# Spectrum Slicing for Multiple Access Channels with Heterogeneous Services
## 5 Non-Orthogonal Multiple Access
### Latency-Oriented System
As the arrival rate increases with \(U_{I}=4\), NOMA becomes the Pareto efficient choice for all points in the latency-throughput trade-off, albeit with a small margin. This is observed in Figure 7c,d, where the Pareto efficient methods are NOMA ALOHA and NOMA TDMA, respectively, with NOMA grouping achieving a close performance. The reason for the better performance of NOMA with high arrival rates is that it allows the intermittent users to access considerably more resources than OMA, which minimizes collisions between them. These collisions are considerably harmful for the system as they cannot be resolved. Therefore, OMA ALOHA becomes infeasible with high arrival rates, whereas OMA TDMA may suffer from queue overflows since intermittent slots are spaced by \(U_{I}T_{\text{int}}\) slots. Next, Figure 8 shows a similar pattern to Figure 7, but with a much better performance of NOMA with respect to OMA. Specifically, NOMA ALOHA and grouping achieve much better trade-offs when compared to OMA TDMA for the considered arrival rates, with the only exception being that NOMA grouping is not viable for \(U_{I}\alpha=0.1\). This is also the case with all the ALOHA methods, which fail for the cases with \(U_{I}\alpha=0.1\) because of the excessive collisions among the intermittent users. Finally, OMA grouping can only achieve the required 90% reliability for the intermittent users with \(U_{I}\alpha=0.1\) by making \(S_{B}=0\). Figure 7: Pareto frontier for \(S_{B}\) and \(T_{90}\) with \(U_{I}=4\). For OMA, \(K^{*}=64\) and \(N^{*}=77\). The case with \(U_{I}=100\), displayed in Figure 9, features a more pronounced differences among the access schemes, indicating that the selection of the access scheme and/or its parameters will be even more critical in massive access scenarios with larger number of users. As in the previous cases, using NOMA becomes more convenient as the total arrival rate increases. OMA ALOHA performs particularly well for low total activation rates, as collisions between intermittent users are rare in this scenario, and in settings that are oriented more towards latency-reliability than broadband user throughput, as increasing the transmission opportunities for the intermittent users can further reduce the probability of collisions between them. In general, it can be concluded that ALOHA schemes perform better under low arrival rates \(U_{I}\alpha\), whereas TDMA schemes perform better when the aggregate arrival rate increases. This may be expected, in particular as the assumed timeliness parameters of interest are rather stringent. The performance of OMA grouping oftentimes lies between that of OMA ALOHA and TDMA for all values of \(U_{I}\). This showcases its robustness to the arrival rate \(U_{I}\alpha\), but also that it is not an ideal option to optimize performance. Instead, NOMA grouping achieves a remarkable performance, oftentimes matching or even surpassing the performance of NOMA ALOHA and NOMA TDMA, even with very high rates. Depending on the scenario, the number of groups is highly variable: if \(U_{I}\alpha=0.1\), the grouping scheme uses the largest possible number of groups (i.e., \(G=50\) with \(U_{I}=100\)), making the scheme closer to TDMA than pure ALOHA. On the other hand, ALOHA is more convenient for lower activation rates, so the best grouping performance will be obtained with \(G=2\). Most interestingly, NOMA schemes outperform OMA under most conditions, with the exception of OMA ALOHA for low arrival rates. This behavior is extremely encouraging for the performance of NOMA in realistic systems, as the collision channel we considered is a worst-case scenario for non-orthogonal access. As observed in our previous work [7], by including the probability of channel capture and intra-collision SIC, the performance of non-orthogonal schemes can only improve. Nevertheless, OMA may can also benefit from capture and intra-collision SIC by mitigating collisions between intermittent users.