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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 Design and performance analysis of a novel trellis coded space-time-frequency transmission scheme(Işık Üniversitesi, 2004-07) Oğuz, Onur; Panayırcı, Erdal; Aygölü, Hasan Ümit; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıThis work is concerned with a transmitter diversity technique for wireless communications over frequency selective fading channels. The proposed technique utilizes orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading sub channels on which space-frequency processing is applied and then additional space-time coding is applied in order to increase the diversity. In order to improve the performance of the system, a convenient trellis coding scheme is investigated and design criteria are established. Performance of proposed space-time-frequency OFDM (STF-OFDM) transmitter diversity technique has been examined by computer simulations.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 Ultra wideband indoor communication(Işık Üniversitesi, 2005-07) Yirmibeş, Elif; Panayırcı, Erdal; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıUltra-wideband communication with its attractive features has received great interest from the industry and the research community. It has the potential for very high data rates with low processing powers. With its extremely wide bandwidth, multiple access capability, enhanced capability to penetrate through obstacles and its robustness to fading invites major advances in wireless communications. However, many important aspects of UWB-based communication systems have not yet been thoroughly examined. The propagation of UWB signals in indoor environments is one of the important issues with significant impacts . The amount of available UWB measurement data is very limited and more information is needed to support a comprehensive channel modeling. This paper describes the time-domain characterization of ultra-wideband propagation in indoor environments and introduces the recent proposed channel models with their effectiveness for indoor communications.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 Performance analysis of space time transmit diversity in UMTS under presence of carrier phase offset(Işık Üniversitesi, 2003-12) Toplu, Mehmet İlker; Panayırcı, Erdal; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıThe reguirements for the next generation of mobile communication systems are primarily driven by the explosive growth of worldwide mobile communication subscribers. Additionaly, 3G systems will allow various of new services including wireless Internet acess, high -speed mobile data services and other multimedia applications. The expected growth of subscibers and the provided services indicate the need for increased system capacity of the future mobile communication systems. This thesis deals with Transmit Diversity techniques used for capacity enhancement in the downlink. The principle of Transmit Diversity is to send the same information with two antennas at the base station. If the signals from the different antennas fade independently, diversity at the receiver is achieved. Therefore, the ratio between the desired signal and the noise plus interference from other base stations can be enhanced. This means that there is less transmission power required to meet the same signal quality as in the single antenna case. The investigations within the scope of this work focus on Space-Time Transmit Diversity concept specified in the UMTS standard. I consider the application of Alamouti's Space-Time Transmit Diversity tevhniques in different scenarios. It's performance is evaluated by the calculation of the Average Error at the receiver depentent on the Signal to Noise Ratio. The performance of this method is compared by means MATLAB simulations.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 A novel channel estimation algorithm for ultra wideband communication systems(Işık Üniversitesi, 2005) Çağlar, Ekrem; Panayırcı, Erdal; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıUltra WideBand (UWB) is an emerging technology that is currently investigated as one viable solution for high capacity, short range wireless indoor systems. Its robustness in dense multipath environments, low cost and low power implementation together with high bit rates achievable are its unique promising features. However UWB signals occupy extremely large bandwidth and experience different propagation characteristics than conventional narrowband signals. Thus to evaluate the performance of UWB systems, an innotive channel model and corresponding channel estimation algorithm is required. This work is concerned with channel models developed so far for the UWB systems and appropriate channel estimation techniques. In this work the IEEE 802.15.3a channel model which best decribes the propagation of UWB signals is chosen as working model and a novel channel estimation method based on sequence estimation via EM algorithm is derived.
