# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Power Domain NOMA
#### 3.2.4 Antenna Selection-Based MIMO-NOMA:  
In this strategy, antenna selection helps to maintain a variety of a MIMO systems [112] in terms of capital and operational expenditure, complexity, and power consumption using numerous antennas at the same time. Researchers [112] have utilised the MIMO method in OMA systems, but owing to the inter-user interference, they were unable to obtain a significant gain when compared to MIMO-NOMA. As a result, the following alternatives are recommended to address this issue.  
_TAS-NOMA:_ The performance in terms of the sum-rate of a downlink MISO-NOMA system is explored in [113] using transmit antenna selection (TAS) at the BS, where the BS transmitter and each mobile user's receiver have multiple antennae and a single antenna, respectively. Essentially, the finest antenna at the BS with the better SINR is chosen in the TAS-OMA method. A similar antenna at the BS that provides the largest sum rate is chosen in the TAS-NOMA system in [113]. In addition to applying an efficient TAS technique, in a two-user huge MIMO-NOMA system, a user-selecting technique is used to maximise the possible sum rate in [114] for two users with a single band and multiple users with multiple band cases.  
_NOMA-space shift keying (SSK):_ This is a method that combines NOMA with SSK to elevate the SE of a cell-edge user. In contrast to typical modulation techniques, it is an approach that sends information using the antenna index [115]. SSK has several advantages, the most important of which is that it decreases transmitter overhead and receiver complexity. As a result, the employment of SSK in conjunction with NOMA is employed to elevate the SE of spatial modulation networks. This represents a promising technique for future wireless networks.  
NOMA-generalised (G)SSKThis method is used to resolve the limitation of SSK in the transmit numbers of antennas. The authors of [116] proposed that NOMA-GSSK to elevates the SE of cell-edge users. Unlike NOMA-SSK, this concept has multiple transmit antennas. In contrast to MIMO-NOMA and NOMA-SSK, this system multiplexes the users in both the spatial and power domains, providing better EE and SE, and a low BER.  
PD-NOMA-SSKBecause of the varying level of power that users require for operation in the PD-NOMA system, the amount of resources available for group communication is restricted. As a result, cryptographic keys are inefficiently dispersed between users. After evaluating this issue, the authors of [117] combined the SSK modulation method with PD-NOMA to meet the requirement for high SE. The authors further used MU-MIMO methods to multiplex PD-NOMA-SSK to improve system throughput.  
NOMA-hybrid automatic repeat request (HARQ)In [118], the authors investigated the single-user MIMO (SU-MIMO) system along with an HARQ design. This technique was suggested by the authors in order to eliminate inter-stream and inter-user interference. The effects of precoding matrix adaptation, user pairing, and transmission power assignment (TPA) ratio adaptation on retransmission in MIMO-NOMA systems was also investigated. The study demonstrated that NOMA has a considerably greater HARQ probability than OFDMA, and that variable retransmission techniques could improve the performance.