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Öğe EpiGenetic Algorithm for Optimization: Application to Mobile Network Frequency Planning(Springer Heidelberg, 2016) Biroğul, SerdarDaha fazla Genetic algorithms (GA) has been used as a successful algorithm for many problems. GA has been redesigned with different methods or used in hybrid algorithms to solve different problems and improve solutions. In this study, epigenetic algorithm (EGA) design has been made by adapting epigenetic concepts to the classical GA structure. GA is counted as a heuristic research algorithm, and there is randomness in the function of genetic operators. However, owing to some serious research in medical field, it has been shown that through the epigenetics, randomness of crossover and mutation operators can be defined. With regards to this information in the field of medicine, in this study design of EGA, how epicrossover, epimutation operators, and epigenetic factors are made and how they do work and also how the epigenetic inheritance is possible have been told. Our designed EGA has been applied on base stations' BCCH frequency planning in GSM network that is a constrained optimization problem. Real base station's data have been used in solving the problem. EGA and GA coding have been made by using C# programming. In order to analyze the success of EGA than the classical GA, both algorithms have been used in solving of this problem. Because of this, EGA gave better results in a shorter time and less iteration than classical GA's.Daha fazla Öğe Frequency and voltage-dependent electrical and dielectric properties of Al/Co-doped PVA/p-Si structures at room temperature(Iop Publishing Ltd, 2014) Yücedağ, İbrahim; Kaya, Ahmet; Altındal, Şemsettin; Uslu, İbrahimDaha fazla In order to investigate of cobalt-doped interfacial polyvinyl alcohol (PVA) layer and interface trap (D-it) effects, Al/pSi Schottky barrier diodes (SBDs) are fabricated, and their electrical and dielectric properties are investigated at room temperature. The forward and reverse admittance measurements are carried out in the frequency and voltage ranges of 30 kHz-300 kHz and -5 V-6 V, respectively. C-V or epsilon '-V plots exhibit two distinct peaks corresponding to inversion and accumulation regions. The first peak is attributed to the existence of Dit, the other to the series resistance (R-s), and interfacial layer. Both the real and imaginary parts of dielectric constant (epsilon ' and epsilon '') and electric modulus (M ' and M ''), loss tangent (tan delta), and AC electrical conductivity (sigma(ac)) are investigated, each as a function of frequency and applied bias voltage. Each of the M ' versus V and M '' versus V plots shows a peak and the magnitude of peak increases with the increasing of frequency. Especially due to the Dit and interfacial PVA layer, both capacitance (C) and conductance (G/w) values are strongly affected, which consequently contributes to deviation from both the electrical and dielectric properties of Al/Co-doped PVA/p-Si (MPS) type SBD. In addition, the voltage-dependent profile of Dit is obtained from the low-high frequency capacitance (CLF-CHF) method.Daha fazla Öğe Electrical and photovoltaic properties of Au/(Ni, Zn)-doped PVA/n-Si structures in dark and under 250 W illumination level(Elsevier Sci Ltd, 2014) Altındal, Şemsettin; Tunç, Tuncay; Tecimer, Hüseyin; Yücedağ, İbrahimDaha fazla Electrical and photovoltaic properties of Au/(Ni, Zn)-doped PVA/n-Si structures were investigated in dark and under 250 W illumination level using forward and reverse bias current voltage (I-V) measurements at room temperature. Reverse saturation current (I-0), ideality factor (n), and zero-bias-barrier height (phi(B0)) values were found as 1.18 x 10(-8) A, 2.492 and 0.705 eV in dark (low region); 9.10 x 10(-7) A, 7.515 and 0.597 eV in dark (high region); and 1.05 x 10(-6) A, 6.053 and 0.593 eV under 250W illumination level. The forward bias semi-logarithmic I-V plot in dark was described using a two diode model, indicating two current-transport mechanisms acting in the diode. The first mechanism can be attributed to recombination of carriers between Au/(Ni, Zn) and doped PVA, and second one can be attributed to recombination in the depletion region. In addition, the energy density distribution profile of surface stares (N-ss) was extracted from the forward-bias I-V data by raking into account the voltage dependent of the effective barrier height(phi(e)), ideality factor n(V), and series resistance (R-s). (C) 2014 Elsevier Ltd. All rights reserved.Daha fazla Öğe Electrical and dielectric properties and intersection behavior of G/omega-V plots