# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Non Orthogonal Multiple Access
#### 3.4.2 Power Allocation Issues  
In the power allocation scheme, users who have more channel gain are given less power, while users who have less channel gain are given more power. In NOMA, the users with various PA are multiplexed in one RB, which will increase the system's performance, but inefficient PA methods cause interference and also reduce the system's performance. Therefore, proper selection of power allocation methods plays an important role and depends on many parameters such as CSI, QoS, total system power, and channel conditions. Therefore, many optimization techniques are proposed for PA fairness [124; 125; 126; 127; 128; 129; 130]. The PA is optimised by maximising the maximum and average sum rates using physical layer QoS [124; 125]. The optimal PA was proposed by minimising the power consumption [126; 127; 128]. Different PA techniques have been proposed for maximising the capacity rate and total power [129; 130]. PA based on expenditure or cost is explained using graph theory [131; 132; 133; 134; 135; 136; 137]. In Ref. [131], the authors proposed a simulated annealing to  
Figure 12: NOMA with a SIC receiver. optimize the PA and to maximise the throughput. In Ref. [132], two user concepts were considered. In this method, power is allocated to two users or only one user, whichever has a higher power gain. In Ref. [133], the authors proposed a price-based PA optimization algorithm for two users based on their QoS. The data rate is maximised using an optimization algorithm [134]. In Ref. [135], the authors proposed an auction-based game design by bidding on the transmitting power. The spectrum scarcity issues in both licensed and unlicensed bands for NOMA were discussed in Refs. [136; 137]. The joint PA and user pairing problems are solved by using an optimization algorithm [138; 139; 140; 141; 142; 143; 144; 145; 146]. To identify the best multiplexed users in NOMA, a greedy algorithm, and to optimise PA, an optimization algorithm was proposed in Ref. [138]. A joint PA and user pairing optimization has been used to maximise the transmitted power and reduce the computational complexity of the NOMA system [139]. In Ref. [140], the authors proposed a proportional fair scheduling algorithm to reduce the complexity of a single carrier NOMA system. A dynamic PA scheme was used to optimise the PA in both UL and DL and also to maximise the system throughput [141]. The proposed fractional error factor method will increase the energy efficiency and also maximise the performance gain of both the UL and DL of UE [142]. A swarm optimization algorithm has been proposed to find the outage rate and average data rate of all the user pairs [143]. The spectral efficiency and error probabilities are improved by using a sort-based user pairing method [144]. This method was used to maximise the system capacity and reduce the interference. The user pairing and PA were dynamically varied to identify the system capacity of an individual and paired users [145]. The system capacity for pairing users was better than for individual users. In Ref. [146], the bit error rate is considered along with the individual and paired user's capacities. As summarised in Table 12, various PA algorithms and joint user pairing and PA algorithms have been proposed in recent research to address the coverage and capacity issues in 5G networks.