Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/260
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Article Citation - WoS: 24Citation - Scopus: 26Scintillation and Ber for Optimum Sinusoidal Gaussian Beams in Weak Non-Kolmogorov Turbulence(Elsevier Science Bv, 2014) Gercekcioglu, Hamza; Baykal, YahyaThe scintillation index and the average bit error rate (BER) are evaluated for the optimum sinusoidal Gaussian beams in weak non-Kolmogorov turbulence. The beam parameters that minimize the scintillation index and the average BER are stated and such beams are denoted as the optimum beams. For the collimated Gaussian, cos- and cosh-Gaussian beams, the scintillations increase as the power law exponent, a increases. Cos- and cosh-Gaussian beams that have larger absolute displacement parameters are found to exhibit larger scintillations especially at small a. Larger focal length and larger source size of cos-Gaussian beams induce reduction in the scintillations. When the propagation distance is large, the power law exponent is small and the source size is large, the scintillations of the optimum beams tend to decrease. Small power law exponent and large source size reduce the average BER. The optimum beam is shown to exhibit the smallest average BER for any a. (C) 2014 Elsevier B.V. All rights reserved.Article Citation - WoS: 18Citation - Scopus: 16Ber of Annular and Flat-Topped Beams in Strong Turbulence(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, YahyaThe 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.Article Citation - WoS: 19Citation - Scopus: 17Average Transmittance in Non-Kolmogorov Turbulence(Elsevier Science Bv, 2013) Ata, Yalcin; Baykal, Yahya; Gercekcioglu, HamzaAverage transmittance in non-Kolmogorov turbulence is evaluated. Our recently published equivalent structure constant formulation is employed in our numerical evaluations. At the fixed propagation distance and wavelength, and at the corresponding equivalent structure constant, as the power law exponent of the non-Kolmogorov spectrum increases, the on-axis transmittance is found to decrease. At the same power law exponent of the non-Kolmogorov spectrum, the off-axis transmittance is obtained to be smaller than the on-axis transmittance. Off-axis transmittance variation versus the power law exponent shows that similar to the on-axis case, increase in the power law exponent eventually causes the off-axis transmittance to decrease, however this decrease occurs at larger power law exponent for larger off-axis distance. (C) 2013 Elsevier B.V. All rights reserved.Article Citation - WoS: 34Citation - Scopus: 39Intensity Fluctuations of Flat-Topped Beam in Non-Kolmogorov Weak Turbulence(Optical Soc Amer, 2012) Gercekcioglu, Hamza; Baykal, Yahya; Gerçekciolu, HamzaResults obtained on the intensity fluctuations of flat-topped Gaussian beams in weakly turbulent non-Kolmogorov horizontal atmospheric optics links are represented. Effects on the scintillation index of the power law a that describes the non-Kolmogorov spectrum are examined. Our results correctly reduce to the existing intensity fluctuations of flat-topped beams in Kolmogorov turbulence. Variation of the scintillation index against non-Kolmogorov power law alpha exhibits a peak at the worst power law alpha(w), which happens to be smaller than the Kolmogorov power law of 11/3. If the power law is smaller (larger) than alpha(w), increase in alpha will increase (decrease) the intensity fluctuations. Evaluation of the scintillation index at the worst power law results in smaller fluctuations for a Gaussian beam at short propagation distances; however, at long propagation distances flatter beams happen to possess smaller fluctuations. The scintillation change versus the source size follows a similar trend regardless whether the flat-topped beam propagates in a Kolmogorov or non-Kolmogorov medium. (C) 2012 Optical Society of AmericaArticle Citation - WoS: 9Citation - Scopus: 11Application of Equivalent Structure Constant in Scintillations and Ber Found for Non-Kolmogorov Spectrum(Elsevier Science Bv, 2014) Baykal, Yahya; Gercekcioglu, HamzaThe evaluation of system parameters in the non-Kolmogorov turbulent atmosphere involves the structure constant valid at the relevant non-Kolmogorov power law exponent. In some of the existing results, the comparisons of system parameters found under the Kolmogorov and non-Kolmogorov turbulences were made by using the same structure constant for all the power law exponents of the non-Kolmogorov spectrum. In this paper, we evaluate the scintillations and the average Bit Error Rate (< BER >) for the flat-topped and the