Modulation formats suitable for ultrahigh spectral efficient WDM systems

We analyze the performance of four different modulation formats in ultrahigh spectral efficient wavelength-division multiplexing (WDM) systems. Besides standard nonreturn-to-zero intensity modulation, we analyze duobinary, differential phase-shift keying (DPSK), and carrier-suppressed return-to-zero (CSRZ)-DPSK and, for each of them, optimize several system parameters, such as filtering and dispersion map. First, we optimize transmitter and receiver parameters in back-to-back condition, then we study the performance in a multispan scenario and its dispersion map optimization. Duobinary, DPSK, and CSRZ-DPSK show high suitability for ultrahigh spectral efficient WDM systems and high resilience to dispersion compensation tolerances and fiber nonlinearities. The investigation is based on detailed simulative analyses.