# Hybrid Ring- and Tree-Topology RoF Transmission System with Disconnection Protection
## 3 Experimental Results and Discussion
Figure 2: Spectrum of downlink light signals after penetrating (**a**) SBOADM1, (**b**) SBOADM2, (**c**) SBOADM3 and (**d**) SBOADM4. In terms of uplink signal transmission, the optical signals generated from each BS are injected into the Add/Drop_1 and Add/Drop_2 of the connected SBOADM after passing through a 16 \(\times\) 2 optical coupler. As discussed in the previous section, the uplink optical signals routed to the Add/Drop_2 of the SBOADM will be transmitted back to the CO along the CW direction of the main ring fiber link and consumed completely at the SW in the CO. By contrast, the uplink optical signals injected into the Add/Drop_1 of SBOADM will be routed to the I/O_P1 of the SBOADM and be sent back to the CO along the CCW direction of the main ring fiber link. Given that each SBOADM only drops down a dedicated optical signal to the connected BS-Group during this routing process, all uplink optical signals can successfully penetrate the other SBOADMs and pass along the SMF1 to the CO end when transmitted in the main ring-fiber link. The optical spectrum of each uplink signal received at the CO end of each BS-Group is overlapped and shown in Figure 5. The power levels of the uplink optical signals generated from the BSs at the BS-Group1, BS-Group2, BS-Group3, and BS-Group4 were \(-\)7.06, \(-\)10.7, \(-\)14.38, and \(-\)18 dBm, respectively. The power differences between the two adjacent sets of uplink optical signals were 3.64, 3.68, and 3.62 dB. These values are roughly equivalent to the insertion loss (3.46 dB) when the signal penetrates from I/O_P2 of the SBOADM to I/O_P1. Similarly to the downlink transmissions, all four sets of the uplink RoF signals were also received successfully and converted back to baseband format. Figure 6a-d shows that the transmitted signals are obtained clearly in the spectrum diagrams and the measured eye diagrams are open and clear. Figure 4: Electro-spectrum and eye diagram of frequency down-converted downlink RoF signals received by (a) BS-Group1, (b) BS-Group2, (c) BS-Group3 and (d) BS-Group4. In addition to observing the spectra and eye diagrams, the raw bit error rate (BER) performances for the BSs in BS-Group1 to BS-Group4 (indicated as BS-G1, BS-G2, BS-G3, and BS-G4) in the back-to-back (B2B) and downlink transmissions scenarios and in the B2B and uplink transmissions scenarios are illustrated in Figure 7a,b. Instead of estimating the BER value based on a few measured moments of the probability density function, such as Q-factor or error-vector magnitude (EVM), the raw BER is the ultimate quality measure in optical communication links [29]. To obtain the raw BER values, the down-frequency RF signals were analyzed by an Anritsu MP1632a and their BER values are calculated by dividing the number of errors by the total number of bits sent. It is clear that each set of the \(\alpha\)-values of the received signals is the same as the \(\alpha\)-values of the received signals. Figure 5: Overlapped uplink optical spectra measured at the CO end of each BS-Group. Figure 6: Electro-spectrum and eye diagram of frequency down-converted uplink RoF signals received at the CO end of (**a**) BS-Group1, (**b**) BS-Group2, (**c**) BS-Group3 and (**d**) BS-Group4. of signals can enjoy an error rate lower than \(10^{-9}\) when the receiver powers are larger than \(-21.5\) to \(-21.2\) dBm for downlink transmissions and around \(-19.5\) to \(-19\) dBm for uplink transmissions. Compared with the B2B scenario, the power penalty of the four sets of signals was all less than 0.5 dB for the downlink and uplink transmissions. The clear eye diagram and error-free transmissions provide considerable evidence of the advanced transmission performance of the proposed system. In verifying the breakpoint protection function of the proposed transport system, we assume that the SMF3 located between the SBOADM2 and SBOADM3 is interrupted for any reason. As a result, the SBOADM3 and the SBOADM4 will lose connections with the CO end.