Millimeter Wave Hybrid Beamforming Systems

Abstract

The motivation for this thesis is the design of millimetre wave (mmWave) hybrid beamforming systems for supporting high user density. mmWave systems with hybrid digital-to-analogue beamforming (D-ABF) have the potential to fulfil 5G traffic demands. However, the capacity of mmWave systems is severely limited as each radio frequency (RF) transceiver chain in current sub-array mmWave base station (BS) architectures support only a particular user. Therefore, two new algorithms have been proposed for broadband mmWave systems. The algorithms operate on the principles of selection combining (SC) and principal component (PC). SC is a spatio-temporal hybrid D-A BF which has been designed to exploit multipath diversity,which is a characteristic feature of broadband propagation at mmWave. A novel low-complexity variant of SC, called low-complexity selection combining (LC-SC) has also been proposed for supporting high user density for such sub-arraymm-Wave BS. mmWave lens-antenna systems are an emergent beamforming technology. They are novel because they eliminate the requirement of traditional analog beamformers. In this context, a low-complexity beam allocation (LBA) algorithm, proposed in an earlier research, has been applied to solve the challenging problem of maximizing sum data-rates in switched-beam mmWave systems. However, there are practical limitations, such as restrictions in the number of available RF chains at the BS, sensitivity to sidelobe interference and the beam generation techniques. Using generalized beam-patterns, the maximum sum data-rates achievable in switched-beam mmWave systems is compared to fixed-beam systems by applying LBA. Then, the impact on maximum sum data-rates of actual beam-patterns, obtained from a practical mmWave lens-antenna, which have higher and non-uniform sidelobes compared to the theoretical beams, is assessed. Non-orthogonal multiple access (NOMA) relay with hybrid digital-to-analog precoding (D-A P) as a promising solution for supporting high user densities in overloaded millimeter wave (mmWave) systems is investigated. To support high user densities in current mmWave hybrid D-A P systems, an idea based on exploiting the concept of NOMA relay to support 2K users per RF chain is proposed, where 2K ≥ M. To design the hybrid D-A P systems, the SC and PC algorithms are combined with NOMA relay to support significantly higher user densities. In future research, performance impairments in beamforming assisted mmWave NOMA systems due to far-user’s angle-of-departure (AoD) divergence with respect to the near-user is being investigated. This investigation is novel since most literature in NOMA considers both the near-user and far-user pairs static with respect to one another