# Social-Aware Relay Selection for Cooperative Multicast Device-to-Device Communications
## 2 Literature Review
Recently, several analyses of user behaviors and their social relationships are addressed by considering the most popular content shared on social networks such as Facebook, Instagram, YouTube, and WhatsApp, as well what the influences of other friends, media, bloggers and advertising are. Different probabilistic models are proposed to predict the rate of downloading and the requests' evolution of some content. The new vision is to join the social relationships information and proximity-based communications to build an autonomous, trustworthy network of smart IoT devices. The impact of social relationships on the overall throughput of a D2D communication system is considered in [18] with the stop-wait approach for relay selection. The distributed resource allocation, mainly based on cooperative game theory to exploit diverse social relationships on a physical domain, is analyzed in [19]; a Bayesian model for social relationships and a coalitional graph game for efficient data distribution is proposed in [20]; the social characteristics are used to help ad hoc peer discovery in [21]; a neighbor discovery method and a dynamic detection of overlapping social communities is highlighted in [6]. [PERSON] et al. [22] propose a model for the delivery of content in the online social level jointly with the optimization of a traffic offloading process for the D2D communications layer. In [23], the sociality among IoT devices is used to model the trustworthiness for successful D2D-based content delivery to significantly reduce the impact of malicious behavior. In [18], the optimal stopping policy for relay selection is proposed for the case of cooperative D2D relaying, but the multicast context is not considered. This policy, suitably elaborated for the multicast case, is considered as the benchmark for our throughput performance. In [24], a cooperative multicast scheme with underlaid D2D communications is proposed, in which social relationships drive the relay selection. The selected relay nodes receive broadcast messages from the BS on downlink (DL) band and then forward these to multicast users in the uplink (UL) band via D2D communications to increase the multicast transmission rate. In the proposed paper, a cooperative multicast scenario is also considered as in [24] with the difference that the best relay is selected to optimize the global end-to-end delivery delay metric. Moreover, the same content broadcast from the BS to the relay is distributed to the users of the multicast group by assuming that one message is delivered only when all the users in the multicast group have received it. We would like to stress that our focus is on a social-physical scenario, for which, in particular, the social level trust is considered to have a more general meaning with respect to classical real-world networks. In particular, the social level trust of a device is related to its need or interest in receiving a given information flow, for example, according to the novel paradigm of Fog computing/networking and applications in which clusters of smart devices collaborate toward a common goal. As a consequence, the end-to-end delivery delay is considered as the most important parameter to evaluate the optimal social-aware relay selection mechanism in the case of multicast transmission. Therefore, in this paper, the best relay is selected among the different D2D devices according to the sociality index, the proximity distance to the multicast users and the links' propagation conditions.