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Yayın Cooperative strategies, achievable rates and resource allocation for OFDMA channels(Işık Üniversitesi, 2011-07-28) Bakım, İsmail Sezi; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıThe design of next generation wireless communication systems brings along new challenges, since the degrading factors such as fading and multi-user interference become harder to deal with as the number of users and the bandwidth require- ments increase. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple accessing technique which provides a solution to both of the problems above: it provides a relatively simple way of assigning available bandwidth to users, while avoiding interference; and at the same time, it converts a frequency selective fading channel, to parallel flat fading subchannels, hence reducing the effects of intersymbol interference. However, in wireless channels, what is tradi- tionally considered as interference is in fact side information, and combined with the diversity created by the orthogonal subchannels in OFDMA, this side infor»mation can be carefully taken advantage of to increase the rates achievable by the users. In our thesis, without imposing any prior constraints on subchannel allocation, we investigate cooperation strategies, achievable rates and resource allocation for OFDMA channels. We propose new cooperative encoding strategies for wireless communication net- works över OFDMA channels. We particularly focus on a two user cooperative OFDMA system, based on block Markov superposition encoding (BMSE). We obtain expressions of the resulting achievable rate regions for ali proposed coroperative encoding strategies. We show that, by allowing for re-partitioning and re-encoding of the cooperative messages across subchannels, it is possible to better exploit the diversity created by OFDMA, and higher rates can be achieved. In order to take full advantage of the diversity created by OFDMA, we then introduce a channel adaptive cooperation strategy for OFDMA, and optimize the transmit powers as a function of the channel states. We provide the optimality conditions that need to be satisfied by the powers associated with the users' codewords and derive the closed form expressions for the optimal powers.Yayın Metamutator: Its Realizations and Its Applications(Işık Üniversitesi, 2019-07-22) Minayi, Elham; Göknar, İzzet Cem; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıMutators became very popular after 1971, when Leon Chua realized the memristor, his postulated fourth circuit element, using them. The reason for the popularity of mutators, which are easily realizable 2-port devices, lies in the fact that they render possible the simulation/emulation of, hence experimentation with, postulated non-existing (not o the shelf available) elements like memstors.1 On the other hand, in the literature when simulating/emulating elements with 2- ports, many 4-port \generalized mutator-like" realizations that nobody has been able to identify are being used. These underlying 4-ports, and their ability to act as a mutator when two of the ports are properly terminated, have been thus named metamutator. In this thesis, in addition to introducing some of metamutator realizations, newly designed metamutator circuits with one or two active devices are introduced. Also, a new active device, with only twelve transistors, named Additive and Differential IC (AD-IC), is proposed with its layout and use in metamutator circuit design. In addition, many 1-port and 2-port circuit realizations using metamutators have been introduced. 1-port applications are memstor simulation/emulation, oating and/or grounded impedance scaling which comprise inductance simulation, capacitance multiplication, oscillators and Frequency Dependent Negative Resistor (FDNR) simulation. 2- port applications cover Voltage Mode Multiple Input Single Output (VM-MISO) and Current Mode Single Input Multiple Output (CM-SIMO) universal lters and implementations of transconductance and transimpedance ampli ers. Also, two di erent applications of AD-IC: ADIC based analog multiplier and AD-IC based full-wave recti er are proposed in the thesis.Yayın Ring array processing for forward-looking intravascular and intracardiac ultrasonic imaging(Işık Üniversitesi, 2010-06-08) Tekeş, Coşkun; Karaman, Mustafa; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıForward-looking (FL) catheter-based imaging systems are highly desirable for guiding interventions in IVUS and ICE applications. One of the main challenges in the array based FLIVUS systems is the large channel count which results in increased system complexity. Synthetic phased array processing with reduced firing count simplifies the front-end and hence can enable 3-D real-time imaging. Recently, we have investigated dual-ring arrays suitable for IVUS and ICE imaging. In this dissertation, we present and explored two different optimized array designs based on dual circular and hexagonal rings that are suitable for synthetic phased array processing with reduced number of firings. We used simulated annealing to optimize the wideband PSF of the array system. The optimal firing set from different Tx-Rx elements eliminates most of the redundant spatial frequencies in the coarray and produces low side lobes and number of firings. The optimization procedure starts with the calculated wideband PSF of the full set coarray including all Tx-Rx firing combinations. In every iteration step, a predefined coarray with only one element change is compared with the previous set in terms of cost function. For the two designs, we obtained optimal reduced set by minimizing both the peak side lobe level and the integrated side lobe energy. For two test cases, we obtained reduced sets of 512 and 256 coarray elements, and constructed 2-D PSFs. The simulation