oceanic turbulent medium, the effect of anisotropy on the received intensity fluctuations is investigated. For different anisotropy factors, the variations of the scintillation index vs. the ratio that determines the relative strength of temperature and salinity in the index fluctuations, the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, viscosity, link length and the wavelength are plotted. It is found that, for all the oceanic turbulence and the link parameters of interest, as the medium becomes more anisotropic, the intensity of the optical spherical wave fluctuates less. It is concluded that the performance of an optical wireless communication systems (OWCS) operating in anisotropic oceanic turbulence is better than the performance of OWCS operating in isotropic oceanic turbulence.
Citation - WoS: 106
Citation - Scopus: 115
Effect of Eddy Diffusivity Ratio on Underwater Optical Scintillation Index
(Optical Soc Amer, 2017) Elamassie, Mohammed; Uysal, Murat; Baykal, Yahya; Abdallah, Mohamed; Qaraqe, Khalid
The performance of underwater optical wireless communication systems is severely affected by the turbulence that occurs due to the fluctuations in the index of refraction. Most previous studies assume a simplifying, yet inaccurate, assumption in the turbulence spectrum model that the eddy diffusivity ratio is equal to unity. It is, however, well known that the eddy diffusivities of temperature and salt are different from each other in most underwater environments. In this paper, we obtain a simplified spatial power spectrum model of turbulent fluctuations of the seawater refraction index as an explicit function of eddy diffusivity ratio. Using the derived model, we obtain the scintillation index of optical plane and spherical waves and investigate the effect of the eddy diffusivity ratio. (C) 2017 Optical Society of America
Citation - WoS: 6
Citation - Scopus: 9
Effects of Adaptive Optics on Bit Error Rate of M-Ary Ppm Oceanic Optical Wireless Communication Systems With Aperture Averaging in Strong Turbulence
(Iop Publishing Ltd, 2021) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin Caner
Scintillation is the result of oceanic turbulence reducing the bit error rate (BER) performance of oceanic optical wireless communication (OWC) systems. The scintillation, also known as intensity fluctuations, occurs due to the turbulence-induced wavefront deformations. The correction of deformations by adaptive optics (AO) reduces the scintillation effect of turbulence and results in improved BER performance. In this paper, an oceanic OWC (OOWC) system that has a Gaussian laser beam at the transmitter, finite-sized circular aperture at the receiver, employing M-ary pulse position modulation (PPM) and operating in strong oceanic turbulence, is considered. Improvement in the BER performance of the OOWC system is examined with the implementation of AO correction. Comparison of BER performances between the AO and non-adaptive optics OOWC systems is shown by calculating the metric defined. BER of M-ary PPM OOWC links is evaluated over gamma-gamma fading channels. The modified Rytov theory together with the Zernike filter functions is used to find the AO corrected aperture averaged scintillation index where extended Huygens-Fresnel technique is used to obtain the average received signal power.
Citation - WoS: 50
Citation - Scopus: 55
Expressing Oceanic Turbulence Parameters by Atmospheric Turbulence Structure Constant
(Optical Soc Amer, 2016) Baykal, Yahya
The parameters composing oceanic turbulence are the wavelength, link length, rate of dissipation of kinetic energy per unit mass of fluid, rate of dissipation of mean-squared temperature, Kolmogorov microscale, and the ratio of temperature to salinity contributions to the refractive index spectrum. The required physical entities such as the average intensity and the scintillation index in the oceanic medium are formulated by using the power spectrum of oceanic turbulence, which is described by oceanic turbulence parameters. On the other hand, there exists a rich archive of formulations and results for the above-mentioned physical entities in atmospheric turbulence, where the parameters describing the turbulence are the wavelength, the link length, and the structure constant. In this paper, by equating the spherical wave scintillation index solutions in the oceanic and atmospheric turbulences, we have expressed the oceanic turbulence parameters by an equivalent structure constant used in turbulent atmosphere. Such equivalent structure constant will help ease reaching solutions of similar entities in an oceanic turbulent medium by employing the corresponding existing solutions, which are valid in an atmospheric turbulent medium. (C) 2016 Optical Society of America
Citation - WoS: 13
Citation - Scopus: 13
Field Correlation of Flat-Topped Beams in Anisotropic Non-Kolmogorov Turbulent Atmosphere
(Taylor & Francis Ltd, 2019) Baykal, Yahya; Ata, Yalcin
Field correlation of flat-topped beams in anisotropic non-Kolmogorov turbulent atmosphere is formulated and evaluated. Larger anisotropic factor causes higher field correlations. Smaller field correlations are seen when the transverse distance at the receiver plane increases. Smaller field correlations are observed at large off-axis transverse receiver points, which are valid for any anisotropic factor and for any power-law exponent of non-Kolmogorov turbulence. When the flat-topped beam is composed of large number of Gaussian beams, the field correlation becomes smaller. In anisotropic non-Kolmogorov turbulence, longer propagation distances, larger structure constants, smaller inner scales and smaller source sizes decrease the field correlation. Larger power law exponent of non-Kolmogorov turbulence increases the field correlations at any anisotropic factor.
