# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Small Cells  
Small cells are low power short range transmission systems or BS deployed to cover a small geographical area. The characteristics of small cells are similar to the conventional BSs. These small cells can be deployed in indoor or outdoor regions and it will provide high data rates and low latency. These small cells are classified into three types based on the area covered by the BS, the maximum number of user's covered, and the maximum  
Figure 3: 5G coverage enhancement techniques. power transmitted [47]. They are femto cells, pico cells, and micro cells, which are shown in Figure 4. The classification of small cells and their parameters are shown in Table 4.  
5G requires small cells to achieve its key parameters such as enhanced mobile broad band, URLLC, massive connections and enhanced spectral efficiency. Small cells use low power transmitting stations and are very easy to deploy because of their low complexity in hardware installation. These small cells can be mounted on a wall for indoor applications and small towers or lamp posts can be used for outdoor applications. In small cells, the coverage and data rate depends on the operating frequency, transmitted power, and height and tilt angle of an antenna. The radius of the coverage area is always proportional to the transmitted power [48]. The small cells have many advantages such as high data rates, faster deployment, cost effective, they can work with low powers, and require less installation space. To achieve high data rates and large capacity, the 5G network is integrated with small cells, MIMO, beam-forming, and mmWaves. The mmWave, i.e., from 30 GHz to 300 GHz, is a much less utilised spectrum and it provides very high speed communication. However, the problem with high frequency mmWave is that it collides with obstacles such as trees, buildings, etc., in the free space, \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Parameter** & **Femto Cell** & **Pico Cell** & **Micro Cell** \\ \hline Maximum coverage radius & 50 m & 250 m & 2.5 Km \\ \hline Maximum transmitted power & 100 mW & 250 mW & 5 W \\ \hline Maximum number of users & 16 & 64 & 200 \\ \hline Type of backhaul & Fibre, Wired & Fibre, wired & Fibre, wired, microwave \\ \hline Cost & Low & Low & Medium \\ \hline Advantages & Offload network congestion, extended coverage and high data rates & Extended throughput and coverage & Extended coverage \\ \hline Applications & Residential & Small enterprise & Smart cities, smart metro \\ \hline Location & Indoor and outdoor & Indoor & Outdoor \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of small cells. Figure 4: Small cell techniques. and causes multipath fading, which leads to reduced signal strength. Therefore, mmWaves are the best frequency bands for short distance communication and can be used in small cells. A geographical area without small cells and with small cells is shown in Figure 5. In Figure 5, the mmWave signal is facing multipath fading but in Figure 5, with the use of small cells, the multipath fading is avoided and it provides better coverage than compared to the conventional method. In the literature, researchers deployed the femto, pico, and micro cells and identified the network capacity, data rates, and coverage area [49; 50; 51; 52; 53; 54; 55; 56; 57]. In Ref. [49], the authors considered a stochastic geometry theorem for the optimization of small BS density in a ultra dense Hetnets. The optimized small BS density enhances the energy efficiency for a particular number of UEs. In Ref. [50], the authors deployed a small cell in a moving bus and estimated the coverage area as 300 m in an urban environment. For indoor applications such as residential, hospitals, schools, offices, and shopping malls, the femto cells are preferred because of their easy installation, cost effectiveness, and low power requirements [51].