results show that the optimized dual circular and hexagonal ring arrays provide up to 10-dB and 5-dB improvements, respectively, in peak near side lobe level with no widening in main lobe width. Our test results show that the side lobe levels of the dual circular and hexagonal arrays can be remarkably and controllably reduced by using simulated annealing optimization, and the resulting optimal array designs can meet the resolution and speed requirements of 3-D real time IVUS and ICE imaging.Yayın Design and optimization of multiuser cooperative protocols for wireless networks(Işık Üniversitesi, 2014-09-12) Edemen, Çağatay; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıRecently, the main trend in wireless communications has shifted from voice transmission to data communication. Naturally, this shift caused an increase in the data rate and bandwidth requirements. Being a limited and thus expensive resource, wireless spectrum needs to be used efficiently. For higher spectral and data rate efficiency, new spectrum allocation policies as well as new transmission techniques are needed. Cooperative and cognitive radios are promising emerging technologies, which can enable efficient spectrum resource utilization as well as high data rate transmission in the next generation wireless networks. This thesis addresses wireless relay networks consisting of multiple cooperative and/or cognitive nodes. The main contribution of this thesis is the extension of cooperative strategies from two users to three user settings, focusing on mutual cooperation. In the first part of this dissertation, we concentrate on a superposition block Markov encoding based three user cooperative communication scheme for a fading Gaussian multiple access channel. We consider all possible channel conditions between users and propose a channel adaptive block Markov encoding strategy. In the second part of this thesis, for the same three user MAC model we discuss a new channel non-adaptive superposition block Markov encoding structure which enables all three users to cooperate collectively as well as in pairs. The proposed cooperation models not only provide increased diversity to all participating users, but also contain as special cases the multiple relay channel and the multiple access relay channel. In the third part, we focus on the joint use of cognition and cooperation. In particular, we consider overlay and underlay cooperative-cognitive radio models with one primary and two secondary users. We extend the cooperative encoding models developed for the three user MAC to the cognitive set-up. In all three problems studied in this thesis, we obtain new and improved achievable rate regions, by cognition and/or cooperation.Yayın Circuit models with mixed lumped and distributed elements for passive one-port devices(Işık Üniversitesi, 2006-01-23) Şengül, Metin; Aksen, Ahmet; Yarman, Bekir Sıddık Binboğa; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıIn this thesis, to model measured data obtained from an actual passive one-port device, a circuit modeling method with mixed lumped and distributed elements is proposed. Namely, measured data is modelled by means of its Darlington equivalent, in other words, as a lossless two-port terminated with a resistance. Two network topologies are examined. The first topology is ladder networks connected with unit elements and the second one is cascaded separate lumped and distributed networks. In the proposed modeling method, analytic expression of the input reflection coefficient of the two-port model is obtained by using gradient method, and then, after synthesizing this two-variable function, the model is reached. Thus, for the first time in the literature, a two-variable circuit modeling method is presented.Yayın Solid-state arrays and beamformers for side-looking intravascular ultrasonic imaging(Işık Üniversitesi, 2010-06-08) Şişman, Alper; Karaman, Mustafa; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıCurrent solid-state intravascular ultrasound (IVUS) devices use a one-dimensional (1-D) rectangular array wrapped around the cylindrical catheter. Beamforming in these devices is limited to synthetic phased array processing, and the image quality is degraded by low signal to noise ratio (SNR) and lack of focusing in elevation. We propose new solid-state reconfigurable cylindrical array configurations, based on non-uniform aperture sampling, enabling realization of low-cost integrated phased array beamformers based on the latest IC and transducer integration techniques. We employed non-uniform (Fresnel) aperture sampling with identical differential delays between adjacent elements to reduce dynamic beamformer complexity. For this purpose, the angles of the element boundaries and centers are determined by equalizing path difference of neighboring elements. For Side Looking-IVUS (SLIVUS), the image plane is scanned through stepping the active aperture, and the active array must be reformed in each step. Fresnel sampled ring array leads to a simplified phased array beamformer with N/2 identical differential delays. The same logic applies to a two-dimensional (2-D) array with NL×NE-elements over a cylindrical patch. We compared the proposed and existing array designs through numerical point spread function (PSF) simulations. The uniform and Fresnel sampled 1-D ring arrays perform similarly in terms of beam quality while the latter improves image SNR by a factor of ?NL due to synthetic vs full phased array processing. The Fresnel sampled 2-D array performs dramatically better in elevation compared to 1-D ring array. The additional contribution of 2-D array on image quality is high SNR produced by phased array beamforming. The proposed Fresnel sampling of cylindrical aperture simplifies phased array beamformer by halving the required delay element count and transforms the complex 2-D beamformer