## 2 Literature Review  
### Development of 5G Optical Networks  
The age of 5G is also the age of the Internet of Everything. The successful realization of concepts such as the IoT and artificial intelligence is dependent on the establishment of a comprehensive 5G network system. Currently, much of the traditional public network is already \"fiberized\" [9]. In other words, the development of 5G will deeply affect and change the development patterns and pace of the optical communication industry. To adapt to new technical and application requirementsgenerated by 5G, the technology and standards of the new generation of information communication technology (e.g., optical transport and optical access networks), the establishment and deployment of such networks, as well as the use of new optical devices and modules, will be essential.  
Additionally, because of characteristics inherent to the age of 5G (e.g., the mobility of users' devices, real-time data transmission, and bandwidth requirements of big data), network control methods in the age of 5G require a higher level of ultrareliable and low-latency communications compared with conventional network control methods. This places an incredibly high demand on the transmission and processing of wireless communication in 5G fronthaul networks. The adaptation, transmission, and exchange of wireless communication in optical networks must be completed in an extremely short time, which constitutes a great challenge faced by 5G fronthaul optical networks [6]. Therefore, some optical communication discussions have been initiated to solve the bandwidth and latency problems faced by future 5G networks. Solutions that have been proposed include a dense wavelength-division-multiplexed passive optical network [14], orthogonal frequency division multiplexing [15; 16], and space division multiplexing [17]. Future 5G optical network operators should work toward establishing the large bandwidth and low latency required for the efficient transmission of user data. Furthermore, a key goal of a 5G optical network is to achieve the efficient transmission of big data through the use of the multidimensional (i.e., time, wavelength, and space dimensions) multiplexing technology of optical networks.  
Today, technical investments in 5G optical networks are constantly growing. In Europe alone, 5G will generate US$363-606 billion in growth for network equipment manufacturers, and comparable investment can be expected in North America and Asia. Creating optical backhaul and fronthaul networks for 5G may represent the largest opportunity for optical equipment and cable manufacturers [9]. Given the considerable potential business opportunities 5G and IoT technologies could engender, governments worldwide are actively facilitating technology development and establishing related infrastructure [18]. The topic of incorporating optical network technologies into the development and application of 5G has sparked the attention of both academia and industry [6; 7; 8; 9]. For example, an Institute of Electrical and Electronics Engineers (IEEE) working group on Time-Sensitive Networking (TSN) included the application of optical networks in its research agenda [19]. Given the development potential of 5G optical networks, an in-depth observation of technologies in this domain is imperative. Because 5G optical networks exhibit diverse future applications, and patents are the concrete outputs of industrialized technologies, the researchers conducted a patent analysis, with a focus on 5G optical networks, to identify the key technologies and development trends of 5G optical networks, and employed the network analysis method to explore identified key technologies. This network analysis method is explained further in the following subsection.