In this paper the performance of FDM systems with a capacity of the order of a few hundreds of Gbit/s and a span up to 500 km is analysed by means of an analytical model. These systems can be important in the future high speed digital optical network.The comparison between IM-DD and coherend systems gives quite different results if transoceanic single channel systems or very high capacity FDM systems with a span of a few hundreds of km (300-500 km) are considered. While in the first case it is well known that IM-DD exhibits better performance the situation is different in the second case. Coherent systems exhibit better performance: up to 300 Gbit/s can be transmitted over a distance of 500 km adopting homodyne PSK technique.

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