Novel system designs for future mmwave communication networks

The advent of next-generation communication systems, has spurred a demand for innovative approaches to overcome the limitations of traditional wireless technologies. This thesis investigates novel system designs for future millimeter-wave (mmWave) communication networks, focusing on improving bit error rate (BER), achievable rate, energy efficiency, and communication coverage. With mmWave technology promising enhanced bandwidth and data throughput, the challenges posed by severe path loss and signal attenuation in these high-frequency bands necessitate the use of advanced techniques, such as large antenna arrays, index modulation (IM), and reconfigurable intelligent surfaces (RISs). The first part of this thesis introduces cluster index modulation (CIM), a novel IM scheme tailored for more realistic mmWave environments compared to existing IM schemes. CIM leverages the spatial separation of clusters and indexes distinct propagation paths to improve BER performance. This concept is extended with a hybrid beamforming architecture and further explored within an RIS-aided framework, demonstrating the potential of CIM to enhance spectral and energy efficiency in complex mmWave environments. To address the high cost and power demands associated with large antenna arrays, this thesis proposes the Plug-In RIS, a fully passive and low-complexity solution tailored for blockage mitigation in mmWave networks. This standalone RIS operates without the need for decoupling end-to-end channel, providing a cost-effective alternative for coverage enhancement in dead zones. Additionally, a comprehensive design framework for both BS-side and UE-side RIS deployments is presented, offering insights into optimal placement strategies for different application scenarios and highlighting practical design considerations in RIS control and configuration. The latter part of the thesis focuses on beyond-diagonal RISs (BD-RISs), an advanced RIS structure integrated with the base station to enable efficient passive beamforming. A BD-RIS provides high beamforming gains comparable to those of active antenna arrays but with significantly lower energy consumption, making it a valuable solution for 6G applications requiring enhanced coverage and energy efficiency. Although primarily focused on communication performance, BD-RIS is also shown to support applications in single-anchor localization, where high beamforming gain is crucial for accurate positioning. In summary, this thesis contributes to the development of novel system designs for mmWave networks by presenting innovative modulation schemes, energy-efficient RIS designs, and advanced beamforming techniques. The findings provide a foundation for future research in scalable, cost-effective solutions that meet the stringent requirements of next-generation wireless networks.