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Doktora Tezleri

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Browsing Doktora Tezleri by browse.metadata.publisher "Çankaya Üniversitesi"

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    Doctoral Thesis
    Beam shaping effects on MIMO free-space optical communication systems
    (Çankaya Üniversitesi, 2016) Gökçe, Muhsin Caner
    Multiple Input Multiple Output (MIMO) systems are employed in Free Space Optical (FSO) communication links to improve the link reliability in the presence of atmospheric turbulence. In this thesis, we consider a MIMO FSO system with practical transmitter and receiver configurations that consists of a radial laser array with Gaussian beams and a detector array with Gaussian apertures. Using the extended Huygens-Fresnel principle in weak atmospheric turbulence, we have derived formulations to find the average power and the power correlations on the finite sized detectors. This lets us to quantify the performance metrics such as the power scintillation index, the aperture averaging factor and the average bit error rate () as a function of system parameters, i.e., transmitter and receiver ring radius, number of Gaussian laser beams, number of detectors, laser source size, detector aperture radius, degree of source coherence, link distance and the structure constant of atmosphere. At first, by the help of the derivations the performance of multiple-input single-output (MISO) FSO system is investigated using both for coherent and for partially coherent Gaussian sources. Then, we improve our derivations and investigate the performance of MIMO FSO systems. In this way, the performance of MIMO FSO system is compared to that of MISO FSO, single-input multiple-output (SIMO) FSO and single-input single-output (SISO) FSO systems. MISO systems are then employed in underwater wireless optical communication (UWOC) links to mitigate the degrading effects of oceanic turbulence. To quantify the scintillation index of the MISO UWOC system, the Huygens - Fresnel principle is used with the novel equivalent structure constant of atmosphere. The oceanic turbulence parameters such as rate of dissipation of mean-squared temperature, rate of dissipation of kinetic energy per unit mass of fluid, Kolmogorov microscale, the ratio of temperature to salinity contributions to the refractive index spectrum, link distance and the wavelength which are expressed by the novel equivalent structure constant of atmosphere. Using the Matlab program, we present graphs and investigate the effect of system parameters on the performance metrics.
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    Doctoral Thesis
    Design of high performance low latency rateless codes
    (Çankaya Üniversitesi, 2017) Abdulkhaleg, Nadhir Ibrahim
    Luby Transform (LT) codes are one of the best rateless codes mainly designed for binary erasure channel. The characteristics of such codes perfectly performing when used with bulk data files, however a performance degradation has been observed when using them with short length messages. In this thesis, we present a new design for rateless codes, particularly an efficient LT codes using robust soliton distribution (RSD) as a degree generation method and tested in both binary erasure channel (BEC) and noisy channels like the additive white Gaussian noise (AWGN) channel. First, a new proposed decoding technique is defined as belief propagation-pattern recognition (BP-PR) is implemented to enhance the decoding ability of the conventional (BP) algorithm to overcome the problem of losing degree-one coded symbols which caused early decoding termination. The simulation results approve the improvement of the BP-PR when used with LT-RSD and outperforms the bit error rate (BER) records for the state of art techniques like memory-based robust Soliton distribution using conventional BP (LT-MBRSD-BP) or the Gaussian elimination assisted belief propagation (LT-RSD-BP-GE) and improve the records for the BER when used with MBRSD, ISD and optimal degree distribution (ODD), to form the new code called (LT-MBRSD-BP-PR),(LT-ISD-BP-PR) and (LT-ODD-BP-PR) respectively. Second, a new efficient deterministic encoding technique using deterministic degree generator with random data selection (LT-DE) is applied for extremely short data lengths. The degree generation method is based on creating the degrees in a repeated frame with a limited upper value called repetition period (𝑅𝑝) and the data symbols are chosen sequentially from a truncated data file. The data file is truncated to segments of length (𝑅𝑝) and each segment is chosen based on a random sequence. Testing this (LT-DE) against (LT-RSD-BP-PR), (LT-MBRSD-BP-PR) and (LT-ODD-BP-PR) in a BEC environment had approved the superiority of such code over all the other mentioned techniques. It has the lower error floor and higher successful decoding rate with minimum overhead and computational cost. The formation of this (LT-DE) associates a mutual relation between the successive coded symbols which motivate us to present a new sequential decoding technique mainly used over (AWGN) channel. With such new encoding-decoding technique LT codes can approach the decoding complexity cost of Raptor codes with smaller overhead and less encoding complexity as well.
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    Doctoral Thesis
    Scattering of evanescent waves by reflectors
    (Çankaya Üniversitesi, 2017) Kara, Mustafa
    In this thesis, the scattering integral of electromagnetic incident waves scattered by a cylindrical parabolic perfectly electric conducting reflector is evaluated asymptotically. For the determination of the reflected fields the method of stationary phase, and for the diffracted fields the edge point method, whose details are given in appendix, are employed. A plane wave with an arbitrary angle is assumed as incident on the reflector. The evaluated reflected and diffracted fields are examined numerically for any observation point. In the evaluation of diffracted fields, the non-uniform diffracted field expression encountered is overcome by means of Fresnel function. For the numerical analysis, basically two cases are taken into consideration. One of them is the incident angle being real, and the other is the complex value of it. For these values, the effects of all possible combinations including the complex conjugates are examined and plotted numerically.