Elektrik - Elektronik Mühendisliği Bölümü Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12469/47

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Citation - WoS: 1
Citation - Scopus: 1
Across Dimensions: Two- and Three-Dimensional Phase Transitions From the Iterative Renormalization-Group Theory of Chains
(2020) Keçoğlu, İbrahim; Berker, A. Nihat
Sharp two- and three-dimensional phase transitional magnetization curves are obtained by an iterative renormalization-group coupling of Ising chains, which are solved exactly. The chains by themselves do not have a phase transition or nonzero magnetization, but the method reflects crossover from temperaturelike to fieldlike renormalization-group flows as the mechanism for the higher-dimensional phase transitions. The magnetization of each chain acts, via the interaction constant, as a magnetic field on its neighboring chains, thus entering its renormalization-group calculation. The method is highly flexible for wide application.
Citation - WoS: 5
Citation - Scopus: 5
Analysis of Mixed-Element Structures Formed With Shunt Capacitors Separated by Transmission Lines
(IEEE-Inst Electrical Electronics Engineers Inc, 2019) Şengül, Metin Y.; Çakmak, Gökhan
In this brief, the analysis of mixed-element structures formed with shunt capacitors separated by commensurate transmission lines is performed first time in the literature. First, a low-pass lumped-element ladder network is considered. Then the series inductors are replaced with commensurate transmission lines. As a result, a practically important mixed-element structure is obtained. Then the description of the structure by means of two frequency variables (one for shunt capacitors and one for transmission lines) is detailed: Explicit expressions for the coefficients of the descriptive two-variable polynomials in terms of the coefficients of the single variable boundary polynomials are derived for various numbers of elements, which are obtained first time in the literature. Finally, a mixed-element broadband matching network is designed to illustrate the usage of the obtained expressions. If it is preferred not to have shunt capacitors, they can be replaced with open-ended stubs via Richard's transformation. So the resultant circuit is extremely suitable for microstrip fabrication.
Citation - WoS: 26
Citation - Scopus: 34
Broadband Decoupling and Matching of a Superdirective Two-Port Antenna Array
(IEEE-INST Electrical Electronics Engineers Inc, 2008) Volmer, Christian; Şengül, Metin Y.; Weber, Joern; Stephan, Ralf; Hein, Matthias A.
Decoupling and matching networks may be used to improve the performance of compact antenna arrays where mutual radiator coupling has caused a degradation of the diversity capabilities. A popular network consists of a 180 degrees rat-race directional coupler which decouples the antenna ports followed by impedance matching networks at each port. Researchers however usually neglect the presence of losses both within the antenna array and the decoupling and matching network. For this reason we have built various narrowband and broadband matching networks and compare their performances with the help of calibrated far-field measurement data.
Citation - WoS: 5
Citation - Scopus: 6
Code Shift Keying Impulse Modulation for Uwb Communications
(IEEE-INST Electrical Electronics Engineers Inc, 2008) Erküçük, Serhat; Kim, Dong In; Kwak, Kyung Sup
In this paper the system performance of M-ary code shift keying (MCSK) impulse modulation is studied in detail and compared to M-ary pulse position modulation (MPPM) under single- and multi-user scenarios. For that bounds on the semi-analytic symbol-error rate (SER) expressions are derived and simulation studies are conducted. When practical implementations of MCSK and MPPM are considered it is shown that MCSK can provide about 2 dB performance gain over MPPM as it reduces the effects of multipath delays on the decision variables by randomizing locations of the transmit pulse.
Citation - WoS: 9
Citation - Scopus: 9
Complete Density Calculations of Q-State Potts and Clock Models: Reentrance of Interface Densities Under Symmetry Breaking
(Amer Physical Soc, 2020) Artun, E. Can; Berker, A. Nihat
All local bond-state densities are calculated for q-state Potts and clock models in three spatial dimensions, d = 3. The calculations are done by an exact renormalization group on a hierarchical lattice, including the density recursion relations, and simultaneously are the Migdal-Kadanoff approximation for the cubic lattice. Reentrant behavior is found in the interface densities under symmetry breaking, in the sense that upon lowering the temperature, the value of the density first increases and then decreases to its zero value at zero temperature. For this behavior, a physical mechanism is proposed. A contrast between the phase transition of the two models is found and explained by alignment and entropy, as the number of states q goes to infinity. For the clock models, the renormalization-group flows of up to 20 energies are used.
