Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Wireless Physical Layer Network Coding for Multiple Antenna Systems(01. Izmir Institute of Technology, 2020) İlgüy, Mert; Özbek, BernaWireless networks are prone to interference due to their broadcast nature. In the design of most of the traditional networks, this broadcast nature is perceived as a performance-degrading factor. However, physical layer network coding (PNC) exploits this broadcast nature by enabling simultaneous transmissions from different sources and facilitates an increase in the spectral efficiency of the wireless networks. Besides, the massive multiple input multiple output (MIMO) is considered as one of key technologies to improve the spectral efficiency for wireless communication systems. The combination of PNC and multi-user massive MIMO in the sixth generation (6G) networks can increase further the spectral efficiency. In this thesis, PNC based systems are examined via bit error rate (BER) and coverage probability by focusing on the BER of the network coded symbol (NCS). Hence, PNC based systems are compared with network coding (NC) and conventional schemes. The influence of the signal-to-noise ratio (SNR) differences of the users are examined on the BER performances. Thereby, an alternative method to estimate NCS is proposed for the MIMO-PNC systems without using log likelihood ratio (LLR). We derive a closed form expression for the coverage probability in PNC based multi-user massive MIMO systems by employing zero forcing (ZF) equalization. The non-orthogonal multiple access (NOMA) based PNC system is proposed. We show the applicability of the PNC in the NOMA based MIMO systems by giving the the BER performance results.Master Thesis Secure Beamforming for Millimeter Wave Communications(Izmir Institute of Technology, 2020) Erdoğan, Oğulcan; Özbek, BernaOver the last decade, many advancements have been made in the field of wireless communications. Among the major technology enablers being explored for the fifth-generation (5G) networks at the physical layer (PHY), a great deal of attention has been focused on millimeter-wave (mmWave) communications, massive multipleinput multiple-output (MIMO) antenna systems and beamforming techniques. These enablers bring to the forefront great opportunities for enhancing the performance of 5G and beyond-5G networks, concerning throughput, spectral efficiency, energy efficiency, latency, and reliability. At the meantime, the wireless communication is prone to information leakage to the unintended nodes due to its open nature. Hence, the secure communication is becoming more critical in the wireless networks. To address this challenge, the concept of Physical Layer Security (PLS) is explored. In this thesis, we examine the statistical mmWave transmission through linear beamforming techniques for PLS based systems. We propose the secure multiuser (MU) MIMO mmWave communications by employing hybrid beamforming at the base stations (BS), legitimate users and eavesdroppers. Using a 3 Dimensional mmWave channel model for each node, we employ the artificial noise (AN) beamforming to jam the channels of eavesdroppers and to enhance the secrecy capacity of the overall communication system. We investigate the secrecy performance on different scenarios including the single cell and multicell mmWave MU-MIMO downlink communications and reveal the key points directly related to the system security.Master Thesis Beam Selection Techniques in Millimeter Wave Communications(Izmir Institute of Technology, 2019) Cumalı, İrem; Özbek, BernaMillimeter wave (mmWave) communication is an advantageous technology which is capable of meeting the needs of future mobile networks. On the other hand, the propagation characteristics and system requirements are the restrictive factors for utilization of mmWave communication. Hybrid and digital beamforming architectures can be evaluated as worthy candidates to utilize mmWave communication. In the hybrid architecture, selection of a few number of beams by exploiting the sparse structure of the beamspace channel provides high spectral efficiency with low complexity. In this thesis, the multi-user mmWave communication in sparse and dense environments are investigated. Beam selection algorithms presented in the literature are performed for the sparse environment. While the number of users is equal to the number of radio frequency (RF) chains in a sparse environment, the number of RF chains is less than the number of users in a dense environment. Therefore, an algorithm which performs beam and user selection for the dense environment is proposed. The user selection in the proposed beam and user selection algorithm is performed based on the correlation among users’ channels. Since the users’ channels are highly correlated in mmWave communication, the proposed beam and user selection algorithm improves the spectral efficiency considerably. Furthermore, a non-uniform rectangular array (NURA) antenna configuration for mmWave communication is investigated when the digital beamforming architecture is employed. Then, a user selection algorithm is proposed under the case of lower number of antennas. The simulation results demonstrate the improvement in sum data rate through the proposed user selection algorithm in mmWave communication with NURA configuration.Master Thesis Offloading Strategies for Heterogeneous Wireless Networks(Izmir Institute of Technology, 2016) Tuna, Evren; Özbek, BernaThere has been a tremendous increase in the usage of multimedia services with the rapid penetration of mobile devices. In parallel to the technological developments in hardware and software of communication devices, users demand to have higher quality and more reliable services. The developments in network technologies are towards forming a converged structure that mobile, fixed