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Gercekciolu H.,General Maritime | Baykal Y.,Cankaya University
IEEE Journal on Selected Areas in Communications | Year: 2015

On-axis intensity fluctuations of laser array beams are evaluated when they are used in a weakly turbulent non-Kolmogorov atmosphere. Our formulation of the scintillation index is based on the Rytov method, which in the limiting case, correctly reduces to the known Gaussian beam scintillation index in weak Kolmogorov turbulence. When the radius of the ring (on which the array beamlets are placed), number of beamlets forming the laser array, source size of the beamlets, propagation distance or the wavelength is fixed, a decrease in the power law exponent of the non-Kolmogorov spectrum is found to decrease the scintillation index. Examining for any realization of the non-Kolmogorov spectrum, it is observed that an increase in the ring radius, number of beamlets, wavelength, and decrease in the source size of the beamlet, propagation distance reduces the intensity fluctuations. © 2015 IEEE. Source


Although numerous studies have examined factors that influence smartphone addiction, few have analyzed the potential protective factors inherent in individuals that may benefit future intervention programs for smartphone addiction. Thus, this study established a model for analyzing the mediating effect that learning self-efficacy and social self-efficacy have on the relationship between university students' perceived life stress and smartphone addiction. Sampling 387 Taiwanese university students, we distributed scales surveying for university students' life stress, learning self-efficacy, social self-efficacy, and smartphone addiction. Data retrieved from the scales were analyzed using structural equation modeling (SEM). The SEM path analysis yielded the following results: (1) Academic stress had negative predictive power for social and learning self-efficacies, and interpersonal relationship stress had negative predictive power for social self-efficacy. (2) Social self-efficacy had positive predictive power for smartphone addiction. (3) Family and emotional stresses had positive predictive power for smartphone addiction. Generally, the results of this study could be used to significantly predict the life stresses that influenced university students' smartphone addiction. In addition, social self-efficacy can be considered a cognitive mechanism that mediates the relationships between academic stress and smartphone addiction and between interpersonal relationship stress and smartphone addiction. Finally, we discussed the research results and offered relevant suggestions for schools, university students, and future studies. © 2014 Elsevier Ltd. All rights reserved. Source


Gercekcioglu H.,General Maritime | Baykal Y.,Cankaya University
Optics Communications | Year: 2013

The average bit error rate () 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, 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, obtained for the annular and the flat-topped beams in strong turbulence is advantageous over the Gaussian beam values. © 2013 Elsevier B.V. All rights reserved. Source


Gercekcioglu H.,General Maritime | Baykal Y.,Cankaya University
Optics Communications | Year: 2014

The 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, α increases. Cos- and cosh-Gaussian beams that have larger absolute displacement parameters are found to exhibit larger scintillations especially at small α. 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 α. © 2014 Elsevier B.V. Source


Baykal Y.,Cankaya University | Gercekcioglu H.,General Maritime
Optics Communications | Year: 2014

The 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. © 2013 Elsevier B.V. Source

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