Analysis of Millimeter-Wave Networks: Blockage, Antenna Directivity, Macrodiversity, and Interference

Due to its potential to support high data rates at low latency with reasonable interference isolation because of signal blockage at these frequencies, millimeter-wave (mmWave) communications has emerged as a promising solution for next-generation wireless networks. MmWave systems are characterized by the use of highly directional antennas and susceptibility to signal blockage by buildings and other obstructions, which significantly alter the propagation environment. The received power of each transmission depends on the direction the corresponding antennas point and whether the signal’s path is line-of-sight (LOS), non-LOS (i.e., partially blocked), or completely blocked. A key challenge in modeling blocking in mmWave networks is that, in actual networks, the blocking might be correlated. Such correlation arises, for example, when single transmitter tries to broadcast to pair of receivers that are close to each other, or more generally when they have a similar angle to the transmitter. In this situation, if the first receiver is blocked, it is likely that the second one is blocked, too. This dissertation explores four related but distinct issues associated with mmWave networks: 1) Analytical modeling of networks consisting of user devices and blockages with fixed or random, but independent, locations, 2) The careful characterization of correlated blocking and analysis of its impact on the performance of mmWave networks, 3) The proposed use of macrodiversity as an important strategy to mitigating correlated blocking in mmWave networks and the corresponding analysis, and 4) The proposed use of networks of unmanned aerial vehicles (UAVs) to provide connectivity in urban deployments. This work provides insight into the performance of variety of applications of mmWave communications, ranging from wireless personal area networks (WPAN), device-to-device networks, traditional terrestrial, cellular networks, and the UAV-based networks where the UAVs act as the cellular base stations. A common thread throughout this dissertation is the development of new tools based on stochastic geometry and their application to modeling and analysis. The analysis presented in this dissertation is general enough to find application beyond mmWave networks, for instance the results may also be applicable to systems that use free-space optical (FSO) signaling technologies

Design and optimization of reconfigurable intelligent surfaces for enhanced wireless communication systems

Reconfigurable intelligent surface (RIS) has been identified as a promising disruptive innovation to realize a faster, safer and more efficient communication system in the coming 6th generation (6G) era. The RIS is a meta-material composed surface comprising a large number of passive scattering unit cell (UC) elements. Each element independently controls incident signals by dynamically adjusting their amplitude and/or phase shifts. The reflected signals from all elements are coherently combined and directed towards specified directions, enabling selective electromagnetic (EM) properties. By densely deploying RISs and intelligently coordinating them within wireless propagation environments, it is possible to achieve reconfigurable and programmable end-to-end wireless channels. This innovation has the significant potential to revolutionize wireless communication by enhancing signal quality, coverage, and capacity in a cost-effective and energy-efficient manner. This thesis aims to systematically study the design of RIS to address potential challenges in its practical deployment for wireless communication enhancement. An overview of basic technologies that may be encountered in RIS-assisted systems has been first studied. To address the inaccurate and complex channel estimation and ensure sufficient and stable power gain, a RIS-aided broadbeam design is then proposed. The design proposed in this thesis will mainly include the RIS beamforming design of generating single and multiple flat beams to cover any arbitrary sector regions. Meanwhile, the thesis also tends to define cooperation modes of base stations (BSs) concerning whether they reach an agreement on collaboratively utilising RISs and sharing resources. The resource allocation scheme between cooperative and non-cooperative BSs will be investigated. Lastly, the thesis also aims to design a RIS codebook in the wideband system leveraging the beam squint effect. The design of a codebook can largely reduce computational complexity. To conclude, the work presented in this thesis provides insight into the design of RIS for broadbeam design, which can be viewed as an initial step towards achieving channel estimation. The investigation of non-cooperative BSs and the design of RIS codebooks also provide guidance for further theoretical study and practical implementation of RIS for enhancing wireless communication systems