Currently, Wi-Fi and Bluetooth provide some D2D communications functionality, but because these work in unlicensed band, the interference can be come uncontrollable. Another drawback is that unlike cellular network, they cannot provide security and QoS guarantee.
One of the features of 5G network is support for device-to-device (D2D) communications. When a device communicates via the base station, it is operating at the macro cell level; if a device communicates directly with another device, it is operating at the device level. In congested areas and at the cell edge, devices can create an ad hoc mesh network so that they can communicate with each other. There are four types of device level communication.
- Device relaying with BS-controlled link information. This is for a device that is at the cell edge, where the signal strength might be poor. The device communicates with the BS by relaying information through another device. Doing this helps the device to maintain a high QoS and battery life. The BS communicates with the relaying device for partial or full control link formation.
Device relaying with BS-controlled link information (Gupta and Jha, 2015) 