into a separable beamformer that requires simple 1-D beamformers. The resulting 1-D and 2-D SLIVUS array configurations could be realizable using monolithic integration of the analog beamformers with transducer arrays.Yayın Multiband and concurrent matching network design(Işık Üniversitesi, 2020-02-05) Yıldız, Serkan; Aksen, Ahmet; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıMultiband network design has become a big issue due to the current improvements in RF and MW technologies. Since the new wireless protocols come up fast such as 3G, 4G, 5G etc., the RF and MW hardware must be designed to provide services to all distinct technologies in simultaneous regimes. To avoid interference, noise, harmonics, etc., the use of multiband matching or ?ltering at the front-end or back-end of RF/MW devices is an inevitable work. Particularly multiband matching in a multiband system is always necessary to be sure of proper power transfer between each multiband units. In this thesis, the multiband matching network design problem is addressed under the analytical perspective. Therefore, a deep investigation on network theoretical foundations is performed. In addition, only passive networks are concerned for providing concurrent operation. One of the well-known semi-analytical network design methods, which is called real frequency technique, is taken as a main domain to develop the thesis contributions. Two main approaches are elaborated. The ?rst approach is the generation of proper frequency mapping functions and determination of prototype networks to design multiband matching network. The proposed frequency transformation approach is useful and practical for multiband ?lter and matching network designs. As a second research track of the thesis; a synthesis based approach is aimed to be developed for generalized multiband matching network designs. A proper integration of the proposed approach with RFT matching techniques and examination of the implementation results are provided.Yayın Co-training using prosodic, lexical and morphological information for automatic sentence segmentation of Turkish spoken language(Işık Üniversitesi, 2018-01-15) Dalva, Doğan; Güz, Ümit; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıSentence segmentation of speech aims detecting sentence boundaries in a stream of words output by the speech recognizer. Sentence segmentation is a preliminary step toward speech understanding. It is of particular importance for speech related applications, as most of the further processing steps; such as parsing, machine translation and information extraction, assume the presence of sentence boundaries. Typically, statistical methods require a huge amount of manually labeled data, which is time and labor consuming process to prepare. In this work, novel multiview semi-supervised learning strategies for the solution of sentence segmentation problem are proposed. The aim of this work is to and effective semi-supervised machine learning strategies when only a small set of sentence boundary labeled data is available. This work proposes three-view co-training and committee-based strategies incorporating with agreement, disagreement and self-combined strategies using lexical, morphological and prosodic information, and investigates performance of the proposed learning strategies against baseline, self-training and co-training. The experimental results show that the proposed learning strategies highly improve the sentence segmentation problem, since data sets can be represented by three redundantly suffcient and disjoint feature sets.Yayın New approaches for channel estimation in wireless communications(Işık Üniversitesi, 2006-03-10) Şenol, Habib; Bayazıt, Uluğ; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıThis thesis first proposes a conputationally efficient, pilot-aided linear minimum mean square error (MMSE) batch channel estimation algorthm for the orthogonal frequency division multiplexing OFDM systems in unkonown wireless fading channels. The batch linear MMSE will be converted to the sequential linear MMSE estimator due to fast convergence property and the simple structure. In addition to OFDM systems, focusing on transmit diversity (OFDM) transmission through frequency selective channels, this thesis pursues a channel estimation approach in time-domain for both space-frequency OFDM (SF-OFDM) and space-time OFDM (ST-OFDM) systems. This thesis also proposes a computationally efficient, pilot aided linear minimum mean square error (MMSE) time domain batch channel estimation algorithm for OFDM systems with transmit diversity in unknown wireless fading channels. The proposed batch approaches (with or without transmit diversity) employ a convenient representation of the discrete multipath fading channel based on the Karhunen Loeve (KL)orthogonal expansion and finds MMSE estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MMSE estimators . Moreover, optimal rank reduction is achieved by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the estimation algortihm. The performance of the proposed approaches is studied through analytical and experimental result. Then, in order to explore the performances, the stochastic Cramer- Rao lower bounds are considered for the proposed approaches . The effect of a modelling mismatch on the performance of the estimator is also analyzed.In order to explore the performance of the transmit diversity OFDM systems, the closed-form expression for the average symbol error rate (SER) probability is also derived for the maximum ratio receive combiner (MRRC) in these systems. Simulations result confirm our theoretical analysis, and illustrate that the proposed channel estimation algorithms for OFDM systems with and without transmit diversity are capable of tracking fast fading and frequency selective fading, respectively and improving overall performance.
