[8] G.P. Agrawal. Nonlinear Fiber Optics , 3rd Ed. Academic Press, San Diego, 2001. [9] E. Mateo, L. Zhu, and G. Li. Impact of XPM and FWM on the digi- tal implementation of impairment compensation for WDM transmission using backward propagation. Optics Express . vol. 16, no. 20, pp. 16124- 16137, 2008. 12