for Al/Co-PVA/p-Si (MPS) structures at temperatures below room temperature(Korean Physical Soc, 2014) Yücedağ, İbrahim; Kaya, Ahmet; Altındal, Şemsettin; Uslu, İbrahimDaha fazla Both the electrical and the dielectric properties of the Al/Co-doped polyvinyl alcohol/p-Si metal-polymer-semiconductor (MPS) structure have been studied using temperature-dependent admittance-voltage (C/G-V) measurements at temperatures below room temperature at 300 kHz. The C-V plot indicates two peaks for each temperature corresponding to inversion and accumulation regions, respectively. The first peak was attributed to a particular distribution of interface traps (D (it) ), and the second was attributed to the series resistance (R (s) ) and interfacial polymer layer. G/omega-V plots show almost U-shape behavior for all temperatures and a crossing at almost 3 V. Such behavior of the G/omega-V plots may be attributed to the lack of free charge at low temperatures. After this intersection point, while the value of the capacitance (C) starts decreasing, the G/omega continues to increase. The temperature-dependent real and imaginary parts of the dielectric constant (epsilon', epsilon aEuro(3)) and of the electric modulus (M', MaEuro(3)), as well as the ac electrical conductivity (sigma (ac) ), of structure were obtained using C and G data before and after the intersection point (at 2 and 6 V), respectively. Experimental results show that the epsilon', epsilon aEuro(3), loss tangent (tan delta), sigma (ac) , M', and MaEuro(3) values were strong functions of the temperature and the applied bias voltage. In addition, G/omega-T and epsilon aEuro(3)-T plots show two different behaviors, one before and the other after the intersection point.Daha fazla Öğe Dimensional optimization of 6-DOF 3-CCC type asymmetric parallel manipulator(Taylor & Francis Ltd, 2014) Toz, Metin; Küçük, SerdarDaha fazla In this paper, dimensional optimization of a six-degrees-of-freedom (DOF) 3-CCC (C: cylindrical joint) type asymmetric parallel manipulator (APM) is performed by using particle swarm optimization (PSO). The 3-CCC APM constructed by defining three angle and three distance constraints between base and moving platforms is a member of 3D3A generalized Stewart-Gough platform (GSP) type parallel manipulators. The dimensional optimization purposes to find the optimum limb lengths, lengths of line segments on the base and moving platforms, attachment points of the line segments on the base platform, the orientation angles of the moving platform, and position of the end-effector in the reachable workspace in order to maximize the translational and orientational dexterous workspaces of the 3-CCC APM, separately. The dexterous workspaces are obtained by applying condition number and minimum singular values of the Jacobian matrix. The optimization results are compared with the traditional GSP manipulator for illustrating the kinematic performance of 3-CCC APM. Optimizations show that 3-CCC APM have superior dexterous workspace characteristics than the traditional GSP manipulator.Daha fazla Öğe Dielectric properties and electric modulus of Au/PPy/n-Si (MPS) type Schottky barrier diodes (SBDS) as a function of frequency and applied bias voltage(World Scientific Publ Co Pte Ltd, 2015) Yücedağ, İbrahim; Ersöz, Gülçin; Gümüş, Ahmet; Altındal, ŞemsettinDaha fazla Au/PPy/n-Si Schottky barrier diodes (SBDs) were fabricated by forming polypyrrole (PPy) organic layer on n-Si using the spin coating technique. Frequency-dependent dielectric constant (epsilon'), dielectric loss (epsilon ''), loss tangent (tan delta), real and imaginary parts of electrical modulus (M' and M '') and AC electrical conductivity (sigma(ac)) parameters of the structure were investigated in the frequency range of 10-500 kHz. It was found that the values of the epsilon', epsilon '' and tan delta, in general, decrease with increasing frequency while an increase is observed in sigma(ac), M' and M ''. The tan delta and M '' also exhibit a peak at about zero-bias voltage, while peak intensity weakens with increasing frequency. The values of epsilon' and M' decrease with increasing voltage while an increase is observed in epsilon '', tan delta sigma(ac) and M ''. These changes in epsilon', epsilon '', tan delta, M', M '' and sigma(ac) values was attributed to surface charge polarization and the particular density distribution of surface states localized at PPy/n-Si interface.Daha fazla Öğe Development of derivation of inverse Jacobian matrices for 195 6-DOF GSP mechanisms(Tubitak Scientific & Technical Research Council Turkey, 2016) Toz, Metin; Küçük, SerdarDaha fazla One of the key issues in robotics