  We present a study of the influence of dispersion induced phase noise for
CO-OFDM systems using FFT multiplexing/IFFT demultiplexing techniques (software
based). The software based system provides a method for a rigorous evaluation
of the phase noise variance caused by Common Phase Error (CPE) and
Inter-Carrier Interference (ICI) including - for the first time to our
knowledge - in explicit form the effect of equalization enhanced phase noise
(EEPN). This, in turns, leads to an analytic BER specification. Numerical
results focus on a CO-OFDM system with 10-25 GS/s QPSK channel modulation. A
worst case constellation configuration is identified for the phase noise
influence and the resulting BER is compared to the BER of a conventional single
channel QPSK system with the same capacity as the CO-OFDM implementation.
Results are evaluated as a function of transmission distance. For both types of
systems, the phase noise variance increases significantly with increasing
transmission distance. For a total capacity of 400 (1000) Gbit/s, the
transmission distance to have the BER < 10^-2 for the worst case CO-OFDM design
is less than 800 and 460 km, respectively, whereas for a single channel QPSK
system it is less than 1400 and 560 km.
