Towards 6G: Unified RoF Fronthaul Using a DML-Based Centralized Free-Running Laser for Simultaneous VLC and mmWave Transmission

This work experimentally demonstrates, for the first time, a unified Radio-over-Fiber (RoF) fronthaul for simultaneous visible light communication (VLC) and millimeter-wave (mmWave) transmission, enabled by a directly modulated laser (DML) in a centralized free-running configuration. Photonic heterodyning of dual optical carriers is used to generate high-frequency signals with cost-effective and scalable operation, suitable for 6G fronthaul networks. Two hybrid scenarios are validated: (i) a Light-Emitting Diode (LED)-based VLC/mmWave link for low-bandwidth Internet of Things (IoT) -oriented services and (ii) a LASER-based VLC/mmWave link for high-speed indoor communications. Optimized power levels achieve error vector magnitude (EVM) values of 2.1% for VLC and 3.7% for mmWave, supporting data rates up to 1.65 Gbps and 692 Mbps, respectively. The inherent tunability of the free-running approach allows straightforward extension to sub-THz frequencies, positioning this architecture as a cost-efficient and future-proof solution for optical-wireless convergence toward 6G networks.