Optimizing the subcarrier granularity of coherent optical communications systems

Abstract

In this paper, we use numerical simulations to show that the symbol rate has a significant effect on the nonlinearity-limited performance of coherent optical communication systems. We consider the case where orthogonal subcarriers are used to maximize the spectral efficiency. Symbol rates from 0.78125 Gbaud to 100 Gbaud and links of up to 3200 km, without inline dispersion compensation, were simulated. The results show that the optimal symbol rates for the 800-km link and 3200-km link were 6.25-Gbaud and 3.125-Gbaud respectively. The optimal baud rate decreases as the length of the link is increased. After 3200 km, the performance of the 100-Gbaud system was worst in the nonlinearity-limited regime producing a received Q 2.4-dB lower than the 3.125-Gband system. The variation in the nonlinearity-limited performance is explained by using Cross-Phase-Modulation (XPM) theory and by considering the RF spectra of the intensity fluctuations of the signal along the link. The findings of the paper suggest that the maximum capacity of nonlinear dispersive optical links can only be achieved by using multiple subcarriers carrying a few Gbaud each, and not by high symbol rate systems.