# Hybrid Ring- and Tree-Topology RoF Transmission System with Disconnection Protection
## 1 Introduction  
Various applications, such as virtual reality and 4K video on-demand, increase the bandwidth demand of wireless communications, and radio over fiber (RoF) transmission systems are, therefore, being developed to meet the rapidly increasing demand [1, 2, 3, 4, 5, 6, 7, 8]. With the upgrading of wireless broadband networks, such as from 4G networks to 5G networks, the signal coverage range of each base station (BS) will be reduced greatly and the density of the BS distribution will be increased significantly in metropolitan areas [9, 10, 11, 12, 13]. However, in a tree-topology network structure, the number of BSs that can be connected to each feeder fiber is limited to between 64 and 128 (\(2^{\mathrm{x}}\) to) [14, 15]. Embedding wavelength division multiplexing (WDM) technique into time-division multiplexing (TDM) based tree-topology network will be a suitable solution for overcoming this limitation [16]. If an optical multiplexing/de-multiplexing component, such as an arrayed waveguide grating (AWG), is added into the remote node (RN) of the tree-topology RoF transport systems to route different wavelengths of light carriers to different feeder fibers and significantly increase the number of BSs that can be served per feeder fiber [17, 18]. The main drawbacks of such hybrid architecture are the limited scalability and flexibility of extending or reorganizing AWG's optical routing characteristics. In parallel with utilizing AWGs to form the hybrid TDM and WDM RoF network mentioned above, some scholars have proposed forming hybrid TDM and WDM RoF networks with better flexibility by utilizing an integrated ring and tree structure network instead of using only a tree structure [18; 19; 20]. Such hybrid network architecture provides a potential self-disconnection repair capability that can overcome unexpected fiber-link failure by using an optical add/drop multiplexer (OADM) to bridge optical wavelengths transmitted between the ring and tree topology. Each OADM node can utilize an alternate fiber loop for signal transmissions to avoid breakpoints to ensure that users will not experience any usage problems during fiber line repair, and the original bandwidth connection can also be maintained [21; 22]. Nevertheless, a traditional OADM can only direct and route the downstream and upstream optical signals in the same direction of the ring-fiber link. Additional optical switches (SWs) are required to be embedded into each OADM node to form a bidirectional OADM (BOADM) and a spare ring-fiber link is necessary to achieve self-disconnection repair capabilities [23; 24]. If the abovementioned two-wire BOADM can be changed to a single-line BOADM (SBOADM), then the entire ring network architecture only requires a single fiber ring structure to achieve the same self-disconnection protection effect. Therefore, good performance can be achieved in terms of system complexity, fiber laying costs, and logistics maintenance. Recently, some experts and scholars have proposed the use of SWs, array waveguides, optical circulators, and optical amplifiers, etc., to construct SBOADMs and utilize such SBOADMs to accomplish bidirectional WDM ring networks [25; 26]. Such SBOADMs can utilize a single ring fiber link to achieve bidirectional transmissions properly. However, the SWs and optical amplifiers in such SBOADM are not passive components and require additional power supply to function properly. This scenario is not ideal in terms of system cost, architecture scalability, and system management and maintenance. Thus, to optimize and simplify the structure of the published SBOADMs, we have developed a new generation SBOADM to accomplish bi-directional add/drop multiplexing functionality by utilizing four passive optical circulators (OCs) and only two sets of fiber Bragg gratings (FBGs) [27].