Channel Limits in Noisy Physical Media: Fibre Nonlinearity, the Seawater Skin Effect, and Ocean Absorption Against the Shannon Formula

The Shannon formula is compared with physics-bound capacities for three stated channels: a dual-polarisation C-band optical fibre channel limited by Kerr nonlinear interference, a 1 kHz radio channel through seawater limited by the skin effect, and a 1-11 kHz underwater acoustic channel at 10 km range limited by frequency-dependent absorption (Thorp's formula). In each case the capacity with the binding physical effect included is computed from the accompanying script and compared with the value from the unmodified formula. Model calculations: simulated, not measured.