Novel over-the-air methods and practical approaches for reconfigurable intelligent surfaces system designs and analyses

The rapid evolution of wireless communication networks towards 6G and beyond requires innovative solutions to enhance spectral efficiency, improve system reliability, and manage the growing complexity of modern communication systems. This thesis explores the integration of reconfigurable intelligent surfaces (RIS) with communication systems to address fundamental challenges in wireless networks, such as challenges in the channel, overall system efficiency, system scalability, practicality and more. By leveraging RIS technology, the wireless propagation environment can be dynamically reconfigured to optimize signal transmission, mitigate interference, and enhance network performance. This thesis presents novel over-the-air (OTA) approaches to RIS-aided communication systems, considering both passive and active RIS architectures as well as their practical implications in real-world deployments. Chapter 2 investigates an virtual OTA equalization technique enabled by RIS, demonstrating how RIS phase alignment can effectively eliminate inter-symbol interference (ISI) and convert frequency-selective fading channels into frequency-flat channels. Chapter 3 includes a study proposing RIS-aided uplink non-orthogonal multiple access (NOMA) schemes, optimizing resource allocation and improving outage probability performance for two users. Chapter 4 introduces a hybrid RIS architecture for OTA index modulation (IM), integrating passive and active RIS elements to achieve superior spectral efficiency and reduced detection complexity. Chapter 5 focuses on network-independent RIS architectures, proposing a user-controlled RIS system that enhances practicality, signal quality and energy efficiency in a simple and practical manner without relying on centralized network control. Chapter 6 discusses RIS identification techniques in mobile networks, addressing critical challenges related to RIS security, authentication, and interference mitigation, introducing a new and important concept for RIS deployment to the literature. Chapter 7 presents a power-differentiated dual-code index modulation (RIS-PD-DCIM) scheme that improves spectral efficiency and performance through optimal RIS partitioning strategies. The contributions of this thesis provide a comprehensive framework for RIS-aided communication systems, addressing key limitations in existing wireless technologies and paving the way for the seamless integration of RIS into 6G networks. The proposed methodologies and system models demonstrate the potential of RIS to revolutionize wireless communications by offering enhanced system designs, improved spectral and energy efficiency, and robust performance in dynamic environments. These findings contribute to the ongoing efforts in the research community to develop practical, scalable, and intelligent wireless networks for the future.