Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Subject: search.filters.subject.Wireless communications

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  • Master Thesis
    Cell Selection Algorithms for Conventional Narrow Band Wireless Systems
    (Izmir Institute of Technology, 2017) Yılmaz, Saadet Simay; Özbek, Berna
    Public safety organizations provide a stable and secure environment for the society. Wireless communication between public safety officers is very important to transmit voice or data during an emergency crisis. When the public communication networks can not provide service during crisis, disaster and high traffic cases, Professional Mobile Radio systems (PMR) such as conventional Association of Public Safety Communications Officials (APCO25) and trunked Digital Mobile Radio (DMR) systems are needed to improve the service quality and to provide uninterrupted service provided to the users. While providing continuous voice and data service, it is very important to efficiently select the base station to be served and to ensure that a mobile user can seamlessly attach from one base station to another base station while moving within a cell. In this sense, it is critical to determine the base station to be served by efficient cell selection algorithms. Cell selection is the process of deciding the base station which provides services to the users. Cell selection plays an important role in balancing the system load and thus overall system performance. By means of efficient cell selection algorithms, it is aimed to reduce the waiting time and to connect a base station as soon as possible while establishing reliable transmission link for PMR systems in emergencies.
  • Master Thesis
    Multiple Antenna Based Phyical Layer Security Wireless Systems
    (Izmir Institute of Technology, 2017) Özdoğan Şenol, Özgecan; Özbek, Berna
    In the last decade, the demand for wireless services increases at unprecedented rates. Due to the inherent open nature of radio propagation, wireless transmission is vulnerable to various attacks despite its popularity. Therefore, communication security in wireless networks is becoming more critical than ever. Conventionally, cryptographic techniques are deployed on upper layers of network protocols as a solution. As a complement to the traditional cryptographic techniques, physical layer (PHY) security exploits the characteristics of wireless channels to enable secure wireless communications. The aim is to limit the amount of information that can be extracted by any unauthorized users via utilizing inherent randomness of noise and communication channels. The design of PHY security schemes is not based on the premise that eavesdropper has limited computational power contrary to upper layer secrecy techniques. In fact, the eavesdropper may have infinite computational power. Nevertheless, secure communication can be achieved by the combination of appropriate coding and transmit precoding design with the usage of available channel state information. PHY security methods can work independently from upper layer encryption techniques. Thus, PHY security techniques can be used to leverage the secrecy of already existing communication systems. In this thesis, PHY security enhancement mechanisms, especially in multiuser multiple antenna systems with a limited feedback link are investigated. Four different system models under secrecy consideration with different channel conditions including quasi-static fading channels, temporally correlated fading channels are presented. In order to disrupt the reception of any potential eavesdropper, artificial noise (AN) beamforming scheme is employed. The effects of lack of perfect channel state information (CSI) at the transmitter and the AN leakage that is caused by limited CSIT are analyzed. The thesis proposes a reduction in feedback load using receiver side selection criterion with special codebook design and appropriate beamforming. Our approach is capable of enhancing the security of wireless communications by selecting the users with favorable channel conditions and quantizing channel direction information (CDI) by a special codebook. Also, inter-user interference is utilized as a jamming method when eavesdropper’s CSI unknown by the transmitter. Simulation results demonstrate the feasibility of the proposed PHY security mechanisms by examining the achievable secrecy rates.