The core of the global telecommunication network consists of wavelength division multiplexed (WDM) optical transmission systems. WDM is the technology of choice as it allows for a high spectral efficiency. We propose an effective way to counter the nonlinearities like Four Wave Mixing and Cross Phase Modulation to achieve the spectral efficiency of 4 b/s/Hz using CSRZ modulation format. We use the concept of non-uniform channel spacing and non-uniform power assignment between adjacent channels of the WDM system. We have simulated an 8 channel DWDM lightwave system with bit rate of 100 Gb/s and channel spacing of 25 GHz.