# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Non Orthogonal Multiple Access
#### 3.4.4 Security 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{4}{*}{[PERSON] et al. [150]} & A sub-optimal iterative resource allocation (RA) algorithm is considered to provide a perfect balance between the overall system sum rate and its complexity. & Enhanced the total power saved with the increased number of users and reduced the total power consumption. The spectral efficiency \& gain is enhanced by reducing the overall power consumption. & The outage probability is low at low transmit powers. \\ \hline \multirow{4}{*}{[PERSON] et al. [151]} & Proposed a robust optimization scheme for DL-NOMA system. A non-convex problem is converted into a convex problem by using a complementary geometry programming and arithmetic geometric mean approximation schemes. & The proposed scheme achieves high transmit power savings compared to the non robust scheme. Overall & Average transmit power is improved if the number of users are facing poor channel gain issues and do not satisfy the minimum cost scheme. \\ \hline \multirow{4}{*}{[PERSON] et al. [152]} & An outage probability based user scheduling scheme was proposed for a DL-NOMA system under imperfect SIC. The non-convex KPA problem was solved by optimising the user scheduling through a matching theory algorithm and branch and & The proposed method minimised the total transmit power required. & The required transmit power increases with the residual interference. The increased residual interference deteriorates the outage probability of the users. \\ \hline \multirow{4}{*}{[PERSON] et al. [153]} & Two imperfect CSI schemes such as channel distribution information and channel estimation uncertainty are considered for the MIMO-NOMA system to enhance the sum rate and also an efficient SCA algorithm is used to optimise the sum rate. & The overall system sum rate and system utility are maximised. & Due to the imperfect CSI, the overall interference increases and there is a trade-off between system complexity and throughput. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [154]} & Imperfect SIC is considered for a DL MC-NOMA system to maximise the weighted sum rate. A low complexity PA algorithm is proposed based on the perfect and imperfect SIC. & The weighted sum rate decreases with the SIC errors and power factor. & As the number of users or number of iterations increases, weighted sum rate also increases, which increases the total power required. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [155]} & A dynamic decoding order SIC receiver is considered for the UL-NOMA system. Analytical expression for outage probability is derived based on the three user scenario. & The outage probability of user 1 and user 2 decreases but it increases with a dynamic SIC receiver provides very low outage probability with the fixed SIC. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [156]} & Initially a slow fading channel was considered to define a outage achievable rate, then a statistical channel is used to estimate the SIC errors in a NOMA system. & Enhanced the outage achievable rate of NOMA system compared to the conventional OMA system and also achieved the higher data rates. & A perfect SIC receiver is assumed in this paper, but in practical cases an imperfect SIC needs to be considered to get accurate results. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [157]} & A single cell DL-NOMA system is considered for high speed railways in order to provide high data rates. A partial CSI with Rician fading channel is assumed to estimate the performance of the Rician factor \(K\). & The proposed system enhanced the average outage probability compared to the conventional OMA system. & In high speed railways, Doppler shift arises due to the mobility, which leads to channel estimation errors and inter channel interference. \\ \hline \multirow{4}{*}{[PERSON] et al. [158]} & Multi user DL-NOMA system with limited CSI feedback channel is considered to achieve high sum rate. Random beamforming and zero forcing techniques are also considered to reduce the interference between the users. & Random beam-forming technique is more suitable for a limited CSI feedback channel and it achieves higher system sum rate than compared to the OMA system. & In this paper, only limited CSI feedback channel is considered, which reduces the overall system performance. \\ \hline \multirow{4}{*}{[PERSON] et al. [159]} & The effect of beam-forming technique in a multi input single output up-link NOMA system is considered with one bit feedback to estimate the system performance. & Enhanced the system performance and also achieved the high coding gain. & The overall performance of the enhanced number of transmitting antennas. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [161]} & A single cell DL-NOMA system performance is studied under imperfect CSI and CSI based on order statistics. Analytical expression for outage probability and average sum rate are derived. & Enhanced the performance of NOMA system compared to the conventional OMA system under two proposed CSI scenarios. & Perfect CSI gives better performance compared to the proposed two CSI scenarios because it is an ideal case. \\ \hline \multirow{4}{*}{[PERSON] [162]} & A repetition based NOMA system is considered to achieve high diversity gain. An analytical expression for outage probability was derived based on the key parameter. & The proposed method enhanced the average error probability and outage probability. & The average error probability depends on the code length. \\ \hline \hline \end{tabular}
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