Incoherent Optical Beamformer for ARoF Fronthaul in Mm-Wave 5G/6G Networks

Analog beamforming is a key technology to enable millimeter-wave (mm-wave) mobile communications. Nonetheless, the most widespread beamforming solutions are based on electrical implementation, which is inefficient in terms of power consumption, signal bandwidth, and losses. Optical beamforming is a promising alternative for future mm-wave mobile communications due to its inherent benefits, such as large bandwidth, low cross-talk, and low losses. Optical beamforming networks (OBFN) are an outstanding technique to simultaneously generate and radiate multiple beams in an effective manner. True time delays (TTDs) based on optical waveguides are an attractive solution since they offer constant delay in the spectrum to avoid beam squint and allow highly scalable OBFN implementation. In this work, for the first time to the best of the authors' knowledge, an incoherent 4x4 OBFN based on optical waveguides and capable of simultaneously generating four beams is presented, including a thorough explanation of its operating principles and providing a detailed characterization. The presented OBFN is fabricated on Si $_{3}$ N $_{4}$ photonic integrated circuit (PIC), and designed for transmission at 27.5 GHz, within the n257 5G band. Furthermore, the building blocks forming the OBFN PIC are deeply explained, providing their corresponding theoretical formulation and key design parameters. The fabricated PIC is exhaustively characterized through all its building blocks, showing in detail the realized measurement procedure. The experimental measures match the design parameters with minimal error, showing the feasibility of fabricating future OBFNs with the same technology and topology for larger antenna arrays. The experimental results corroborate the viability of the presented OBFN architecture as an excellent technology to consider in future mm-wave 5G/6G networks. The contribution of this work paves the road to turn optical beamforming into a mature, scalable, and efficient technology.