# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Device to Device Communication
#### 3.3.4 Security and Privacy  
In D2D communication, device discovery was done using a centralised, distributed, and semi-distributed scheme. The centralised and distributed schemes are paired in a semi-distributed scheme to identify neighbouring users, which causes some security issues in both cellular networks and ad-hoc wireless networks. These issues will affect the availability, confidentiality, and authenticity of the devices on the network. Therefore, to overcome these issues, some privacy- and security-based algorithms are proposed [108; 109; 110; 111]. In Ref. [108], a user sub-channel matching algorithm has been proposed to address the spoofing and interference between the CUEs and DUEs. The proposed algorithm will reduce the interference and improve the overall sum rate. In Ref. [109], an optimization algorithm has been proposed to reduce the interference due to eavesdroppers. This proposed method will reduce interference and enhance the physical layer security of CUs and throughput, but it will protect only one CU from multiple eavesdroppers. In Ref. [110], two privacy-preserving authentication protocols are proposed to provide a secured link between the devices in D2D communication. These methods will improve the efficiency and security by reducing the interference between the DUs. In Ref. [111], a secure key generation algorithm has been proposed to secure the DUs from malicious users, which leads to improved information confidentiality between the DUs. Only a few authors proposed algorithms to solve the security and privacy issues. Different security and privacy algorithms have been used in recent research to resolve the coverage and capacity issues in 5G networks, as summarised in Table 10.  
The future challenges relating to privacy and security issues are as follows:  
* Lack of Standardisation: To provide a secured connection between the DUs, there are no standard rules for data transmission, the amount of data to be transferred, feedback signalling, security policies, and so on. Therefore, the standardisation of these parameters needs to be addressed; \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 [PERSON] et al. [108] & In a D2D underlaid communication system, physical layer security is discussed and an optimal threshold is used to enhance the security of a CU. & The proposed scheme effectively protects the CUs from the eavesdroppers. & In this method only one CU is protected from the eavesdroppers and D2D pairs. However, in practical multiple CUs, protection is required. \\ \hline [PERSON], [PERSON] et al. [109] & Proposed a key agreement and privacy preserving authentication protocols to provide the secured D2D communication. & Provides a secured D2D communication from the malicious users and also protects from the internal attacks. & Estimation of success probability is further required. \\ \hline [PERSON], [PERSON] et al. [110] & Relay based cooperative trusted and semi-trusted and non trusted environments are used to provide a physical layer security. The optimal relay pairs are selected based on the room trusted environments. & The trusted environment will provide a better security when compared with the semi and non trusted environments. & The average user security decreases with the distance threshold. \\ \hline [PERSON], [PERSON] et al. [111] & A secured key is generated through a bit string based on the enhanced identity encryption scheme. The leakage of UE’s identity can be reduced with this method. & The time required for encryption and decryption process is much less. The UE’s identity can be hidden by using the Lagrange coefficient. & As the number of UE’s messages increases, the time required for encryption, decryption, and authentication also increases. \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Summary of security and privacy algorithms in D2D communications. * Security Threats: A lot of security threats are happening in D2D communications.