# Multipath Interference Impact Due to Fiber Mode Coupling in C+L+S Multiband Transmission Reach
## 4 Transmission Reach Assessment
### Transmission Reach Assessment with MPI  
In this section, we present some results regarding the MPI impact on C+L+S MB transmission scenario considering QPSK and 16-QAM signals with a 64 GBaud symbol rate. As in G.654E SMFs, the S-band signals are below the wavelength cut-off; the coherent MPI impact, due to mode coupling, only affects transmitting signals in this band [14]. Signals from the L- and C-bands are not affected by this phenomenon as their wavelengths are above the cut-off wavelength [14]. Nevertheless, for comparison purposes, we consider the expected MPI impact on the L- and C-bands, as if their wavelengths were not above the cut-off wavelength.  
Figure 7 represents the GSNR for the three studied bands as a function of the number of spans for three MPI scenarios--no MPI, \(-\)34 dB/span, and \(-\)28 dB/span--for a typical splice loss of, respectively, 0.034 dB and 0.068 dB [14]. A 1 dBm optical launch power was used so that the GSNR is maximized, as shown in the previous section. As can be seen in Figure 7, without MPI, the L-band has a greater reach than the C- and S-bands, as already pointed out, whereas when the MPI is considered, this trend continues. It can also be observed that the transmission reach in the S-band decreases when MPI is considered. For a 2000 km link (20 spans), the S-band suffers a 0.2 dB GSNR penalty for a \(-\)34 dB/span MPI, and for a more restrictive MPI per span, e.g., \(-\)28 dB/span, a 0.7 dB penalty is achieved. If the L-band suffered from the MPI impact, a larger GSNR penalty would be obtained, respectively, 0.7 dB and 2.2 dB for \(-\)34 dB/span and \(-\)28 dB/span, as observed in Figure 7.  
Figure 7: GSNR as a function of the number of spans for the three bands and three MPI scenarios—no MPI, \(-\)34 dB/span, and \(-\)28 dB/span.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Modulation Format**} & **Bit Rate** & \multicolumn{2}{c}{**ROSNR in**} & \multirow{2}{*}{**RSNR in the Signal Bandwidth (dB)**} \\  & **(Gbit/s)** & & **the Reference Bandwidth** \\  & & **of 12.5 Ghz (dB) [31]** & \\ \hline QPSK & 200 & 16 & 8.9 \\
16-QAM & 400 & 24 & 16.9 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Practical ROSNRs for various modulation formats, considering a BER equal to \(2\times 10^{-2}\).  
Figure 8 represents the number of spans as a function of the MPI/span for the three bands, considering two modulation formats, QPSK and 16-QAM. These two signal formats are considered feasible if the estimated GSNR is higher than the RSNR given in Table 3 (a fixed 2 dB system margin is considered). From Figure 8, it can be observed that the impact of the MPI for the S-band scenario is mild compared with the hypothetical impact in the L-band scenario. For the hypothetical impact in the L-band scenario, an approximately 52% reach decrease is estimated when the MPI/span increases to \(-26\) dB/span for QPSK signals, whereas for 16-QAM signals, an approximately 60% decrease is observed. The S-band scenario is less impacted by the MPI due to the reduction in the signal power induced by the ISRS effect in this band: an approximately 25% reach decrease is observed for QPSK signals.