# Social-Aware Relay Selection for Cooperative Multicast Device-to-Device Communications
## 2 Literature Review  
D2D communications are foreseen to be of paramount importance in 5G systems for improving system capacity and for offloading traffic from a BS. In this context, devices can autonomously establish direct connections sharing the spectrum of cellular systems (underlaid D2D communications) and resource allocation, and management approaches have to be handled to provide proximity services [3; 4]. Different research proposes interference-avoiding schemes under the management of network infrastructure to prevent harmful interference among D2D and cellular users or to analyze autonomous D2D data transmissions with guaranteed limited and tolerable interference on cellular users. Others challenges faced regard the significant reduction of traffic load and communication delay [5] and the neighbor discovery methods to detect proximity users in cellular networks with underlaid D2D communications [6]. As users can self-organize with direct connections, D2D cooperation can be the main means to improve throughput and energy efficiency. Unfortunately, when D2D users cooperate, the devices acting as relays expend extraordinary energy for data transmissions, even over short distances. Hence, it is necessary to select D2D relays among multiple devices by considering whether a device has already been selected as a relay many times or if it has to handle too many cooperative users to avoid excessive energy consumption of the relay and to decrease the life-time of the system [7]. In D2D communications networks, the mobile devices are intermittently connected in an ad hoc manner, and the topology of the network can be highly dynamic. In [8], a novel opportunistic network routing protocol based on social rank and intermeeting time is considered. Cooperative multicast transmissions, for which different devices with good channel conditions are selected as relay nodes, improve the achievable data rate with respect to the direct transmission from the BS, which suffers from the constraint of the worst propagation channel as a result of the long distance, for example. In the literature, various relaying technologies are considered, such as decode and-forward (DF) and amplify-and-forward (AM) relaying. Different relaying strategies have been proposed, such as relay selection, in which multiple relay nodes allow for a more efficient use of the system resources through the selection of the best source-relay and relay-destination channel, and incremental relaying, in which feedback from the destination about the success or failure of direct source-destination transmission is used. Full duplex (FD) relays offer high spectral efficiency but suffer from strong self-interference and loop interference if multiple antennas are installed on each relay. On the other hand, half-duplex (HD) relays, for which the source transmits information to relays in the broadcast phase and then relays this forward to destinations in the successive time slots, causes multiplexing loss. Many methods have recently been proposed to overcome the multiplexing loss in HD relaying. Successive relay techniques [9] are analyzed to improve the spectral efficiency of HD relays, in which a pair of relays is selected, and while one relay receives data from the source, the other transmits the previously received data to the destination. The use of a buffer at the relay nodes, \"buffer-aided relaying methods\", as in [10; 11; 12; 13; 14; 15], increases the spectral efficiency with respect to HD relaying without a buffer. This approach allows for selecting the best HD buffer-aided relay among the various relay-destination links (opportunistic relaying schemes), to transmit the data if the channel conditions are good or buffer it otherwise, as in max-max relay selection by [PERSON] et al. [16] or the max-link relay selection scheme by [PERSON] et al. [17].