Through intensive computer simulations, ultra-long-haul transmission characteristics of 100-Gbit/s polarisation-multiplexed 16-quadrature-amplitude-modulation (16-QAM) signals are studied, which are densely wavelength-division multiplexed (WDM) on the 25 GHz spaced frequency grid. The dispersion-unmanaged link consisting only of large-core singlemode fibres has strong tolerance to cross-phase modulation among WDM channels, and 2000 km transmission is enabled with the digital coherent receiver.