Wideband Intelligent Reflecting Surfaces for 5G mmWave Coverage Extension in Urban Areas

This paper presents a novel multi-frequency design approach for passive intelligent reflecting surfaces (IRSs) with wideband beam steering capabilities, specifically optimized for the n258 (24.25 – 27.5 GHz) and n257 (26.5 – 29.5 GHz) 5G frequency bands. The proposed method synchronizes the mean phase response of unit cells across two distinct frequencies, creating a unified reflection phase profile that depends on the physical parameters of the unit cells. Additionally, at the IRS phase level, a pioneering 2D mean phase distribution of multiple frequencies is applied across the IRS aperture rather than a single frequency, enhancing beam steering precision across the entire 24 to 30 GHz range. To validate this approach, an IRS is designed to redirect signals from a 5G base station, assumed 15 meters away at a 15<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> angle from the IRS normal line, towards an area with weak 5G signal coverage at a 37<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> angle. The IRS panel comprises 40 × 40 polarization-insensitive unit cells with periodicities of 0.41<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _\rm {26GHz}$</tex-math></inline-formula> and 0.44<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _\rm {28GHz}$</tex-math></inline-formula>, distributed across the passive IRS. Both simulations and measurements confirm that this multi-frequency IRS design effectively spans the n258 and n257 5G spectrum bands, delivering up to 3 dB higher reflection amplitude and up to 6.2 dB reduction in specular reflection lobe levels compared to conventional single-frequency designs.