We study the robustness of a nonlinear frequency-division multiplexing (NFDM) system, based on Zakharov-Shabat spectral problem (ZSSP), that is comprised two independent quadrature phase-shift keyed (QPSK) channels modulated in discrete spectrum associated with distinct eigenvalues. Among many fiber impairments may limit this we focus determining limits due to third-order dispersion, Raman effe...