The other papers that I considered for the discussed topic are following: “Spectral and Energy Efficiency Considerations in Mixed-Line Rate WDM Networks with Signal Quality Guarantee” Aleksejs Udalcovs, Paolo Monti, Vjaceslavs Bobrovs, Richard Schatz**, et al. Institute of Telecommunications, Riga Technical University; “QPSK wvith Coherent Detection over Ultra-Long Distance Improved by Nonlinearity Mitigation” Gabried Chariet, Alcatel Lucent Research & Innovation.