Higher bitrates and advanced modulation formats facilitate overlay upgrades of installed submarine systems

Upgrades of undersea communication systems beyond their design capacity is an attractive alternative to deploying new cables. Increasing the per wavelength channel bitrate is an obvious way to facilitate this. Nevertheless, this comes with tightened performance requirements, such as improved noise and dispersion tolerance, which will reduce any available margins. Applying new modulation formats is a route to mitigate these effects. This paper discusses the technical challenges for carrying out upgrades, either as overlays or on dark fibres, and also presents recent research results on how to add capacity to systems currently operating with Nx10.7 Gbit/s RZ-ASK signals over transoceanic distances. Experiments and simulations are used to analyse performance and limitations, with 20 Gbit/s RZ-DPSK being a strong candidate for upgrades.