## 2 Materials and Methods
### Shared Fiber Topology  
As a next step, we considered a shared fiber link for classical and quantum transmission. Specifically, the packetized classical data flow propagated from the BBU node and the 5G Terminal Node (and vice versa with symmetrical transport) and the photon transmission from Alice to Bob is performed over a shared SSMF link, as illustrated in Figure 3. This shared use of fiber installations would boost the compatibility of quantum communications with existing optical infrastructures and lead to a significant improvement in terms of cost-effectiveness and addressable market for QKD [45]. In this proposed coexistence setup, the performance of the quantum channels suffers mainly from the nonlinear effect of Spontaneous Raman Scattering (SpRS), arising from the inelastic scattering of the strong bidirectional classical data signal with fiber material. The forward scattered Raman power, generated by the downlink, is obtained by [46; 47]  
\[P_{f} =P_{\rm dl}\rho_{\rm dl}\Delta\lambda\frac{e^{-\alpha_{q}L}-e^{- \alpha_{\rm dl}L}}{\alpha_{\rm dl}-\alpha_{q}}, \tag{4}\]  
where \(P_{\rm dl}\) is the launch power of the downlink, \(\rho_{\rm dl}\) is the effective Raman cross-section, \(\Delta\lambda\) is the quantum receiver spectral bandwidth, which is set to 0.8 nm, L is the propagation distance and \(\alpha_{\rm dl}\), \(\alpha_{q}\) are the fiber attenuation coefficients for the downlink's wavelength and the quantum channel's wavelength respectively. The uplink generates backward scattered Raman photons with power given by [46; 47]  
\[P_{b} =P_{ul}\rho_{ul}\Delta\lambda\frac{[1-e^{-(\alpha_{ul}+\alpha_{q})L} ]}{\alpha_{ul}+\alpha_{q}}, \tag{5}\]  
where \(P_{ul}\) is the launch power of the uplink, \(\rho_{ul}\) is the effective Raman cross-section and \(\alpha_{ul}\) is fiber attenuation coefficients for the uplink's wavelength. In this coexistence topology, Equation (1) has to be modified to account for the contribution of Raman noise to the SKR, by adding the Raman photon detection probability (calculated by expressing the forward and backward Raman power levels in terms of noise photons per ns (gate duration time) [23; 35; 48]) to the other detection probabilities. To be able to select the quantum information transmitted through the shared fiber, we considered a bandpass optical filter at Bob's side, introducing a wavelength independent loss of 2.7 dB. This filter is assumed to spectrally isolate the quantum passband, therefore no leakage photons due to classical power can be recorded due to the crosstalk. This assumption can be easily met, by keeping the channel spacing between the downlink and the quantum signal large enough (-400 GHz [38]). Raman Scattering mechanism covers an ultra-broadband window and gets stronger as the propagation distance increases. Strictly speaking, it is maximized at a specific propagation distance, depending on the fiber attenuation value (>20 km for each wavelength allocation we encountered), longer than distances for which SKR can be established for shared topologies, after which it decreases slowly. It is also worth mentioning that in C-band topologies, besides the Raman scattering, the Kerr-based effects (e.g., FWM) should be taken into consideration, since they could contribute a significant amount of contamination photons which would increase the QBER performance [48; 49; 50; 51]. Figure 3: Quantum-secured eCPRI transport layer interconnecting the BBU and the 5G terminal nodes. One Standard Single-Mode Fiber (SSMF) link is used for both packetized classical data flow from the BBU node and the 5G Terminal Node (and vice versa) and for the photon transmission from Alice to Bob. In order to investigate in depth the magnitude of contamination of QKD links by Raman scattering photons in realistic network topologies, we considered two different wavelength allocations for the classical signals. In the first case, the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm (downlink) and at 1490 nm (uplink).