Recent coherent optical communication systems address modulation and detection techniques for high spectral efficiency and robustness against transmission impairments. This paper demonstrates the generation and transmission of beyond 400Gb/s transmission system over the standard 50GHz ITU-T grid at a net spectral efficiency of 8.4b/s/Hz. Coherent detection employing QAM modulation formats along with DSP algorithm have become one of the most promising technologies for next generation high speed transmission systems due to the high power and spectral efficiencies. This paper shows that, the use of near ideal Nyquist pulse shaping, spectrally-efficient high-order modulation format, and distributed Raman amplification, coherent equalization may enable us to operate future 400G systems over the standard 50GHz-grid optical network. Keywords-Wave length division multiplexing (WDM); coherent detection; digital signal processing (DSP); QAM; Nyquist pulse.I.