Advanced radio-over-fiber interfaces and high-speed optical links toward 6G large-capacity fronthaul networks [Invited]

The exponential growth in mobile data traffic necessitates substantial advancements in fronthaul networks to support next-generation wireless communications. This paper comprehensively reviews two critical aspects for achieving large-capacity fronthaul networks: high-speed optical links and advanced radio-over-fiber (RoF) conversion interfaces. We first overview several fronthaul network architectures, including direct fiber connection, various wavelength-division multiplexing solutions, and point-to-multipoint architectures. Next, we explore optical transceiver modules for fronthaul networks, focusing on the evolution from none-return-to-zero to four-level pulse amplitude modulation format to achieve 50 Gb/s and beyond data rates and the standardization progress of these optical transceiver modules. We then analyze emerging challenges in high-speed optical fronthaul links, particularly chromatic dispersion, multi-path interference, and four-wave mixing, along with their respective mitigation strategies. Furthermore, we review advanced RoF conversion interfaces beyond conventional standardized digital interfaces, including digital RoF with data compression and unequal bit protection, analog RoF with enhanced robustness against noise, and hybrid digital–analog RoF with efficient signal-to-noise ratio scaling and low-complexity implementation. These technologies collectively enable the development of large-capacity fronthaul networks essential for supporting future 6G wireless communications.