is finding high-performance manipulator structures. To evaluate the performance of a parallel manipulator, researchers mostly use kinematic performance indices (such as condition number, minimum singular value, dexterity, and manipulability), which are based on inverse Jacobian matrices. Driving the inverse Jacobian matrix of even one parallel manipulator is a very cumbersome process. However, in this paper, general equations for the inverse Jacobian matrices of 195 GSP mechanisms are symbolically derived by considering 4 basic leg types having 1 angular and 4 distance constraints. With the help of these general equations, the development of the inverse Jacobian matrix for a GSP mechanism can be achieved by defining only the leg connection points on the base and moving platforms with minimum cost. Having derived the inverse Jacobian matrices, one can directly compute kinematic performance indices to measure and compare the manipulator performance of the 195 GSP mechanism. These analyses may yield new high-performance GSP mechanisms for use in engineering, medical device design, and other applied branches. Two different mechanisms (symmetrical and asymmetrical) are given as examples to describe the methodology for deriving the inverse Jacobian matrices. Finally, 2 numerical examples are given for illustrating the practical applications of the procedure.Daha fazla Öğe Dexterous workspace optimization of an asymmetric six-degree of freedom Stewart-Gough platform type manipulator(Elsevier, 2013) Toz, Metin; Küçük, SerdarDaha fazla In this paper, an asymmetric Generalized Stewart-Gough Platform (GSP) type parallel manipulator is designed by considering the type synthesis approach. The asymmetric six-Degree Of Freedom (DOE) manipulator optimized in this paper is selected among the GSPs classified under the name of 6D. The dexterous workspace optimization of Asymmetric parallel Manipulator with tEn Different Linear Actuator Lengths (AMEDIAL) subject to kinematics and geometric constraints is performed by using the Particle Swarm Optimization (PSO). The condition number and Minimum Singular Value (MSV) of homogenized Jacobian matrix are employed to obtain the dexterous workspace of AMEDLAL. Finally, the six-DOF AMEDLAL is also compared with the optimized Traditional Stewart-Gough Platform Manipulator (TSPM) considering the volume of the dexterous workspace in order to demonstrate its kinematic performance. Comparisons show that the manipulator proposed in this study illustrates better kinematic performance than TSPM. (C) 2013 Elsevier B.V. All rights reserved.Daha fazla Öğe DATA CLUSTERING BASED ON FUZZY C-MEANS AND CHAOTIC WHALE OPTIMIZATION ALGORITHMS(Yildiz Technical Univ, 2019) Arslan, Hatice; Toz, MetinDaha fazla Clustering is the process of sub-grouping data according to certain distance and similarity criteria. One of the most commonly used clustering algorithms in the literature is the Fuzzy C-Means (FCM) algorithm based on the fuzzy clustering principle. Although FCM is an efficient algorithm, random selection of initial cluster centers is a disadvantage since it easier trap the algorithm into local optimum. This problem can be solved by approaching the clustering problem as an optimization problem. In this article, Whale Optimization Algorithm (WOA), a global optimization algorithm developed by inspiration from hunting behaviors of humpback whales, has been improved with chaos maps using an adaptive normalization method and chaotic WOA algorithms are proposed. They are then hybridized with FCM algorithm. The performances of the proposed chaotic optimization algorithms are tested with thirteen different benchmark functions. Results are evaluated with means and standard deviations of the objective function values and with the Wilcoxon Sign Rank Test at 0.05 significance level. The clustering performances of the proposed hybrid algorithms measured according to the objective function, the Rand Index and the Adjusted Rand Index values and compared with the K-Means, FCM and some of the other hybrid algorithms for six different data sets selected from the UCI Repository database. In addition, the new hybrid clustering algorithms are improved by using Chebyshev distance function instead of the classical Euclidean distance for the FCM algorithm in order to increase their data clustering performances. As a result, it has been seen that the used chaos functions improve the optimization performance of WOA algorithm, integrating chaotic WOA algorithms with FCM algorithm enhances the disadvantages of FCM algorithm and changing the distance function increases clustering performance of the proposed algorithms.Daha fazla