# Hybrid Ring- and Tree-Topology RoF Transmission System with Disconnection Protection
## 3 Experimental Results and Discussion
In this case, the interrupted BS-Groups can be reconnected to the CO end by simply changing the SW in the CO end from a parallel-bridge state to a cross-bridge state. Figure 8 shows that the downlink optical signals in this case are separated into two paths through a \(1\times 2\) optical splitter after passing through output port2 of the SW. One set of downlink optical signals is routed back to the input port2 of the SW and then passed through the SW in the cross-bridge state and the SMF1 to the SBOADM1 and the SBOADM2 in a CW direction (the route path under normal conditions). The other set of downlink optical signals emitted from the \(1\times 2\) optical splitter is routed to the SMF5 and directed to the SBOADM4 and SBOADM3 in a CCW direction. With the bi-directional transmission characteristics of the new-generation SBOADM, the target optical signals can still be intercepted at the corresponding BS-Group as the downlink optical signal is injected into the SBOADM via I/O_P1 or the I/O_P2. Meanwhile, the non-target optical signals can penetrate the SBOADM directly. When the optical signals are fed into the SBOADM via I/O_P2, the detailed routing pass way of the optical signals inside the SBOADM is indicated in the inset of Figure 8.  
Figure 9a-d show the spectrum measured when the downlink optical signal penetrates the SBOADM1 and SBOADM2 in the CW direction and penetrates the SBOADM3 and SBOADM4 in the CCW direction when a breakpoint occurs. Figure 10a-d shows the spectra measured at the receiving end of BS when the downlink optical signal is captured by SBOADM to the BS in BS-Group1 to BS-Group4 when a breakpoint occurs. The spectra of the signal penetrating SBOADM1 and SBOADM2 are roughly similar to the data measured under the condition of no breakpoints (Figure 2). However, each light carrier will have extra attenuation because of the additional passing of the \(1\times 2\) optical splitter and SW during the transmission process. In the spectrum penetrating SBOADM3 and SBOADM4, the downlink signal is injected into the SBOADMs through the I/O_P2 of the SBOADMs. The results of the initial experiment on SBOADM show that the insertion loss of penetrating light signals was approximately 3.46 dB, which was 1.8 dB more than when injected through I/O_P1. Therefore, the maximum power of the non-target optical signal in Figure 9d is  
Figure 7: BER performances for the BSs in BS-Group1 to BS-Group4 in (**a**) B2B and downlink transmissions scenarios as well as in (**b**) B2B and uplink transmissions scenarios. lower than the value in Figure 9a. In addition, a comparison of the non-target optical signal power that penetrates SBOADM4 and SBOADM3 shows that the difference between these two values was approximately 3.7 dB (1.92 \(-\) (\(-\)1.78) = 3.70 dB). Figure 10a-d also shows that the power levels of the target optical signal were 28 dB to 32 dB higher than that of the non-target optical signal when each group of downlink optical signals is intercepted by each BS-Group. Thus, the transmission of the RoF signal can be received and restored successfully to the electrical domain. Figure 8: Diagram of the routing of downlink optical signal in the hybrid ring- and tree-topology RoF transmission system when breakpoints occur. Figure 3:0 The spectrum measured when the downlink optical signal penetrates (**a**) SBOADM1 and (**b**) SBOADM2 in CW direction and penetrates (**c**) SBOADM3 and (**d**) SBOADM4 in the CCW direction when a breakpoint occurs. In the transmission portion of the uplink signals, the uplink signals of each BS are routed to the Add/Drop_1 and Add/Drop_2 of the connected SBOADM at the same time and passed in the CW and CCW directions to the main ring-fiber link. The CW transmission of optical uplink signals via SBOADM1 and SBOADM2 and the CCW transmission of optical uplink signals via SBOADM3 and SBOADM4 are blocked by breakpoints from returning to the CO end.