# Social-Aware Relay Selection for Cooperative Multicast Device-to-Device Communications
## 1 Introduction
* The _offline mobile communications network layer_, which determines the wireless connections subject to the channel propagation conditions. For example, a high level of relationships can exist between two users of the social layer because they belong to the same group, such as family or colleagues, but the same connection cannot exist in the physical layer because there is no proximity or there are bad propagation conditions of the link, as shown in Figure 1.  
Figure 1: Social–physical layers for cooperative device-to-device (D2D) relaying. The social-trust level can be evaluated through the analysis of the contents and account information shared, for example, by tracking online social network services and calculating the ranking of the user to spread the content (this is related to the number of friends). The social relationships can be retrieved by matching mobile phones' contact books: if two users are colleagues or members of the same family, it is very likely that they have many phone contacts in common. Moreover, when the D2D relay has received popular content from the BS, it can share this to other users in proximity through D2D communications, and a new entry in the group of friends can make available this content, considerably decreasing the traffic load of the BS. The geographical location can impact this social transfer of the same data; for example, in a school building, it is highly likely that different students download the same viral video, and thus a proximity area can be a characteristic for content delivery. Current researches analyze the cooperative communication gain for proper relay selection by considering a weighted tradeoff between the social trust information and the physical constraints in order to maximize the throughput with respect to the direct transmission, mainly for scenarios with only one destination. This paper presents a multicast scenario with a source (either the eNodeB or a D2D user), which transmits the same data content to many destinations through the selection of the best relay among different alternatives, by taking into account the relay social-trust level and propagation condition links to optimize the end-to-end delivery delay. This is defined as the time required to deliver data from the source through the relay to all the destinations belonging to the multicast group (i.e., the social community). The relay itself is interested in receiving data content because it can belong to a multicast group. The social-trust level is related to the social relationships of the relay with the source and represents the part of the transmit power that the relay is available to give _as friend_ to forward the data to the multicast community and as a consequence, the availability to consume its own battery. The performance of this social cooperative multicast system is analyzed by considering that the direct link among the source and destinations is not available because of path loss and shadowing effects and that all the D2D multicast devices have to receive all the data. The remainder of the paper is organized as follows. Section 2 provides a literature survey. In Section 3, the system model is introduced, and in Section 4, we provide the simulation results for the end-to-end delivery time performance evaluation of the integrated social multicast D2D-based system. Finally, the conclusions are drawn.