# Cooperative Power-Domain NOMA Systems: An Overview
## 3 Types of NOMA
### Code Domain NOMA  
_Low-density spreading code-division multiple access (LDS-CDMA)_: LDS-CDMA is based on the original CDMA principle which is meant to reduce the interference in traditional CDMA systems by utilising LDS instead of traditional spreading sequences. LDS-CDMA's essential idea has been described in [60; 61]. Furthermore, Ref. [60; 62] discussed an iterative multi-user detection (MUD) based an MPA that is less complicated than the optimum detector. The authors in [63] proposed a systematic method for developing LDS codes for LDS-CDMA, with the fundamental concept of mapping a spreading matrix containing the spreading sequences of constellation signature. In addition, an information theoretical analysis was used to compute the region of capacity in [61].  
_Low-density spreading-aided OFDM (LDS-OFDM)_: OFDM and multi-carrier (MC)-CDMA have many similarities, when it comes to spreading in frequency-domain, which distributesusers' symbols between all OFDM subcarriers. If a chip's quantity of codes is equal to the subcarriers' quantity. Then, across all subcarriers, numerous mobile equipment may be accommodated by overlaying the distinct, spreading sequences for the specific users on top of one another. The authors of [24] presented the LDS-OFDM system's concept and characteristics, including its frequency variation order, receiver architecture, and capacity to function under rank-deficient situations with more users than chips. In order to manage receiver complexity, the authors of [64] set a limitation on the numbers of users for each subcarrier. In addition, in [65; 66], LDS-OFDM was compared to both OFDMA and single-carrier (SC)-FDMA in order to evaluate the peak-to-average-power ratio (PAPR), with system and link-level performance.  
Sparse code multiple access (SCMA):Another significant NOMA strategy is the recently suggested SCMA technology, which is based on code-domain multiplexing derived from the fundamental LDS-CDMA scheme. SCMA was thoroughly explored through the multiplexing features, transmission features, factor graph representation, and message-passing algorithm-based receiver design. The message-passing method, which is similar to LDS-CDMA, may also be utilised by the receiver of SCMA for MUD. The receiver complexity, on the other hand, may become extreme. Improved variations of the message forwarding method have been developed in [67; 68] to overcome this issue.The authors of [67] presented an alternate search method that depends on traditional signal entropy theory for removing all redundant conditional channel probability calculations while maintaining decoding speed. In contrast, Ref. [69] proposed a simple combination of repeated decoder for achieving a convincing balance between complexity and performance. On the other hand, the strong turbo-principle has been introduced in [68] for enabling communication between the decoder of the channel and SCMA detector in order to increase the BER performance of SCMA.  
Pattern-division multiple access (PDMA):In this technique, multiplexing may be performed in the code, spatial, and power domains [70]. Non-orthogonal patterns are used at the transmitter to minimise user overlaps. The spread of multiple users is defined by the pattern matrix of the subcarrier. MPA can be modified at the receiver to detect the distributed information, and MPA-SIC is used by multiplexed users in both the space and power domains.  
Interleave-division multiple-access (IDMA):Inter-leavers differentiate distinct users in IDMA [71]. The interleaver requires additional bandwidth and memory resources at the receiver and the transmitter. To achieve better performance, an elementary signal estimator is utilised at the receiver. Multiplexing is performed in the power domain in PD-NOMA. At the transmitter, signals from various users are superimposed by assigning the user's power with identical subcarriers, and the succeeding signal is transmitted.  
The kinds of NOMA described above are summarized in Table 2.  
Figure 6: Multiple access techniques: types of NOMA and OMA.