Unit Cell Phase-Frequency Profile Optimization in RIS-Assisted Wide-Band OFDM Systems

TL;DR

This paper introduces a novel RIS design with optimized phase-frequency profiles for wide-band OFDM systems, enhancing data rates and coverage by joint optimization of power allocation and RIS configuration.

Contribution

It proposes a mathematical model for phase-frequency profiles, a method for selecting profiles based on control bits, and a low-complexity algorithm for joint optimization in wide-band OFDM systems.

Findings

Improved achievable rates over existing RIS designs.

Enhanced coverage in wide-band OFDM scenarios.

Effective joint optimization method for power and RIS profiles.

Abstract

The reflection characteristics of a reconfigurable intelligent surface (RIS) depend on the phase response of the constituent unit cells, which is necessarily frequency dependent. This paper investigates the role of an RIS constituting unit cells with different phase-frequency profiles in a wide-band orthogonal frequency division multiplexing (OFDM) system to improve the achievable rate. We first propose a mathematical model for the phase-frequency relationship parametrized by the phase-frequency profile's slope and phase-shift corresponding to a realizable resonant RIS unit cell. Then, modelling each RIS element with $b$ control bits, we propose a method 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 through the joint optimization of (a) power allocations across the sub-carriers and (b) phase-frequency profile for each RIS unit cell from the available set. The analysis is then extended to a multi-user multiple-input single-output (MISO) OFDM scenario. Numerical results show an improvement in the coverage and achievable rates under the proposed framework as compared to single-slope phase-frequency profiles.