Citation - WoS: 13
Citation - Scopus: 17
Conditional Power and Rate Adaptation for Mqam/Ofdm Systems Under Cfo With Perfect and Imperfect Channel Estimation Errors
(IEEE-INST Electrical Electronics Engineers Inc, 2015) Dong, Zhicheng; Fan, Pingzhi; Panayırcı, Erdal; Lei, Xianfu
In this paper a new conditional power and rate adaptation scheme for orthogonal frequency-division multiplexing (OFDM) systems is proposed in the presence of carrier frequency offset (CFO) with perfect and imperfect channel state information (CSI). The conventional adaptive scheme is shown to be a special case of the conditionally adaptive scheme technique that enables the resulting nonconvex optimization problem which is solved in a feasible way. It leads to a solution for optimal power adaptation that maximizes the spectral efficiency of an OFDM system using M-ary quadrature amplitude modulation (MQAM) under average power and instantaneous bit error rate (BER) constraints. Closed-form expressions for the average spectral efficiency (ASE) of adaptive OFDM systems are derived for perfect and imperfect CSI cases. The theoretical results and computer simulations show that range of the conditional adaptation becomes narrow and the performance of constant power and continuous rate is very close to that of the conditionally adaptive power and continuous rate for higher CFO or high signal-to-noise ratio (SNR) values.
Citation - WoS: 9
Citation - Scopus: 9
Construction of Lossless Ladder Networks With Simple Lumped Elements Connected Via Commensurate Transmission Lines
(IEEE-INST Electrical Electronics Engineers Inc, 2009) Şengül, Metin Y.
In this work an algorithm to design lossless ladder networks with simple lumped elements connected via commensurate transmission lines is proposed. After giving the algorithm a lumped-element low-pass Chebyshev filter was transformed to its mixed-element counterpart to illustrate the utilization of the algorithm. The filter designed for a frequency band around 1 GHz was fabricated and experimentally characterized. We find very good agreement between measured and simulated transducer power gain over the entire frequency band of interest.
Citation - WoS: 6
Citation - Scopus: 6
Correlation of Ternary Liquid- Equilibrium Data Using Neural Network-Based Activity Coefficient Model
(Springer, 2014) Özmen, Atilla
Liquid--liquid equilibrium (LLE) data are important in chemical industry for the design of separation equipments and it is troublesome to determine experimentally. In this paper a new method for correlation of ternary LLE data is presented. The method is implemented by using a combined structure that uses genetic algorithm (GA)--trained neural network (NN). NN coefficients that satisfy the criterion of equilibrium were obtained by using GA. At the training phase experimental concentration data and corresponding activity coefficients were used as input and output respectively. At the test phase trained NN was used to correlate the whole experimental data by giving only one initial value. Calculated results were compared with the experimental data and very low root-mean-square deviation error values are obtained between experimental and calculated data. By using this model tie-line and solubility curve data of LLE can be obtained with only a few experimental data.
Citation - WoS: 20
Citation - Scopus: 21
Design of Practical Broadband Matching Networks With Lumped Elements
(IEEE-INST Electrical Electronics Engineers Inc, 2013) Şengül, Metin Y.
It is always preferable to use commercially available software tools to design broadband matching networks for microwave communication systems. However for these tools the matching network topology and element values must be selected properly. Therefore in this paper a practical method is presented to generate matching networks with good initial element values. Eventually the performance of the designed matching network is optimized by employing the commercially available computer-aided design (CAD) tools. An example is given to illustrate the utilization of the proposed method. It is shown that the proposed method provides very good initials for CAD tools.
Citation - WoS: 10
Citation - Scopus: 10
Design of Practical Broadband Matching Networks With Mixed Lumped and Distributed Elements
(IEEE-INST Electrical Electronics Engineers Inc, 2014) Şengül, Metin Y.
