Akademik Veri Yönetim Sistemi
Tezin Türü: Doktora
Tezin Yürütüldüğü Kurum: İstanbul Medipol Üniversitesi, Fen Bilimleri Enstitüsü, Türkiye
Tezin Onay Tarihi: 2024
Tezin Dili: İngilizce
Öğrenci: ABUU BAKARI KIHERO
Danışman: Hüseyin Arslan
Özet:
The vision of modern wireless networks is characterized by the support of diverse applications beyond the conventional connectivity of people through voice/video communication. The focus has widened to include the provision of connectivity between things as we aspire to realize the smart cities/living concept. Such lofty goals have led to the emergence of stringer operational requirements from both network and user equipment sides. For instance, ultimate operational flexibility, high energy and spectral efficiencies, support for massive connectivity, highly reliable, extremely low latency, and secure transmissions, etc. are some of these requirements. One way of meeting these requirements is through robust physical layer designs as the improvement this layer has the spill-over effect that directly benefits the performances of the algorithms in other layers. As such, many revolutionary physical layer techniques have been studied in the literature since the beginning of the 5G research and continue in this era of 5G-Advanced and beyond. Some of the major steps that have been taken include i) the transition to higher frequency bands (millimeter wave and Terahertz) in search of larger spectrums to enable massive user connectivity and sensing capabilities, ii) the Introduction of massive MIMO and extra-large arrays to combat channel effect as well as enhance spatial resolution for both user multiplexing and sensing purposes, İİİ) encouragement of coordinated networks concept to mitigate the cell-edge interference problem, exploit macro-diversity, and improve cell-edge users' experience. The coordinated networks concept has been studied under different sub-concepts such as coordinate multipoint, centralized radio access networks, distributed MIMO systems, cell-free MIMO, etc., and iv) search for new waveforms, for example, the introduction of the multi-numerology frame structure as an initial step toward achieving flexible frames has been considered in 5G. This thesis presents some of our contributions to the literature in an effort to revolutionize the physical layer designs. Specifically, our contributions focus on the multi-numerology frame designs, coordinated networks, and physical layer security as well as the development of channel emulators as reliable and cost-efficient testbeds for testing prototypes and physical layer algorithms prior to their adoption to the networks. Under the study on channel emulators, a simple and effective channel emulator that features a benchtop-sized (small-sized) reverberation chamber (RVC), radio frequency (RF) cable, and power controller is proposed for emulating Rician propagation characteristics with a flexibly controllable k-factor. The developed emulator can precisely introduce the Rician propagation effect (with the desired k-factor) to the input RF signal. The proposed emulator is cost-efficient as it utilizes off-the-shelf simple RF components and it is suitable for algorithm/prototype testing and educational purposes in wireless laboratories. The next study explores the inter-numerology interference (INI) problem in 5G's multi-numerology frame structure. 5G radio access technology (RAT) standard has allowed the coexistence of multiple frame structures with different subcarrier spacings in one frequency band in order to facilitate flexibility in serving multiple users with varying and often conflicting requirements. Though efficient in providing the required flexibility, this approach introduces a new kind of interference into the system known as INI. In this study, a novel cyclic prefix (CP) insertion technique (referred to as common CP) for a multi-numerology system is mathematically analyzed in terms of the INI problem and its extensive comparison with the conventional CP configuration standardized for the 5G multi-numerology systems is presented. An in-depth discussion of various critical issues concerning multi-numerology systems such as frequency domain multiplexing, time domain symbol alignment, and orthogonality between subcarriers of different numerologies is presented in the light of both, conventional and common CP configurations. The analyses reveal that common CP has the advantage of restructuring the INI pattern in the system in a manner that paves the way for developing better techniques of avoiding or minimizing INI in future generations. Afterward, we explore synchronization problems in Coordinated Networks which involve multipoint to multipoint transmissions. Specifically, time and frequency synchronization between multiple devices and multiple transmission and reception points are among the critical challenges in coordinated networks. It is generally not possible for a single device to establish perfect synchronization with multiple transmission/reception points simultaneously. Our work focuses on developing spectral efficiency ways of facilitating timing synchronization in such systems. Specifically, a novel and flexible hybrid guard duration (HGD) based on zero tail (ZT) and cyclic prefix (CP) concepts is proposed to create room for accommodating any timing mismatches between a device and the transmission/reception points it is communicating with without extending the total symbol duration. This is unlike the traditional extended CP (E-CP) approach that degrades the system's spectral efficiency (SE) and elevates the transmission latency due to the extended symbol duration. HGD maintains the usage of the normal CP size and the total symbol duration, thereby ensuring better SE performance and relatively low latency. The proposed HGD is evaluated and compared with the traditional E-CP in terms of bit error rate, SE, and latency. Finally, the thesis sheds light on the physical layer security (PLS) concept from a wireless channel perspective. Unlike traditional cryptographic methods, PLS utilizes the random characteristics of wireless channels to secure the entire communication process against various attacks. Emerging technologies like Reconfigurable Intelligent Surfaces (RIS), Machine Learning (ML), and sensing are expected to enhance these networks by introducing new channel features that can be readily exploited for PLS. Our work in this domain examines these new features and their potential for PLS implementation. It also highlights important considerations when selecting channel features for PLS, emphasizes the role of channel control and sensing technologies, and discusses security attacks targeting channel characteristics. Additionally, it explores the Secret Key Generation (SKG) process in the context of array non-stationary channel characteristics in Extremely Large MIMO (XL-MIMO) systems.