### Prior Work  
In [8], the authors implemented the OFDMA scheme and proposed a joint user association and power allocation algorithm to obtain an energy-efficient HetNet environment. The authors achieved an energy-efficient solution with compromised performance and smaller power constraints for the sub-channel allocations. In OFDMA, a frequency resource is allocated to each user, even to a receiver with relatively low SE and throughput. In addition, with conventional methods that introduce scheduling, a receiver with a fair channel condition has a higher chance of being considered than one with a diminished channel condition. This leads to fairness problems with increased latency. PD-NOMA assigns the same frequency resource to multiple users to improve the SE. In addition, multiuser detection (MUD) with SIC is implemented at the user end to manage interference. However, in [9] the researchers presented a resource-allocation scheme under distributed clustering with OFDMA and PD-NOMA schemes. The authors have worked on various cluster sizes and analyzed with numerical and simulation results that large clusters can provide better performance in terms of better spectral efficiency. In addition, considering user association, they discussed the importance of both schemes in achieving improved gains for both uplinkand downlink communication, though more work could also be performed to show the overall improvement in the system capacity and throughput with the proposed cluster formations and user association under the PD-NOMA scheme. PD-NOMA has gained considerable research interest in HetNets and has been implemented as an efficient resource allocation strategy compared to the conventional OFDMA technique. As in [10], a distributed resource allocation is presented for a self-backhauled small-cell network in full-duplex mode. The authors of this paper contributed to joint user scheduling, mode selection, and power allocation in a two-tier network, intending to maximize the network sum rate and the quality of service (QoS) and capacity constraints by implementing PD-NOMA over the small-cell network in the full-duplex mode. However, the authors did not study increased capacity with ultra-dense random user deployments. In [11], PD-NOMA is utilized for user association and resource allocation in a dense heterogeneous network and improved connectivity in 5G systems. This work is presented by considering various case studies to demonstrate the effectiveness of PD-NOMA in ultra-dense networks. It emphasizes that the development of techniques that offer robust connectivity of massively deployed devices to improve energy efficiency with managed user deployments through increased network capacity with various distant user allocations has not been taken under consideration. In [12], PD-NOMA was implemented with beamforming to gain energy-efficient resource allocation and user association. The network is considered to be an ultra-dense user-centric heterogeneous network. In addition, in [13], the authors deemed multi-access edge computing and worked on offloading techniques to improve the performance in heterogeneous networks by reducing delays and achieving user association and resource sharing through a game-based algorithm. Moreover, in [14], the complete details of the PD-NOMA scheme and its practical implementation are discussed in detail. In [14], the analysis and discussion of PD-NOMA were mainly performed as an efficient future radio multi-access technique. In [15], the author conducted a comprehensive survey on PD-NOMA and analyzed it as a proficient scheme to enhance the network capacity with efficiency and ultimately improve the network performance along with a few future research directions in the said domain by combining PD-NOMA with other schemes. Moreover, in [5], [PERSON] et al. utilized PD-NOMA for user association and sub-channel assignment to accomplish an effective radio resource management strategy.