# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.6 Clustered-Based MIMO-NOMA  
The clustered-based users and the proper beams for each cluster are designed using clustered-based MIMO-NOMA (CB-MIMO-NOMA). Through the use of an appropriate transmit detector and precoder, CB-MIMO-NOMA efficiently reduces inter-cluster interference. This method ensures that the beam connected with a certain cluster is orthogonal to other clusters users. The disparity between the users' channel conditions grows as a result of cluster isolation. The SIC method of NOMA may also be used to reduce intra-cluster interference. NOMA spatial modulation (SM):SM is an energy-efficient method in which the information bits are sent using an index of the transmit antennas and amplitude-phase modulation. In [126], the authors combined NOMA and SM in a downlink MU-MIMO situation. Inter-user interference may be minimised using this system by using the SIC. Furthermore, the authors of [126] presented an optimal power allocation scheme analysed on the basis of symbol rate errors to obtain high SE. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**NOMA Variant** & **Problem Discussed** & **Metric** & **Advantages** & **Open Issues** \\ \hline TAS-NOMA [127] & Multiple antennas at transmitter & SR & Reduces the cost, complexity and power & Imperfect CSI \\ \hline NOMA-SSK [115] & Low SE of cell-edge users & EE BER SE & Reduces decoding complexity & Power allocation scheme \\ \hline NOMA-GSSK [116] & Low SE of cell-edge users & EE BER SE & Reduces computa-tional complexity & SIC analysis \\ \hline PD-NOMA-SSK [117] & Secerecy & BER SE & Improves network throughput & Power allocation problem \\ \hline NOMA-HARQ [118] & Inaccurate MCS selection & SE & & Extension of MU-MIMO \\ \hline NOMA-BF [120] & Inter-cluster, inter-user interference & SR & Improves QoS & Imperfect CSI \\ \hline Random-BF-NOMA [121] & Inter-cluster, inter-beam interference & Th & Reduces CSI feedback & Analysis of imperfect SIC \\ \hline ZFBF-NOMA [122] & Inter-cluster interference & SE Th & Maximises overall throughput & Multicell scenario \\ \hline ROBUST-BF-NOMA [123] & Inter-beam, inter-cluster interference & SR & Maximises worst case sum rate & Extension of MU-MIMO \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of NOMA based on MIMO. _Signal alignment (SA)-NOMA:_ In this technique, interference between clusters is removed in the MIMO-NOMA system. The authors of [128] used this approach to investigate the OP of downlink and uplink MIMO-NOMA broadcasts in a single-cell setting. Different QoS criteria were met using the fixed power allocation (FPA) approach, but the CR-inspired power allocation technique ensures the higher QoS requirements of users. \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**NOMA Variant** & **Problem Discussed** & **Metric** & **Advantages** & **Open Issues** \\ \hline C-BF-NOMA [124] & Inter-cell, inter-cluster interference & Th & Increases throughput of cell-edge users & Imperfect SIC \\ \hline NOMA-MRT [125] & Sum rate & SR & Maximises weighted sum rate & Extension of mMIMO \\ \hline NOMA-SM [126] & Inter-user interference & SE EE & Enhanced SE & Imperfect CSI \\ \hline SA-NOMA [128] & Inter-cluster interference & OP & Provides large diversity gain & Imperfect CSI and SIC \\ \hline PH-NOMA [129] & Inter and intra-cluster interference & SR OP & Minimises total power consumption & MIMO-NOMA scenario \\ \hline H-NOMA [73] & Transmission power & OP & Reduces the transmission power & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._  
Figure 11: Types of hybrid NOMA in MIMO communication systems. Hybrid (H)-NOMA:In comparison to ZFBF, H-NOMA is a low-complexity precoding algorithm. It depends on the concept of the quasi-degradation idea and may be used in real-world networks. A MISO-NOMA system with two users was investigated in [73] under the sequential user pairing technique.