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.
