# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.1 Cooperative Relaying Communication
The relay in [85] sends a superposition coded composite sign using NOMA after receiving the transmitted symbols in parallel from both sources with exclusive allocated powers. In comparison to CRS-NOMA-ND and CRS-NOMA, the benefit of NOMA-RS is that it has numerous sources. FD-NOMA-RSFD-NOMA-RS is composed a of cooperative relay-based FD NOMA, with two source-destination pairs sharing the same full-duplex relaying (FDR) [86]. With a processing delay \(\tau\), FDR demultiplexes these symbols and concurrently sends an overlaid composite signal to the end destinations via downlink NOMA. The scheme's performance was evaluated in terms of OP, ergodic sum capacity (ESC), and outage total capacity. They authors conducted an analytical analysis of their method by utilising both ideal and imperfect SIC circumstances to demonstrate its performance. Space-time block code (STBC)-NOMAA type of cooperative DF with a two-phase relaying method based on Alamouti STBC-NOMA [87] was considered. To investigate the system's performance, the authors of [87] used the separate Rayleigh fading channels, and they also characterised the asymptotic approximations for ESC, OP, and outage SC. When compared to the standard CRS, it achieved substantial performance gains utilising NOMA and typical DF relaying methods. Dynamic relay selection (DRS)-fixed power allocation (FPA) and DRS-dynamic power allocation (DPA)For cooperative NOMA, these DF dual-relay selection methods employ distributive space-time coding [88]. In this method, the system power is fixed under the DRS-FPA scheme. One relay demultiplexes all users signals, and the optimal relay is chosen using the max-min criterion. The DF dual-relay selection technique using dynamic power allocation was utilised in DRS-DPA, with DPA being applied to both hops instead of only the second hop. The effectiveness of both approaches was assessed using a closed-form formulation of OP. Dynamic decode-and-forward (DDF) NOMAIn this system, cooperative NOMA employs a DDF relay to improve anonymous users' reception dependability. In DDF-NOMA [89], the user closest to the BS uses partial reception to demultiplex the overlaid signal combination received by users and then delivers the signal to the user who is the furthest away. Random user pairing was utilised to eliminate the requirement of channel status information at the BS. Randomly employed algorithms, and algorithms based on distance user-pairing are two user-pairing algorithms that require feedback and user location information, respectively [89]. _Two-way relay (TWR)-NOMA:_ In TWR-NOMA systems, two distinct NOMA groups for communication used HD-DF to relay with each other [90]. The SINR of users was calculated using the effects of having perfect SIC and imperfect SIC. The authors also assessed the closed-form OP expressions in an asymptotic manner and an exact solution manner. The major goal of this system was to solve decoding order problems caused by perfect SIC. _Buffer-aided (BA)-relay:_ In BA-NOMA, transmission and reception of data packets through relays when the BS-R and R-D connections are disabled [91] boosts the system's throughput by increasing the dependability of the relaying systems. Furthermore, the authors of [92] presented a one-source, two-user, buffer-aided relaying approach in which information was sent to two users via a dedicated relay. _Multi-relay selection:_ An AF and multi-relay selection approach was used to investigate the performance of cooperative NOMA by the authors of [93]. In this approach, the BS interacts with users through a chosen relay and establishes direct connections with them. The results demonstrate that as the relays quantity increases, so does the performance gain; however a constant performance is achieved with more than two relays in the better SNR area. _Cooperative full-duplex relaying (CFR) NOMA:_ Here, an incomplete SIC FD relaying system is used by the users close to the BS and functioned as an FD relays to facilitate relay to further users. This system was discussed in [94].