and internet access technologies are able to operate together. Heterogeneous wireless networks have a critical role in order to meet dramatically increasing traffic demand. As a result of better operation of the systems with the help of heterogeneous wireless networks, it is possible to serve subscribers with higher performance with the help of offloading which transfer the traffic load from a network to another one. Various strategies are used in order to offload traffic between different wireless communication technologies. The main objective of this thesis is to examine offloading strategies which provides operation of different wireless communication technologies efficiently in heterogeneous wireless networks. The performance evaluations of different offloading strategies in various scenarios are implemented. The comparisons of strategies which are user initiated and network initiated are provided by considering their overhead load.Master Thesis Cell Selection and Interference Coordination Techniques for Heterogeneous Wireless Networks(Izmir Institute of Technology, 2015) Mahmuda, Subaha; Özbek, BernaThe rapid growth of traffic demands during past years, has led to the immense deployment of heterogeneous wireless networks consisting large-scale macro cells overlaid with multiple tiers of small cells. This is conceived as the major capacity and performance enhancement coordinator by means of increasing the spectral efficiency per unit area. However, heterogeneous networks implementation comprises new technical challenges related to interference issues and throughput deterioration. Advanced interference coordination techniques are introduced to handle these challenges. The usage of range expansion allows captivating more users and hence attaining performance improvement, however causes extra downlink interference. This becomes exquisite for higher bias values; hence the benefits convert into significant deterioration. To overcome these issues, range expansion should be jointly designed with inter-cell interference coordination. The main objective of this thesis is to analyze the concept of heterogeneous network, the cell selection strategies including range expansion, interference coordination schemes and energy efficiency. The performance evaluations are obtained to different macro-pico base stations deployment scenarios for heterogeneous network by using various cell selection algorithms with and without interference coordination depending on frequency allocation schemes to figure out their impact on the system performance for different contours.Master Thesis Space-Time Coding for Cdma-Based Wireless Communication Systems(Izmir Institute of Technology, 2002) Taban, Alper; Altınkaya, Mustafa AzizMultiple transmit antennas giving rise to diversity (transmit diversity) have been shown to increase downlink (base station to the mobile) capacity in cellular systems.The third generation partnership project (3GPP) for WCDMA has chosen space time transmit diversity (STTD) as the open loop transmit diversity technique for two transmit antennas.On the other hand, the CDMA 2000 has chosen space time spreading (STS) and orthogonal transmit diversity (OTD) as the open loop transmit diversity.In addition to all the standardization aspects, proposed contributions such as space time coding assisted double spread rake receiver (STC-DS-RR) are exist.In this thesis, open loop transmit diversity techniques of 3GPP, CDMA 2000 and existing contributions are investigated.Their performances are compared as a means of biterror- rate (BER) versus signal-to-noise ratio (SNR).Master Thesis Effects of Transmission Rate on the Performance of Ieee 802.11 Dcf Based Multi-Hop Wireless Networks(Izmir Institute of Technology, 2012) Sancaklı, Sibel; Aydoğdu, CananMulti-hop wireless networks offer promising applications for future communications and they differ from widespread single-hop networks. Throughput (node-to-node successfully transmitted bits per second) is an important performance metric in single-hop wireless net- works, whereas goodput (end-to-end successfully delivered bits per second) becomes an indication of the performance in multi-hop networks. Energy-efficiency is another impor- tant performance metric due to the limited battery life of mobile devices. Main factors that affect the network performances are packet collisions that occur due to the hidden terminal problem and blocking of packets at interface queues at intermediate nodes in multi-hop networks. In this dissertation, the effect of transmission rate on goodput, throughput and energy performances of IEEE 802.11g DCF based on multi-hop wireless networks are in- vestigated over a large range of traffic loads. The performances are observed under direct transmission and multi-hop transmission, considering MAC contention such as binary ex- ponential backoff, retransmissions, collisions and overhearing of nodes. IEEE 802.11g DCF is used because it supports high data rates and has interoperability with the older version of IEEE standards. Network Simulator 2 is modified to compute the goodput, throughput and energy per bit (EPB) performance metrics under perfect channel condi- tions. The results reveal that varying the data rate has no effect on goodput, throughput and energy under light traffic loads. Under moderate-to-heavy traffic loads, goodput and energy efficiency performances drop sharply whereas throughput remains constant. Hid- den terminals and interface queue blocking is observed to be the reason for performance reduction of increasing goodput and EPB which increase with traffic load under moderate- to-heavy traffic loads. This suggests that a rate adaptation algorithm, which discriminates the reason of packet drops and keeps the transmission rate at the maximum can improve goodput and energy performances significantly for multi-hop