Full-duplex transmission without an uplink light source for a millimeter-wave radio over a free-space optical system

The radio over free-space optical (RoFSO) system is a promising alternative technique for mobile fronthaul networks such as aerial base stations where fiber link is unavailable to deploy. However, the full-duplex transmission imposes challenges on the design of the RoFSO system due to the additional structural complexity when ease-of-installation is a mandatory request. The interplay between the uplink and downlink for a simplified structure has been less investigated in related works. In this paper, we propose a full-duplex RoFSO transmission system structure in which the uplink light source is physically saved. The optical center carrier of the double-sideband (DSB) modulated signal in the downlink is extracted as the light source for the uplink, and thus the system complexity is reduced. In order to mitigate the permanence degradation of uplink transmission due to the attenuated light source, channel estimation and power pre-compensation are implemented without additional training symbols by utilizing the characteristics of the channel correlation between uplink and downlink and the information independence of optical center carrier in the DSB. The proposed channel estimation algorithm is verified theoretically and experimentally. The results show that the difference between uplink and downlink channel attenuation is -0.7dB. In case of the excessive power compensation and the resulting effects, a power control mechanism is introduced to the power pre-compensation. By using the power pre-compensation mechanism, the bit error rate (BER) can reach 1.58×10^-3 and 3.8×10^-3, respectively, under the channel conditions of medium turbulence and weak turbulence. Owing to the power control mechanism, the power saving rates of downlink and uplink are up to 78% and 67%.