# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Non Orthogonal Multiple Access
#### 3.4.5 NOMA--Future Challenges and Research Issues
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Author \& Ref. No.** & **Approach** & **Advantages** & **Limitations** \\ \hline \multirow{5}{*}{[PERSON] et al. [163]} & \multirow{5}{*}{An optimal PA and power splitting ratio selection algorithms are used to maximise the security sum rate (SSR) of the physical layer in a DL-NOMA system.} & Enhanced the overall system performance with the optimal PA and power splitting ratio methods. The power splitting method provides the highest SSR compared to the PA method. & \multirow{5}{*}{The SSR of the system is affected by the individual users’ minimum required energy.} \\ \cline{3-4}  & & Enhanced the SSR using the large protected zone radius. Users with good channel conditions will get higher SSR and fairness than compared with the users with poor channel conditions. & \multirow{5}{*}{SSR is affected by the poor users. If the number of poor users increases, then the SSR and fairness decreases.} \\ \cline{3-4}  & & Enhanced the SSR of a UAV enabled multicast-unicast transmission system is considered for a DL-NOMA to maximise the SSR. & \multirow{5}{*}{The multi-casting SSR decreased with the power because more powers are required to transmit the multi-cast information.} \\ \cline{3-4}  & & Enhanced the SSR of a UAV enabled NOMA system and also increased the uni-casting secrecy transmiting powers. & \multirow{5}{*}{The multi-casting SSR decreased with the power because more powers are required to transmit the multi-cast information.} \\ \hline \multirow{5}{*}{[PERSON] et al. [166]} & \multirow{5}{*}{Optimal NOMA scheme is designed to maximise the transmit power based on the SSR and QoS requirements.} & Enhanced the performance of the NOMA system regardless the number of users. & \multirow{5}{*}{Eavesdroppers are assumed as performance, but an eavesdropper will always deteriorate the overall performance.} \\ \cline{3-4}  & & The performance of the system depends on the users position from the BS. Enhanced secrecy outage probability is achieved when the malicious users are far away from the BS. & \multirow{5}{*}{Secrecy outage probability deteriorates if the number of participants’ near the BS increases.} \\ \cline{3-4}  & & A technique called stochastic geometry is used to locate the users in NOMA system. Secrecy performance of the proposed technique is estimated based on single and multiple antennas. & \multirow{5}{*}{Performance of the network is overestimated due to the assumption of perfect SIC.} \\ \hline \multirow{5}{*}{[PERSON], [PERSON] et al. [169]} & \multirow{5}{*}{A novel transmission outage constrained scheme is considered in NOMA system to provide security and reliability. Outage probability and diversity order are estimated.} & The proposed scheme provide the secured transaction without leakage of any confidential information & \multirow{5}{*}{The increased number of eavesdroppers will reduce the efficiency.} \\ \cline{3-4}  & & CR-NOMA network with multiple PUs and SUs is considered. Analytical expressions for outage probability and throughput are derived by assuming PU as a trusted user and SU as an eavesdropper. & \multirow{5}{*}{Enhanced the secrecy performance by pairing the PUs with better channel conditions.} & \multirow{5}{*}{SUs will not improve the secrecy performance and throughput because they are assumed to be eavesdroppers.} \\ \cline{3-4}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Summary of optimization and analytical algorithms used to resolve the security issues in NOMA systems. There are still some challenges and issues with the implementation of NOMA that need to be addressed. The major concerns that need to be addressed in the future are:  
* User Pairing in NOMA: In the NOMA implementation, multiple users are multiplexed into one RB, and the SIC receiver will decode the paired users at the receiver. We have discussed pairings of two or three users in previous sections, but not multiple users.