| Περίληψη: | Moving from fifth-generation (5G) into Beyond 5G (B5G) and sixth-generation (6G) wireless networks, both academia and industry around the world have already started to investigate advanced technologies. More stringent requirements such as ultra high data rate, high energy-efficiency, extremely high reliability, ultra low latency, global coverage and connectivity are considered in future networks. Reassessment of key performance indicators (KPIs) and definition of new KPIs in future use cases are required, since not only novel 6G technologies are introduced but also changes in the architecture of the conventional 5G cellular networks are expected.
Massive multiple-input-multiple-output (MIMO), the main technology of 5G, is a reality and research community investigates new promising physical layer technologies. Although massive MIMO resolves many basic problems of wireless communications, several open problems remain. The usage of multiple antennas enables beamforming, thus increasing the received signal power and liming interference issue at the end user. However, this does not apply for each served user. Large signal variations are observed in a real system especially for cell-edge users, due to possible blockages that are placed between the communication link. Furthermore, interference problem arises from other neighbor cells. Apart from performance metrics related to the user quality of experience, a base station (BS) equipped with a massive number of antennas features with high power consumption, that comes into conflict with the energy-efficient future networks. Several technologies such as Reconfigurable Intelligent Surfaces (RISs), Cell-Free MIMO and Orthogonal Time Frequency Spatial (OTFS) modulation are emerging in 6G aiming to solve these open problems.
RISs are envisioned as a new physical layer technology in 6G wireless communication systems. A RIS is a two-dimensional, low-cost surface that is strategically placed in the infrastructure and introduces an optimal phase shift to the incident signal, thus reflecting the electromagnetic wave in the desired direction of the user. Therefore, RIS installation can be used to achieve smart propagation environments by turning the wireless channel from a passive actor into a service. Properties of the meta-surface can be altered and controlled in real-time, while low power consumption is the main benefit of RIS node compared to other physical layer technologies. These features explain the great interest of the research community on this novel technology. Various possible use cases are presented in this work, proving the potential of RIS technology in future wireless communication networks.
In this thesis, we study the efficiency of power iteration-based methods in the received signal power maximization problem, assuming a RIS-aided Orthogonal Frequency Division Multiplexing (OFDM) system. Apart from state-of-the-art passive beamforming methods, general techniques that solve the uni-modular quadratic program (UQP) problem, are also examined. Binary phase shifts with unbalanced amplitudes and mutual coupling are considered, while a method that is easily implemented on hardware is proposed. Furthermore, proposed architectural optimizations, related to memory and loop dependencies management, lead to a low latency-oriented solution. Thus, the implementation of the proposed method, on a Zynq UltraScale+ multiprocessor system on a chip (MPSoC) device, results in an extremely low execution time.
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