## 4 Results and Discussion
### Research Challenges and Future Trends  
Although the NOMA system is generally recognized as one of the most important multiple access strategies for 5G and beyond, many unresolved difficulties must be investigated to advance the NOMA system's implementation. As a consequence, this part examines prospects for resolving NOMA's research challenges and provides a short overview of some of NOMA's research concerns. 1. NOMA with faulty CSI: The present analysis of the NOMA system assumes a perfect CSI to execute multi-user SIC at the user receiver or resource distribution at the base station (BS) [124; 125; 126; 127]. In addition, this assumption cannot be implemented practically inside the NOMA system. Thus, real-time channel estimate errors occur in NOMA systems. Therefore, the NOMA system's theoretical analysis should address channel estimation errors and incorrect CSI. Hence, improved approaches and algorithms are necessary for actual NOMA systems to get an optimum channel estimate [128; 129; 130; 131]. 2. Imperfect SIC implementation: Due to incorrect extraction and discovery, there is an increase in time and residual intrusion of previously recognized users during actual SIC operations [131; 132; 133; 134; 135]. This results in error propagation inside the SIC receiver (i.e., an imperfect SIC receiver) and decreases the performance of the NOMA system. So, for a successful SIC implementation, it is necessary to construct hardware devices that minimize processing complexity, reduce latency time, and improve NOMA performance. Hence, effective methods for performing the SIC process are developed in the uplink and downlink NOMA systems [136; 137; 138; 139]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Unresolved Issues** & **Future Directions** \\ \hline We are optimizing resource allocation and offloading using NOMA and MEC & NOMA-based MEC founded smart city \\ \hline Pricing Tactics and the Effects of user & Trust, privacy, and Openness with Blockchain Technology \\ \hline Increasing capacity with multipath networking and multi-layer heterogeneous architecture & Mobility solution for smart agriculture \\ \hline Content delivery with NOMA-based MEC & NOMA-based MEC: Aiding and using technology in Industry 6.0 \\ \hline Regulatory policies and Standardization & Mobility solution for smart vehicular network \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of research issues and future research. 3. A heterogeneous network (HetNet) is a wireless network that uses several network topologies and operating systems [140]. Also, it has nodes with varied transmission capabilities and coverage regions. By incorporating tiny cells within the coverage of macrocells, the HetNet may reduce the energy consumption of future wireless networks and improve user QoS [31; 32; 33; 34]. Real-time NOMA allows networks of different sorts to exchange resources. In cooperative communication, NOMA-based HetNet improves downlink coverage substantially [36; 37; 38; 39; 40]. In HetNets, it is recommended to employ effective heterogeneous suitable communication techniques using NOMA to reduce interference issues and accomplish spectrum splitting since there is mutual interference between numerous users and a limited spectrum resource. In light of this, several studies [41; 42; 43; 44; 45] must build an effective NOMA-based HetNet. 4. NOMA in millimetre wave (mm-Wave) communication: Millimeter-wave (mm-wave) communications and the NOMA technology are essential for meeting the high data rate requirements of 5G and beyond [141; 142; 143; 144]. NOMA approaches become increasingly interesting at mmWave frequencies since the mmWave band is fundamentally suitable for multi-Gbps 5G wireless technologies. In addition, NOMA approaches may boost data transfer speeds while supporting multiple user connections [145; 146; 147; 148] Despite its promise, the use of NOMA in mmWave communications is still in its infancy and applicable in various circumstances, including IoT and cloud-assisted vehicle systems.