hi asif…
sry i misinterpretted it…
actually negative power penalties do occurs in OOFDM transmission signals…..the resulting negative power penalty is independentof both cyclic prefix and signal modulation format. Furthermore the negative power penalty is controllable by applying adaptive modulation and/or variation in DMLoperating condition.The negative power penalty is a direct result of the negative dispersion fiber-enabled compensation of the DML positive transient frequency chirp.
The physical mechanisms behind the negative power
penalty can be explained as follows: The optical-domain OOFDM signal phase cannot be preserved perfectly in the electrical domain due to subcarrier intermixing upon square-law photon detection in the receiver. As the unwanted subcarrier intermixing effect decreases with decreasing the optical phase at the input facet of the PIN, thus the MetroCor fiber-induced reduction in the DML-modulated
OOFDM signal phase gives rise to the observed negative power penalty. Whilst, for coherent OOFDM MetroCor transmission systems, the OOFDM signal phase can be preserved perfectly in the electrical domain, no negative power penalties exist.
The above physical explanations imply that a partial elimination of the subcarrier intermixing effect should be able to increase the obtained power penalty and simultaneously extend the transmission distance over which the minimum power penalty is observed.
i have also attached a pdf file..
hope this will help

with regards