In this paper we analyze the impact of the spontaneous Raman scattered noisegenerated from multiple optical classical channels on a single quantum keydistribution channel, all within the telecom C-band. We experimentally measurethe noise generated from up to 14 continuous lasers with different wavelengthsusing the dense wavelength division multiplexing (DWDM) standard, in bothpropagation directions in respect to the QKD channel, over different standardSMF-28 fiber lengths. We then simulate the expected secure key generation ratefor a decoy-states-based system as a function of distance under the presence ofsimultaneous telecom traffic with different modulation techniques, and show asevere penalty growing with the number of classical channels present. Ourresults show that, for in-band coexistence, the telecom channels should bedistributed as close as possible from the quantum channel to avoid the Ramannoise peaks. Operation far from the zero dispersion wavelength of the fiber isalso beneficial as it greatly reduces the generation of four-wave mixing insidethe quantum channel. Furthermore, narrow spectral filtering on the quantumchannels is required due to the harsh limitations of performing QKD under realtelecom environments, with the quantum and several classical channelscoexisting in the same ITU-T C-band.
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