LTE-A Access, Core, and Protocol Architecture for D2D Communication

The term device-to-device (D2D) communications refers to direct short-range communications between terminals of a mobile network, without the intermediate transmission to a base station (BS). Differing from conventional approaches, such as Bluetooth and WiFi-Direct, D2D communications utilize licensed spectrum with quality of service (QoS) guarantees, while no manual network detection-selection is required. Compared to the very appealing cognitive radio communications, where secondary transmissions are allowed in parallel with cellular (primary) transmissions, D2D communications are established by cellular (primary) users, reaping the benefits of being synchronized and controlled by the BS. The introduction of D2D communications in cellular networks is expected to be beneficial from a variety of perspectives, shifting the current cellular communications to a more flexible and dynamic state. The short distance between D2D transmitter and receiver provides better link conditions and, thus, more efficient connection with lower energy consumption. From the network’s perspective, the use of spectrum and processing resources is reduced, since the intermediate transmissions to the BS are avoided. Moreover, the coexistence of cellular and D2D transmissions in shared spectrum bands can lead to higher spectrum utilization, offloading at the same time the cellular network. From the operators’ point