Centralized bidirectional heterogeneous fiber-FSO-mmWave-converged networks for 6G dense cellular network deployments

To address the unprecedented technical challenges arising from ultra-dense cellular network deployment for applications in densely populated urban areas envisioned for 6G, this paper proposes and experimentally demonstrates a novel, to our knowledge, centralized bidirectional heterogeneous access network with advanced baseband unit (BBU) pooling and cost-effective remote radio head (RRH) designs free from both lasers and digital signal processing (DSP). The network supports flexible deployments of fiber, free space optical (FSO), and millimeter wave (mmWave) segments, thus ensuring ubiquitous network connectivity. More importantly, it also seamlessly converges various network segments (fiber, FSO, and mmWave) and enables their uplink (UL) and downlink (DL) signals to concurrently and continuously flow between the BBU and user equipment (UE) without requiring optical-electrical/electrical-optical conversions and/or DSPs at any intermediate nodes. In the proposed network, DL mmWave signals are generated and detected using a free-running laser and a passive envelope detection. For the UL case, conventional electrical local oscillators and mixers are used for mmWave up-conversion and down-conversion. The performances of the proposed networks, including UL/DL channel interferences and achievable throughputs, are experimentally evaluated over a fiber-FSO-mmWave setup with 10 km fiber, 1.8 m FSO, and 3 m mmWave links (39 GHz/0.4 Gbit/s for DL, 36.5 GHz/0.2 Gbit/s for UL). The experimental results show robust bidirectional transmissions with negligible UL/DL interferences and minimal impacts from Rayleigh and Brillouin backscattering.