# Hybrid Ring- and Tree-Topology RoF Transmission System with Disconnection Protection
## 3 Experimental Results and Discussion  
In this experiment, the optical powers of the downlink optical signals before being injected into the 26 km SMF1, and the uplink optical signals before being injected into the 16 \(\times\) 2 optical coupler, are 15 and 5 dBm, respectively. Based on the characteristics of the FBGs within each SBOADM, the 1547.4 nm light carrier is intercepted into the BS-Group1 when the downlink signals are transmitted to SBOADM1 through the 26-km SMF1. The remaining 1549.06, 1550.67 and 1552.19 nm optical signals are captured by SBOADM2, SBOADM3, and SBOADM4 in sequence and then sent to BSs in BS-Group2, BS-Group3, and BS-Group4, respectively. The wavelength spacing between each two adjacent optical signals is 1.66, 1.61, and 1.52 nm. Since these four light sources have sufficiently wavelength-spaced (larger than 19-GHz frequency-spaced) from its neighbor, the interference caused in the main channel from the neighboring channel is insignificant [28]. Figure 2a-d shows the spectra of four downlink RoF signals measured as they penetrate from SBOADM1 to SBOADM4. In this experiment, the reflectivity of each FBG inside SBOADMs is not identical but is roughly around 97%. Thus, the power difference between the captured light carrier and the penetrated light carrier was inconsistent after penetrating through each SBOADM. The power differences are approximately 14.6, 13.9 and 16.1 dB after penetrating SBOADM1, SBOADM2, and SBOADM3, respectively. After penetrating SBOADM4, residual optical signals, as shown in Figure 2d, can be considered as noise for the entire transmission system, but this noise can be consumed naturally by the 2 \(\times\) 1 optical coupler and SW in CO after passed through the SMF5. When the downlink signal is extracted by each SBOADM and injected into each BS-Group, the spectrum measured in front of the PDs of BSs at the BS-Group1 to BS-Group4 are shown in Figure 3a-d, respectively. In the four groups, the optical signal power levels to be received were \(-\)7.09, \(-\)8.94, \(-\)10.81 and \(-\)12.67 dBm, respectively. The power difference between the two adjacent BS-Groups is roughly 1.85 dB, which is approximately equivalent to the insertion loss when the signal penetrates from I/O_P1 of SBOADM to I/O_P2. Although parts of the untargeted signals are also dropped and directed to the BSs, their power levels are much lower than the targeted signal. Comparing the peak power difference among the targeted optical signal and the untargeted residual signals, the optical signal-to-noise power ratios (OSNRs) for the four BS-Groups were 29.1, 28.6, 32.2 and 31.67 dB, respectively. When the four sets of downlink optical signals are received by the PD of each BS, they can be successfully detected and the obtained RF signals can be converted back to baseband format. The electro-spectrum and eye diagram of the frequency down-converted downlink RoF signals received by BS-Group1 to BS-Group4 are indicated in Figure 4a-d. The frequency down-converted 1.25 Gbps NRZ signal is clearly presented \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Optical Carriers** & **RF Signal (Anritsu** & **Baseband Signal** & **Modulation** & **Optical Spectrum** \\
**(Santec TSL210)** & **MG3697c)** & **(Anritsu MP1632a)** & **Analyzer** \\ \hline
1547.40 nm & & & & \\
1549.06 nm & 10 GHz & 1.25 Gbps (PRBS) & Mach Zender & \\
1550.67 nm & & & Modulators (MZMs) & Anritsu MS9740A \\
1552.19 nm & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: System parameters. in the baseband area. This means that all four sets of RoF signals are received successfully by each BS-Group. Besides, when the received NRZ sequence is fed into an oscilloscope, open and clear eye diagrams are also obtained. This means that very little intersymbol interference (ISI) is presented and the interference caused in the main channel from the neighboring channel is ignorable. Figure 3: Spectrum measured in front of the PDs of BSs at the (**a**) BS-Group1, (**b**) BS-Group2, (**c**) BS-Group3 and (**d**) BS-Group4.