Thesis Defence: A Reconfigurable-Intelligent-Surface Unit Cell With Tunable Phase and Phase-Frequency Slope For Wide-Band OFDM Systems

Omran Abbas, supervised by Dr. Anas Chaaban, will defend their thesis titled “A Reconfigurable-Intelligent-Surface Unit Cell With Tunable Phase and Phase-Frequency Slope For Wide-Band OFDM Systems” in partial fulfillment of the requirements for the degree of Master of Applied Sciences in Electrical Engineering.

An abstract for Omran Abbas’ thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in person defences.

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ABSTRACT

The reflection characteristics of reconfigurable intelligent surfaces (RIS) depend on the phase response of the constituent unit cells, which is necessarily frequency-dependent. This work investigates two topics: the role of RIS unit cells with different phase-frequency profiles in a wide-band orthogonal frequency division multiplexing (OFDM) system to improve the achievable rate, and the implementation of a unit cell with independent control over the slopes and phase of the phase-frequency profile.

Firstly, that is we propose a mathematical model for the phase-frequency relationship of a realizable resonant RIS unit cell parametrized by the resonance frequency and resonant phase slope. Then, modelling each RIS element with b control bits, we propose an algorithm for selecting the parameter pairs to obtain a set of 2^b phase-frequency profiles. The proposed method yields an RIS design that outperforms existing designs over a wide range of user locations in a single-input, single-output (SISO) OFDM system. We then propose a low-complexity optimization algorithm to maximize the data rate that is comprised of the joint optimization of (a) power allocations across the sub-carriers and (b) the phase-frequency profiles for each RIS unit cell from the available set. We then study a multi-user multiple-input single-output (MISO) OFDM scenario and show an improvement of (90%) in the coverage and achievable rates under the proposed framework as compared to single-slope phase-frequency profiles.

Secondly, we design an electrically tunable linearly polarized array at 2.5 GHz that yields low reflection losses and simultaneously controllable reflection phase and phase-frequency slope. The proposed design consists of three layers of unit cells with dog-bone-shaped elements in the top layer and patch elements in the bottom layer that are placed over a ground plane. Each patch and dog-bone element is loaded with a varactor, whose reverse bias voltage is controlled to provide a phase-frequency profile with a continuous slope. Simulation results show slope values between 10 degrees/MHz and 1 degrees/MHz, with a range of 340 degrees phase shift for each slope at 2.5 GHz. The experimental measurements obtained using a prototype comprised of 16 unit cells showed continuous slope values between 12 degrees/MHz and 1.25 degrees/MHz, with a phase shift range of 300 degrees at 2.4 GHz.