M.Sc Thesis

M.Sc StudentShulkind Gal
SubjectIdentification and Compensation of Nonlinear
Interference in Coherent Optical OFDM Fiber
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Moshe Nazarathy


Coherent optical OFDM has recently become the transmission technique of choice for fiber-optic communication. However, in order to improve the tradeoffs between reach and capacity the transmission power should be driven higher - thus nonlinear fiber impairments are becoming the limiting factor in photonic transmission. To address the nonlinear bottleneck in coherent optical transmission, multiple methods for nonlinear-interference mitigation have been proposed based on transmit or receive side signal processing algorithms partially cancelling the nonlinear interference. Any nonlinear compensation (NLC) approach requires a-priori characterization of the nonlinear interaction in the fiber, for which measurement techniques are not currently available.

We propose an efficient nonlinear system identification method for the nonlinear coherent fiber-optic channel. In our data-aided nonlinearity measurement algorithm, an OFDM-like training sequence is transmitted and signal processing of the received signal extracts the Volterra Series Transfer Function (VSTF) description of the fiber link non-linearity. Our nonlinear optical performance monitoring algorithm is highly efficient in acquisition time and computational complexity and is scalable - enabling to tradeoff complexity with identification accuracy. The second part of the work improves a previous Volterra-Series Decision-Feedback nonlinear compensator design, increasing its accuracy and extending its applicability to multi-span fiber links. The novel Volterra compensator offers high quality nonlinear mitigation and its computational complexity is significantly reduced by more than an order of magnitude vs. conventional Split-Step-Fourier based compensators. Finally, we derive a fully-analytic statistical analysis of the nonlinear performance of the ‘DFT-Spread’ OFDM modulation format, which has recently been increasingly adopted, as a fix for the large peak-to-average-power ratio of OFDM. DFT-Spread was shown to be useful in mitigating nonlinear interference in coherent optical OFDM links but the mechanism of nonlinear interference generation has not been understood. Our analytic model accurately accounts for the mitigation performance of this method. In the process, we extend the ‘DFT Spread’ modulation format to a generalized class of ‘Unitary Spread’ transmission schemes, and identify as a special case an OFDM variant we refer to as Wavelet-Spread (WAV-S), which provides a different tradeoff of performance vs. complexity.