# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Small Cells
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Authors \& Ref. No.** & **Approach** & **Advantages** & **Limitations** \\ \hline [PERSON] et al. [49] & The energy efficiency of an ultra dense HetNets is estimated based on the stochastic geometry theorem. & The small cells are optimised and enhanced the energy efficiency. & The energy efficiency is constant with a further increase of UEs and it reduces the overall system performance. UE density optimization is required to enhance the efficiency. \\ \hline [PERSON] et al. [50] & Small cells are deployed in a public transport bus to verify the traffic to and from the macro cells. Traffic offloading is verified dynamically and the DL outage probability expressions are derived. & The overall system coverage and outage probability gain are enhanced. Power consumption is reduced. & The pre-coding signals are to be perfectly formatted to avoid any security issues. \\ \hline [PERSON], [PERSON] et al. [52] & The impact of outdoor UEs on the traffic offload is analysed based on the cell densification concept. & Enhanced the UEs and cell edge throughput, overall system capacity, and coverage. & The overall deployment cost increases due to the deployment of additional pico cells. \\ \hline [PERSON], [PERSON] et al. [54] & The interference effect of the HCS networks and the outage probability of femto cells are analysed. & Enhanced the outage probability spectral efficiency of a HCS network. & Carrier to interference ratio between the femto cells in the HCS network to be avoided. \\ \hline [PERSON], [PERSON] et al. [55] & Pico cells are deployed with macro cells to improve the overall system performance. & Enhanced the coverage capacity and cell edge coverage. Traffic density at the macro cell is reduced. & Identifying the fixed locations of pico cells is very important in order to reduce the traffic density at the macro cell and to improve the cell edge coverage. \\ \hline [PERSON] et al. [56] & The coverage area of a macro cell is added with the pico cells to improve the overall system throughput and the received signal power is used to estimate the path loss. & Enhanced the cell edge spectral efficiency, coverage capacity, and throughput. & The system complexity and deployment cost increases with the number of pico cells deployed. \\ \hline [PERSON] [57] & A macro cell and pico cell cooperative scheduling scheme is considered to reduce the UL and DL interference due to the macro cell on pico cell UEs. & Enhanced the traffic offloading, DL throughput to 36\%, UL throughput to 7\% and overall system performance by reducing the & Need to improve the UL throughput and reduce the interference between the pico cell UEs and macro cell. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of small cell-based approaches of 5G networks. of the important factors when deploying them in indoor or outdoor regions. The correct way to test a small cell is to improve the quality of service and also to satisfy the subscribers demands. The user traffic, mobility and handovers, and overall system load should be considered as the testing parameters. These parameters should be improved by proper testing. So, AI based algorithms need to be proposed to test the small cells automatically. Identifying the correct testing method is also one of the future challenging issues.