  We present a comparative study of the influence of dispersion induced phase
noise for CO-OFDM systems using Tx channel multiplexing and Rx matched filter
(analogue hardware based); and FFT multiplexing/IFFT demultiplexing techniques
(software based). An RF carrier pilot tone is used to mitigate the phase noise
influence. From the analysis, it appears that the phase noise influence for the
two OFDM implementations is very similar. 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) and this, in turns,
leads to a BER specification. Numerical results focus on a CO-OFDM system with
1GS/s QPSK channel modulation. Worst case BER results are evaluated and
compared to the BER of a QPSK system with the same capacity as the OFDM
implementation. Results are evaluated as a function of transmission distance,
and for the QPSK system the influence of equalization enhanced phase noise
(EEPN) is included. For both types of systems, the phase noise variance
increases significantly with increasing transmission distance. An important and
novel observation is that the two types of systems have very closely the same
BER as a function of transmission distance for the same capacity. For the high
capacity QPSK implementation, the increase in BER is due to EEPN, whereas for
the OFDM approach it is due to the dispersion caused walk-off of the RF pilot
tone relative to the OFDM signal channels. For a total capacity of 400 Gb/s,
the transmission distance to have the BER < 10-4 is less than 277 km.
