M.Sc Thesis

M.Sc StudentWeidenfeld Rakefet
SubjectNonlinear Impairments in Coherent Optical OFDM Systems and
their Mitigation
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Moshe Nazarathy


In this work we analytically analyze OFDM  propagation along the optical link, accounting for the combined effect of dispersion and nonlinearity, along with fiber loss and optical amplifiers noise. We solve the Nonlinear Schrodinger Equation  in the perturbation approach, under the undepleted pump approximation. We further introduce a more intuitive solution - the so-called integral approach, whereby the nonlinear signal is evaluated as a sum of elementary nonlinear contributions generated along the optical link. Our analytical results show a good agreement with the numerical simulation of the optical link, realized in the Split Step Fourier method.

In the second part, we introduce a novel method for nonlinear compensation. We synthesize an approximation of the nonlinear signal, accounting for the interplay between dispersion and nonlinearity, as analyzed in the first part, then subtract it from the received signal. Our method offers a good compromise between complexity and performance, yielding a 2 dB improvement over a previous compensation method which ignores dispersion completely. The compensation scheme is modeled in a Volterra-based technique, driven by a preliminary decision feedback. Essentially, we perform frequency shaping of the preliminary decision set, prior to synthesizing an emulated version if the nonlinear signal. We optimize the frequency shaping weights, in order to synthesize the best approximation of the received nonlinear signal.

We distinguish between the two types of nonlinear processes, ‘coherent’ (self phase modulation, cross phase modulation) and ‘noncoherent’ (four wave mixing) and compensate for each type separately.  This separation yields a major reduction in error propagation, hence improves performance significantly.

Finally, we suggest an innovative realization which requires only a baud-rate sampling, instead of an over sampling rates (which might be a serious bottleneck in any practical realization). The suggested implementation can be also integrated into other nonlinear compensation schemes.