The intra-channel nonlinearity is inherent damage in an optical transmission system and is originated from the Kerr effect. When the rate of the single channel reaches 40-60 Gbits/s or more, the pulses within the same channel will be greatly widened and overlapped each other due to the effect of dispersion, and with the effect of the nonlinearity, energy exchange will occur between the overlapped pulses. In such a case, even though the residual dispersion in the link was compensated for at the receiving side, the system would still be severely nonlinearly damaged. The effect of nonlinearity within the channel on the system includes: timing jitter, signal amplitude fluctuation, and generation of ghost pulse.
In recent years, as the rise of the capacities of optical transmission systems, more complex multidimensional modulation technologies, instead of simple intensity modulation formats, have been a hot spot in studies. For ensuring that a complex modulation has a sufficient signal to noise ratio, it is needed that a link system has higher input power, and this undoubtedly results in the increase of cost of nonlinearity of the system.
For a long-haul optical communication system, how to compensate for or mitigate the cost of nonlinearity within a channel is an important question for study. Studies have been done with respect to design of link, DSP processing of receiver and coding of transmitting signal. A method for mitigating nonlinearity by subtracting nonlinear perturbation at a receiver side has been proposed in the prior art. Such a method is based on double oversampling, wherein a perturbation item is equal to a weighted sum of products of a series of three items (symbol information data of three moments), and the weighted value is decided by the dispersion, gain/attenuation and nonlinear coefficient of the link. The advantage of the method exists in the reduction of complexity, and especially in a PSK system, a pre-compensated waveform may completely be realized by means of addition and subtraction.
However, in the implementation of the present invention, the inventors found the disadvantages of the prior art exist in: the performance is not good for NRZ (Not Return to Zero) code, and the performance of the system needs to be further improved.
Following documentations are listed for better understanding of the present invention and the prior art, which are incorporated herein by reference, as they are fully stated herein.    [Reference 1]: IEEE PTL Vol. 12, No. 4, 2000, Antonio Mecozzi et. al.    [Reference 2]: L. Dou, Z. Tao, L. Li, W. Yan, T. Tanimura, T. Hoshida, and J. C. Rasmussen, “A low complexity pre-distortion method for intra-channel nonlinearity,” in Proc. OFC/NFOEC2011 Conf., Los Angeles, U.S.A., March. 2011, paper OThF5.