Öğe Temperature and voltage dependences of dielectric properties and ac electrical conductivity in Au/PVC plus TCNQ/p-Si structures(Elsevier Sci Ltd, 2014) Yücedağ, İbrahim; Kaya, Ahmet; Tecimer, Hüseyin; Altındal, ŞemsettinDaha fazla Dielectric properties and ac conductivity (sigma(ac)) of Au/PVC+TCNQ/p-Si structures have been investigated in the wide temperature range of 80-420 K using capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at 1 MlHz. It has been found that the forward bias C-V plots exhibit a distinct peak especially at a high temperature. Effects of the series resistance (R-s), interfacial PVC+TCNQ layer and the density distribution of interfaces'traps (D-it) on the electrical peak and the dielectric properties were investigated in detail. All of the dielectric properties such as the real and imaginary parts of dielectric constants (epsilon', epsilon''), electric moduli (M' and M"), loss tangent (tan delta), and sigma(ac), values were found to be strong functions of temperature and applied bias voltage. These changes become considerably high especially in the depletion region. In addition, the voltage dependent Rs values were obtained and they increased with decreasing temperature C-V-T and G/omega-V-T measurements confirmed that R-s, D-it, and PVC+TCNQ layers are very important parameters that strongly influence both dielectric properties and sigma(ac) of structures. (C) 2014 Elsevier Ltd. All rights reserved.Daha fazla Öğe Temperature and Interfacial Layer Effects on the Electrical and Dielectric Properties of Al/(CdS-PVA)/p-Si (MPS) Structures(Springer, 2018) Demir, Gülçin Ersöz; Yücedağ, İbrahim; Kalandaragh, Yashar Azizian; Altındal, ŞemsettinDaha fazla In the present study, cadmium sulphide (CdS) nanopowders were prepared by using a simple physical ball milling technique, and their x-ray diffraction (XRD) analysis confirmed the formation of hexagonal wurtzite structure of CdS. The morphology of CdS nanopowders was characterized by scanning electron microscope (SEM). Dielectric and electrical properties of the manufactured Al/(CdS-PVA)/p-Si (MPS) type structures were investigated by capacitance-voltage (C-V) and conductance-voltage (G/-V) measurements as functions of temperature and applied bias voltage at 500kHz. Some main parameters of the structure such as real and imaginary parts of complex dielectric constants, epsilon(=epsilon-j epsilon), loss tangent (tan), a.c. electrical conductivity (sigma(ac)), and real and imaginary parts of complex electric modulus, M*(=M+jM) of the structure were investigated in the temperature range between 230K and 340K. Ln(sigma(ac))-q/kT curve showed a linear behavior. The value of activation energy (E-a) was obtained as 0.0601eV at 5.0V from the slope of this curve. Moreover, argand diagrams of complex modulus were studied to determine relaxation process of these structures.Daha fazla Öğe TRAFİK DUYARLI KABLOSUZ VÜCUT ALAN AĞLARININ BAŞARIM ANALİZİ(Pamukkale Univ, 2015) Çalhan, AliDaha fazla Mikro elektronik, kablosuz haberleşme, tümleşik devreler ve algılayıcı ağların sağlık alanında birlikte kullanılması, Kablosuz Vücut Alan Ağları’nın (Wireless Body Area Networks, WBANs) ortaya çıkmasını sağlamıştır. WBAN ile insan vücuduna yerleştirilmiş küçük boyutlu, enerji tüketimi düşük kablosuz algılayıcı düğümler sayesinde insan vücudunun gözetim altında tutulması amaçlanmaktadır. Günümüzde WBAN sağlık, spor, eğlence, askeri uygulama gibi birçok alanda kullanılmaktadır. Ağı oluşturan algılayıcı düğümler kablosuz olarak çalıştıkları için sınırlı enerji kaynaklarına sahiptirler. Bu sebeple, WBAN’lar için kullanılacak ortam erişim protokollerinin enerjiye duyarlı olması gerekmektedir. Ayrıca WBAN’ların farklı veri trafiklerine (normal, isteğe bağlı ve acil) sahip olması, koordinatör düğümlerde veri işlem sırasının belirlenmesinin önemini arttırmıştır. Bu çalışmada, veri trafiğine duyarlı bir WBAN yapısı tasarlanmıştır. Tasarlanan yapının başarım analizi için uçtan-uca gecikme ve iş çıkarma oranı sonuçları incelenmiştirDaha fazla Öğe On the frequency dependent negative dielectric constant behavior in Al/Co-doped (PVC plus TCNQ)/p-Si structures(World Scientific Publ Co Pte Ltd, 2014) Yücedağ, İbrahim; Kaya, Ahmet; Altındal, ŞemsettinDaha fazla The dielectric properties, electric modulus and ac electrical conductivity (sigma(ac)) of Al/Co-doped (PVC + TCNQ)/p-Si structures have been investigated in the wide frequency and voltage range of 0.5 kHz-3 MHz and (-4 V)-( 9 V), respectively, using the capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at room temperature. The real and imaginary parts of dielectric constant (epsilon ', epsilon ''), loss