Computer-aided design (CAD) tools are always preferred for designing broadband matching networks. However these tools give excellent results when the suitable matching network topology and initial element values are provided. Therefore in this brief a new initialization algorithm is proposed to get suitable network topology and element values for CAD tools. Then the power transfer capability of the matching network can be improved by using any CAD tool. It is clear from the example studied that the new method generates excellent initials.
Citation - WoS: 27
Citation - Scopus: 34
Design of Practical Matching Networks With Lumped Elements Via Modeling
(IEEE, 2007) Yarman, Sıddık Binboğa; Şengül, Metin Y.; Kılınç, Ali
It is a common practice to utilize commercially available software tools to design matching networks for wireless communication systems. Most of these tools require a properly selected matching network topology with good initial element values. Therefore in this paper a practical method is presented to generate matching networks with initial element values. In the implementation process of the proposed method first the driving point immitance data for the matching network is obtained in a straight forward manner without optimization. Then it is modeled as a realizable bounded-real input reflection coefficient which in turn yields the desired matching network with reasonable element values. Eventually the initial design is improved by optimizing the performance of the matched system employing the commercially available computer-aided design (CAD) packages. An example is given to illustrate the utilization of the proposed method. It is shown that new method provides excellent results as a front-end when utilized together with CAD tools.
Citation - WoS: 11
Citation - Scopus: 11
Devil's Staircase Continuum in the Chiral Clock Spin Glass With Competing Ferromagnetic-Antiferromagnetic and Left-Right Chiral Interactions
(Amer Physical Soc., 2017) Caglar, Tolga; Berker, A. Nihat
The chiral clock spin-glass model with q = 5 states with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations is studied in d = 3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature antiferromagnetic bond concentration p random chirality strength and right-chirality concentration c. The system has a ferromagnetic phase a multitude of different chiral phases a chiral spin-glass phase and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase bordered by fragments of regular and temperature-inverted devil's staircases are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.
Citation - WoS: 10
Citation - Scopus: 11
Energy Efficient Robust Scheduling of Periodic Sensor Packets for Discrete Rate Based Wireless Networked Control Systems
(Elsevier, 2020) Farayev, Bakhtiyar; Uçar, Seyhan; Şadi, Yalçın; Coleri, Sinem
Wireless networked control systems (WNCSs) require the design of a robust scheduling algorithm that meets the stringent timing and reliability requirements of control systems, despite the limited battery resources of sensor nodes and adverse properties of wireless communication for delay and packet errors. In this article, we propose a robust delay and energy constrained scheduling algorithm based on the exploitation of the mostly pre-known periodic data generation nature of sensor nodes in control systems. We first formulate the joint optimization of scheduling, power control and rate adaptation for discrete rate transmission model, in which only a finite set of transmission rates are supported, as a Mixed-Integer Non-linear Programming problem and prove its NP-hardness. Next, we propose an optimal polynomial-time power control and rate adaptation algorithm for minimizing the transmission time of a node subset. We then design a novel polynomial-time heuristic scheduling algorithm based on first determining the concurrently transmitting node subsets and then distributing them uniformly over time by a modified Karmarkar-Karp algorithm. We demonstrate the superior performance of the proposed scheduling algorithm in terms of robustness, delay and runtime on the Low-Rate Wireless Personal Area Network (LR-WPAN) simulation platform, which we developed in network simulator-3 (ns3).
Citation - WoS: 11
Citation - Scopus: 14
Fault-Tolerant Training of Neural Networks in the Presence of Mos Transistor Mismatches
(IEEE-INST Electrical Electronics Engineers Inc, 2001) Öğrenci, Arif Selçuk; Dündar, Günhan; Balkır, Sina
Analog techniques are desirable for hardware implementation of neural networks due to their numerous advantages such as small size low power and high speed. However these advantages are often offset by the difficulty in the training of analog neural network circuitry. In particular training of the circuitry by software based on hardware models is impaired by statistical variations in the integrated circuit production process resulting in performance degradation. In this paper a new paradigm of noise injection during training for the reduction of this degradation is presented. The variations at the outputs of analog neural network circuitry are modeled based on the transistor-level mismatches occurring between identically designed transistors Those variations are used as additive noise during training to increase the fault tolerance of the trained neural network. The results of this paradigm are confirmed via numerical experiments and physical measurements and are shown to be superior to the case of adding random noise during training.
