D2D Communication in 5G Networks

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.

1. 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)
   

D2D Communication in LTE-A Networks

LTE-A radio interface uses techniques such as carrier aggregation, MIMO, millimeter waves, low-power nodes (e.g. pico cells eNB, relays) and device-to-device (D2D) communications. Combining various technologies seamlessly makes it necessary to support LTE-A heterogeneous network (HetNet) deployment. HetNet consists of various radio access technologies (e.g., LTE, HSPA, Wi-Fi, CDMA2000), multiple cell formats (multi-tier cells) with different cell sizes and power levels, different backhaul technologies etc.

D2D communication is a form of device relaying. Device refers to any user-owned mobile device with cellular connectivity. Device relaying makes it possible for devices in a network to function as transmission relays for each other and form a massive ad hoc mesh network.

D2D allows data to be routed directly between spatially close mobile user equipment (UE). Data between a UE pair do not traverse via the eNB or the core network. The short communication range improves energy efficiency, throughput, delay and spectrum efficiency. Possible D2D applications are content sharing, gaming, connectivity extension and traffic offloading.

The diversity of radio access available on UE (at the moment, cellular network, Bluetooth and Wi-Fi) provides D2D communications with flexibility in link establishment, resource allocation, energy efficiency, and applications and services. D2D pairs may be located in the same cell or in different cells. They may communicate as an underlay or overlay to the existing LTE-A network. They may use the licensed or unlicensed band.

D2D communications among multi-tier cells (Liu et al, 2015)