Hybrid RoF-RoFSO System Using Directly Modulated Laser for 24 – 26 GHz 5G Networks

We present an experimental hybrid radio over fiber (RoF) and radio over free space optics (RoFSO) system employing a directly modulated laser (DML) for a radio frequency band of 24 – 26 GHz. Moreover, a 3.6 m long radio free space link is combined with a pair of wideband antennas. We outline the specific settings of the proposed high-frequency system for deployment in the centralized fronthaul networks as part of the 5 ^th generation wireless technologies. We show a potential extension in terms of the added optical loss when using a 100 MHz signal bandwidth. In order to determine the optimum system performance, the setting of directly-modulated laser bias current is investigated. Measured results illustrate that the performance of the proposed system is less susceptible for up to medium turbulence conditions by maintaining an error vector magnitude (EVM) value below the limit of 9 %, given for 64- quadrature amplitude modulation (64-QAM). The lowest recorded EVM of the whole system for 64-QAM and a 100 MHz bandwidth at a frequency 24 GHz is 4.1 % while using a band pass filter to reduce amplified spontaneous emission noise in the system.