Abstract :
Coherent optical orthogonal frequency-division multiplexed (OFDM) systems have been shown to be vulnerable to fiber nonlinearity, four-wave mixing (FWM) in particular. Consequently, transmission of OFDM symbols with a low peak-to-average power ratio (PAR) is widely recommended. In this paper, we examine the correlation between PAR and the intensity of light generated through FWM for two different systems. It is demonstrated that the two quantities are strongly correlated in the region of high PAR values and, furthermore, OFDM symbols with very large PAR are likely to induce unusually strong FWM noise. On the other hand, the correlation is less pronounced for moderate PAR values, especially in the presence of a large amount of uncompensated fiber dispersion. Selective mapping (SLM), a simple coding method to avoid high-PAR symbols, is then introduced and its impact on the overall FWM generation process is analyzed. It is shown that such a reshaping of the PAR distribution eliminates undesirable bursts of FWM-induced noise, enhancing the overall system tolerance to fiber nonlinearity at a very small sacrifice in data rate. A concatenation of SLM with a previously proposed nonlinearity compensation scheme is also investigated. It is demonstrated that the benefits provided by the two methods can be easily combined, leading to further performance improvements.
