  We model the transmission of ultrawideband signals, including
wavelength-dependent fibre parameters: dispersion, nonlinear coefficient and
effective fibre core area. To that end, the inter-channel stimulated Raman
scattering Gaussian noise integral model is extended to include these
parameters. The integrals involved in this frequency-domain model are
numerically solved in hyperbolic coordinates using a Riemann sum. The model
implementation is designed to work on parallel GPUs and is optimised for fast
computational time. The model is valid for Gaussian-distributed signals and is
compared with the split-step Fourier method, for transmission over standard
single-mode fibre (SSMF) in the O-band (wavelengths around the zero-dispersion
wavelength), showing reasonable agreement. Further, we demonstrated SNR
evaluation over an 80~km SSFM single-span transmission using 589x96 GBaud
channels, corresponding to almost 59 THz optical bandwidth, fully populating
the O, E, S, C, L and U bands (1260-1675 nm). The SNR evaluation is completed
in just 3.6 seconds using four Nvidia V100 16GB PCIe GPUs. Finally, we used
this model to find the optimum launch power profile for this system achieving
747 Tbps of potential throughput over 80 km fibre and demonstrating its
suitability for UWB optimisation routines.
