Upgrades of Submarine Systems using Higher Bitrates and Advanced Modulation Formats

Upgrading undersea communication systems beyond their design capacity is an attractive alternative to deploying new cables and increasing the bitrate is an obvious way to facilitate this. Nevertheless, this comes with tightened performance requirements, such as reduced noise and dispersion tolerance, which will reduce any available margins. Applying new modulation formats is a route to mitigate this effect. 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 N×10.7 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.