## 3 Experimental Setup of the CDM Transmission System  
In this section, we introduce the experimental setup of the CDM field trial using 800 Gb/s OTN transceivers over deployed MCF cable, which is shown in Figure 2. At the transmitter station, four sets of 80-wavelength C+L band OTN transmitters were utilized to generate four channels of WDM signals for CDM transmission in the 4-core fiber. For each set of OTN transmitters, a pair of C-band and L-band 800 Gb/s optical transponder units (OTU) was employed to generate modulated optical signals for transmission testing. The 800 Gb/s OTUs have a modulation rate of up to 130 Gbaud, which is enabled by the OE-MCM packaging technique. In this packaging approach, the driver is directly mounted on the photonic integrated circuits (PICs) through flip-chip welding, and the DSP die as well as the PICs are packaged on the same substrate, which can effectively reduce the length of high-speed signal transmission and thus ensure enough modulation bandwidth. Thanks to the high modulation rate, the 800 Gb/s OTUs can adopt a more noise-tolerant PCS-16 QAM modulation format instead of the previous PCS-64 QAM modulation format to achieve the target bit rate, which makes them more suitable for long-span unrepeated transmission and long-haul transmission. The central wavelengths of the C-band and L-band OTUs are tunable within the ranges of 1524.89 nm to 1571.65 nm and 1575.78 nm to 1625.77 nm, respectively, with a channel spacing of about 1.2 nm (150 GHz). The OTN transmission system can transmit up to 80 wavelength channels simultaneously, with 40 C-band and 40 L-band OTUs for each fiber core. Due to the lack of enough OTUs, dummy light (DL) generated by the C-band and L-band amplifier spontaneous emission (ASE) noise sources was utilized to fill the remaining wavelength channels of the C+L band. In addition to simulating fully loaded 80-wavelength transmission, the filling wavelength channels can also maintain the stability of the broadband transmission system under the influence of stimulated Raman scattering (SRS), which is crucial for practical broadband WDM transmission systems where the add/drop operations of wavelength channels occur frequently. This is mainly because the SRS will cause power transfer from short-wavelength signals to long-wavelength signals, and the power transfer is related to the filling situation of the wavelength channels [5]. In practical broadband transmission systems with frequent \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Core \#1** & **Core \#2** & **Core \#3** & **Core \#4** \\ \hline Span loss & 33.6 & 33.3 & 32.9 & 33.0 \\ Total crosstalk & \(-\)41.4 & \(-\)44.6 & \(-\)42.1 & \(-\)45.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Span loss and total inter-core crosstalk of the 106 km 4-core CDM link (unit: dB). add/drop operations of wavelength channels, the SRS will cause a dynamic power change of the received signals at different wavelengths. Filling all the vacant wavelength channels using DL keeps the power transfer caused by the SRS stable, and the impact of the SRS can be balanced through power pre-compensation. The high-speed modulated signals and the DL were multiplexed through a C+L band integrated wavelength selection switch (WSS), and a pair of C-band and L-band EDFAs were followed to amplify the signals. The C-band and L-band amplified signals were then multiplexed by an optical band multiplexer (OBM) and further coupled to the 4-core fiber through a FIFO device. At the receiver station, four sets of C+L band 800 Gb/s OTN receivers were employed to demultiplex and coherently detect the four channel-received signals from the 4-core CDM transmission link. The long-span unrepeated transmission link was the above-mentioned 106 km CDM link, which consists of a segment of single-span 106 km 4-core fiber and a pair of FIFO devices. For each set of OTN receivers, a pair of C-band and L-band EDFAs were first utilized to amplify the received signals.