Modulation formats suitable for ultra high 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-tozero 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 CSRZDPSK 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.