Reconfigurable intelligent surface aided wireless systems

Abstract

In this thesis, we investigate the family of spectral-, hardware- and energy-efficient reconfigurable intelligent surfaces (RIS) architectures. Firstly, we propose a channel estimation method relying on a low pilot overhead, which exploits the spatial correlation of the cascaded two-hop channels of multi-cell multiple-input and multiple-output (MIMO) RIS-aided systems. Then, we investigate RIS-aided beamforming architectures, where a two-stage channel estimation and beamforming scenario is considered. In the first stage, the instantaneous channel is estimated for the RIS configuration. In the second stage, once the RIS configuration has been determined, the equivalent channel spanning from the user equipment (UE) to the base state (BS) is estimated for configuring the BS beamforming weights. Additionally, spectral-efficient RIS beamforming architectures, including the non-diagonal RIS and simultaneous transmitting and reflecting (STAR) RIS, are analyzed. In the nondiagonal RIS architecture, the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment, which increases the flexibility in the design of RIS phase shifts, hence, potentially improving the system performance. In the STAR-RIS architecture, we investigate the ergodic spectral efficiency of STAR-RIS aided systems relying on spatial division multiple access (SDMA), while considering realistic imperfect channel state information (CSI) and transceiver hardware impairments (HWIs). Furthermore, a hardware-efficient RIS-based modulation schemes is proposed for the single-user systems. Both amplitude-phase shift keying (A-PSK) and quadrature amplitude-phase shift keying (QA-PSK) are conceived. As for multiple-user systems, we conceive a transmitter relying on a single radio frequency (RF) chain for low-complexity RIS-aided multi-user communication. Finally, we investigate energy-efficient RIS based holographic beamforming architectures. We propose two novel reconfigurable holographic surface (RHS) aided architectures, namely a phase-controlled RHS-aided beamformer and a switch-controlled RHS-aided beamformer, which are based on the configuration of the phase shift and of the ON-OFF state of each RHS radiation element, respectively. Afterwards, an intelligent omni-surface (IOS) assisted holographic multiple-input and multiple-output (HMIMO) architecture is conceived for 360⁰ full-space coverage at a low energy consumption. Furthermore, the theoretical ergodic rate's lower bound is derived based on the moment matching approximation method. Cell-free networks are also considered. Specifically, we propose a hybrid beamforming architecture for the single-layer RHS and then the near-field spectral efficiency of the RHS in cell-free networks is derived theoretically by leveraging a stochastic geometry approach.

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Identifiers

ORCID for Qingchao Li: orcid.org/0000-0003-4928-334X

ORCID for Mohammed El-Hajjar: orcid.org/0000-0002-7987-1401

ORCID for Lajos Hanzo: orcid.org/0000-0002-2636-5214

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