###### Abstract  
In the current era of exponentially growing demand for user connectivity, spectral efficiency (SE), and high throughput, the performance goals have become even more challenging in ultra-dense 5G networks. The conventional orthogonal frequency division multiple access (OFDMA) tech-niques are mature but have not proven sufficient to address the growing user demand for high data rates and increased capacity. Therefore, to achieve an improved throughput in an ultra-dense 5G network with an expanded network capacity, the unified non-orthogonal multiple access (NOMA) technique is considered to be a more promising and effective solution. Throughput can be improved by implementing PD-NOMA, as the interference is managed with the successive inter-ference cancellation (SIC) technique, but the issue of increased complexity and capacity with compromised data rate persists. This study implements the clustered PD-NOMA algorithm to enhance user association and network performance by managing the users in clusters with fewer users per cluster with the implementation of the cooperative PD-NOMA within the clusters. In this study, we enhanced the user association in a network and ultimately improved the throughput, sum rate, and system capacity in an ultra-dense heterogeneous network (HetNet). By imple-menting the proposed clustered PD-NOMA scheme, the system throughput has improved by 23% when compared to the unified PD-NOMA scheme and 65% when compared to the OFDMA scheme with a varied number of randomly deployed users, along with an improvement in system capacity of 8% as compared to the unified PD-NOMA and almost 80% as compared to the conventional OFDMA scheme in a randomly deployed ultra-dense multi-tier heterogeneous network. Thus, we improved the network performance with the proposed algorithm and achieved increased capacity, throughput, and sum rate by outperforming the unified PD-NOMA scheme in an ultra-dense heterogeneous network. heterogeneous network; power domain NOMA; orthogonal multiple access; high power small cell base stations; low power small cell base stations; capacity; sum rate; successive interference cancellation +
Footnote †: journal: Applied Sciences  
shown in [2], the author has improved user association and data rate with increased density but with a highly complex solution using the cluster-based OFDMA technique. Moreover, in OFDMA, each sub-channel can serve only one user at a definite time slot, frequency band, or code to avoid interference and enable single-user detection under a simple receiver scheme. Ultimately, it is not expected to provide an adequate spectrum efficiency. Therefore, a less complex mechanism is required with which efficient spectrum use can be achieved along with improved fairness, high data rate, increased capacity, and enhanced user connectivity. This could be achieved by simultaneously serving multiple receivers with various channel conditions as with the non-orthogonal multiple access (NOMA) technique. Researchers have widely used NOMA schemes because of their competence in increasing the system capacity and accommodating the network requirement of increased spectral efficiency (SE) and throughput with low complexity measures. NOMA is implemented as either a power domain (PD-NOMA) scheme or a code domain (CD-NOMA) scheme; in this paper, we considered PD-NOMA. In PD-NOMA, different users are assigned different power coefficients under the respective channel conditions to attain improved system performance. This scheme has a simple implementation, does not require considerable changes to the existing network architecture, and does not require additional bandwidth to improve the SE [4].