High data rate, ultra-low latency, and massive connection requirements in 5G services have substantially promoted the development of telecommunications. The development of 5G signals through fibre-FSO-wireless communications (see Fig. 1) has greatly accelerated this global trend. In this demonstration, we report the deployment of two-way fibre-FSO-5G wireless communications using polarisation-orthogonal modulation. By implementing polarisation-orthogonal modulation, the polarisation of the central carrier (x-polarisation) is orthogonal with the optical sidebands modulated with 5G MMW and sub-6 GHz signals (y-polarisation). It shows a 5G communication system with a low-complexity configuration to split the central carrier for upstream carrier and optical sidebands for downstream transmission. The successful establishment of the two-way FSO-based interface between fibre and 5G communication marks a crucial step in the implementation and development of 5G MMW/sub-6 GHz communications. It has a major impact on the integration of fibre optics, FSO, and MMW/sub-6 GHz communications and the use of employing polarisation-orthogonal modulation.

The architecture of 5G MMW/sub-6 GHz signals through two-way fibre-FSO-wireless communications employing polarisation-orthogonal modulation is demonstrated in Fig. 2. The light sent out from a distributed feedback laser diode is supplied to a Mach-Zehnder modulator (MZM) through a polarisation rotator. The MZM is worked at the minimum transmission point and driven by integrated 1-Gbps/2.2-GHz and 10-Gbps/19-GHz 16-QAM-OFDM signals through the modulator driver. Polarisation rotator rotates the polarisation direction of a polarised light by θ angle. Due to the electro-optical characteristics of LiNbO3 crystal, the half-wave voltage V_π in the x-direction is approximately 3.58 times larger than that in the y-direction. Owing to higher V_π in the x-direction, the y-polarised light is modulated as it passes through the LiNbO3 crystal. The x-polarised light, on the other hand, remains unmodulated. Since the MZM is biased at the minimum transmission point, y-polarised light is modulated in an optical carrier suppression form. The y-polarised downstream sidebands are optically converted from a 1-Gbps/2.2-GHz signal to 1-Gbps/4.4-GHz 5G sub-6 GHz signal, and from a 10-Gbps/19-GHz signal to 10-Gbps/38-GHz 5G MMW signal. Whereas the x-polarised light is transmitted and reused as an upstream carrier. For upstream, the x-polarised central optical carrier split by the polarisation beam splitter is reused and modulated by an MZM with integrated 1-Gbps/1.85-GHz and 10-Gbps/13-GHz 16-QAM-OFDM signals. The MZM is worked at the minimum transmission point as well, which leads to the x-polarised upstream light being modulated in an optical carrier suppression form. The x-polarised upstream sidebands are optically converted from a 1-Gbps/1.85-GHz signal to 1-Gbps/3.7-GHz 5G sub-6 GHz signal, and from a 10-Gbps/13-GHz signal to 10-Gbps/26-GHz 5G MMW signal. Over 25-km SMF, 1-km FSO, and 20-m/10-m 5G wireless transmission, sufficiently low BERs and EVMs are obtained for downstream/upstream transmissions.

Figure 3(a) shows the downstream/upstream BERs at different received MMW/sub-6 GHz powers over 25-km SMF, 1-km FSO, and 20-m (MMW)/10-m (sub-6 GHz) RF wireless cascaded-medium. For 10-Gbps/38-GHz and 1-Gbps/4.4-GHz 16-QAM-OFDM signals transport (y-polarisation; downstream), we achieve a 3.8x10⁻³ (FEC limit) BER at -26.7 and -28.5 dBm received MMW/sub-6 GHz powers, and we attain a 4.7x10⁻⁵ (< 3.8´10⁻³ FEC limit) BER at -24.6 and -26.8 dBm received MMW/sub-6 GHz powers. For 10-Gbps/26-GHz and 1-Gbps/3.7-GHz 16-QAM-OFDM signals transport (x-polarisation; upstream), we achieve a BER of 3.8x10⁻³ at -27.4 and -28.8 dBm received MMW/sub-6 GHz powers, and we attain a BER of 4.7x10⁻⁵ at -25.4 and -27.4 dBm received MMW/sub-6 GHz powers. In addition, the measured EVMs at different received MMW/sub-6 GHz powers are exhibited in Fig. 3(b). Over 25-km SMF, 1-km FSO, and 20-m (MMW)/10-m (sub-6 GHz) 5G wireless transports, the EVMs of four 16-QAM-OFDM signals are less than the 12.5% 3GPP limit as the received MMW/sub-6 GHz powers are higher than -28.7 (y-polarised 10-Gbps/38-GHz; downstream), -29.4 (x-polarised 10-Gbps/26-GHz; upstream), -30.5 (y-polarised 1-Gbps/4.4-GHz; downstream), and -31 (x-polarised 1-Gbps/3.7-GHz; upstream) dBm, respectively.

A two-way fibre-FSO-5G wireless communication system employing polarisation-orthogonal modulation is offered and realized. For downstream transmission, y-polarised intensity-modulated 10-Gbps/38-GHz and 1-Gbps/4.4-GHz 16-QAM-OFDM signals through fibre-FSO-5G wireless communication is practically built. For upstream transmission, x-polarised intensity-modulated 10-Gbps/26-GHz and 1-Gbps/3.7-GHz 16-QAM-OFDM signals transport through FSO-fibre-5G wireless communication is practically constructed. 3.7 and 4.4 GHz carriers are adopted for 5G signal transmission to meet 5G sub-6 GHz frequency band (410 MHz-7.125 GHz) demands, and 26 and 38 GHz carriers are adopted for 5G signal transmission to satisfy 5G MMW frequency band (24.25 GHz-71 GHz) requirements. With an in-depth observation of two-way fibre-FSO-5G wireless communication system, good performance of low BER and EVM are achieved through a distance of 25 km SMF, 1 km FSO, and 20 m/10 m 5G wireless. The successful establishment of the two-way FSO-based interface between fibre and 5G communication marks a crucial step in the implementation and development of 5G MMW/sub-6 GHz communications. It has a major impact on the integration of fibre optics, FSO, and MMW/sub-6 GHz communications and the use of employing polarisation-orthogonal modulation.