# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Device to Device Communication
#### 3.3.3 Interference Management
[104], a massive MIMO system operating in FDD mode has been proposed to reduce the feedback overhead. In Ref. [105], the authors proposed a relay-based D2D system to improve the system's capacity and performance by avoiding interference. The relay can use the untapped resources of the CUE to enhance the spectrum's efficiency. In Ref. [106], a fractional frequency reuse and a blank sub-frame concept were used to avoid interference between the CUEs and DUEs. The numerical results show that the interference level of the DUEs decreased as compared to the conventionalmethods. This method increases the system throughput and efficiency. In Ref. [107], a dynamic distance-based algorithm was proposed to reduce the interference. Using this algorithm, outage probability, SINR, and cell density are estimated for D2D-enabled hetnets. Different interference management algorithms have been used in recent research to resolve the coverage and capacity issues in 5G networks, as summarised in Table 9.  
The future challenges in the interference management that need to be addressed are  
* D2D in mmWave Communication: Multi-cell D2D communication is possible with the small cell concept and mmWave in 5G networks. The mmWave improves the data rates, network capacity, and latency but different interferences occur due to the penetration losses of mmWaves. Therefore, the interference mitigation algorithms for 5G mmWave need to be addressed to provide multicell D2D communication;  
\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. [98]} & Proposed a NOMA based multi interference cancellation scheme using the optimal resource allocation and signal detection. Optimised the power consumption. & The proposed scheme reduces the overall system complexity up to 13 dB compared to the OFDM and conventional NOMA schemes. & Latency is increased and the fairness factor is decreased with the increased number of D2D pairs. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [99]} & Proposed a Lunius system to optimise the local file sharing between the peer to peer users. It improves the user experience and offloads the traffic at BS in a D2D communication system. & Throughput, data rates, and spectral efficiency are maximised and the traffic load at the BS is reduced. & The Lunius system performance depends on the cooperation between the D2D links and the error rates. \\ \hline \multirow{4}{*}{[PERSON], [PERSON] et al. [100]} & MIMO with minimum mean square error & \multirow{4}{*}{Data rates are increased and the bit error rates are reduced.} & Only a two-users scenario is considered but in practical cases the number of users in a given cellular region are more than two. \\ \cline{1  
* Cell Densification and Offloading: The small cell and macro cell concepts of 5G are integrated with D2D communication to provide multi-cell D2D transmissions. Multicell D2D is used to improve the spectral efficiency and overall system performance of D2D communication. Therefore, the resource allocation and interference control algorithms for multicell D2D communication are challenging future issues.