# Social-Aware Relay Selection for Cooperative Multicast Device-to-Device Communications
## 1 Introduction  
In the incoming fifth-generation (5G) system, device to device (D2D) and relaying communications are envisaged as enablers to face the huge amount of data traffic due to novel and advanced applications and services. In particular, the emerging trend of sharing photos, video, and music among friends requires a large amount of data at a high data rate. In D2D communications, the devices can share the same relevant contents or help the neighbours to deliver data by establishing a direct link without (or with limited) involvement of a base station (BS) or eNodeB. As the D2D communications occur over short distances, they can support a higher data rate with respect to infrastructured communications. Furthermore, D2D communications enhance the spectral efficiency, lighten areas with elevate traffic and improve the user quality of experience. In the foreseen integrated 5G and Internet of Things (IoT) infrastructures, short-range D2D communications can interconnect heterogeneous devices with lower-energy consumptions guaranteeing proximity services for 5G/IoT networks. The D2D concept allows for a direct connection between D2D pairs of devices, considerably (or fully) reducing the exchange of traffic requests with the BS. The D2D approach is also useful in the multicast context, where multiple cellular/IoT devices have to receive the same data from the BS. Various devices can self-organize into clusters, and some ofthese can be selected to act as a relay to help the forwarding of data, particularly to the end-nodes of the networks and the offloading of traffic from the BS. Relay-assisted communications have been actively studied and are already considered in the standardization process of mobile broadband communication systems, such as in the Third Generation Partnership Program (3 GPP) Long-Term Evolution Advanced (LTE-Advanced), IEEE 802.16j and the IEEE 802.16m [1,2] to improve the cell-edge coverage radius and to provide high data rates to the users located in the cell-edge or in coverage holes. In addition, cooperative multicast relaying can improve data rates as a result of the shorter distance from multicast devices with respect to the direct transmission, for which the worst propagation channel limits the available transmission rate. In the next generation, 5G networks, cognitive relaying and cooperative D2D relaying will continue to be the main players. In cooperative D2D communications, the common concept is that all the devices can relay to each other, but, for example, the battery charging can often limit data forwarding assistance because of the energy consumption. If D2D communication is under a partial operator control, the user acting as a relay can have a bill reduction, or the user can offer his battery and bandwidth consumption only if he wants to help his friend. Recently, the increasing demand for social applications, such as Facebook, Twitter, YouTube, Instagram, and so on, has suggested integrating the social behavior on a cooperative D2D/relaying design, in which the D2D users can communicate directly with each other and can exchange content mainly if they are friends. Different social communities, in which each participant has the same interest in content, can be formed, for example, by tracking friends, kin and colleagues that share content frequently by online social networks. Social media networking indeed represents a disruptive paradigm leading the transition to the \"Web2.0\". The proliferation of popular applications for smartphones points out a constant trend for geo-referenced services with an increased level of integration among different communities, thus making the content dissemination more pervasive and instantaneous. Generally, the social trust model for cooperative D2D communications is built by two different layers that interact each other:  
* The _online social network layer_, which indicates the different levels of relationships among the D2D users.