  We investigate (a) spontaneous Raman scattering of classical signals in long
optical fiber links leading to significant Raman noise in quantum channels
propagating in the same fiber and (b) impact of chromatic dispersion on the
performance of high quantum bit rate QKD systems. One way of decreasing Raman
noise is to decrease the classical signal power. This may be achieved by the
use of coherent modulation formats and the deployment of Forward Error
Correction technology. Additionally, low loss fiber with small Raman gain at
telecom wavelengths enables the simultaneous transmission of multiple quantum
channels and one or a few classical channels at 100 Gb/s rate over tens,
possibly hundreds, of kilometers.
