Fig. 5: 5G (5G, fifth-generation) MMW (MMW, millimetre-wave)/sub-6 GHz signals through two-way fibre–FSO (FSO, free-space optics)-wireless communications.

a Architecture of 5G MMW/sub-6 GHz signals through two-way fibre–FSO–wireless communications employing polarisation-orthogonal modulation technique. b A photo of experimental setup. DFB LD distributed feedback laser diode, PR polarisation rotator, 16-QAM-OFDM 16-quadrature amplitude modulation-orthogonal frequency-division multiplexing, MZM Mach–Zehnder modulator, EDFA erbium-doped fibre amplifier, VOA variable optical attenuator, OC optical circulator, SMF single-mode fibre, OBPF optical band-pass filter, PC polarisation controller, PBS polarisation beam splitter, PA power amplifier, PTFE polytetrafluoroethylene, HA horn antenna, LO local oscillator, LNA low noise amplifier, DSO digital sampling oscilloscope.