Citation - WoS: 9
Citation - Scopus: 8
Flat-Topped Beam Transmittance in Anisotropic Non-Kolmogorov Turbulent Marine Atmosphere
(Spie-soc Photo-optical instrumentation Engineers, 2017) Ata, Yalcin; Baykal, Yahya
Turbulence affects optical propagation, and, as a result, the intensity is attenuated along the path of propagation. The attenuation becomes significant when the turbulence becomes stronger. Transmittance is a measure indicating how much power is collected at the receiver after the optical wave propagates in the turbulent medium. The on-axis transmittance is formulated when a flat-topped optical beam propagates in a marine atmosphere experiencing anisotropic non-Kolmogorov turbulence. Variations in the transmittance are evaluated versus the beam source size, beam number, link distance, power law exponent, anisotropy factor, and structure constant. It is found that larger beam source sizes and beam numbers yield higher transmittance values; however, as the link distance, power law exponent, anisotropy factor, or structure constant increase, transmittance values are lowered. Our results will help in the performance evaluations of optical wireless communication and optical imaging systems operating in a marine atmosphere. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)
Citation - WoS: 13
Citation - Scopus: 14
Fourth-Order Mutual Coherence Function in Oceanic Turbulence
(Optical Soc Amer, 2016) Baykal, Yahya
We have recently expressed the structure constant of atmospheric turbulence in terms of the oceanic turbulence parameters, which are the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of kinetic energy per unit mass of fluid, rate of dissipation of the mean-squared temperature, wavelength, Kolmogorov microscale, and link length. In this paper, utilizing this recently found structure constant and the fourth-order mutual coherence function of atmospheric turbulence, we present the fourth-order mutual coherence function to be used in oceanic turbulence evaluations. Thus, the found fourth-order mutual coherence function of oceanic turbulence is evaluated for the special case of a point source located at the transmitter origin and at a single receiver point. The variations of this special case of the fourth-order mutual coherence function of oceanic turbulence against the changes in the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of the mean-squared temperature, the wavelength, and the Kolmogorov microscale at various link lengths are presented. (C) 2016 Optical Society of America
Citation - WoS: 23
Citation - Scopus: 24
Higher-Order Laser Beam Scintillation in Weakly Turbulent Marine Atmospheric Medium
(Optical Soc Amer, 2016) Baykal, Yahya
The atmosphere above the sea or ocean, known as the marine atmosphere, affects optical waves propagating through it in a different manner than the atmosphere above land. Like other system design parameters, intensity fluctuations of laser light propagating in marine atmosphere, quantified by the scintillation index, also show different variations. The on-axis scintillations of higher-order laser beams are formulated and evaluated when such excitations are employed in a weakly turbulent marine atmospheric medium. Variations of the scintillation index with respect to the changes in the Gaussian beam size of the higher-order mode, link length, wavelength, and structure constant are reported. Our results can be used in the design of an optical wireless communication link design operating in marine atmospheres. (C) 2016 Optical Society of America
Citation - Scopus: 9
Improvement of Underlay Cooperative Cognitive Networks Bandwidth Efficiency Under Interference and Power Constraints
(Ksii-kor Soc internet information, 2019) Preveze, Barbaros; Alkhayyat, Ahmed; Al-Mishmish, Hameed R. M.
The definition of the bandwidth efficiency (BE) of cognitive cooperative network (CCN) is the ratio between a number of the licensed slot(s) or sub-channel(s) used by the unlicensed users to transmit a single data packet from the unlicensed transmitter to unlicensed destination, and from unlicensed relay(s) to unlicensed destination. This paper analyzes and improves the BE in the underlay CCN with a new reactive relay selection under interference and power constraints. In other words, this paper studies how unlicensed cooperative users use the licensed network slot(s) or sub-channel(s) efficiently. To this end, a reactive relay selection method named as Relay Automatic Repeat Request (RARQ) is proposed and utilized with a CCN under interference and power constraints. It is shown that the BE of CCN is higher than that of cooperative transmission (CT) due to the interference and power constraint. Furthermore, the BE of CCN is affected by the distance of the interference links which are between the unlicensed transmitter to the licensed destination and unlicensed relay to the licensed destination. In addition, the BE for multiple relays selection over a CCN under interference and power constraints is also analyzed and studied, and it is shown that the BE of CCN decreases as the number of relays increases.
Citation - WoS: 6
Citation - Scopus: 6
Intensity Correlation of Collimated Gaussian Beams Propagating in Biological Tissues
(Taylor & Francis Ltd, 2021) Baykal, Yahya
Intensity correlations in turbulent biological tissues are evaluated when collimated Gaussian laser beam is used. Variations of the intensity correlations in biological tissues are investigated versus the diagonal distance from the origin of the observation plane for various tissue distances, biological tissue types, strength coefficients of the refractive-index fluctuations, starting points at the observation plane. Also, intensity correlations in biological tissues for various biological tissue types versus the strength coefficient of the refractive-index fluctuations, small length-scale factor, fractal dimension. It is found that intensity correlations become smaller at larger diagonal distance, tissue distance, strength coefficients of the refractive-index fluctuations, and at smaller source size, small length-scale factor and fractal dimensions. Behaviour of the intensity correlations is found to depend on the type of the biological tissue. Our results can be used in tissue identification and to find the disorders in biological tissues.
Citation - WoS: 65
Citation - Scopus: 70
Intensity Fluctuations of Multimode Laser Beams in Underwater Medium
(Optical Soc Amer, 2015) Baykal, Yahya
In an oceanic optical communications link, the received intensity fluctuations, quantified by the scintillation index, are formulated and evaluated when a multimode laser is used. The variations of the scintillation index versus the oceanic turbulence parameters are examined for different multimode laser structures. Oceanic turbulence parameters used are the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the Kolmogorov inner scale, and the parameter w that defines the ratio of temperature to salinity contributions to the refractive index spectrum. The results in this paper can be used to improve performance in the design of oceanic optical communications links. (C) 2015 Optical Society of America

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