The subsea fiber as a Shannon channel
Since many years, the Q-budget table (normalized by the ITU-T G.977) has been widely used to characterize the transmission performance of subsea cables: this table detailed the margin allowance breakdown for any modulated wavelength. The fiber achievable transmission capacity was then deduced from the wavelength spacing and the system operating bandwidth. However, the emergence of coherent detection and Digital Signal Processing (DSP) capabilities has enabled the deployment of a wide range of modulation schemes featuring various bit rate, FEC encoding, constellation and spectral shaping, non-linear effect mitigation, thus leading to a transponder-dependent fiber transmission capacity. Combined to the recent trend of the industry to deploy “open” cables it is now time to define a new method to characterize the subsea fiber performance independently of the transponder type. This is emphasized by the introduction of Space Division Multiplexing (SDM) systems equipped with a high fiber pairs count, bringing the granularity at the fiber level: easy to swap, to sell and to manage. Cable capacity will be evaluated via the sum of fiber capacities deduced from any SLTE (Submarine Line Terminal Equipment) at any time with any margin. The proposed method for non-dispersion-managed undersea systems, relies on the General Signal to Noise ratio (GSNR) to remove the effect of baud rate, which is changing rapidly in each generation of SLTE. These have been metrics already widely debated at conferences/publications. Topics such as accuracy, Gaussian Noise (GN) model, assumptions, and measurability, are discussed to clarify definitions and a methodology. Finally, the paper reviews and discusses fiber capacity based on a given GSNR-based performance budget and various transponder types.