Experimental Validation of a Bidirectional Multi-Wavelength MIMO A-RoF/FSO Hybrid Link

Analog radio-over-fiber (A-RoF) and free-space optics (FSO) are key enablers for future extreme-bandwidth wireless systems. A-RoF offers a low-cost and scalable means of transporting radiofrequency (RF) signals over long distances without repeated analog-to-digital converter/digital-to-analog converter and RF up/down-conversion, while FSO delivers high-capacity, cable-free fronthaul with highly directive optical links for last-mile connectivity. Despite extensive theoretical and simulation work, experimental demonstrations that jointly integrate bidirectional, multi-wavelength A-RoF and FSO with multiple-input multiple-output (MIMO) and direct modulation and direct detection remain scarce. This paper presents an experimentally validated bidirectional hybrid communication architecture, intended for heterogeneous network deployments. The multi-wavelength midhaul link is implemented via coarse wavelength division multiplexing (WDM) and the fronthaul link is implemented via a FSO link. The proposed architecture employs direct modulation and direct detection, coarse WDM over a 12.5 km single-mode fiber for the midhaul, an 11 m FSO fronthaul, a fifth-generation of mobile network transceiver implemented on Universal Software Radio Peripheral (USRP), and adaptive MIMO processing. The metrics used are root mean squared error vector magnitude (EVMRMS), throughput, and spectral analysis. Results demonstrate robust bidirectional operation at very low RF input powers: at -29 dBm, the system maintained EVMRMS ≈ 10.5% with ≈ 31 Mbps throughput in downlink and EVMRMS ≈ 7% with ≈ 10 Mbps throughput in uplink; at -11 dBm, the transceiver reached its maximum capacity (in the downlink, 130 Mbps and in the uplink, 30 Mbps), without observable spectral distortion. These findings confirm that the joint use of A-RoF, FSO and MIMO to create a hybrid network provides a practical, low-cost, and scalable midhaul/fronthaul solution and serves as a viable experimental step toward sixth generation of mobile network (6G)/Terahertz (THz) architectures.