# Real-Time Spatial-Division Multiplexing Transmission with Commercial 400 Gb/s Transponders
## 2 Real-Time SDM Transmission over Single-Mode WC-MCFs
At each output of the fan-out device, the WDM signals are demultiplexed by the other three WSSs and then detected by the corresponding 400 G receivers. The photo of the experimental setup and fibers under test is shown in Figure 2b. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Input Cores of 60 km MCF**} & \multicolumn{8}{c}{**Output Cores**} \\ \cline{2-9}  & **Core 1** & **Core 2** & **Core 3** & **Core 4** & **Core 5** & **Core 6** & **Core 7** \\ \hline Core 1 & \(-\)14.00 & \(-\)50.20 & \(<\)\(-\)60 & \(-\)50.88 & \(-\)51.40 & \(-\)51.36 & \(-\)51.00 \\ \hline Core 2 & \(<\)\(-\)60 & \(-\)15.70 & \(<\)\(-\)60 & \(-\)50.36 & \(-\)52.00 & \(<\)\(-\)60 & \(-\)51.48 \\ \hline Core 3 & \(<\)\(-\)60 & \(<\)\(-\)60 & \(-\)15.60 & \(-\)51.10 & \(<\)\(-\)60 & \(-\)52.80 & \(-\)52.70 \\ \hline Core 4 & \(-\)51.15 & \(-\)52.10 & \(-\)52.34 & \(-\)13.90 & \(-\)51.30 & \(-\)52.13 & \(-\)52.40 \\ \hline Core 5 & \(-\)50.30 & \(-\)53.50 & \(<\)\(-\)60 & \(-\)50.68 & \(-\)14.60 & \(<\)\(-\)60 & \(<\)\(-\)60 \\ \hline Core 6 & \(-\)48.48 & \(<\)\(-\)60 & \(-\)50.40 & \(-\)49.10 & \(<\)\(-\)60 & \(-\)16.00 & \(<\)\(-\)60 \\ \hline Core 7 & \(<\)\(-\)60 & \(-\)52.33 & \(-\)51.75 & \(-\)50.80 & \(<\)\(-\)60 & \(<\)\(-\)60 & \(-\)15.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Insertion loss (diagonal data) and inter-core crosstalk (nondiagonal data) of the 60 km MCF with FIFO (Unit: dB). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Input Cores of**} & \multicolumn{8}{c}{**Output Cores**} \\ \cline{2-9}  & **Core 1** & **Core 2** & **Core 3** & **Core 4** & **Core 5** & **Core 6** & **Core 7** \\ \hline Core 1 & \(-\)16.20 & \(-\)54.40 & \(-\)52.80 & \(-\)54.20 & \(-\)52.50 & \(-\)48.40 & \(-\)53.00 \\  
The OSNR and BER performance versus input power of core 1, 2 and 4 at 193.4 THz in two cases are depicted in Figure 3a. The launched power varies from \(-20\) dBm to 7 dBm by adjusting the output power of the EDFAs online. After each adjustment of launching power, the system will run for 5 min before recording data. The OSNR of each channel increases with the input power. The three channels of the single-roll scheme and center-core channel in the 20-roll scheme have similar OSNR performance, while the outer-core channels in the 20-roll system have an OSNR penalty of about 4 dB compared to other channels because of the extra insertion loss. The insertion loss comes from the 19 fusion splice points with slightly angular misalignment. Figure 3b exhibits the pre-forward-error-correction (FEC) BER performance of the six channels in the two schemes. As can be seen, BER of all channels are below the FEC threshold (\(4.75\times 10^{-2}\)) of the 400 G receivers. At the sensitivity limit of the receiver, the 20-roll MCF transmission system ran for 6 h, and the BER was still below the threshold of \(4.75\times 10^{-2}\) during this time. It should be noted that there is no hardware or software modification on the transponders, which means that the two weakly coupled MCF systems are highly compatible with commercial single-mode transceivers.