wireless networks.Master Thesis A Study on Wireless Channel Models: Simulation of Fading, Shadowing and Further Applications(Izmir Institute of Technology, 2008) Arsal, Ali; Özen, SerdarIn this thesis, we simulate multipath fading which is assumed to have Rayleigh or Rician distribution under the non-line-of sight or line-of-sight condition respectively; as well as spatial shadowing process, assumed to be log-normally distributed. We propose a low-complexity high performance Rayleigh fading simulator, an autoregressive moving average (ARMA)(3,3) model. This proposed method is a variant of the method of filtering of the white Gaussian noise where the filter design is accomplished in the analog domain and transferred into the digital domain. The proposed model is compared with improved Jakes. model, autoregressive (AR) filtering and inverse discrete Fourier transform (IDFT) techniques, in performance and computational complexity. The proposed method outperforms AR(20) filter and modified Jakes. generators in performance. Although IDFT method achieves the best performance, it brings a significant cost in storage which is undesirable.The proposed method achieves high performance with the lowest complexity.Additionally, we apply the quantized filter extension of our proposed filter design, since quantized filters are generally used in hardware implementations due to their minimum power consumption, minimum heat generation and their computational efficiency. We simulate spatial shadowing process, via the simulation method proposed by Patzold and Nguyen. This method is derived from a reference model by using the sum of sinusoids principle. There are two methods enabling the fitting of the simulation model to the reference model with respect to the probability density function (pdf) of the received signal strength as well as to a given autocorrelation function with a decaying exponential shape.Furthermore we use our predicted autocorrelation function obtained via the site-specific radio propagation prediction software named Wireless InSite in order to determine the model parameters.Master Thesis Implementation of Relay-Based Systems in Wireless Cellular Networks(Izmir Institute of Technology, 2010) Çınar, Melih; Özbek, BernaThe wireless cellular networks are limited by interference and coverage issues where the users at the edge of the cell usually do not receive enough signal energy. To combat these problems and provide higher signal to interference noise ratio and capacity without increasing the transmit power, the idea of using relays in cellular networks was explored and evaluated in the literature. On the other hand, multiple input multiple output (MIMO) antenna systems have great potential to increase capacity and reliability of a wireless cellular network compared to single input single output systems. Hence, the integration of MIMO systems in the relay-based cellular networks has great potential to meet the growing demands of future communication. In this thesis, we explore the performances in conventional and relay-based wireless systems with single and multiple antennas by ad justing the frequency reuse factor as one and four. We consider wireless cellular based networks where six fixed relays are placed evenly in each cell in a hexagonal layout. A user chooses to receive the transmitted signal either directly from the base station or via one of the relays by employing selection algorithms. Throughout this thesis, we first determine the optimum relay locations considering different relay powers. Then, we investigate the system capacity for the cell with and without relays. Next, we examine the capacity performances by changing the cell diameter and the relay power. Finally, we explore the performances of relay based networks with multiple antennas.Master Thesis Differential and Coherent Detection Schemes for Space-Time Block Codes(Izmir Institute of Technology, 2002) Oruç, Özgür; Altınkaya, Mustafa AzizThe single most important technique providing reliable communication over wireless channels is diversity. One of the diversity techniques is space diversity. Typical examples of space diversity are multiple transmit and/or receive antenna communications. By employing multiple antennas either at the transmitter or at the receiver, multiple antenna communications introduce a space diversity to combat fading without necessarily sacrificing bandwidth resources; thus they become attractive solutions for broadband wireless applications.Space-time coding is a new diversity method for communication over wireless channels using multiple transmit antennas. Two types of space-time coding have been studied for a few years: trellis and block coding. Space.time trellis codes perform extremely well at the cost of relatively high complexity. Owing to this, in this thesis, space-time block codes are investigated in detail. Not only these codes support an extremely simple maximum likelihood detection algorithm based only on linear processing at the receiver, but also they can be used for multiple transmit antenna differential detection.In this thesis, coherent and differential detection schemes for space-time block coding are investigated in detail over non-dispersive Rayleigh fading channels. In coherent detection systems, Alamouti's and Tarokh's codes with two, three and four transmit antennas are simulated for BPSK and QPSK modulation. Furthermore, Tarokh's and Hughes' differential detection schemes with two transmit antennas are investigated and a unifying approach to these structures is developed with a new design criteria for optimal unitary group codes. Then, the codes chosen by Tarokh and Hughes are simulated using a generalized differential detection scheme for BPSK and QPSK modulation. Moreover, codes of Hochwald are also simulated in order to compare them to the codes of Tarokh.