  In this paper we analyze the impact of the spontaneous Raman scattered noise
generated from multiple optical classical channels on a single quantum key
distribution channel, all within the telecom C-band. We experimentally measure
the noise generated from up to 14 continuous lasers with different wavelengths
using the dense wavelength division multiplexing (DWDM) standard, in both
propagation directions in respect to the QKD channel, over different standard
SMF-28 fiber lengths. We then simulate the expected secure key generation rate
for a decoy-states-based system as a function of distance under the presence of
simultaneous telecom traffic with different modulation techniques, and show a
severe penalty growing with the number of classical channels present. Our
results show that, for in-band coexistence, the telecom channels should be
distributed as close as possible from the quantum channel to avoid the Raman
noise peaks. Operation far from the zero dispersion wavelength of the fiber is
also beneficial as it greatly reduces the generation of four-wave mixing inside
the quantum channel. Furthermore, narrow spectral filtering on the quantum
channels is required due to the harsh limitations of performing QKD under real
telecom environments, with the quantum and several classical channels
coexisting in the same ITU-T C-band.
