Modified Nonlinear Decision Feedback Equalizer for Long-Haul Fiber-Optic Communications

Abstract

In a long-haul optical fiber communication system, fiber attenuation, dispersion and nonlinearity combined with nondeterministic noise from optical amplifiers used for periodic regeneration cause adverse effects on system performance. Several optical and electrical signal processing techniques have been proposed and implemented to extract the transmitted data; some provide better performance than others, but at a cost of higher computational complexity. We present a modified nonlinear decision feedback equalizer designed for use in a legacy optical communication system with periodic dispersion compensation. The effects of noise and nonlinearity on the equalizer coefficients are investigated, and a suboptimal convergence algorithm to reduce such effects is proposed and verified. Our equalizer provides performance comparable to that obtained using digital backpropagation while being computationally simpler, compensating linear and nonlinear physical impairment effects effectively even at high power levels where fiber nonlinearity is significant. Performance prediction of the designed DFE is also discussed, using a numerical method, with and without error propagation.

Efficient nonlinear equalizer for intra-channel nonlinearity compensation for next generation agile and dynamically reconfigurable optical networks

“…It should be noted that in the case of other symmetric pulses the perturbation coefficients still satisfy the properties stated in Eqs. (10) and (11). We also verified Eqs.…”

“…We also verified Eqs. (10) and (11) by numerical integration of Eq. (6) for a root raised cosine (RRC) pulse shaping filter.…”

“…However, their major drawback is their high complexity [7,8]. A Wiener-Hammerstein equalizer (for OFDM transmission) [9] and modified nonlinear decision feedback equalizer [10] was proposed as a simpler alternative. These algorithms adaptively compensate for all fiber impairments according to following formulas (presented in time-domain):…”

“…Therefore, their performance was investigated for single polarization systems with inline dispersion compensation and a short channel nonlinear memory length (i.e. fiber with small dispersion parameter) [9,10].…”

“…The perturbation-based nonlinear pre-compensation (or post-equalization) technique compensates the accumulated nonlinearities with only one computation step and can be implemented with one sample per symbol [11][12][13]. Typically, perturbation based NLCs have lower computational complexity than blind nonlinear equalization algorithms [7][8][9][10]. However, perturbation based NLC algorithms requires prior knowledge of fiber optic transmission system parameters in order to calculate perturbation coefficients [11] and 50% chromatic dispersion (CD) pre-compensation at the transmitter for efficient implementation [12,13].…”