# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.3 Hybrid NOMA  
To improve the SE of sensors in IoT, PD-NOMA multiplexing, which is based on the hybrid TDMA method, was developed in [105]. By characterising the throughput and OP, the authors assessed the BackCom-assisted IoT network's performance. Several types of Hybrid NOMA are classified in Figure 11.  
_MISO and MIMO NOMA:_ Currently, MISO-NOMA and MIMO-NOMA networks are widely used in 5G networks and have sparked a surge in academic and industrial interest. Systematic research on the development of NOMA in 5G networks began in 2014 [105; 106]. Those efforts were focused on SISO systems to improve user fairness in the early stages of research. The flexibility of NOMA in serving numerous users utilising the same frequency-time resources is the reason for its adoption in SISO. Using NOMA in MISO and MIMO systems, on the other hand, improves spectrum reuse efficiency by adding diversity in the power domain. Signals from multiple users are mapped depending on the specified power value and then broadcast using the corresponding channel. Using the SIC technique, each user can identify the desired signals. MIMO-NOMA, in contrast to NOMA is a single-antenna system that uses high-dimensional power and beamforming [34] that introduces new features to power coefficient computation. The benefits of MIMO-NOMA can be summarised as follow:  
* _Spectrum efficiency:_ MIMO-NOMA is able to minimise power consumption by utilising the power domain for user multiplexing. The SIC condition ensures that the received interference can meet the data rate requirement following the signal decoding. As power consumption decreases, more users may be supplied concurrently, improving spectrum efficiency. * _Enhanced user cooperation:_ This scheme can preserve service quality and fairness by adjusting power allocation between different types of users. By allocating more power to weak users, MIMO-NOMA may increase cell-edge quality and hence improve the cell-edge user effectiveness. * _Many wireless transmission scenarios:_ As the MIMO-NOMA scheme can easily be integrated with different modern MIMO technologies, such as cooperative multi-point (CoMP) technology [107] and cloud radio networks [108], MIMO-NOMA is often implemented as a cooperative method either inside a single base station (BS) or across numerous BSs. Interference in MIMO systems lowers the network SE and QoS of the cell-edge user. Hence, NOMA is combined with MIMO, wherein the user must be careful with the allocation of the power in NOMA. A MIMO-NOMA comparison is provided in Table 4. As a result, in the MIMO-NOMA system, effective antenna selection, beamforming, and clustered based NOMA approaches are necessary. _Back-scatter-aided NOMA:_ Ambient back-scatter communication (ABC) and NOMA have recently been combined, and this latest combination has showed significant promise in connecting large-scale IoT in future unmanned aerial vehicle (UAV) networks. ABC's main goal is to eliminate the need for batteries by harnessing the power of WiFi, TV towers, cellular base station, and UAV RF signals that are already in use. To modulate and reflect data between wireless devices, ABC uses smart sensor tags. On the other hand, NOMA enables simultaneous communication between multiple IoT devices [109]. The underlying concept of ABC is the potential for wireless device communication using currently existing ambient radio frequency waves. With better spectrum efficiency and extensive connectivity, NOMA has recently been studied extensively. In order to reduce the overall transmission power of ABC-NOMA cooperative vehicle-to-everything networks (V2 XneT) while maintaining service quality, a new optimization approach was proposed in [110,111]. To be more precise, initially, the BS sends an overlaid signal to connected roadside units (RSUs). The RSUs then use the decode-and-forward protocol to deliver an overlaid signal to serving vehicles.