# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Device to Device Communication
#### 3.3.1 Mode Selection
\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 \hline \multirow{6}{*}{[PERSON] et al. [78]} & \multirow{6}{*}{A joint optimal mode selection and resource allocation and power allocation schemes are proposed for a D2D undelayed cellular networks.} & The system sum rate is improved through the optimization of mode selection schemes. An optimised resource allocation scheme reduces the co-channel interference. & D2D links will suffer from the co-channel interference from the cellular links. \\ \cline{2-4}  & & & Provides high system sum rate and low interference between DUE and CUE. The sum rate achieves only 10 bits/s/Hz for six antennas. \\ \hline \multirow{3}{*}{[PERSON] et al. [80]} & \multirow{3}{*}{An optimization model for resource allocation based on non-transferable utility and distributed coalition formation algorithms are proposed.} & Traffic offload, system performance, and average sum utilities are enhanced. & A single cell scenario is used in this paper but a multi cell scenario is required in 5G systems for better coverage. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [81]} & DL throughput optimization techniques are proposed for D2D communications. The multi hop relay aided in-band D2D, game based power adjustment and greedy algorithms are discussed. & DL throughput, energy efficiency, and coverage capacity are enhanced using the proposed algorithms. & The co-channel interference increases with the increased number of UEs. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [82]} & \multirow{3}{*}{Optimization of power allocation, resource allocation, and mode selection based relay assisted D2D communication system is considered.} & Power allocation complexity is reduced and overall system throughput is maximised. & At low powers the interference between the DUs is increased and the system sum rate is reduced. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [83]} & \multirow{3}{*}{Proposed a Markov chain theory for single user and greedy heuristic algorithm for multi user scenarios for mode selection in D2D communication.} & Outage probability, system sum rate, and average user SINR are improved without increasing the system complexity and signalling overhead. & At low SINR, the DUE is unable to select the best mode of operation and the system performance is affected by the synchronisation errors. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [84]} & Resource allocation and mode selection algorithms are proposed to enhance the system performance. & Spectral efficiency and system throughput are enhanced. & Multiplexing resources increases the interference between the UEs. \\ \hline \multirow{3}{*}{[PERSON] et al. [85]} & \multirow{3}{*}{Proposed a throughput optimal joint mode selection selection user scheduling algorithm and discrete rate adaptation scheme for D2D communication system.} & User fairness increases with the optimal mode selection. Cell throughput increases with the multi user diversity. & Joint rate power adaptation modes for partial CSI are to be addressed. \\ \hline \multirow{3}{*}{[PERSON] et al. [86]} & \multirow{3}{*}{D2D enabled NOMA cellular network resource allocation and mode selection issues are discussed. The expression for optimal resource allocation was derived based on game theory approach.} & Enhanced the system sum rate, coverage capacity, and spectral efficiency. & Inter cell interference increases with the increased number of BSs. \\ \hline \hline \multirow{3}{*}{[PERSON] et al. [87]} & \multirow{3}{*}{To protect the PUs, an optimal mode selection algorithm is analysed.} & The success probability increases and interference between the DUs is reduced. & Optimal guard zone radius is required to improve the system capacity. \\ \hline \end{tabular}
\end{table} 
Table 7: Summary of mode selection, power allocation, and resource allocation algorithms in D2D communications.