## 4 Experimental Results of the Field Trial  
In this section, we further introduce the experimental results of our field trial utilizing 800 Gb/s OTN transceivers with the system setup shown in Figure 2. The BER values under different optical signal-to-noise ratios (OSNRs) at the receiver were first measured to evaluate the OSNR threshold of the 800 Gb/s CDM system. The OSNR was measured according to Equation (1) defined by ITU-T Recommendation G.Sup39:  
\[OSNR=10 log\frac{P_{i}}{N_{i}}+10 log\frac{B_{m}}{B_{r}}, \tag{1}\]  
where \(P_{\mathrm{i}}\) is the optical signal power of the \(i\)-th channel and \(N_{\mathrm{i}}\) is the ASE noise power of the \(i\)-th channel measured within the noise equivalent bandwidth \(B_{\mathrm{m}}\). Both \(P_{\mathrm{i}}\) and \(N_{\mathrm{i}}\) are expressed in the linear unit of watt. \(B_{\mathrm{r}}\) is the reference optical bandwidth, which is 0.1 nm, corresponding to 12.5 GHz in the C-band. The OSNR at the receiver was adjusted by adding white noise through an ASE noise source. The measured results at 1549.72 nm and 1589.57 nm are shown in Figures 3a and 3b, respectively. The results in the back-to-back (B2B) configuration are also illustrated for reference. We found that the C-band and L-band signals showed similar BER-OSNR characteristics. Under the soft-decision forward error correction (SD-FEC) limit, which was 3.3 \(\times\) 10\({}^{-2}\), the OSNR threshold was lower than 23.2 dB for each fiber core channel under 106 km transmission. This relatively low OSNR threshold indicates that the CDM system utilizing 800 Gb/s OTN transceivers has good noise tolerance, which is mainly owing to the PCS-16 QAM modulation format. Compared to the results in the B2B configuration, the degradation of the OSNR threshold induced by fiber transmission was no more than 0.3 dB, which demonstrates that the coherent receiver can compensate well for the impacts brought about by fiber transmission. However, it needs to be acknowledged that compared with the results in our previous work, which was conducted over the same MCF cable with dual-polarization quadrature phase shift keying (DP-QPSK) 400 Gb/s OTN transceivers, the OSNR threshold has increased by approximately 7.2 dB [25]. This is mainly because the noise tolerance of the PCS-16 QAM modulation format is worse than that of the DP-QPSK modulation format. We further investigated the BER values of each fiber core at the 80 wavelength channels by adjusting the central wavelengths of the OTUs at both the transmitter and receiver stations. The single-wavelength input power of the C-band and L-band signals was +11 dBm and +9 dBm, respectively. The input power of the C-band signal was 2 dB higher  
Figure 3: Measured BER values as a function of OSNR at (**a**) 1549.72 nm and (**b**) 1589.57 nm of the CDM transmission system using 800 Gb/s OTN transceivers. than that of the L-band signal, which is mainly because in our field trial, under an input power of around +10 dBm, approximately 2 dBm of power transferred from the C-band signals to the L-band signals induced by the SRS. Therefore, under the same input power, the received power of the C-band signals would be approximately 4 dB lower than that of the L-band signals. Further, considering that the noise figure of the L-band amplifier is approximately 2 dB worse than that of the C-band amplifier, setting the input power of the C-band signals to be 2 dB higher than that of the L-band signals will approximately balance the transmission performance of the signals in the two bands. The experimental results are shown in Figure 4. It was found that the BERs all remained lower than the SD-FEC limit for each fiber core over the C+L band after unrepeated 106 km CDM transmission. These results indicate that the PCS-16 QAM 800 Gb/s OTN transmission system is capable of tolerating a span loss of over 33.6 dB and achieving long-span unrepeated optical transmission.