## 5 Performance Evaluation
Figure 7 also shows that the reliability becomes again nearly independent of the number of UEs that are being supported. The degradation of reliability experienced from \(\Delta t=\Delta t_{1}\) to \(\Delta t=\Delta t_{avg}\) is again due to a major relevance of the effect of self-collisions when we compute the reliability. Expressions in (25) and (26) show that \(P_{sc}\) also depends upon the latency requirement \(L\). The effect of self-collisions is more relevant when the latency requirement is stricter. For example, Figure 7 shows that the effect of self-collisions already limits the maximum reliability that can be achieved when \(K=4\) if the latency requirement is equal to 0.5 ms. Latency requirements significantly influence the reliability levels that can be satisfied. This is the case because latency requirements limit the number \(K\) of replicas that can be sent for each packet. Figure 4 shows that the maximum reliability level that can be guaranteed depends on the latency requirements when only considering \(P_{c}\). Figure 7 also shows  
Figure 7: Reliability for different latency requirements \(L\) and number of repetitions \(K\) (\(U=6\)): (**a**) \(\lambda=0.1\) packets; (**b**) \(\lambda=1\) packet. that the effect of self-collisions becomes more relevant with stricter latency requirements. These results show that it is a challenge guaranteeing high reliability demands with very low latency levels. The results in Figure 7 demonstrate that current 5G NR grant-free scheduling with _K_-repetitions and shared resources cannot guarantee some of the more demanding reliability and latency levels. However, it is important emphasizing that other proposals cannot meet such requirements either, and these actually perform worse than the implementation analyzed in this study. This is actually the case for the proposals that transmit the first copy of a packet in dedicated resources for the UEs. These resources can be reserved using grant-based scheduling (such as in [14]) or semi-persistent scheduling (such as in [13]). Grant-based scheduling requires the UE to send an SR to the BS, and wait for the BS to reply with a grant message. The exchange of these messages between the UE and the BS is illustrated in Figure 8 This handshaking generates a non-negligible \(T_{total}\) latency that is equal to:  
\[T_{total}=2\ T_{L1/L2}+T_{align}+2\ T_{proc}+3\ T_{tx}=2.3\ \text{ms} \tag{27}\]  
where \(T_{L1/L2}\) is the \(L1/L2\) processing latency at the BS and the UE, \(T_{align}\) is the alignment latency (the alignment latency is the time elapsed from the moment the UE is ready to transmit to the actual time the transmission starts), \(T_{proc}\) is the processing latency (this latency represents the latency between the reception of the SR and the transmission of the grant message), and \(T_{tx}\) is the time required to transmit the SR and grant messages. Following [24], we consider \(T_{L1/L2}\) = \(T_{align}\) = \(T_{tx}\) = 1 TTI, and \(T_{proc}\) = 2.33 TTI. These values are a best-case scenario, since they represent reduced processing times that can be achieved with 3 GPP Release 15 compared to Release 14. Equation (27) shows that the total latency (2.3 ms) introduced by the grant-based scheduling process to assign dedicated resources to UEs is higher than the latency achieved with the 5G NR grant-free scheduling implementation analyzed in this study. For example, Figure 7 shows that this implementation can guarantee latency levels below 1 ms (this latency is guaranteed with a reliability up to 1-10\({}^{-5}\) when \(K\) = 4, \(\lambda\) = 0.1 packets, \(U\) = 6, and \(\Delta t\) = \(\Delta t_{avg}\)). The alternative to grant-based scheduling is Semi-Persistent Scheduling (SPS). In this case, UEs are assigned dedicated resources for a period of time. During this period, UEs can utilize the resources without requesting permission from the BS. This avoids the latency introduced by grant-based scheduling. However, semi-persistent scheduling inefficiently utilizes the radio resources when the traffic is aperiodic.