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| Ref. | Objective | Solution approach | Category | Tech. |
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| [26] | Improve reliable detection, maximum diversity gain, and reduce system complexity. | The highest diversity gain with minimum outage probability achieved by cooperative PD-NOMA. User pairing is used as a promising solution to reduce system complexity. | Single carrier power domain | Co-PD-NOMA |
| [27] | Achieve the fairness performance of the NOMA scheme better than TDMA under perfect and average CSI. | Investigated power allocation techniques that ensure fairness by formulating the research problems as nonconvex optimization. | Single carrier power domain | PD-NOMA |
| [28] | Further improve the outage performance of MIMO-NOMA. | Improvement achieved by implementing detection and precoding matrices for MIMO-NOMA. | Single carrier power domain | MIMO-NOMA |
| [29] | Resource allocation algorithm design for multicarrier NOMA systems. Multiple half-duplex uplink and downlink users simultaneously served by a full-duplex base station. | An algorithm is designed for multiple half-duplex uplink and downlink users simultaneously served by a full-duplex base station. Used weighted sum system throughput maximization from the solution of a nonconvex optimization problem. | Multicarrier power domain | MC-NOMA |
| [30] | For the downlink NOMA system, optimized power allocation and subchannel assignment to increase energy efficiency. | For subchannel multiplexed users, a low-complexity suboptimal algorithm is presented, which comprises power proportional factor determination and energy-efficient subchannel assignment. | Single carrier power domain | PD-NOMA |
| [31] | Improve the link-level performance of SCMA in highly overloaded scenarios. | Proposed an iterative multiuser SCMA receiver by employing channel coding which uses the coding gain and diversity gain. | Multicarrier code domain | SCMA |
| [32] | Maximize the mutual information in sparse code multiple access (SCMA). | Maximize the mutual information between continuous output and discrete input using an iterative codebook optimization algorithm. | Multicarrier code domain | SCMA |
| [33] | Substantially minimize the hurdles of the message passing algorithm (MPA) scheme. | For uplink SCMA systems, a shuffled-message passing algorithm (S-MPA) scheme is proposed, based on a serial message update strategy. | Multicarrier code domain | S-MPA |
| [34] | Reduce the decoding hurdles of the current message passing algorithm. | Based on list sphere decoding (LSD), a low-complexity decoding algorithm is proposed. The LSD only works with signals inside a hypersphere by evading the extensive search for all possible hypotheses. | Multicarrier code domain | LDS |
| [35] | Minimizing the hurdles of the SCMA decoding. | Proposed a Monte Carlo Markov Chain- (MCMC-) based SCMA decoder. Benefiting from the linearly increasing complexity of the MCMC method. | Multicarrier code domain | MCMC |
| [36] | Maximize the sum rate subject to QoS and system-level constraints like power constraints. | Multiple users utilized the same SCMA codebook, and for user signal nonorthogonality, the PD-NOMA scheme is utilized. | Power & code domain | PD-SCMA |
| [37] | For random signature selection, allowed grant-free transmission to achieve high overloading. | Introduced a blind multiple user detection for MUSA systems by using a special blind detection algorithm. | Single carrier code domain | MUSA |
| [38] | For the paired users, optimized the modulated symbol mapping. | Performance of MUSA with SIC has been considered by using mirror constellation bit error ratio (BER). | Single carrier code domain | MUSA |
| [39] | A family of short length complex sequences is selected to permit an easy multiuser interference cancellation. | Successive/parallel interference cancellation with minimum mean square error (MMSE-SIC/PIC) has been investigated for appropriate MUSA receivers. | Single carrier code domain | MMSE-SIC/PIC |
| [40] | Increase user overloading and minimize multiuser interference. | Enlarge the pool of the spreading sequences by using nonorthogonal dense spreading sequence to increase user overloading and reduce multiuser interference. | Single carrier code domain | MUSA |
| [41] | To further enlarge the coverage area and improve transmission reliability. | With forward relay and half-duplex decode, an uplink cooperative PDMA (co-PDMA) scheme is suggested. | Single carrier code domain | Co-PDMA |
| [42] | Increase the performance of PDMA uplink system by using diversity gains and coding potentials. | By using diversity gains and coding potentials, an iterative detection and decoding (IDD) algorithm is developed for an advanced PDMA receiver. | Single carrier code domain | IDD |
| [43] | Using the cyclic redundancy check (CRC) to avoid the error propagation. | Based on the MMSE channel decoding and detection, a novel iterative decoding and detection algorithm is proposed, called the SIC iterative processing algorithm. | Single carrier code domain | SIC-MMSE |
| [44] | Proposed the power allocation and pattern assignment in the downlink PDMA system. | To optimize the overall throughput of total users based on the optimum Iterative Water-Filling (IWF) algorithm, a joint pattern assignment and power allocation (JPPA) scheme is offered. | Single carrier code domain | JPPA & IWF |
| [45] | Improve security by changing the signal’s identity. | Physical layer security system is suggested based on constellation scrambling (CS) and multiple parameter weighted fractional Fourier transform (MP-WFRFT). | Single carrier code domain | MP-WFRFT |
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