# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.7 Reconfigurable Intelligent Surface (RIS)-Aided NOMA  
Intelligent reflecting surfaces are well accepted as possible physical 6G wireless platform support. With their capacity to alter the nature of interacting electromagnetic waves by carefully controlling the phase alterations of reflections, RISs have shown promise for increasing the SE of RIS-NOMA. RISs can be implemented to increase channel gains and ensure excellent network performance. The property of NOMA to provide greater user fairness and RIS properties of best channel gain can be theoretically utilised in combination to ensure that all devices receive the same data rate. Significant worries about the power consumption, energy efficiency, and the coverage area of future networks have been raised as a result of the rising demands for higher data rates and better wireless communications. NOMA networks supported by the RIS-related work is presented in this subsection. Some significant RIS-NOMA systems discussed in literature are given in Table 5.  
Throughput in RIS-NOMA:The optimization approach introduced in [130] is frequently used to alternatively establish the phase shifts of RIS and the transmit beamforming at the BSs because of the optimization problems non-convexity defined for improved throughput. The authors of [131] initially showed that phase shift changes are not required for uplink and downlink NOMA systems, which helps to solve the issue and improves the signalling overhead, when considering a wireless-powered RIS-NOMA system with several devices/users. In [132], beamforming at the BS and phase shifts at the RIS were optimised in addition to assigning the channel and the NOMA users decoding order to increase the system throughput. The RIS system's sum rate was investigated in [133] in order to evaluate the NOMA, FDMA, and TDMA approaches. Additionally, in RIS-NOMA systems ([134; 135]), the utilization of a BS with multiple antennas was taken into consideration. Plans for optimising the active and passive beamforming at both the BS and RIS were then discussed in order to increase the sum rate. RIS-NOMA in mmWave and massive MIMO communicationsMillimeterwave [136], massive MIMO [137], and other major 6G communication system technologies are all very compatible with RIS-NOMA. In order to maximise the system sum rate, [PERSON] et al. [136] considered the architecture of mmWave networks and presented an optimization approach in which the power distribution and reflection coefficients were alternatively tuned. The outcomes displayed in [136] demonstrate the value of RIS integration in mmWave-NOMA systems. Leveraging upon the fact that mmWave massive MIMO systems were created owing to the narrow wavelength of mmWave transmissions which allows for the deployment of many antennas, the unique approach has shown potential of mitigating the effects of imperfect SIC and take advantage of polarization diversity as a result. The use of RIS-NOMA with mmWave and mMIMO systems was taken into consideration by [PERSON] et al. [137], where the antenna power and leakage were concurrently explored. Regarding imperfect SIC in NOMA, the extent to which RISs can control wave polarization in dual-polarized MIMO-NOMA networks was explored in [138]. RIS-NOMA in THz Communications:Terahertz communications are discussed as a possible enabling influence for achieving ultra-high information rates and providing a sufficient transmission capacity for 6G wireless networks. In order to serve applications that call for accurate localization and environment sensing, future wireless communication networks will use higher frequencies [139]. Due to the restricted number of propagation channels available for such high frequencies (mostly because of high penetration losses, high path loss values, and low scattering), sensing and localization accuracy may suffer. An effective way to address this issue, particularly in NLOS communication circumstances, is by creating a smart radio environment utilising RIS [140].