# Supporting Heterogenous Traffic on Top of Point-to-Multipoint Light-Trees +
## 4 Illustrative Results
### Performance Evaluation of DSCM and OCS for Optical P2 MP  
For evaluation purposes, we implemented a MATLAB-based simulator. For OCS, a single carrier 64-QAM@64 GBd DP OCS signal was generated, whereas for DSCM, we implemented 64-QAM@64 GBd DP with 16 SCs each operating at 4 GBd. The signal is then sampled and sent through a root-raised cosine pulse shaper with a roll-off factor of 0.06. For DSCM, after applying frequency shift, the SCs were multiplexed. The signal was launched onto an 80 km long fiber link with \(N\) spans; an optical amplifier with a noise figure of 4.5 dB compensates for the fiber losses after each span. To simulate ASE noise, additive white Gaussian noise is injected after each span. The fiber channel was simulated using standard single-mode fiber with the following parameters: fiber loss = 0.21 dB/km, dispersion \(D\) = 16.8 ps/(km-nm), and nonlinear coefficient = 1.14 W\({}^{-1}\) km\({}^{-1}\). The payload was generated using \(2^{13}\) pseudorandom symbols. The signal was then transmitted using the symmetric split-step Fourier method [21], which solved the nonlinear Schrodinger equation for signal propagation in the fiber channel. The signal was received coherently, and an ideal chromatic dispersion filter was applied. For DSCM, the signal was first demultiplexed and passed through a matched filter before performing the down sampling.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**OCS \#** & **\#CP** & **Throughput [Gb/s]** & **SE [\%]** & **CE [\%]** \\ \hline
1 & 32 & 250 & 83.3 & 41.7 \\ \hline
2 & 8 & 37.5 & 50.0 & 6.3 \\ \hline
4 & 4 & 12.5 & 33.3 & 2.1 \\ \hline
5 & 16 & 100 & 66.7 & 16.7 \\ \hline Total & & 412.5 & – & 68.80 On this simulation scenario, we first analyze the performance of both OCS and DSCM with 16 SCs. First, Figure 6 depicts the spectra of the single carrier and the DSCM signals, where we observe that both signals use the same bandwidth. To study the performance, we simulated a light-tree with two leaves with 8 and 10 spans, respectively. We vary the launch power in the range \([-4,+2]\) dBm. Figure 7 presents the average bit error rate (BER) of both polarizations as a function of the power for a light-tree with two leaves with 8 and 10 spans, respectively. We observe that DSCM provides a better performance compared to OCS at higher launch powers where non-linearities are significantly present. This is because each SC operates on a reduced baud rate of 4 GBd. An important point to note here is that although for eight spans the performance of DSCM is better than OCS, for optimal power, the BER remains lower than the FEC threshold of \(1\times 10^{-2}\) in both the cases and the post-FEC performance of both systems remains the same. Therefore, if the main goal is to reduce non-linearities, i.e., for long haul systems, DSCM is a superior solution.  
Figure 6: Spectrum of the single carrier signal used for OCS (**a**) and 16 SC DSCM (**b**).  
Complementing Figure 7, Figure 8 details the optical performance of OCS for each individual polarization (Figure 8a) and the performance of each SC of the DSCM system (Figure 8b), considering for eight spans. The BER of both polarizations are almost the same in OCS. However, the performance of each SC at \(-1\) dbm reveals that the SCs in the center are affected more than the SCs at the extremes because of the SC interaction mainly due to the non-linearities. However, the BER remains under the FEC threshold, thus guaranteeing the same post-FEC performance of each SC.