Citation - WoS: 3
Citation - Scopus: 3
Frustrated Potts Model: Multiplicity Eliminates Chaos Via Reentrance
(Amer Physical Soc, 2020) Türkoğlu, Alpar; Berker, A. Nihat
The frustrated q-state Potts model is solved exactly on a hierarchical lattice, yielding chaos under rescaling, namely, the signature of a spin-glass phase, as previously seen for the Ising (q = 2) model. However, the ground-state entropy introduced by the (q > 2)-state antiferromagnetic Potts bond induces an escape from chaos as multiplicity q increases. The frustration versus multiplicity phase diagram has a reentrant (as a function of frustration) chaotic phase.
Citation - WoS: 126
Citation - Scopus: 151
Ieee 802.15.7r1 Reference Channel Models for Visible Light Communications
(IEEE-Inst Electrical Electronics Engineers Inc, 2017) Miramirkhani, Farshad; Narmanlıoğlu, Ömer; Baykas, Tuncer; Uysal, Murat; Panayırcı, Erdal
The IEEE has established the standardization group 802.15.7r1 "Short Range Optical Wireless Communications", which is currently in the process of developing a standard for visible light communication (VLC). As with any other communication system, realistic channel models are of critical importance for VLC system design, performance evaluation, and testing. This article presents the reference channel models that were endorsed by the IEEE 802.15.7r1 Task Group for evaluation of VLC system proposals. These were developed for typical indoor environments, including home, office, and manufacturing cells. While highlighting the channel models, we further discuss physical layer techniques potentially considered for IEEE 802.15.7r1.
Citation - WoS: 19
Citation - Scopus: 32
Iterative Channel Estimation and Decoding of Turbo Coded Sfbc-Ofdm Systems
(IEEE, 2007) Doğan, Hakan; Çırpan, Hakan Ali; Panayırcı, Erdal
We consider the design of turbo receiver structures for space-frequency block coded orthogonal frequency division multiplexing (SFBC-OFDM) systems in the presence of unknown frequency and time selective fading channels. The Turbo receiver structures for SFBC-OFDM systems under consideration consists of an iterative MAP Expectation/Maximization (EM) channel estimation algorithm soft MMSE-SFBC decoder and a soft MAP outer-channel-code decoder. MAP-EM employs iterative channel estimation and it improves receiver performance by re-estimating the channel after each decoder iteration. Moreover the MAP-EM approach considers the channel variations as random processes and applies the Karhunen-Loeve (KL) orthogonal series expansion. The optimal truncation property of the KL expansion can reduce computational load on the iterative estimation approach. The performance of the proposed approaches are studied in terms of mean square error and bit-error rate. Through computer simulations the effect of a pilot spacing on the channel estimator performance and sensitivity of turbo receiver structures on channel estimation error are studied. Simulation results illustrate that receivers with turbo coding are very sensitive to channel estimation errors compared to receivers with convolutional codes. Moreover superiority of the turbo coded SFBC-OFDM systems over the turbo coded STBC-OFDM systems is observed especially for high Doppler frequencies.