tangent (tan delta), sigma(ac) and the real and imaginary parts of electric modulus (M ', M '') were found strongly function of frequency and applied voltage especially at low frequencies. The epsilon '-V plot shows an anomalous peak in the forward bias region due to the series resistance (R-s), surface states (N-ss) and interfacial layer (PVC + TCNQ) effects for each frequency and then it goes to negative values known as negative dielectric constant (NDC) at low frequencies (f <= 70 kHz). Such observation of NDC is important result because it implies that an increment of bias voltage produces a decrease in the charge on the electrodes. The amount of negativity epsilon ' value increases with decreasing frequency and this decrement in the NDC corresponds to the increment in the epsilon ''.Daha fazla Öğe On the Anomalous Peak in the Forward Bias Capacitance and Dielectric Properties of the Al/Co-Doped (PVC plus TCNQ)/p-Si structures as Function of Temperature(Amer Scientific Publishers, 2015) Yücedağ, İbrahim; Kaya, Ahmet; Altındal, ŞemsettinDaha fazla The temperature and voltage dependence of electric and dielectric characteristics and ac electrical conductivity (sigma(ac)) of Al/Co-doped (PVC + TCNQ)/p-Si structure in the temperature range of 200-360 K and voltage range of (-4 V)-(9 V) have been investigated in detail by using experimental C-V and G/omega -V measurements at 500 kHz. The value of dielectric constant (epsilon'), dielectric loss (epsilon ''), dielectric loss tangent (tan delta), the real and imaginary parts of electric modulus (M' and M ''), and the sigma(ac) were strongly dependent applied bias voltage and temperature, especially in the depletion and accumulation regions. Such a behavior in these parameters can be explained on restructuring and reordering of charges at interface traps/states. The forward C-V plots exhibit an anomalous peak for each temperature and the peak position shift towards lower voltages with increasing temperature due to the particular density distribution of interface traps (D-it) and series resistance (Rs) of structure. Therefore the plots of dielectric properties and also sigma(ac) indicate two different behaviors before and after intersection point. Before this intersection point, while the values of the epsilon', epsilon '', and sigma(ac) increase, the tan delta decreases, after this intersection point, while the value of the epsilon', epsilon '', and sigma(ac) decrease, the tan delta increases. The ln (sigma(ac))-q/kT plot shows two linear regions both for the 2 V and 9 V which are corresponding to below room temperature (200-300 K) and above room temperature (320-360 K) and the corresponding activation energy (E-a) values were called as Ea((I)) and Ea((II)), respectively. Thus the E-a values were obtained from the slope of these Arrhenius plot as 182 meV and 4.7 meV for 2 V and 22 meV and 0.6 meV for 9 V, respectively.Daha fazla Öğe Investigation of photo-induced effect on electrical properties of Au/PPy/n-Si (MPS) type schottky barrier diodes(Springer, 2017) Demir, Gülçin Ersöz; Yücedağ, İbrahim; Bayrakdar, Sümeyye; Altındal, Şemsettin; Gümüş, AhmetDaha fazla Au/PPy/n-Si (MPS) type Schottky barrier diodes (SBDs) were produced and their current-voltage (I-V) characteristics were measured in the positive and negative bias regions at 300 K. The basic electronic quantities such as reverse-saturation current (I-o ), ideality factor (n), zero-bias barrier height (Phi(B0) ), series (R-s) and shunt resistances (R-sh) were obtained by using I-V data in total darkness and illumination (100 W/m(2)). The values of these parameters were found as 7.79 x 10(-9) A, 5.41, 0.75 eV, 1 k Omega and 130 M Omega in dark) and 4 x 10(-9) A, 4.89, 0.77 eV, 0.9 k Omega and 1.02 M Omega under illumination), respectively. Also the energy density distribution behaviors of surface states (N-ss ) have been acquired by calculation of effective barrier height (Phi(e) ) and ideality factor n (V) depending on voltage in total darkness and illumination. The values of N (ss) show an exponentially increase from the mid-gap of Si to the lower part of conduction band (E-c ) for two conditions. The possible current conduction mechanisms were determined by plotting of the double logarithmic I-V plots in the positive voltage zone and the value of current was found proportional to voltage (I similar to V (m) ). The high values of n and R-s were ascribed to the certain density distribution of N-ss localized at semiconductor /PPy interface, surface conditions, barrier inequality, the thickness of PPy interlayer and its roughness. The open-circuit voltage of the photodiode was found as 0.36 V under 100 