# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.1 Cooperative Relaying Communication  
To achieve better signal strength between the BS and destination, cooperative communication employs one or more relays. It employs two time frames: direct phase transmission is used in the first frame and forwarding information from relays are relayed to ultimate destinations in the second frame, as illustrated in Figure 8. It has numerous benefits, listed as: (i) it expands the coverage area, (ii) it reduces the multi-path fading effect, (iii) it increases system capacity, (iv) it eliminates the difficulty of antenna terminals mounting process, and (v) QoS is enhanced for cell edge users. Relays employ DF and/or AF protocols to send data from one end (transmitter) to the other end (receiver) in this technique. These are categorised as either half-duplex (HD) or full-duplex (FD) systems depending on the relaying process. Researchers have utilised NOMA with cooperative communication to enhance the networks' spectral efficiency. Reduced redundancy in systems, fairness, and increased weak users diversity gain are all advantages of this integration. Moreover, many cooperative NOMA versions are explored next, based on the aforementioned benefits. _Cooperative relaying NOMA (CR-NOMA):_ Recent work has investigated the diversity gain in order to make use of the NOMA technique's existing knowledge [72]. In this approach, a user with good channel conditions decodes other users' messages and works as a relay to facilitate the weak users (with bad channel conditions) for better reliability of reception. It increases the gain in diversity for all users. Nonetheless, it has been found that during the cooperative phase, when data are transferred using relay towards weak users, extra time slots are required to meet the objective because relaying operations are carried out serially. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**NOMA** & **Domain Multiplexing** & **Received Type** & **Advantage** & **Open Issues** \\ \hline LDS [60; 61; 62; 63; 64; 65; 66] & Spreading codes & MPA & Without CSI & redundancy in coding \\  & & & & issue \\  & & & &  
NOMA, with its cooperative ability, is being explored in many fields. Even though it has been discovered that the geometric-based stochastic channel model (GBSM) offered better, more useful, and more realistic channel properties, the analysis of CR-NOMA with a mMIMO system is dependent on channel models, which are theoretically explained in a manner similar to the correlated-based stochastic channel model (CBSM). Research on GBSM channel models with the effective properties of CR-NOMA's large antenna arrays and coding techniques has received little attention. As a result, it is crucial to investigate mMIMO CR-NOMA that considers channel features such as path loss and tilt angle. Further research into the coexistence of big antenna transmitters and coding methods is required [74]. In [75], the authors considered a shared DF relay for a cooperative relay sharing (CRS) network based on NOMA, which allowed two sources to connect with their associated users at the same time and frequency. Using a dynamic decoding order technique based on the max-min criteria, a new transmission idea was presented to reduce network outage probability at the expense of less complexity and overhead. The work also characterised outage probability expression for the proposed scheme. In [76], the effects of generic fading channel on cooperative NOMA networks with residual transceiver hardware impairments (RTHIs) was investigated. Imperfect CSI and SIC were considered. More specifically, the non-cooperative and cooperative scenarios were presented as two representative NOMA scenarios. For cooperative NOMA, it was shown that cooperative NOMA performed better than the non-cooperative NOMA in high SNR scenario as demonstrated by the authors findings in [76]. In [77], the effectiveness of a terrestrial satellite network consisting of a satellite as a source and its numerous terrestrial primary receivers and paired users placed on the ground, was examined.