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We have numerically investigated transmission properties of four transmission formats. We established that while conventional WDM systems with large channel spacing benefit from shorter RZ and CRZ pulses that help to cope with nonlinearity, the newer generation of DWDM systems with spectral efficiencies greater than 40% will need pulses with narrower spectrum to fight the channel crosstalk. We showed that a good candidate for use in such system is a long duty cycle RZ format. Its spectrum at the receiver is nearly as narrow as the one of NRZ format but the transmission properties are better. We found that by optimizing the pulse width (duty cycle) of RZ pulses we can match the performance of CRZ signal in the system with the large channel spacing (50 GHz) and show better performance than the one of NRZ or CSRZ signal for system with tight channel spacing (25 GHz).