# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Multiple Input and Multiple Output
\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{3}{*}{[PERSON], [PERSON] et al. [176]} & Massive MIMO technology was considered for the 5G and 6G networks to enhance the throughput and data rates. & High data rates of 100 Gbps are achieved using the mmWave frequency band of 28 GHz. & Multiple M-MIMO systems are used to enhance the throughput. This will increase the system complexity. \\ \hline \multirow{3}{*}{[PERSON] et al. [177]} & A cylindrical antenna structure for the MIMO system was proposed to enhance the system capacity and SINR. & The system capacity is enhanced by 2.1 times compared to the planner structure. This will also enhance the coverage radius of up to 100 km. & Optimization of antennas will further enhance the system capacity. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [178]} & Beam division multiple access technique was considered for the M-MIMO system to enhance the system capacity. & System capacity is enhanced 10 times and energy efficiency is enhanced 100 times by reducing the bit error rate. & Spectral efficiency and data rate enhancement is required to improve the overall system performance. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [179]} & A cell free MIMO UF receiver was considered with the zero forcing, MMSE, and maximum ratio combining detectors. & The spectral efficiency is enhanced using the zero forcing and MMSE detectors compared to the cellular M-MIMO and small cell M-MIMO systems. & The maximum ratio combining detector reduce the spectral efficiency. \\ \hline \multirow{3}{*}{[PERSON], [PERSON] et al. [180]} & A triangular lattice structure of antennas was considered for M-MIMO system between the frequency bands of 24.25 GHz and 29.5 GHz. & Higher spectral efficiency is achieved by maintaining the minimum distance between the antennas. & The optimal distance should be estimated to improve the spectral efficiency and to achieve higher data rates. \\ \hline \multirow{3}{*}{[PERSON] et al. [181]} & The trade-off between spectral efficiency and energy efficiency is used to reduce the transmit power based on multi-objective optimization in a M-MIMO system. & Spectral efficiency and energy efficiency are maximised by minimising total energy consumption. & The number of antennas is fixed in this paper. \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Summary of M-MIMO techniques used to enhance the data rates and capacity of 5G networks. estimation for a M-MIMO system with a large number of antennas becomes very difficult and is impossible to carry out in real-world applications. TDD Mode Channel Estimation: The problem during the DL channel estimation in FDD mode is solved by using TDD mode. In this mode, with the advantage of channel reciprocity, the BS estimates the DL channel with the use of UL CSI. During DL, the user will forward the orthogonal pilot signals to the BS, and the BS will estimate the CSI to the user terminal based on these pilot signals;
* Pilot Contamination: In M-MIMO systems, the BS requires the user terminal's channel response in order to estimate the channel. When the user terminal delivers orthogonal pilot signals to the BS, the BS estimates the uplink channel. Additionally, the BS calculates the downlink channel to the user terminal using M-MIMO's channel reciprocity property. The BS can accurately estimate the channel if the pilot signals in the home cell and nearby cells are orthogonal. However, because there are only a limited number of orthogonal pilot signals available in a given bandwidth and period, adjacent cells must reuse the orthogonal pilot signals, which leads to pilot contamination. Hence, the pilot signal contamination is one of the challenges for future research in M-MIMO systems;
* Energy Efficiency: Energy efficiency is defined as the ratio of spectral efficiency to the total transmitted power.