# Device-To-Device Communication in 5G Environment: Issues, Solutions, and Challenges
## 2 Device Discovery  
The device discovery process occurs when the devices transmit a discovery signal through a base station to discover the neighboring devices. There are several integrating technologies related to communication that are being considered by 5G as having potential in helping the discovery process. A device discovery procedure can be divided into centralized and distributed device discovery [16]. These categories are the basis of all the remaining technique functions [17]. For the centralized device discovery, a centralized entity will assist the devices in discovering one another, usually at an access point or a base station. The intended device informs the base station about its purpose to connect with adjacent devices. The base station needs to acquire specific information such as channel conditions, power and also the interference control policy that depends on the system prerequisites.  
The full or partial participation of the BS during device discovery depends on the predesigned protocols. The device is not permitted to initiate device discovery with another device if the BS is also included. The BS facilitates all the discovery signals among each device. For this situation, to start the device discovery process, the devices use the discovery signals that had been transmitted by BS and transmitted the discovery signal back to the BS. For the partial involvement of BS, the device transmitted the discovery signal without obtaining prior authorization from the BS. Nonetheless, BS is  
\begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Challenges**} & \multicolumn{10}{c}{**References**} \\ \cline{2-11}  & **[1]** & **[5]** & **[6]** & **[8]** & **[9]** & **[10]** & **[11]** & **[12]** & **[13]** & **[14]** & **[15]** & **Ours** \\ \hline Device & & X & & & X & & X & X & & X \\ Discovery & & & X & & X & & X & & X & \\ Interference & & X & & & X & X & & & X & \\ Management & & & & & & & & & X & \\ Privacy and & & X & & & & & & & & X & X \\ Security & & & & X & X & & X & & & & X \\ Power & & & & X & X & & X & & & & X \\ Control & & & & & & X & & & & & X \\ Mode & & & & & & X & & & & & X \\ Selection & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison with existing related surveys.  
included to exchange the quantity of Signal-to-Interference Noise Ratio (SINR) and gain path of each device. This mechanism will assist the BS in deciding the communication plausibility for each device. Finally, both devices will be asked by the BS to begin the correspondence.  
For the distributed device discovery, the devices have permission to discover other devices without including the BS. The device communicates with the control signals in irregular intervals for the purpose of discovering the neighboring devices. However, the problems that always associate in the distributed mode are interference, power of discovery signal and synchronization issues. That is why the in-band device discovery is considered to be more efficient in D2D design. There are many device discovery schemes that have been proposed based on centralized and distributed device discovery. The in-band category includes network-assisted discovery, beacon-based discovery, and direct discovery, while the remaining is for the out-band category. Table 2 shows the comparative summary of device discovery schemes in D2D communications.