# Spectrum Slicing for Multiple Access Channels with Heterogeneous Services
## 7 Conclusions  
In this work, we investigated the performance trade-offs with orthogonal and non-orthogonal spectrum slicing in a multiple access system with broadband and intermittent users. We derived closed-form expressions for both PAoI and latency-reliability for the intermittent users, along with throughput for the broadband user, in a time-slotted system in which the users share a single frequency channel.  
The results illustrate that, by implementing an erasure code at the broadband user, the choice between OMA and NOMA depends on the specific features of the considered scenario and on the objectives of the system designer. In particular, the number of intermittent users and their aggregate arrival rate have major impacts on the preferred slicing and access method for latency-oriented systems. In these cases, TDMA was clearly preferable for the higher arrival rates, whereas ALOHA performed remarkably well with low to medium arrival rates. Interestingly, the opposite effect can be seen for the choice of the access scheme, as NOMA outperformed OMA with higher arrival rates, and orthogonal allocation worked better for lower arrival rates. The NOMA ALOHA scheme presents a case of particular interest, as by correctly tuning the coding parameters for the broadband user, it could oftentimes achieve the best performance trade-offs with low to medium arrival rates in the extreme cases--that is, when the intermittent users required the lowest latency and when the broadband users required the highest throughput. On the other hand, NOMA TDMA is clearly the best access method for latency-reliability with high arrival  
Figure 9: Pareto frontier for \(S_{B}\) and \(T_{90}\) with \(U_{I}=100\). For OMA, \(K^{*}=64\) and \(N^{*}=77\).  
rates. The PAoI results show that the two access methods are almost equivalent, as long as they are configured properly, and the main driver of performance is the packet generation process. However, OMA TDMA does show significant advantages with respect to the other OMA schemes, as it avoids collisions entirely, whereas the other OMA schemes may still have collisions between intermittent users. These results, obtained in the simple collision channel without capture, showcase the potential of NOMA schemes in scenarios with heterogeneous service types as channel capture and intra-collision SIC greatly improve its performance.  
Future work on the subject can be oriented in multiple directions: First, analyzing the system with MPR is definitely a priority, as the worst-case analysis has already shown the advantages of NOMA. Secondly, more realistic systems could be investigated, with time-dependent arrival patterns or with multiple frequency channels, which would add an interesting dimension to the problem by providing parallel resources. The possibility of using packet repetition to increase the intermittent users' reliability is another interesting facet that can be examined, although the complexity of the system may grow beyond the possibility of analytical tools, requiring a simulation-based approach.  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.