Smart - Device - to - Smart - Device Communications

Device-to-device (D2D) communications have been identified by the Third Generation Partnership Project (3GPP) as a potential candidate technology to offload heavily used cellular communication systems. Even though D2D communications has been around in industry and research for the last two decades, the 3GPP decision to include this concept in the cellular context is groundbreaking. There are several fundamental differences between D2D communications in an ad hoc fashion and under cellular control. While the former typically uses a distributed topology, the latter relies on centralized control from a base station assigning communication resources. It also provides a natural market for novel services (including payment models) as well as easing the setup of D2D communication, as node and service discovery is supported or fully carried out by the cellular network. The result is faster setup and less timeand energy-consuming maintenance of the system as a whole. In the 1990s, the D2D communications concept was introduced for ad hoc, sensor, and mesh networks. Ad hoc communication was initially intended to allow fast setup between two peers. Bluetooth is an example of D2D cable replacement solutions. Sensor and meshed networks extended the D2D concept by allowing not only two devices but multiple devices to communicate with each other. While sensor networks focused on accumulation of services (sensor data) from distributed nodes, mesh networks intended to offer coverage extension of devices with the same set of capabilities. The communication among devices was often realized in the industrial, scientific, and medical (ISM) bands in a distributed fashion without the help of any control unit. Currently, D2D communications is realized by logical channels communicating via the base stations of a network (Fig. 1). Long Term Evolution-Advanced (LTE-A) D2D allows direct communications between devices using licensed spectrum if the mobile devices are in close proximity with the overlay network acting as a spectrum broker. This idea was originally introduced in 2006 [1] followed by several publications in that field summarized in [2]. The advantages of cellular controlled D2D communications are many-fold. From the mobile device perspective, the service discovery and node discovery can be performed by the network, which results in large energy savings compared to standard approaches. The reason is that the network is already aware of the position of its devices as well as the services that are needed or advertised by each device. Thus, the network can inform interested devices when to start the search procedure instead of constantly searching for the required service until a suitable device is found. Additionally, cellular controlled D2D can assign devices a series of resources to use, from wireless access to providing security keys. This provides devices with a common broker and frees them from negotiating directly with other devices. This reduces complexity and energy consumption for supporting these capabilities, and enables additional security mechanisms to detect and block malicious devices. For the network, one of the key advantages is the potential to improve spectral reuse. When a ABSTRACT