## 3 Results
### Performance Evaluation of Shared Fiber Topology
For example, for the CMOS photon counters QKD setup coexistence scheme (Figure 10b), for 4 users a rotation time equal to 1.4 s (i.e., 183 bps low limit) can be set even at approximately 9.5 km. However, for 64 users the maximum feasible distance which allows this low-key rotation time is limited to 2 km. On the other hand, as explained in Section 2.2, since both the encoded photons generated from Alice and the Raman noise photons suffer the same loss from the splitter stage, the maximum transmission distance for which secret keys can be distilled is not heavily compromised compared to the P2P shared fiber scenario. More precisely, for the InGaAs photon counters QKD setups, this distance is slightly reduced as the number of users increases (Figure 10a). On the contrary, for the CMOS setups, the maximum distance is almost unchanged, as can be seen from Figure 10b. Figure 9: SKR as a function of fiber length, for the P2P shared fiber link, for both InGaAs and upgraded (i.e., with total efficiency equal to 20%) upconverted CMOS-based QKD setups, where: (**a**) the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm (downlink) and at 1490 nm (uplink); (**b**) the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm for both downlink and uplink. The horizontal lines with different styles correspond to the AES limits for key refresh times equal to 5.14 min (0.83 bps), 1 min (4.3 bps) and 1.4 s (183 bps), respectively. To emphasize on the role of the photon counter features to the overall performance of QKD layer in the coexistence scheme, we take a closer look at the contribution of dark counts and Raman noise photons to the total Quantum Bit Error Rate (QBER) recorded at the maximum fiber length (i.e., at the point where SKR approaches zero). To identify their contributions, we need to decouple the QBER into separate error components. In our topology, the main error components contributing to QBER performance are: the Raman noise photons (\(\text{QBER}_{\text{Raman}}\)), the DCR (\(\text{QBER}_{\text{dark}}\)), the Bob's Mach-Zehnder Interferometer's contrast (\(\text{QBER}_{\text{opt}}\)) and the detector's afterpulsing effect (\(\text{QBER}_{\text{after}}\)). Therefore, QBER is well approximated by [61] (pp. 134-135)  
\[\text{QBER}=\text{QBER}_{\text{Raman}}+\text{QBER}_{\text{dark}}+\text{QBER}_ {\text{opt}}+\text{QBER}_{\text{after}}. \tag{6}\]  
With our assumed setup parameters and considering the configuration where the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm for both downlink and uplink (see Figure 10), when SKR approaches zero, QBER (calculated following [35; 48]) reaches its maximum value, \(\sim\)7.5%. Since \(\text{QBER}_{\text{opt}}=(1-V)/2=1\%\) is constant and the afterpulsing effect contribution is practically negligible compared to the other error components at the maximum fiber length, we can conclude that  
\[\text{QBER}_{\text{Raman}}+\text{QBER}_{\text{dark}}\cong 6.5\%. \tag{7}\]  
In Figure 11, we have plotted the \(\text{QBER}_{\text{Raman}}\) and the \(\text{QBER}_{\text{dark}}\) for \(N=1\), \(N=4\), \(N=16\) and \(N=64\) terminal users, at the point where SKR approaches zero for each case. For the case of using InGaAs photon detectors, the contribution of the DCR to the total QBER becomes stronger as the number of terminal users increases, since the optical losses associated to the cascaded splitter stages lead to lower number of total registered incoming photons at the Bob station. Therefore, in order for Equation (7) to be satisfied, Raman noise contribution should be lower (Figure 11a), which means that, as described in Section 2.2, the maximum propagation distance should be subsequently shorter. On the contrary, since the DCR of the CMOS photon counters is much lower compared to the InGaAs counterparts, its contribution to the total QBER is preserved practically constant as the number of terminal users increases and consequently, the same holds for the Raman noise (Figure 11b).