## 5 Discussion and Technological Prospects  
### Discussion and Technological Prospects of the 800 Gb/s OTN Transceivers  
In our field trial, we demonstrated a real-time single-span 106 km CDM field trial utilizing commercial 800 Gb/s OTN transceivers over deployed 4-core MCF for the first time. These results are mainly due to the fact that the 130 Gbaud 800 Gb/s OTN transceivers utilizing the PCS-16 QAM modulation format are more noise-tolerant and capable of withstanding larger span loss compared to previous PCS-64 QAM 800 Gb/s OTN transceivers. To further discuss the differences of 800 Gb/s OTN systems using different modulation formats, the performance of single-span 106 km CDM transmission systems using different modulation formats at 1549.72 nm was evaluated based on the existing experimental and simulation results, which are shown in Table 3. The previous experimental results using 130 Gbaud QPSK 400 Gb/s OTN transceivers are also presented for reference [25]. The span loss was assumed to be 33.6 dB, which is the maximum span loss. The single-wavelength input power of the C-band signals was set to be +11 dBm, which is consistent with our experiments. Moreover, for better comparison, the single-wavelength input power of other 800 Gb/s signals using different modulation formats was adjusted to ensure that the power spectral density was consistent with that in our experiment. The input power of the L-band signals was still 2 dBm lower than that of the C-band signals. The OSNR thresholds were obtained from the field trial and previous laboratory tests. It should be emphasized that the OSNR margins presented in Table 3 are the results at 1549.72 nm, which should generally be greater than 2 dB to ensure that the signals in the entire C+L band can be transmitted. This is because there are inevitable differences in the performance of the signals in the C+L band. From Table 3, we can obtain that the 400 Gb/s and 800 Gb/s signals using the QPSK modulation format have the maximum OSNR margin, which indicates that they have the capability to transmit over longer distances. With the help of previous experimental results, i.e., [25], the entire C+L band signals all reserve an OSNR margin of more than 5.5 dB after 106 km transmission, so the transmission distance could be further extended by at least 15 km. Meanwhile, the 800 Gb/s signals using the 16 QAM and PCS-64 QAM modulation formats cannot achieve a single-span 106 km transmission in the experimental environment of a field trial due to the lack of sufficient OSNR margin, which demonstrates the performance advantage of the 130 Gbaud PCS-16 QAM 800 Gb/s OTN transceivers in single-span unrepeated transmission. Therefore, to achieve longer-distance unrepeated 800 Gb/s CDM transmission over MCF cable, it is necessary to conduct research on higher-speed modulators and promote the application of QPSK 800 Gb/s OTN transceivers with a 260 Gbaud modulation rate. While the cost and single-bit power consumption need to be  
Figure 5: Received normalized optical spectrum of the 800 Gb/s 4-core 80-\(\lambda\) transmission system.  
considered comprehensively, the PCS-16 QAM 800 Gb/s OTN transceivers may be a better choice in most scenarios such as metropolitan area networks.