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A simple and illustrative analytical model for the phase noise influence in differential n-level phase shift keying (nPSK) systems is presented. Results for the phase noise sensitivity are given for a dual polarisation system configuration with basic baud-rate of 25 GS/s. Results emphasise the need for highly phase stable optical signal sources in higher constellation configurations as needed for total system capacities of 400 Gbit/s or more. Classical distributed feedback laser diodes with linewidth of 200 kHz-10 MHz are not feasible beyond the order of 200 Gbit/s capacity. 2n-level circular quadrature amplitude modulated (2nQAM) systems may increase the capacity by 50 Gbit/s compared to nPSK with no added phase noise penalty.