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

Dual-Polarized Reconfigurable Intelligent Surface Assisted Broad Beamforming

A reconfigurable intelligent surface (RIS) consists of a large number of low-cost elements that can control the propagation environment seen from a transmitter by intelligently applying phase shifts to impinging signals before reflection. This paper studies an RIS-assisted communication system where a transmitter wants to transmit a common signal to many users residing in a wide angular area. To cover this sector uniformly, the RIS needs to radiate a broad beam with a spatially flat array factor, instead of a narrow beam as normally considered. To achieve this, we propose to use a dual-polarized RIS consisting of elements with orthogonal polarizations and show that the RIS can produce a broad beam if the phase shift configuration vectors in the two polarizations form a so-called Golay complementary sequence pair. By utilizing their properties, we also present a method for constructing configuration for large RISs from smaller ones, while preserving the broad radiation pattern of the smaller RIS. The numerical results corroborate the mathematical analyses and highlight the greatly improved coverage properties.Comment: This letter has been accepted for publication in IEEE Communications Letter

Broad Beam Reflection for RIS-Assisted MIMO Systems with Planar Arrays

While reconfigurable intelligent surface (RIS)-aided user-specific beamforming has been vastly investigated, the aspect of utilizing RISs for assisting cell-specific transmission has been largely unattended. Aiming to fill this gap, we study a downlink broadcasting scenario where a base station (BS) sends a cell-specific signal to all the users located in a wide angular area with the assistance of a dual-polarized RIS. We utilize the polarization degree of freedom offered by this type of RIS and design the phase configurations in the two polarizations in such a way that the RIS can radiate a broad beam, thereby uniformly covering all azimuth and elevation angles where the users might reside. Specifically, the per-polarization configuration matrices are designed in such a way that the total power-domain array factor becomes spatially flat over all observation angles implying that the RIS can preserve the broad radiation pattern of a single element while boosting its gain proportionally to its aperture size. We validate the mathematical analyses via numerical simulations.Comment: To appear in the Proceedings of Asilomar 202

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance