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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 Implementation of cooperative diversity in space frequency coded OFDM system(Işık Üniversitesi, 2007) Yiğit, Can; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıWireless technology is becoming an important part of our daily life. Although earlier wireless technologies were developed to provide voice transmission which did not require large amount of bandwidth, we are now equipped with intelligent mobiles, PDAs and laptop computers each having various applications connecting us to multimedia services that requires high bandwidth utilization. Technology world is converging to IP based systems where reliability and capacity plays an important role. For example the delay periods which can be neglected in voice transmissions can be crucial in services like on demand TV where data transmission have to be smooth. High performance and high reliability requires the bounds of capacity and coding structures to be increased. In this thesis we consider strong coding algorithms and diversity techniques in broadband channels. Space frequency codes which have been proven to achieve high performance in MIMO systems have been employed to single antenna systems by using cooperative diversity where deployment of multiple antennas is difficult. By constructing such a structure we target to achieve available diversities in the channel to improve the performance.Yayın Partner selection and resource allocation in single-cell, multi-cell and cognitive cooperative multiple access channels(Işık Üniversitesi, 2013-06-06) Bakşi, Saygın; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıWireless communication has been widely used across the globe for several years. As the technology advances and gains popularity, more people start using it and more throughput is needed. For that reason, several techniques are being researched such as MIMO systems and cooperative networks. Wireless cooperative networks make use of the natural property of radiation of electromagnetic waves. Since the waves emitted by a user in the network can be heard by all users in the network, one cooperating partner uses this overheard information to increase throughput. However, to maximize the throughput of a system, cooperating partners must be selected intelligently. In this dissertation, first, we will summarize cooperative communication basics, techniques we used in convex optimization and graph theory. Then, we will show how these concepts can be used together to optimally maximize system throughput and propose lower complexity yet nearly-optimal partner selection algorithm will be proposed. Under the light of the results of this work, in the next chapters, we will introduce a novel fractional frequency reuse scheme which encourages users to cooperate and allow system to support more users. In the last chapter, a cognitive scenario will be used for one cell and we will present the optimal partner selection scheme for system throughput maximization.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 Joint modulation and space time block code classification for mimo systems(Işık Üniversitesi, 2014-10-31) Bayer, Özgür; Öner, Mustafa Mengüç; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıIn this thesis, the problems of modulation classification and space time block code (STBC) classification for MIMO systems are investigated. Two existing MIMO modulation type classification algorithms, an average likelihood ratio test, which is optimal in Bayesian sense, and a suboptimal Hybrid Likelihood ratio test, are investigated in detail. Subsequently, we have focused on improving these existing algorithms by using blind channel estimation methods with better performance, such as the Expectation–Maximization algorithm for spatial multiplexing which is known to converge to the maximum likelihood estimate, and a higher order statistic based estimation method in the case of signals which use space time block codes. Subsequently, we have proposed a novel approach for the task of modulation and STBC type classification, in which these two classification problems are considered as a single as joint classification problem, in contrast to the existing works in the literature. Based on this joint classification approach, we have proposed novel likelihood based joint STBC and modulation-type classification algorithms for 2 and 3 antenna cases.Yayın Compress and forward based cooperation strategies in wireless networks(Işık Üniversitesi, 2011-09-09) Güneş, Mehmet; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıCooperative Communications are emerging as an alternative technology to MIMO Communications. In this thesis, we first summarize information theoretical anal- ysis of known channel types and also consider three fundamental relaying tech- niqııes known as Amplify and Forward, Decode and Forward and Compress and Forward. We propose two compress-and forward based cooperation protocols for a two-user multiple access channel, and obtain their achievable rate regions. The perfornıance of the proposed models are is compared with the performance of other two-user cooperative protocols proposed in literatüre.Yayın Channel adaptive user cooperation strategies for fading wireless channels(Işık Üniversitesi, 2006) Edemen, Çağatay; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıIn this dissertation, a channel adaptive method is proposed for a two user halfduplex cooperative communication system under the assumption of perfect channel state information at the transmitters and the receiver. An achievable rate region for the proposed method is characterized, and evaluated by computer simulation.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.Yayın Enabling 5G and 6G technologies through millimeter-wave and VLC integration for enhanced remote health monitoring systems(Işık Üniversitesi, Lisansüstü Eğitim Enstitüsü, 2025-07-01) Dalloul, Ahmed Hany Assad; Miramirkhani, Farshad; Işık Üniversitesi, Lisansüstü Eğitim Enstitüsü, Elektronik Mühendisliği Yüksek Lisans Programı; Işık University, School of Graduate Studies, Electronics Engineering M.S. ProgramThis thesis examines the pivotal role of wireless networks in healthcare, emphasizing the need for high-performance technologies like 5G and emerging 6G to enable efficient data transfer between medical devices such as sensors and remote monitoring equipment. We delve into the current research landscape surrounding 5G mmWave technology in remote health monitoring systems, focusing on its applications, main challenges, and future trends. We explore the wireless connectivity requirements of reconfigurable hybrid optical-radio-based Medical Body Sensor Networks (MBSNs), proposing an extension of conventional MBSNs to more flexible and generic solutions. This thesis introduces a comprehensive literature review across diverse domains including antenna design, small implantable antennas, on-body wearable solutions, and adaptable detection and imaging systems. Our research further investigates methodological approaches in monitoring systems, analyzing channel characteristics, advancements in wireless capsule endoscopy, and sensing and imaging techniques. Additionally, we explore how 6G's framework integrates Visible Light Communication (VLC) in healthcare, demonstrating how VLC-enabled MBSNs can revolutionize remote patient monitoring and real-time health data transmission by accurately estimating VLC channel parameters, such as channel DC gain and RMS delay spread. We introduce a sophisticated ray tracing technique and ML-based algorithm to model channels and estimate path loss and RMS delay spread within different hospital settings such as ICU ward and family-type patient room. The detailed results of the hospital scenarios are listed using various machine learning algorithms such as LSTM, GRU, RNN, Linear Regression SVR, and KNN. The estimation was illustrated and detailed comprehensively by choosing the best-performing ML technique.
