# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Non Orthogonal Multiple Access
#### 3.4.3 Signalling 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{6}{*}{[PERSON] et al. [131]} & & The proposed simulated annealing provides better throughput compared to & \multirow{6}{*}{An equal power allocation scheme is used as an optimal power allocation} \\  & & optimise the power allocation and to enhance the system throughput. & & numerical optimization methods. It also reduces the time complexity for achieving \\ \cline{3-4}  & & A novel price based PA algorithm is considered for a DL-NOMA system with two special cases such as two users scenario and multiple users scenario. & The proposed algorithm maximised the revenue of a BS up to 14.94\% and the total sum rate of the users up to 20.63\%. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [133]} & Stackelberg game theory was used to solve the power allocation problem in NOMA system with QoS constraints. & Optimal price based scheme improved the BS revenue and the number of users allowed to use power. & Power fairness is required to encourage the users with low channel conditions. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [134]} & Glicksberg game theory based PA algorithm is considered for the DL-NOMA system to derive an optimal expression for maximising the utility function. & The average data rate and sum rate are enhanced with the proposed scheme. & As the number of users in a cell increases beyond the threshold, the average data rates are reduced. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [135]} & An auction game based PA scheme was proposed for the hybrid NOMA-OMA system. In this method, BS will act as an auctioneer and sell the powers to the user pairs formed with one strong user and one weak user. & This method achieves very high sum rates compared to the conventional NOMA system. & User pairing plays an important role in this method to provide a power fairness. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [138]} & An optimal PA and an efficient user selection algorithm was proposed to maximise the algorithm performance gain of a DL-NOMA system. & The performance of the proposed optimal power allocation scheme is better than conventional NOMA schemes and also provides the maximum weighted sum rate and performance gain. & The inter/intra cell interference is estimated for an ideal channel and an ideal IC scheme is used to mitigate the interference. \\ \hline \multirow{6}{*}{[PERSON] et al. [139]} & A joint user pairing and PA optimization scheme is proposed to maximise the system performance and to reduce the system complexity. & System complexity is reduced by 62\% and the average user throughput is enhanced by 82\%. & User pairing complexity is reduced but the multi user scheduling complexity is still high. \\ \hline \multirow{6}{*}{[PERSON] et al. [140]} & A low complexity algorithm is designed for a joint PA and user selection scheme. & Overall system complexity is reduced, and cell edge and overall system throughput is enhanced. & In this method only two to three users are paired. If the number of users paired increases then it increases the system complexity. \\ \hline \multirow{6}{*}{[PERSON] et al. [141]} & A joint optimization problem for maximisation of sum throughput is considered based on the power budget, SIC receiver constraints, and minimum rate requirements. & The throughput of UL/DL NOMA system is maximized. & If the cluster size increases then the performance of DL-NOMA system is reduced. \\ \cline{1-1} \cline{2-5}  & A joint user selection and PA scheme is considered for the UL-NOMA system to improve the user fairness. & Enhanced the fairness performance, average data rates, and average outage probability. & The fairness index decreases with the user density. \\ \hline \multirow{6}{*}{[PERSON] et al. [146]} & A dynamic user pairing and PA scheme is considered in a NOMA system to improve the data rates and BER. & System capacity is maximised by satisfying the individual user data rates and BER. & BER is reduced up to some optimal PA value. However, it increases after the optimal PA value. \\ \hline \hline \end{tabular}
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