High-speed reach-extended IM-DD system with low-complexity DSP for 6G fronthaul [Invited]

With the rising traffic demand from the metaverse, holographic communications, and so on, in beyond-5G (B5G)/6G cloud radio access networks (C-RANs), fronthaul (FH) networks that transport radio information between antennas and distributed/central units will become increasingly demanding. Optical technologies are indispensable in supporting 6G FH deployment, which needs to be high-speed and power/cost-effective simultaneously; low latency is also crucial. This work is extended from our invited paper at OFC2023. While the conference version focused on the wired-wireless conversion/converging side [e.g., using digital signal processing (DSP)-enhanced radio-over-fiber (RoF) techniques] of FH, this work contributes to the complementary side, i.e., optical transmission techniques. Considering 6G capacity and cost requirements, beyond- ${200 \text{-Gb/s/}}\lambda$ optics based on intensity-modulation direct-detection (IM-DD) PAM4 and low-complexity DSP are targeted, which transparently support digital RoF techniques like analog-to-digital-compression (ADX)-RoF (split Option 8) or Delta-sigma RoF, and other functional split options. On the other hand, while the reach of 5G FH may be around 10 km, in 6G FH routing and/or rural connectivity cases, distance can be extended to 20 km or even 40 km. In these scenarios, fiber dispersion in the C-band and even edge wavelengths of the O-band pose a significant challenge for the reach extension of 200G IM-DD systems. We validate through extensive experiments the efficient FH reach extension and dispersion mitigation based on a low-complexity optoelectronic feedforward equalization (OE-FFE) technique. We experimentally verify 224 Gb/s single-wavelength and dense WDM transmissions over up to 41 km distances in the challenging C-band with lean digital equalizers and hard-decision forward error correction (HD-FEC), which are among the first demonstrations to our knowledge. Even with split Option 8 and extended reach, the FH systems support up to a 10 Tb/s Common Public Radio Interface (CPRI)-equivalent rate and ${\gt}{300}\;{\rm Gb/s}$ peak wireless data rate. The presented systems can be useful for high-capacity, reach-extended FH scenarios toward 6G.