W/m(2) illumination level. This is evidence that the fabricated sample is very sensitive to illumination. Therefore, it can be put into practice in optoelectronic industries as a photodiode or solar cells.Daha fazla Öğe Investigation of Electrical Characteristics in Al/CdS-PVA/p-Si (MPS) Structures Using Impedance Spectroscopy Method(Ieee-Inst Electrical Electronics Engineers Inc, 2016) Demir, Gülçin Ersöz; Yücedağ, İbrahim; Kalandaragh, Yashar Azizian; Orak, İkram; Altındal, ŞemsettinDaha fazla The cadmium sulfide (CdS) nanopowders have been prepared by ball-milling method, and CdS-polyvinyl alcohol (PVA) nanocompound in the form of film has been deposited on a p-Si wafer as an interfacial layer by spin-coating method. The impedance characteristics of the fabricated Al/CdS-PVA/p-Si (metal-polymer-semiconductor)-type structures were studied in the frequency and voltage range of 5 kHz-5 MHz and +/-1 V, respectively, by considering interface states (D-it), series resistance (R-s), and interfacial layer effects at 300 K. While the voltage and frequency dependence profiles of D-it were evaluated from the low-high frequency capacitance (C-LF-C-HF) and Hill-Coleman methods, R-s profiles were evaluated from the Nicollian and Brews method. Doping concentration atoms (N-A) and barrier height [Phi (B)(capacitance-voltage (C-V))] values were also obtained from the reverse bias C-2 versus V plots for each frequency. While D-it and R-s values decrease with increasing frequency almost exponentially, Phi (B)(C-V) increases linearly. Therefore, both the measured capacitance (C-m) and conductance (G(m)/omega) values were corrected to eliminate the R-s effect. The experimental results show that R-s value is more effective on the impedance measurements at high frequencies in the accumulation region, but D-it is effective at low frequencies in the depletion region.Daha fazla Öğe Comparative study of the temperature-dependent dielectric properties of Au/PPy/n-Si (MPS)-type Schottky barrier diodes(Korean Physical Soc, 2015) Gümüş, Ahmet; Ersöz, Gülçin; Yücedağ, İbrahim; Bayrakdar, Sümeyye; Altındal, ŞemsettinDaha fazla The dielectric properties of Au/PPy/n-Si metal-polymer-semiconductor (MPS)-type Schottky barrier diodes (SBDs) were investigated by using capacitance-voltage (C-V) and conductancevoltage (G/omega-V) measurements at various temperatures and voltages at frequencies of 100 kHz and 500 kHz. Both the real and the imaginary parts of the complex dielectric constant and dielectric loss (epsilon', epsilon aEuro(3)) and of the electric modulus (M', MaEuro(3)), as well as the conductivity (sigma (ac) ), were found to depend strongly on the temperature and the voltage. Both the C and G/omega values increased with increasing applied voltage and had inversion, depletion, and accumulation regions as with a metal-insulator-semiconductor (MIS) type behavior. Both the dielectric constant (epsilon') and the dielectric loss (epsilon aEuro(3)) increased with increasing temperature and decreased with increasing frequency. The loss tangent (tan delta) vs. temperature curve had a peak at about 200 K for both frequencies. The M' and the MaEuro(3) values decreased with increasing temperature and became independent of the frequency at high temperatures. The series resistance (R (s) ) of the diode decreased with increasing temperature for the two frequencies while the sigma (ac) increased. Such behaviors of the dielectric properties with temperature were attributed to the restructuring and reordering of charges at interface states/traps due to the varying temperature, the interfacial polarization, and the interfacial polymer layer. ln(sigma (ac) ) vs. q/kT plots had two distinct linear regions with different slopes for the two frequencies. Such behaviors of these plots confirmed the existence of two different conduction mechanisms corresponding to low and high temperatures. The values of the activation energy (E (a) ) were obtained from the slopes of these plots, and its value at low temperatures was considerably lower than that at high temperatures.Daha fazla Öğe Case study on handoff strategies for wireless overlay networks(Elsevier, 2013) Çalhan, Ali; Çeken, CelalDaha fazla One of the most challenging topics for next-generation wireless networks is the process of vertical handoff since many of wireless technologies overlap each other and build a heterogeneous topology. Several parameters, pertaining to user/application requirements and network conditions, such as data rate, service cost, network latency, speed of mobile, and etc. must be considered in the handoff process of heterogeneous networks along with RSSI information. In this