## 4 Results and Discussion
Moreover, due to the high channel gain difference, the number of users per cluster would be low and thus would result in low interference and eventually improved throughput gain. Figure 9 shows that by implementing clustered PD-NOMA in an ultra-dense HetNet, the overall system capacity improved remarkably compared to the capacity achieved by implementing a unified PD-NOMA and the OFDMA scheme in an ultra-dense heterogeneous network. On performing the analysis, it was found that the overall system capacity with clustered PD-NOMA improved by 8% compared to the unified PD-NOMA scheme and more than 80% compared with the conventional OFDMA scheme. Thus, the proposed clustered PD-NOMA algorithm improved the system capacity by grouping the base stations and user associations with satisfactory data rates, although with clustered PD-NOMA as the average SINR increased, system capacity also improved. Figure 8: Comparative analysis of throughput gain achieved with the proposed scheme and the existing schemes. The potential reason for increased system capacity is the increased channel gain difference due to cluster formations with a lower number of users along with the basic functionality of the PD-NOMA scheme. Thus, with clustered PD-NOMA, the system capacity further improved and eventually will improve the coverage probability in ultra-dense heterogeneous networks. Figure 10 shows an improved sum rate achieved with the proposed algorithm of the clustered PD-NOMA scheme, which hence improved the overall sum rate versus the increased density of associated users within the targeted cell area and eventually resulted in an improved system sum rate achieved with the increased user density, as compared to the sum rate achieved with the unified PD-NOMA and the conventional OFDMA schemes for the same number of users. The results show that the proposed clustered PD-NOMA scheme improved the sum rate by 4 and 14% compared to the unified PD-NOMA and conventional OFDMA scheme, respectively. Therefore, an improved sum rate with an increased system capacity was achieved with the clustered PD-NOMA scheme in an ultra-dense HetNet. Figure 10: Sum rate comparison with maximum number of associated users occupying the same resource block-comparative resource allocation between OFDMA-, power domain NOMA-, and clustered power domain NOMA-enabled schemes in an ultra-dense HetNet. Figure 9: System capacity vs. average SINR. compared to the unified PD-NOMA scheme, and by 80% and 65%, respectively, when compared to the conventional OFDMA scheme, with significant improvement in sum-rate achieved with clustered PD-NOMA implementation. In the future, different cooperative PD-NOMA implementations will be explored, and their impacts on load balancing and system capacity will be studied. In addition, based on the existing literature, there are still so many ways in which PD-NOMA could be implemented to improve the user association and spectral efficiency. Conceptualization, N.H.; formal analysis, N.H. and A.S.; resources, A.S.; supervision, S.R. ; writing--original draft, N.H.; writing--review & editing, N.H. and S.R. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors declare no conflict of interest.