CELL-FREE AND INTELLIGENT REFLECTIVE SURFACES AIDED ARCHITECTURES FOR WIRELESS COMMUNICATIONS

Diluka A Loku Galappaththige, for the Doctor of Philosophy degree in Electrical Engineering, presentedon September 2, 2021, at Southern Illinois University Carbondale. TITLE: CELL-FREE AND INTELLIGENT REFLECTIVE SURFACES AIDED ARCHITECTURES FOR WIRELESS COMMUNICATIONS Major Professor: Dr. Gayan Amarasuriya Aruma Baduge Novel wireless communication technologies are being explored to satisfy the needs of the next generation wireless standards including the ever-increasing demands for higher data speeds, extended coverage, increased reliability, and massive connectivity. Recently, novel cell-free/distributed massive multiple-input multiple-output (MIMO) and intelligent reflective surface (IRS)-assisted wireless architectures have received significant research attention as new physical-layer transmission technologies, which are capable of achieving unprecedented spectral and energy efficiency improvements. To this end, both cell-free massive MIMO and IRS-aided systems have been identified as key candidate technologies for the sixth-generation (and beyond) wireless standard. Thus, this dissertation focuses on designing novel physical-layer transmission technologies for cell-free and IRS-enabled massive MIMO networks by taking into account realistic wireless communication impairments. This dissertation also proposes novel techniques to determine the feasibility of the proposed systems/technologies in fulfilling the requirements of future wireless networks such as enhanced spectrum and energy efficiency, improved reliability, and massive connectivity, through performance analysis. Specifically, new system and channel models, channel estimation techniques, pilot sharing strategies, spectrum utilization techniques, energy harvesting and transmit power allocation schemes, techniques for enabling smart propagation environment, and comprehensive performance analysis frameworks are developed for cell-free/distributed massive MIMO and IRS-assisted communication architectures.Our first work focuses on developing a comprehensive analytical framework for exploiting the feasibilityof enabling simultaneous wireless information and power transfer (SWIPT) in cell-free massive MIMO systems. Specifically, tight approximations to the downlink harvested energy and downlink/uplink achievable rates are derived using a non-linear energy harvesting model for two functional channel state information scenarios. Thereby, max-min fairness-based transmit power control policies are proposed to boost the performance of SWIPT-enabled cell-free massive MIMO by mitigating the deleterious near-far effects in distributed transmission/reception. In our second work, the viability of the coexistence of underlay spectrum sharing in cell-free massive MIMO systems is investigated. To this end, a secondary system is underlaid within the same primary spectrum by exploiting underlay spectrum sharing techniques. A generalized pilot sharing scheme is first formulated, and thereby, the achievable rates are derived for both systems by adopting secondary transmit power constraints for the secondary system. Then, the proposed orthogonal multiple access analytical framework is extended to facilitate non-orthogonal multiple access (NOMA) scenario. The third research contribution investigates the fundamental performance of a distributed IRSs-aided communication system by enabling a smart propagation environment. To be more specific, the statistical characterization of the optimal end-to-end signal-to-noise ratio is derived by controlling the phase-shifts of the incident electromagnetic waves at the IRSs. Thereby, the fundamental performance matrices pertaining to the distributed IRSs-assisted set-up are evaluated for Nakagami-m fading channels. In the fourth work, a multi-hop IRS-assisted communication system is proposed for investigating the feasibility of adopting multi-IRSs in future wireless standards. Finally, the feasibility of adopting an IRS within cell-free wireless communication is investigated. To this end, the fundamental performance matrices including the achievable rate and outage probability are derived over Rayleigh fading channel model. The key design criteria of the above transmission strategies/designs is to efficiently combine the spatial diversity, spatial multiplexing gains, favorable propagation conditions of cell-free massive MIMO with the attributes of NOMA, underlay spectrum-sharing, energy harvesting, and efficient signal processing. Moreover, IRS-assisted transmission schemes are exploited to boost the performance by enabling smart propagation environments. Hence, the fundamental relationships between data rate, coverage, and reliability of the aforementioned transmission strategies are quantified by taking into account the detrimental effects of practical wireless transmission impairments