Baseband-interfaced THz-over-fiber-enabled fronthaul architecture and its uplink experimental demonstration

The forthcoming sixth-generation (6G) radio access network (RAN) is expected to utilize the terahertz (THz) spectrum to meet the increasing demand for ultra-high data rates and dense access. A key enabler is a THz-over-fiber (ToF)-enabled fronthaul that tightly couples a THz wireless edge with optical fiber transport for centralized processing. While substantial progress has been made on ToF downlink (DL) distribution, uplink (UL) aggregation remains comparatively underexplored. It is often constrained by stringent optoelectronic bandwidth and complex optical sideband management in remote active antenna units (AAUs). To address these limitations, we propose a baseband (BB)-interfaced ToF-enabled fronthaul architecture, in which established electrical and optical conversion blocks are organized to realize the fiber-wireless handover in the information-bearing complex BB domain. In the DL of our proposed architecture, the coherent optical receiver first recovers the complex BB signal from the received optical signal after fiber transmission, which is then up-converted by the THz front-end to a THz signal for wireless transmission. In the UL of our proposed architecture, the received THz signal is down-converted to a complex BB signal before optical in-phase/quadrature (I/Q) modulation and fiber transmission, which relaxes the bandwidth requirements of the AAU-side electro-optical interface and improves spectral and power efficiency by avoiding the employment of double-sideband (DSB) modulation. Owing to current hardware availability, we experimentally demonstrate the UL of our proposed architecture and evaluate back-to-back (BTB), 2-m wireless-only, and fully ToF scenarios over 2-m 220-GHz wireless and 0/10/25-km standard single-mode fiber (SSMF) links. With a subsequent digital signal processing (DSP) chain, the proposed UL achieves a bit-error rate (BER) well below the 3.8E-3 threshold for an 8-Gbaud 16-ary quadrature-amplitude-modulation (16QAM) signal after 2-m wireless and 25-km SSMF transmission, and supports 15-Gbaud 16QAM transmission at a gross rate of 60 Gbit/s, below the 2E-2 threshold. These results establish a deployment-oriented baseline for 220-GHz ToF UL and provide a viable pathway toward symmetric, high-speed bidirectional fronthaul in future 6G networks.