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Abbas, Omran

University of British Columbia

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. First, we propose a mathematical model for the phase-frequency relationship of a realizable resonant RIS unit cell parameterized 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 2ᵇ 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.

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2025-01-01

10.14288/1.0438349

Master of Applied Science - MASc

Applied Science, Faculty of; Engineering, School of (Okanagan)

2023-11

Graduate