# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 3 5G Coverage Enhancement Techniques
### Device to Device Communication
#### 3.3.2 Device Discovery
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\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Author \& Ref. No.** & **Approach** & **Advantages** & **Limitations** \\ \hline \multirow{6}{*}{[PERSON] et al. [89]} & \multirow{6}{*}{A random back-off procedure is used to detect a device in D2D communication and analytical expressions for average delay in device discovery are derived.} & Enhanced the device discovery probability. The maximum number of devices discovered are 61.4\% and the success probability is 13.6\%. The occurrence of collisions are reduced in the guard zone of a device discovery. & The proposed model is only applicable to some particular geographic regions such as small roads and urban areas, but not to highways. \\ \hline \multirow{6}{*}{[PERSON] et al. [90]} & \multirow{6}{*}{A VANET based device discovery scheme has been proposed to offload the D2D discovery device discovery is derived.} & VANET enhanced the D2D discovery process efficiently. Achieved low latency in the device discovery process and reduced the traffic due to routing messages of device discovery. & The user density changes dynamically and the prediction based method will not provide an accurate performance of the system at each and every time. \\ \hline \multirow{6}{*}{[PERSON] et al. [91]} & \multirow{6}{*}{A negative device discovery scheme has been proposed to detect the malicious devices can be identified easily using this method.} & Improved the device discovery performance using ML and optimization algorithms. & The user density changes dynamically and the prediction based method will not provide an accurate performance of the system at each and every time. \\ \hline \multirow{6}{*}{[PERSON] [PERSON] et al. [92]} & \multirow{6}{*}{A peer-device discovery scheme based on the deep neural networks has been proposed to detect the malicious devices. Trusted and malicious devices can be identified easily using this method.} & This method will improve the efficiency of detecting the trusted devices. & Channel capacity and SNR are influenced by the instability of the device connections. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [93]} & \multirow{6}{*}{Device discovery maximisation algorithm based on the conversion of half duplex mode of threshold was proposed.} & The device discovery rate is significantly increased with the full duplex mode compared to the conventional half duplex mode. & The power consumption increases with the increased number of devices discovered. The collision probability also increases with the increased number of resource blocks assigned per user. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [94]} & \multirow{6}{*}{Proposed a neighbourhood device discovery scheme for detecting the moving UEs. This method will estimate the number of devices discovered, which are moving in a straight line between two points.} & Device discovery rate increases with the accuracy density, if the devices are moving with a reduced density change their locations then the proposed method fails to detect the devices. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [95]} & \multirow{6}{*}{Proposed a public safety priority based in-band full duplex frame structure. A time efficient resource allocation scheme is used to convert a half duplex mode of operation to a full duplex mode when the half duplex mode fails to be suppressed the available resources.} & The device discovery rate is enhanced by 37\% compared to the random access method. The number of devices discovered also increased by 13.3\% by reducing the collision rate. \\ \hline \multirow{6}{*}{[PERSON], [PERSON] et al. [96]} & \multirow{6}{*}{Proposed a device discovery and localisation with the use of UAVs in the public safety networks. The MUSIC algorithm is used to locate the UEs in disaster situations.} & The devices affected by a disaster are effectively detected. Enhanced the throughput and reduced the packet error rate. & The packet error rate increases with the increased SNR and with the different modulation and coding schemes. \\ \hline \hline \end{tabular}
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