Free Space Optical Communication (FSO) is a communication method that uses free space, such as the atmosphere and vacuum, as an optical signal transmission medium. Compared with radio frequency, the frequency band used by FSO is the same as the optical fiber communication window, which is the infrared band, so it has the advantages of large bandwidth, high transmission rate, good directivity and unlimited spectrum. Before the 5G era, the requirements for transmission rate and delay were relatively low, and wireless communication could well solve the scenario requirements. Some specific scenarios require high-speed data transmission capabilities, therefore, FSO once again entered the field of research.
For a long time, the Broadband Internet Research Department of China Unicom Research Institute has conducted continuous research and tracking on air-space-ground integrated networking and optical transmission, and is committed to solving future-oriented converged communication technology problems. The “Space-Ground Network Collaboration and Service Convergence Technology Research, Product Development and Test” project includes two teams, the core network and the transmission team. Through the research, the feasibility verification scheme of free space optical communication is clarified. The goal of the first phase of the project is to solve the problem that operators face emergencies, such as natural disasters, or temporarily broadcast large-scale events, or in scenarios where optical cables are not easy to lay, such as islands and mountainous areas, to quickly deploy communication equipment and establish communication between core demand areas and the outside world means to realize functions such as disaster relief, broadcasting and daily communication.
At the beginning of August 2022, China Unicom Research Institute, Beijing Mobile Communications Bureau and Wuhan Liubo Optoelectronics Technology Co., Ltd. carried out a 5G vehicle emergency communication wireless laser data backhaul test. The optical communication method replaces the traditional optical fiber to realize the feasibility of data interaction between the base station and the core network.
This experiment uses point-to-point transmission at a fixed location. One end of the FSO device is connected to the BBU return port, and the opposite end of the FSO device is connected to the ODF rack connected to the core network element. The communication light adopts 1550nm wavelength, within a distance of 100 meters, without turning on the amplifier, the power of the receiving end can be greater than -10dBm, which meets the requirements of the received optical power. This experiment verified both 4G and 5G networks, and the experimental rate can reach up to 10GBps. Due to the limitation of the cable length, the emergency communication vehicle can only travel about 100m, but the equipment can support transmission within 5km. The test network speed of the mobile phone connected to the base station of the emergency vehicle: the 4G downlink peak can reach 61.4Mbps, and the uplink can reach 31.2Mbps; the 5G downlink peak can reach 1008.5Mbps, and the uplink can reach 115.98Mbps. The total round-trip delay is 3ms, which is the same as the return index of the optical cable. Finally, the BER tester was used to continuously test for 117 minutes, and there was no BER.