Citation - WoS: 78
Citation - Scopus: 86
Joint Channel Estimation Equalization and Data Detection for Ofdm Systems in the Presence of Very High Mobility
(IEEE-INST Electrical Electronics Engineers Inc, 2010) Panayırcı, Erdal; Şenol, Habib; Poor, H. Vincent
This paper is concerned with the challenging and timely problem of joint channel estimation equalization and data detection for uplink orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency selective and very rapidly time varying channels. The resulting algorithm is based on the space alternating generalized expectation maximization (SAGE) technique which is particularly well suited to multicarrier signal formats leading to a receiver structure that also incorporates interchannel interference (ICI) cancelation. In order to reduce the computational complexity of the algorithm band-limited discrete cosine orthogonal basis functions are employed to represent the rapidly time-varying fading channel by the discrete cosine serial expansion coefficients. It is shown that depending on the normalized Doppler frequency only a small number of expansion coefficients is sufficient to approximate the channel perfectly and there is no need to know the correlation function of the input signal. In this way the resulting reduced dimensional channel coefficients are estimated and the data symbols detected iteratively with tractable complexity. The proposed SAGE joint detection algorithm updates the data sequences serially and the channel parameters are updated in parallel leading to a receiver structure that also incorporates ICI cancelation. Computer simulations show that the cosine transformation represents the time-varying channel very effectively and the proposed algorithm has excellent symbol error rate and channel estimation performance even with a very small number of channel expansion coefficients employed in the algorithm resulting in substantial reduction of the computational complexity.
Citation - WoS: 48
Citation - Scopus: 54
Joint Communication and Computing Resource Allocation in 5g Cloud Radio Access Networks
(IEEE-Inst Electrical Electronics Engineers Inc, 2019) Ferdouse, Lilatul; Anpalagan, Alagan; Erküçük, Serhat
Cloud-radio access network (C-RAN) is regarded as a promising solution to manage heterogeneity and scalability of future wireless networks. The centralized cooperative resource allocation and interference cancellation methods in C-RAN significantly reduce the interference levels to provide high data rates. However, the centralized solution is not scalable due to the dense deployment of small cells with fractional frequency reuse, causing severe inter-tier and inter-cell interference turning the resource allocation and user association into a more challenging problem. In this paper, we investigate joint communication and computing resource allocation along with user association, and baseband unit (BBU) and remote radio head (RRH) mapping in C-RANs. We initially establish a queueing model in C-RAN, followed by formulation of two optimization problems for communication [e.g., resource blocks (RBs) and power] and computing [e.g., virtual machines (VMs)] resources allocation with the aim to minimize mean response time. User association along with the RB allocation, interference, and queueing stability constraints are considered in the communication resource optimization problem. The computing resource optimization problem considers BBU-RRH mapping and VM allocation for small cells, constrained to BBU server capacity and queueing stability. To solve the communication and computing resource optimization problem, we propose a joint resource allocation solution that considers a double-sided auction based distributed resource allocation (DS-ADRA) method, where small cell base stations and users jointly participate using the concept of auction theory. The proposed method is evaluated via simulations by considering the effect of bandwidth utilization percentage, signal-to-interference ratio threshold value and the number of users. The results show that the proposed method can be successfully implemented for 5G C-RANs.
Citation - WoS: 5
Citation - Scopus: 8
Joint Detection of Primary Systems Using Uwb Impulse Radios
(IEEE-INST Electrical Electronics Engineers Inc, 2011) Erküçük, Serhat; Lampe, Lutz; Schober, Robert
Regulation in Europe and Japan requires the implementation of detect-and-avoid (DAA) techniques in some bands for the coexistence of licensed primary systems and secondary ultra wideband (UWB) systems. In a typical coexistence scenario a primary system may have potentially interdependent uplink-downlink communication channels (e. g. simultaneous uplink-downlink communications in a frequency division duplex system) overlapping with the frequency band of a UWB system. If such interdependencies of primary systems' activities are known the UWB system's ability to detect primary systems can be improved. In this study we are interested in determining the possible gains in the detection performance when taking interdependencies into account for practically implementable detection methods. Contrary to selecting the detection thresholds individually for each band as in a conventional detection approach the bands are jointly processed. To this end maximum a posteriori (MAP) decision variables are generated at the receiver and bias terms are introduced to achieve a desired trade-off between the probabilities of detection and false alarm. In addition to finding the optimal detection results based on the Neyman-Pearson (NP) test a suboptimal but practically implementable approach is also considered and the gain compared to conventional independent detection is quantified for various practical scenarios. The results obtained from this study can be used for improving the primary system detection performance of UWB systems as well as for cognitive radios that perform spectrum sensing in multiple bands.

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