Air-Filled SIW Remote Antenna Unit With True Time Delay Optical Beamforming for mmWave-Over-Fiber Systems

Abstract

A low-complexity and efficient mmWave-over-fiber remote antenna unit (RAU) is proposed for broadband transmission and wide-angle squint-free beam steering in the full [26.5–29.5] GHz n257 5G band. It leverages an optical beamforming network (OBFN), implemented on a silicon photonics integrated circuit, and a broadband optically enabled 1x4 uniform linear array (ULA). The antenna elements (AEs) of the ULA are implemented in air-filled substrate-integrated-waveguide technology. They adopt an improved aperture-coupled feeding scheme to achieve high efficiency, high isolation and minimal back radiation over a broad frequency band. Each AE is compactly integrated and co-optimized with a dedicated opto-electrical transmit chain, maximizing the RAU’s performance, including beamforming flexibility and energy efficiency, while minimizing its size. The separately packaged OBFN implements true-time-delay beamforming by means of four switchable optical delay lines that are capable of discretely tuning the delay difference between AEs with a resolution of 1.6 ps, up to a maximum delay of 49.6 ps to fully exploit the ULA’s full grating-lobe-free scan range. The measured AEs are excellently matched in the [25.1–30.75] GHz band, exhibit high isolation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt;$</tex-math></inline-formula> 15 dB) in the operating band, and feature a stable peak gain of 6.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.72 dBi with a beamwidth of at least 95 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> . Additionally, optical beamforming was successfully demonstrated by steering the RAU’s beam towards angles up to 51.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> without grating lobes. The optically enabled 1 × 4 ULA successfully establishes a 64-QAM wireless communication link at 2.2 Gbaud (13.2 Gbps) while beam steering up to 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ ^\circ$</tex-math></inline-formula> with an error vector magnitude below 7.6%.