annular beams in non-Kolmogorov turbulence, this time using the equivalent structure constant which is now different for all the power law exponents. It is observed that the scintillations and the < BER > show completely different behaviour when evaluated with the equivalent structure constant as compared to evaluations with constant structure constant. (C) 2013 Elsevier B.V. All rights reserved.Article Citation - WoS: 32Citation - Scopus: 32Ber of Annular and Flat-Topped Beams in Non-Kolmogorov Weak Turbulence(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, YahyaThe 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.Article Citation - WoS: 69Citation - Scopus: 74Equivalence of Structure Constants in Non-Kolmogorov and Kolmogorov Spectra(Optical Soc Amer, 2011) Baykal, Yahya; Gercekcioglu, HamzaWe find the equivalence of the structure constants in non-Kolmogorov and Kolmogorov spectra in a turbulent atmosphere. As the reference point, the spherical wave scintillation index in a non-Kolmogorov medium is used. Relations of the structure constants are found to be functions of the power law of the turbulence spectrum and the Fresnel zone. It will be useful to employ the equivalence of the structure constants in making performance comparisons found with non-Kolmogorov and Kolmogorov spectra. (C) 2011 Optical Society of AmericaArticle Citation - WoS: 24Citation - Scopus: 23Scintillation Index of Flat-Topped Gaussian Laser Beam in Strongly Turbulent Medium(Optical Soc Amer, 2011) Gercekcioglu, Hamza; Baykal, YahyaIn 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 AmericaArticle Citation - WoS: 5Minimization Effects on Scintillations of Sinusoidal Gaussian Beams in Strong Turbulence(Iop Publishing Ltd, 2011) Gercekcioglu, Hamza; Baykal, YahyaMinimization effects on the on-axis scintillation index of cos Gaussian (cG) and cosh Gaussian (chG) beams are studied in strong turbulence. In our formulation, the unified solution of the Rytov method, which imposes spatial filtering to extend the solution to the strong turbulence regime, is applied. Our solution correctly reduces to the weak turbulence sinusoidal beam scintillations and the strong turbulence Gaussian beam scintillations. The conditions to minimize the scintillations are found to be focused chG beams. Small scale scintillations mainly determine the overall scintillations of cG and chG beams in strong turbulence. In strong turbulence, increase in the source size decreases the scintillations of collimated cG beams but does not change the scintillations of focused cG beams. Collimated cG beams having larger displacement parameters and large focal lengths show smaller scintillations in the strong regime. Change in the displacement parameters for collimated and focused chG beams and the focal length of focused chG beams do not considerably vary their scintillations in strong turbulence.Article Citation - WoS: 37Citation - Scopus: 44Annular Beam Scintillations in Strong Turbulence(Optical Soc Amer, 2010) Gercekcioglu, Hamza; Baykal, Yahya; Nakiboglu, CemA scintillation index formulation for annular beams in strong turbulence is developed that is also valid in moderate and weak turbulence. In our derivation, a modified Rytov solution is employed to obtain the small-scale and large-scale scintillation indices of annular beams by utilizing the amplitude spatial filtering of the atmospheric spectrum. Our solution yields only the on-axis scintillation index for the annular beam and correctly reduces to the existing strong turbulence results for the Gaussian beam-thus plane and spherical wave scintillation indices-and also correctly yields the existing weak turbulence annular beam scintillations. Compared to collimated Gaussian beam, plane, and spherical wave scintillations, collimated annular beams seem to be advantageous in the weak regime but lose this advantage in strongly turbulent atmosphere. It is observed that the contribution of annular beam scintillations comes mainly from the small-scale effects. At a fixed primary beam size, the scintillations of thinner collimated annular beams compared to thicker collimated annular beams are smaller in moderate turbulence but larger in strong turbulence; however, thinner annular beams of finite focal length have a smaller scintillation index than the thicker annular beams in strong turbulence. Decrease in the focal length decreases the annular beam scintillations in strong turbulence. Examining constant area annular beams, smaller primary sized annular structures have larger scintillations in moderate but smaller scintillations in strong turbulence. (C) 2010 Optical Society of America