The traditional passive optical network (PON) architecture faces the problem of difficult fiber deployment. To overcome this problem, free-space optical (FSO) communication technologies have emerged. FSO technology uses a laser as the carrier and an atmospheric channel as the transmission medium, providing a high transmission rate and enabling easy installation. To reduce the construction complexity of PON access systems based on FSO links, wavelength reuse technology is used to realize colorless uplink transmission of the optical network unit (ONU). However, the currently proposed schemes based on devices such as Fabry-Perot laser diodes (FPLDs) and reflective semiconductor optical amplifiers (RSOAs) for wavelength reuse limit the bandwidth of the system. To improve system bandwidth, some scholars have proposed the use of polarization multiplexing technology to realize wavelength reuse, thus entailing the construction of a single light source system. However, system service is singular and cannot meet the diverse needs of users of future broadband access systems. Therefore, researchers have proposed combining multiple services into a single shared infrastructure to enhance system practicality. Nevertheless, these solutions can only achieve one-way multiservice transmission and require complex devices (such as FPLD and RSOA) to achieve colorless ONUs and optical filters to achieve wavelength separation, which severely limits the frequency-adjustment range of the system. In this study, a bidirectional dual-output multiservice transmission system based on an FSO and optical fiber hybrid link is proposed. This system has broad application prospects in broadband wireless communication systems. Under the single-light-source condition, the system can realize the transmission of downlink microwave and millimeter-wave services, as well as the transmission of uplink baseband and microwave services. Simultaneously, the system can be expanded and integrated into a wavelength-division multiplexing (WDM) PON architecture. By simply adjusting the polarizer, the ONU can be made colorless, and a bidirectional dual output can be realized in multiple channels.

In our proposed scheme, the optical carrier generated by the laser diode is sent to a polarization-division multiplexing Mach-Zehnder modulator (PDM-MZM). The system loads 10 GHz 16PSK and 60 GHz 4QAM-OFDM signals onto the downstream PDM-MZM and combines self-coherent detection and digital signal processing technology at the receiving end to realize the downlink transmission of microwave and millimeter-wave services. The angle between the polarizer and the main axis of the modulator was adjusted to 90° to select the downlink 4QAM-OFDM modulated signal as the optical carrier carrying the uplink signal, rendering the ONU colorless. After the uplink baseband signal and 20 GHz 16QAM microwave signal are transmitted through the FSO and optical fiber hybrid link, polarization separation is realized using a polarization beam splitter, and photoelectric conversion is performed to realize the transmission of the uplink baseband and microwave services.

The error vector magnitude (EVM) values of the 10 GHz downlink 16PSK signal and 60 GHz 4QAM-OFDM signal are 3% and 2.98%, respectively, which are less than those of the 3GPP standard, and the constellation diagrams are clearly identifiable (Fig. 3). The EVM value of the 20 GHz 16QAM signal generated in the uplink wis 2.69%, the bit error rate (BER) of the OOK baseband signal is ≪10^-9, and the constellation and eye diagrams are clearly recognizable (Fig. 4). To evaluate the reliability of the system, we analyzed the transmission performance of the system when transmitting different FSO link distances. When the FSO transmission distance is L=2, 3, and 4 km under the corresponding received optical power, the EVM values measured by the 16PSK signal are 4.62%, 5.67%, and 7.58%, respectively, and the EVM values obtained by the 4QAM-OFDM signal are 3.60%, 5.01%, and 6.60%, respectively. The measured EVM is small, indicating that the downlink transmission performance is satisfactory (Fig. 5). The EVM values of the uplink 16QAM signal are 3.41%, 4.39%, and 6.53%, respectively, and when the received optical power is >-17 dBm, the BER of the baseband signal transmission is <10^-9, which indicates that the uplink achieves reliable transmission (Fig. 6). In addition, we examined the link performance under dynamic weather conditions. The longest communication distances that FSO links can transmit in clear, low fog, heavy fog, light rain, and heavy rain are 62.0, 4.8, 1.8, 5.6, and 2.7 km, respectively, and the system has certain advantages in various weather environments (Fig. 8).

A bidirectional multi-service transmission system that can realize the downlink transmission of microwave services, millimeter-wave services, and the uplink transmission of baseband and microwave services is presented. When downlink transmission is applied, the influence of laser phase fluctuation can be avoided, and high-sensitivity detection can be realized. The system can realize a colorless ONU by simply adjusting the polarizer, and, through the use of polarization multiplexing technology, it can further improve the spectral efficiency after it is integrated into the WDM-PON architecture, and bidirectional dual-output multi-service can be realized in multiple channels. The BER of the uplink OOK baseband signal is <10^-9, and the EVM values of the downlink 16PSK 4QAM-OFDM data signals and uplink 16QAM signal are all <3.1%. In addition, the influence of various weather environmental factors and optical fiber transmission loss on the performance of the multi-service transmission system was analyzed, and the results demonstrate the practicability and feasibility of our system.