paper, adaptive fuzzy logic-based vertical handoff decision-making algorithms are presented for wireless overlay networks which consist of GSM/GPRS/Wi-Fi/IJMTS/WiMAX technologies. The parameters data rate, monetary cost, speed of mobile and RSSI information are processed as inputs of the proposed fuzzy-based systems. According to these parameters, an output value, which varies between 1 and 10, is produced. This output value is utilized to determine whether a handoff process is necessary or not and to select the best candidate access point in the vicinity. The results show that, compared to the traditional RSSI-based algorithm significantly enhanced outcomes can be achieved for both user and network as a consequence of the proposed fuzzy-based handoff systems. The simulation results are also compared with those of classical MADM (Multiple Attribute Decision Making) method, i.e. SAW (Simple Additive Weighting). According to the results obtained, the proposed vertical handoff decision algorithms are able to determine whether a handoff is necessary or not, properly, and select the best candidate access network considering the aforementioned parameters. Moreover, fuzzy-based algorithm noticeably reduces the number of handoffs compared to SAW-based algorithm. (C) 2012 Elsevier B.V. All rights reserved.Daha fazla Öğe Artificial Neural Network Based Vertical Handoff Algorithm for Reducing Handoff Latency(Springer, 2013) Çalhan, Ali; Çeken, CelalDaha fazla One of the most challenging topics for next generation wireless networks is vertical handoff concept since several wireless technologies are assumed to cooperate. Plenty of parameters related to user preferences, application requirements, and network conditions, such as; data rate, service cost, network latency, speed of mobile, battery level, interference ratio and etc. must be considered in vertical handoff process along with traditional RSSI information. In this study, a new artificial neural network based handoff decision algorithm is proposed in order to reduce the handoff latency of smart terminal deployed in aforementioned wireless heterogeneous infrastructures. The prominent parameters data rate, monetary cost and RSSI information are taken as inputs of the developed vertical handoff decision system. Performance results of the proposed system are also compared with those of classical Multiple Attribute Decision Making method Simple Additive Weighting, and of some other artificial intelligence based algorithms. According to the results obtained, the proposed neural network based vertical handoff decision algorithm is able to determine whether a handoff is necessary or not properly, and selects the best candidate access network considering the abovementioned parameters. The results also show that, the neural network based algorithm developed significantly reduces the handoff latency while the number of handoffs, which is another vital performance metric, is still reasonable.Daha fazla Öğe A slotted ALOHA-based cognitive radio network under capture effect in Rayleigh fading channels(Tubitak Scientific & Technical Research Council Turkey, 2016) Bayrakdar, Muhammed Enes; Atmaca, Sedat; Karahan, AlperDaha fazla In this paper, a slotted ALOHA-based cognitive radio (CR) network is proposed and the throughput performance of the proposed CR network model under Rayleigh fading channels is examined. Our CR network contains two special groups of users, primary users (PUs) and CR users (CRUs), and they are considered to be sharing a time slotted-based common communication channel. While PUs can access the channel at any time owing to their legal right, CRUs can only access the channel when it is not occupied by the PUs. In the network model developed, PUs access the channel utilizing time division multiple access as a medium access control technique, and CRUs can access the channel by exploiting slotted ALOHA as a random access scheme when the channel is idle. In the proposed network model additive white Gaussian noise and Rayleigh channels is considered for comparison reasons. Taking capture effect into account in Rayleigh fading channels, we have obtained new equations for the throughput of the proposed CR network. We have also developed, modeled, and simulated a sample networking scenario by using MATLAB with the aim of validating the analytical throughput results. Simulation results of the proposed network model precisely match with the analytical results obtained under different network load conditions. Furthermore, it is shown that the overall channel exploitation is increased by utilizing the spectrum holes without interfering with the PUs' transmissions.Daha fazla