## 3 Results
### Performance Evaluation of Fiber-Wireless Topology
Therefore, as explained in Section 3.2, for the InGaAs QKD setup, the maximum feasible transmission distance is longer for the Fi-Wi case (orange line of Figure 13a) compared to the P2 MP configuration serving the same (i.e., 64) terminal users (blue line of Figure 13a). Secondly, the SKR values required for the data and the controller functions encryption, under a key refresh time equal to 5.14 min (3.7 bps), are not heavily increased compared to SKR needed for the data encryption only (0.83 bps). For all the other cases, we find a remarkably similar behavior between the two topologies. However, a much better system performance can be achieved by slightly compromising on the rotation time of the secret keys required for the encryption of the radio controller functions only, while demanding the lowest key rotation time for the data encryption. More specifically, we considered a scenario where a 1.4 s refresh time is set for the data encryption and 1 min for the control layer encryption, respectively. This case demands SKR of at least \((3\times 256+128)\text{bits}/1\ \text{min}+256\ \text{bits}/1.4\ s=198\ \text{bps}\). The comparison between the Fi-Wi topology operated with the above settings and the shared optical fiber P2 MP link serving 64 terminal users is illustrated in Figure 14.  
Assuming the above combination of key rotation times, the Fi-Wi topology offers a significant improvement for both QKD implementation setups, compared to the P2 MP shared fiber link case, since the SKR limits for the two configurations are almost the same (198 bps compared to 183 bps  
Figure 13: SKR as a function of fiber length, for the P2 MP shared fiber link (blue line) and for the Fi-Wi topology (orange line) serving \(N=64\), where the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm for both downlink and uplink, for **(a)** InGaAs; and **(b)** upconverted CMOS-based QKD setups. The horizontal lines with different styles correspond to the AES limits for different key refresh times: 0.83 bps corresponds to 5.14 min refresh time for the data encryption for P2 MP; 3.7 bps corresponds to 5.14 min refresh time for the data encryption and for the controlled functions for Fi-Wi; 183 bps corresponds to 1.4 s refresh time for the data encryption for P2 MP; and 823 bps corresponds to 1.4 s refresh time for the data encryption and for the controlled functions for Fi-Wi. respectively). In more detail, an increase of approximately 5 km (for CMOS) to 6 km (for InGaAs) for the maximum possible fiber distance can be obtained. Finally, it should also be noted that the relative improvement is somewhat more evident for the InGaAs compared to the CMOS setup, since this QKD setup is more sensitive to the decrease in the number of optical terminal nodes which the Fi-Wi provides, respecting the maximum achievable distance, as thoroughly described previously. This behavior of the InGaAs-based QKD setup is more clearly reflected in the case of the relaxed key refresh time of 1 min for the encryption of the controlled functions, where the hybrid topology can offer 2 km extension of the maximum feasible transmission distance. On the other hand, we observe an almost identical behavior between the two topologies in the case of using CMOS detectors, for his lower key refresh time.