We show that dispersion-enhanced phase noise (DEPN) induces performance degradations in both conventional CO-OFDM systems and reduced-guard-interval (RGI) CO-OFDM systems employing RF-pilot phase compensation. After analytically studying DEPN, we show that DEPN causes a 2 to 6 dB optical signal-to-noise ratio (OSNR) penalty at transmission distances of 3200 km and 1600 km for 28 and 56 Gbaud QPSK systems, respectively, using lasers with 2 MHz linewidths. At such distances, DEPN reduces the linewidth tolerance at 1 dB OSNR penalty to 250-500 kHz while in the back-to-back case the tolerance is 1-3 MHz for both systems. When fiber nonlinearity is included, we observe similar performance degradations.
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