On the Effect of FWM in Coherent Optical OFDM Systems

Abstract

We derive the channel capacity of CO-OFDM systems limited by FWM. Simulations confirm that Peak-to-Average-Power-Ratio reduction techniques are suitable for FWM mitigation. These techniques can utilize the decreased capacity to significantly increase the system reach.

Power pre-emphasis for suppression of FWM in coherent optical OFDM transmission

“…In CO-OFDM systems, the exact number of FWM products, both degenerate and nondegenerate, falling on a given subcarrier can be readily calculated [20]. The number of FWM products falling on the i'th subcarrier is:…”

“…Similary, for every degenerate FWM product falling on the g'th subcarrrier we can accordingly find 2 FWM products, which are created with the contribution of this subcarrier. Based on these arguments and taking into account the fact that the number of non-degenerate FWM products is much bigger than the number of degenerate FWM product [20], the number of FWM products created with the contribution of the g'th subcarrier can be calculated approximately:…”

“…CO-OFDM provides an efficient way to compensate for inter-symbol interference caused by both chromatic dispersion (CD) and polarization-mode dispersion (PMD) and uses a simple channel estimation and compensation scheme [17,18]. However, one major drawback of CO-OFDM is that it suffers from a number of nonlinear effects, especially four-wave-mixing (FWM) due to the narrow and equal spacing of subcarriers [19][20][21]. Mitigating the impact of FWM on the performance of CO-OFDM transmission is the motivation of this work.…”

“…It has been shown that in CO-OFDM transmission the power density of FWM noise is higher in the center of the OFDM band than at the edge [20][21][22]. Similarly, the contributions of different subcarriers to the FWM noise, in general, are different.…”

Reduced complexity precoding based peak-to-average power ratio reduction applied to optical direct-detection OFDM

“…An important problem caused by high PAPR is the high dynamic range of power amplifiers and DA/AD-converters required for distortionless transmission. Moreover, recently a correlation between PAPR and the degrading influence of four-wave mixing (FWM) was shown [4]. Consequently, PAPR reduction schemes become an essential part of optical OFDM systems.…”