The demands of the technologies of the future such as ultra-high-definition video conference, mixed reality, autonomous vehicle, and Internet of everything require communications platforms equipped to handle huge quantities of data. Higher frequency communication bands are attractive but have limitations in terms of data loss particularly during wireless transmission. Fibre-FSO communication, which transmits optical signals through free-space by modulating laser light, is one option for better wireless signal delivery. Here we report a platform combining multiple transmission media of 40-km single-mode fibre, with 1.2-km FSO communication, and short range (0.5-2m) radio-frequency wireless. We demonstrate sufficiently low bit error rates (BERs) and error vector magnitudes (EVMs) at sub-terahertz frequencies, satisfying the requirement of 5G new radio (NR) communications applications.

For 5G NR communication, high signal attenuation greatly limits its transmission distance. FSO communication uses laser light to transmit optical signals wirelessly through the air. It compensates for the high signal attenuation of 5G NR MMW/sub-THz communication, thereby providing high-speed connections over long wireless distances. Combining 5G NR and FSO communications at MMW/sub-THz frequencies will provide high data rates over long  distances. Accordingly, the transmission of MMW and sub-THz signals over combined fibre/FSO communication systems for long-haul wireline/wireless transmission (see Fig. 1) shows the potential of providing high data rates. A combined fibre/FSO communication system at MMW/sub-THz frequencies can provide 5G applications not only in dense/metropolitan areas but also in rural/suburban areas. It shows the prospect of 5G applications aiming at dense/metropolitan and rural/suburban areas with high data rate.

In this demonstration, we demonstrate a combined fibre/FSO communication system for long-haul wireline/wireless transmission at MMW/sub-THz frequencies with single optical carrier modulation to effectually decrease dispersion-induced RF fading and optical beating-induced interferences due to multi-carrier (see Fig. 2). Using 50, 100, and 150 GHz frequencies as scenarios, a combined fibre/FSO communication system at MMW/sub-THz frequencies is realized. Each MMW/sub-THz frequency transmits an 18.78-Gbps 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) signal. In system I, Mach-Zehnder modulator optoelectronic oscillator is placed after 40 km SMF transport. In system II, Mach-Zehnder modulator optoelectronic oscillator is placed in front of 40 km SMF Through 40 km single-mode fibre (SMF), 1.2 km optical wireless, and 2 m/1 m/0.5 m RF link, 5G MMW/sub-THz 16-QAM-OFDM signals are transported with impressive performance in terms of sufficiently low BERs and EVMs. This combined fibre/FSO communication system for long-haul wireline/wireless transmission uses multiple transmission media to transport MMW and sub-THz signals with good performance. The successful demonstration of fibre/FSO communication system is an important step towards the realization of 5G MMW/sub-THz communications.

Figure 3a exhibits the measured BERs as a function of optical power transmitted to photo diode (PD)/uni-travelling carrier (UTC)-PD through 40 km SMF, 1.2 km optical wireless, and 2 m/1 m/0.5 m RF link, for systems I and II, respectively. For 18.78 Gbps 16-QAM-OFDM signal at 50, 100, and 150 GHz carrier frequencies, in system I, BER reaches 3.4´10⁻³ (< 3.8´10⁻³ FEC limit) when the optical power transmitted to PD/UTC-PD is -29.8 (50 GHz), -29.2 (100 GHz), and -28.4 (150 GHz) dBm. In contrast in system II, we obtain a BER of 3.4´10⁻³ when the optical power transmitted to PD/UTC-PD is -25.1 (50 GHz), -24.1 (100 GHz), and -22.2 (150 GHz) dBm. Large power penalties of 4.7, 5.1, and 6.2 dB exist between systems I and II. These large power penalties can be attributed to dispersion-induced RF fading because of 40 km SMF transport and optical beating-induced interferences because of multiple carriers. Figs. 3b, 3c, and 3d show the corresponding constellations of 18.78 Gbps 16-QAM-OFDM signal at 50, 100, and 150 GHz carrier frequencies for system I, when BER reaches 3.4´10⁻³. Obviously, clear constellations (Fig. 3b), slightly clear constellations (Fig. 3c), and somewhat clear constellations (Fig. 3d) are achieved. Additionally, to verify the relation between the modulation format and the BERs, we investigate BERs and their correlated constellations of 18.78 Gbps 16-QAM-OFDM signal at 50, 100, and 150 GHz carrier frequencies for system II. When the optical power transmitted to PD/UTC-PD is -27.2 dBm and the modulation format is multi-carrier modulation, degraded BERs of 1.1´10⁻² (Fig. 3e), 4.2´10⁻² (Fig. 3f), and 7.3´10⁻² (Fig. 3g) with blurry constellations are obtained because dispersion-induced RF fading and optical beating-induced interferences are added to the SMF delivered optical carriers.

Figure 4a presents the measured EVMs as a function of optical power transmitted to PD/UTC-PD through 40 km SMF, 1.2 km optical wireless, and 2 m/1 m/0.5 m RF link, for systems I and II, respectively. In system I, the measured EVMs of 18.78 Gbps 16-QAM-OFDM signal at 50, 100, and 150 GHz carrier frequencies are less than the 12.5% requirement at all optical powers transmitted to PD/UTC-PD. In system II, nevertheless, the EVMs are less than the 12.5% requirement when the optical powers transmitted to PD/UTC-PD are higher than -26.5 (50 GHz), -25.4 (100 GHz), and -23.7 (150 GHz) dBm, respectively. The EVM degradation is mainly due to chromatic dispersion produced by the 40 km SMF transport and the interference produced by coherent multi-carrier beating. In addition, to closely correlate with SNR and EVM, the EVM as a function of SNR of the 5G MMW/sub-THz 16-QAM-OFDM signal for system I is shown in Fig. 4b. Obviously, EVM is inversely proportional to SNR. At an EVM threshold of 12.5%, 18.78-Gbps 16-QAM-OFDM signal at 50 GHz carrier frequency operates at a lower SNR than 18.78 Gbps 16-QAM-OFDM signal at 100 and 150 GHz carrier frequencies. At a SNR value of 18.3 dB, 18.78 Gbps 16-QAM-OFDM signal at 50 GHz carrier frequency operates at a lower EVM than 18.78 Gbps 16-QAM-OFDM signal at 100 and 150 GHz carrier frequencies.