# Hybrid Ring- and Tree-Topology RoF Transmission System with Disconnection Protection
## 2 Experimental Setup  
A hybrid ring- and tree-topology RoF transmission system with self-disconnection protection is presented in Figure 1. The transmission system will first connect the CO with each SBOADM through a main ring-fiber link and then bridge each SBOADM with a BS-Group in the tree structure topology. The RF signal in the experiment is generated by modulating a 1.25 Gbps unipolar pseudo-random binary sequence (PRBS, generated by Anritsu MP1632a) with a 10 GHz sinusoidal signal. The downstream RoF signal in CO is simulated by the modulated the 1.25 Gbps/10 GHz signal with a dedicated optical carrier via Mach Zender Modulators (MZMs). The optical carriers for BS-Group1 to BS-Group4 were set at 1547.4, 1549.06, 1550.67 and 1552.19 nm, respectively (the parameters are shown,in Table 1, in detail). These modulated RoF signals are consequently multiplexed by an AWG, amplified by an erbium-doped fiber amplifier (EDFA), and fed into the input port one of a 2 \(\times\) 2 SW, which is in a parallel state, before being injected into the downstream link. Each RoF signal will be routed out of the SW output port1 with 15 dBm optical power and delivered sequentially through a series of single-mode fiber (SMFs) to SBOADM1 to SBOADM4. The distance among each SBOADM was set to 1 km (SMF2, SMF3, SMF4) and the distance between the CO and the SBOADM1 and the CO and SBOADM4 were set to 26 km (SMF1 and SMF5) to simplify the experimental setup. When the RoF signals enter each SBOADM, only a specific RoF signal is intercepted at the add/drop port and routed to the connected BS-Group, while the other RoF signals penetrate. The detailed routing pass way of the optical signals inside the SBOADM is indicated in the inset of Figure 1. Ideally, the desired RoF signal should be dropped by the SBOADM completely. However, the FBGs utilized inside the SBOADM cannot produce 100% reflection. The residual signal will also penetrate the SBOADM and moved forward to the next SBOADM, along with the remained RoF signals. The I/O_P2 of the last SBOADM (SBOADM4) will still have some downlink RoF signals' energy residue and will further inject into output port 2 of the 2 \(\times\) 2 SW in the CO after penetrating SMF5 and a 2 \(\times\) 1 optical coupler. These residual signals will be routed out of input port 2 of the SW and then routed back to the other input of the 2 \(\times\) 1 optical coupler, and repeatedly injected into the SW. As a result, these amounts of residual signal energy will continue to be consumed in the process. When each dedicated downstream RoF signal is dropped by an SBOADM, a 2 \(\times\) 16 optical splitter connected with the two add/drop ports of the SBOADM will distribute the RoF signal to each BS in the connected BS-Group. The downlink RoF signal in a BS will first be routed through an OC to a photodetector (PD) in the receiving unit and restored to a 1.25 Gbps/10 GHz RF signal. The RF signal is then increased, the frequency down-converted, the redundancy noise filtered and analyzed by an oscilloscope (HP63480a), bit error rate analyzer (Anritsu MP1632a), and electro-spectrum analyzer (thinkRF R5500). In the transmission part of the uplink signals, the uplink optical carrier wavelength used by each BS unit can be uploaded successfully to the transmission system via SBOADM by simply being in the FBG reflective wavelength range corresponding to the SBOADM. However, the uplink optical carrier wavelength of each BS is consistent with the downlink optical carrier wavelength to improve the usage efficiency of light wavelengths. Similar to the downstream transmission, the uplink 1.25 Gbps/10 GHz RF signal of each BS is modulated with the uplink optical carrier via MZM. Consequently, the uplink signal in each BS is routed through the OC to the 16 \(\times\) 2 optical coupler and injected simultaneously  
Figure 1: Hybrid ring- and tree-topology RoF transmission system with self-disconnection protection. into the Add/Drop_1 and Add/Drop_2 of the connected SBOADM.