Efficient Uplink Configuration for 6G Fronthaul in Photonic Terahertz Communications via Optical Carrier Sharing Scheme

In this paper, we present an uplink architecture for photonic terahertz (THz) fronthaul based on optical carrier sharing in a 300-GHz optical-heterodyne system. In the considered active remote antenna unit (RAU), a local laser is already required as the optical local oscillator (LO) for THz generation by photomixing. We exploit this mandatory LO in a dual role: one portion is used for downlink photomixing with a uni-travelingcarrier photodiode (UTC-PD), while the remaining portion serves as the optical carrier for uplink data modulation. This removes the need for a dedicated uplink light source and its associated control overhead at the RAU without increasing the number of active components in the uplink branch. To assess bidirectional robustness over a single-fiber link, we develop a closed-form Rayleigh-backscattering (RB) return-power model and derive optical signal-to-interference ratio (SIR) expressions for both the intensity-modulation/direct-detection (IM/DD) uplink and the UTC-PD-based downlink, revealing asymmetric uplink/downlink impairment behavior and the resulting bidirectional operating trade-off. We experimentally validate simultaneous bidirectional transmission using downlink orthogonal frequency-division multiplexing (OFDM) and uplink discrete Fourier transform (DFT)-spread OFDM over a 1-m 300-GHz wireless link and up to 20-km single-mode fiber. Within the stable operating region identified by the RB/SIR analysis, no significant coexistence penalty is observed in either direction. Reliable 20-Gb/s transmission over 20-km SMF is demonstrated in both directions. We further quantify dispersion-induced power fading and its impact on achievable baud rates. Finally, an end-to-end wireless link-budget analysis indicates that the current reach is primarily limited by the THz receiver front-end sensitivity and free-space path loss, rather than by the optical fronthaul itself, suggesting scalability as THz front-end technology improves.