# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.1 Cooperative Relaying Communication
In this work, the nearest NOMA user operated in full-duplex (FD) mode, while the operation of the furthest NOMA user was performed with a decode-and-forward relaying approach. Importantly, the reasonable hypotheses of imperfect SIC based on NOMA and FD-based loop self interference were taken into account. _Collaborative NOMA-aided relaying_: Collaborative NOMA-aided relaying (CNAR) was suggested in [78], and comprises of two NOMA connections referred to as relay-destination (R-D) and collaborative source-relay (S-R) connections. The relay message from the signal of S-R NOMA was extracted, and the power was modified to send the other portion to R-D-linked cell-edge users. The signals from the relay are sent in a specified frequency range in this method. The FD mode was utilised for the S-R and R-D connections to provide high throughput and support numerous users. _HD-cooperative NOMA (CNOMA)/FD-CNOMA_: To address the power allocation issue and to optimize the possible user rate, the authors of [79] developed a hybrid HD/FD method for CNOMA. In addition, for numerous users, a relay selection mechanism was developed. Furthermore, NOMA-HD and FD executed, and allocated optimal power. _FD-NOMA virtual pairing (VP)_: Using a relay, the nearest and furthest users with comparable gain are separated into different NOMA clusters. Over non-overlapping frequency bands, clustering was performed by utilising the VP technique between the nearest and several furthest users. The connectivity in BS and the farthest users is maintained through a specialised FD relay in FD-NOMA-VP, whereas the nearest user interacts to BS. In perfect interference cancellation, the FD-NOMA-VP system outperformed standard MA schemes by a factor of ten [80]. On the other hand, because of the rise in the influence of residual interference, performance suffers as interference cancellation is poor. _NOMA power-line communication (PLC)_: PLC is a NOMA design proposed in [81] with two stages of power allocation. The BS sends data with differing power factors in the first stage, while the superposition-coded signal is received by both the relay and the destination. The signal is decoded at the destination with a greater power level while considering the other signal to be noise. The best power signal is first decoded at the relay and then canceled with the SIC to obtain the second symbol. The relay transmits the second symbol to the target in the second step, provided that it has been correctly decoded. The following are two advantages of utilising the NOMA in a PLC: (i) reduction in power transmitted at PLC modems, which alleviates the problem of compatibility affiliated with PLCs, and (ii) improved user fairness. Cooperative relaying selection (CRS)-NOMAThe authors of [82] improved SE using multiplexed transmissions in the spatial method. The authors investigated the attainable rate for the best channel and found that CRS-NOMA performs better than the standard DF scheme. CRS-NOMA novel design (ND)This scheme was developed around a brand-new receiver design. In general, the source uses the superposition code to concurrently broadcast two signals, and the symbols are first decoded and then relayed with lesser allotted power using SIC [82]. However, in [83], signals either from an in-directed link or a directed link were decoded and then used the maximum-ratio combination and SIC were used to send the signal. N-best relay selection (BRS)This is an enhanced variant of CRS-NOMA in which many relays are utilised instead of just one [84]. The BRS scheme decides which relay can provide the best SNR or SINR at the destination to facilitate signal forwarding. The Rayleigh fading channel's average rate was used to deduce the scheme's performance. NOMA-RS/HD-NOMA-RSA relaying scheme with dual-hop capability is used in this design. A simultaneous interaction between sources was used in this approach to attain their goals.