## 4 Results and Discussion
### Research Challenges and Future Trends
Hence, the NOMA in mm-wave communications proposes developing 5G technologies with high data rate wireless access [149; 150; 151; 152; 153]. 5. Visible light communication (VLC)-based NOMA: Independent of mmWave communications, Visible Light Communication (VLC) has attracted great attention from the standpoint of excellent spectrum resources [154; 155; 156; 157]. The VLC supports several characteristics, including a high data rate, high security, eye safety, license-free broad bandwidth, absence of electromagnetic interference, and low energy usage [158; 159; 160]. In addition, the VLC employs random data signals to power light-emitting diodes (LEDs) rather than the incandescent and fluorescent lights used by traditional light senders. Thus, the VLC is an excellent strategy for 5G networks and beyond. Moreover, the NOMA system may improve the performance of the VLC system. Hence, further study must be conducted on the NOMA VLC system to make VLC-based NOMA advantageous in many situations and places [161; 162]. 6. Mobile edge computing (MEC) based on NOMA is one of the 5G technology's enabling technologies [163; 164; 165]. MEC technology allows user equipment (UEs) to conduct compute-intensive tasks by creating computing capabilities at the network edge and inside wireless access networks. In addition, NOMA systems allow several UEs to share the same resources [166; 167; 168]. Consequently, combining the NOMA system with MEC technology has several advantages, including decreased energy usage, enhanced EE and SE, and a rise in corporate UEs. To solve big data processing in future 5G wireless networks, it is necessary to recommend a NOMA-based MEC in multiple studies [169; 170; 171]. 7. Practical realization of more than two-user pairing: The primary characteristic of the NOMA system is that it serves multiple users with the same resource. Nevertheless, most practical studies use a two-user pairing technique to facilitate the channel gains of two-paired users at the base station and implement effective SIC at receivers. As the need for connected devices such as IoT, machine-to-machine (M2M), and massive machine-type communications increases, new ways for multi-user pairing are also essential. These methods for pairing multiple users must take full use of NOMA. Hence, multi-user pairing strategies enhance the mass connection necessary for the next wireless network. 8. Hybrid NOMA: Integrating hundreds of millions of gadgets via an underlying BS gets challenging due to restricted orthogonal substances with exceptionally reduced-latency mobile communication in situations of minimal SNR. Therefore, the NOMA strategy is inappropriate for this circumstance [171; 172; 173; 174; 175]. To overcome this issue, an integrated MA that relies on a combination of NOMA and OMA approaches offers an improved approach to linking massive equipment in IoT settings, D2D and M2M connections, etc. For MA, a combination of NOMA uses both PD-NOMA and CD-NOMA. Additionally, the combined NOMA provides a higher SE than both NOMA and OMA. As a result, the hybrid NOMA can switch between NOMA and OMA modes of operation and can be utilized in various experiments [176; 177]. * 9. Internet of Things and MIMO-NOMA: IoT, which represents the interconnection of all things, constitutes one of the main components of 6G connectivity. For cellphone IoT, the amalgamation of immense MIMO and NOMA is employed to simplify the integration of many IoT gadgets with a small radio bandwidth [178; 179; 180; 181]. Immense MIMO-NOMA provides excellent transmission effectiveness and system adaptability to support many gadgets with a simple system architecture, which is beneficial for IoT technologies employing affordable, minimal-power, and minimal-complexity gadgets. To show the effects of MIMO-NOMA on IoT demand fulfilment, novel techniques with an optimal multiple-purpose architecture are essential [182; 183; 184].