# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.2 NOMA with SWIPT Protocol  
Besides enhancing SE, energy efficiency (EE) is another significant issue in the 5G wireless networks that must be addressed. The majority of communication devices are supplied with batteries that have a limited lifetime. As a result, energy harvesting (EH) is a better approach for extending the lifetime of these energy-constrained systems as shown in Figure 10. EH methods initially gather energy from sunlight and wind as a renewable resources, but they are unreliable, because they are dependent on the environment. On the other hand, SWIPT is a common EH approach where energy is harvested in the decoding process. It collects energy even from interfering signals. When stronger users serve as relays for weaker users, their batteries drain quickly, according to C-NOMA. As a result, both strong and weak users' signal strength is deteriorated. This challenge inspired academic and industrial researchers to combine SWIPT and C-NOMA to improve network EE in the following way. _Cooperative (C)-SWIPT-NOMA:_ NOMA utilising the SWIPT was originally investigated in [96], in which users were located randomly. In this approach, strong users collect energy from the BS's RF signals during relaying and then forward the weaker users' data. Figure 10: Time-switching (TS) power-splitting (PS) SWIPT Protocol in NOMA system. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**NOMA Variant** & **Problem Discussed** & **Performance Metric** & **Advantages** & **Open Issues** \\ \hline CRS-NOMA [82] & Spatially multiplexing & Average rate & Improves average power & SIC stability Channel estimation with limited feedback MIMO systems \\ \hline CRS-NOMA-ND [83] & Receiver design & Ergodic capacity outage probability & Better ergodic sum rate, better MRC scheme results & Incremental redundancy \\ \hline N-BRS [84] & Rate gain & Average rate & Reduce complexity & Best relay position \\ \hline NOMA-RS [85] & SIC & Sum capacity & Serves a large number of users & Evaluates outage \\ \hline FD-NOMA-RS [86] & Spectral efficiency & Ergodic capacity outage probability & Serves a large number of users & With MIMO-NOMA \\ \hline STBC-NOMA [87] & Reliability & Sum capacity & Boosts the spectral efficiency & Multiple antennas at receiver \\ \hline DRS-FPA-NOMA [88] & Diversity gain & outage probability & Better reception reliability & Nakagami-m fading channel \\ \hline DDF-NOMA [89] & Reception reliability & Outage probability & Better reception reliability & Has limited CSI knowledge \\ \hline CFR-NOMA [94] & Imperfect SIC & Ergodic capacity outage probability & Better fairness, maximises achievable rate & Channel allocation scheme \\ \hline NOMA-RBC [95] & Full duplex relaying & Achievable throughput & Improves weak user rate & Extended to MU-MIMO \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._The authors of [97] built a C-NOMA transceiver with several antennas for one user and a single-antenna BS for the remaining users. At the transmitter side, a zero-forcing (ZF) beamforming architecture was used to analyse the C-SWIPT-NOMA performance with a BS and two users in [98]. Users that are close to the BS function as relays and have numerous antennas, whereas terminal users with a single antenna are farther away from the BS. _C-SWIPT-MISO/SISO-NOMA:_ In [99], the authors utilised the SWIPT-based MISO-NOMA system to implement the transmit antenna selection (TAS) scheme. Criterion-I and criterion-II were used to split the TAS scheme into two categories. They chose an antenna that delivers a channel with better fading conditions between the source and the far user and near user for each of these parameters. respectively. In contrast to [99], the authors of [100] used MISO and SISO methods to execute the C-SWIPT-NOMA transmission scheme. First and foremost, at the EH relays, a power splitting approach was utilized. The relays used this approach to forward information by solely collecting RF signals' energy and limiting their battery usage.