## 5 Latency and Reliability Performance Evaluation in Wearable-Based MCPTT Applications
### Performance Evaluation Results  
After setting the scenarios and the parameters, the next step is to run several simulations and collect the simulation traces that will be used in the performance evaluation. As mentioned in Section 5.1, we use ns-3 updated with models for the support of public safety communications and presented in [41]. These models allow tracing MCPTT mes  
Figure 4: Sidelink channel configurations utilized in the performance evaluation. sages at the application layer and provide output files with these traces. The obtained output files contain the elements that we will use to determine our evaluation metrics. In the latency performance evaluation, we assess the MCPTT access time. According to 3 GPP specifications, the MCPTT access time is defined as \"the time between when an MCPTT user requests to speak and when this user gets a signal to start speaking\" [45]. To determine this metric in group calls, we use the same calculation method detailed in [6]. For the MCPTT private calls, the access time was determined based on the formulas provided in [46]. Figure 5 reports the numerical results for the average access time in MCPTT private (Figure 4(a)) and group calls (Figure 4(b)). The provided results are in function of the PSCCH period and the PSCCH/PSSCH ratio. The values of both parameters are fixed as stated in Section 5.1. The average access time results in private and group calls have different ranges due to the different number of UEs and the different call configurations (e.g., types of messages, counters, and timers). One common observation in Figure 4(a),b is that longer PSCCH periods result in longer access time values. This can be justified by the fact that one important component of the access time is the \"floor request retransmission\" timer, which is equal to the PSCCH period according to the 3 GPP default setting. The impact of the PSCCH/PSSCH ratio on the MCPTT access time is also depicted in Figure 4(a),b. Although this impact is slightly clearer in Figure 4(a), both figures show that preferring PSSCH over PSCCH transmissions (i.e., shorter PSCCH/PSSCH ratio) can provide shorter access time values. The latter result is important in public safety services where even a few milliseconds can make a difference in critical situations. In brief, we can summarize the take-away message from the obtained results in Figure 5 as follows: to guarantee short access times for MCPTT communications, short PSCCH periods with low PSCCH/PSSCH ratios are recommended. The second part of this evaluation is dedicated to reliability performance. The packet delivery ratio is utilized as the reliability performance indicator. It can be defined as the ratio of the number of packets received at the destination to the number of packets sent from the source [47]. Figure 6 reports the numerical results for the average packet delivery ratio in MCPTT private (Figure 5(a)) and group calls (Figure 5(b)). Similar to Figure 5, the results provided in Figure 6 depend on the PSCCH period and the PSCCH/PSSCH ratio. Lower values of the packet delivery ratio are witnessed when using shorter PSCCH periods in both private and group calls. Short PSCCH periods implicate more reconfiguration times, and thus, an increase in the probability of collisions due to the frequent exchange of scheduling messages. Figure 5: Impact of the PSCCH period and the PSCCH/PSSCH ratio on latency performance in MCPTT calls. On top of the PSCCH period and the number of PSCCH subframes, the collision probability of a PSCCH transmission depends on the sidelink allocation size [44]. Therefore, we examine the impact of the sidelink allocation size on the packet delivery ratio in MCPTT private (Figure 6(a)) and group calls (Figure 6(b)). The obtained results in Figure 7 show that high packet delivery ratios can be achieved using long PSCCH periods and a high number of RBs in the sidelink grant.