# Enabling Trustworthy Multicast Wireless Services through D2D Communications in 5G Networks
## 1 Introduction  
Future fifth-generation (5G) networks are expected to support a huge number of heterogeneous connected devices in manifold usage scenarios. Autonomous driving, tactile Internet, personal cloud, disaster alert, video streaming and downloading are among the most challenging 5G use cases. Some technologies are expected to play a key role in helping to satisfy the demanding requirements of the foreseen use cases. Device-to-device (D2D) communications and multicasting are certainly among these. The former, because of its capability to offload cellular data traffic, enhances spectrum efficiency and extends cell coverage [1]. The latter, because of its capability to answer to the increasing user demand for multicast/broadcast multimedia services (mobile TV, IP radio broadcasting, and video streaming [2]). Regarding 5G in general, in the 3 rd Generation Partnership Project (3 GPP), the 5G radio access roadmap foresees two tracks: one is based on the evolution of Long-Term Evolution (eLTE), and the other on the design of the New Radio (NR) access. Thus, the core enhancements/changes of the 5G paradigm are (i) the standardization of a new radio interface (i.e., called New Radio-NR in 3 GPP and other standardization bodies) and (ii) the empowering of the existing LTE systems in order to handle use cases (e.g., mMTC, cMTC, V2X or Sidelink) for which NR is still not mature yet (i.e., the full specification of NR, it was agreed just on the 15 th of June during the 3 GPP RAN#80 plenary). Consequently, in relation to the former case, it is good to point out that LTE constitutes an essential piece of the 5G puzzle and can be considered as a 5G-ready technology due to the variety of enhancements and new features already introduced in Rel-14 and Rel-15 timeframes in 3 GPP. Therefore, even if the research efforts on D2D (i.e., later called Sidelink) and multicast/broadcast services started way before the concept of \"5G\" gained momentum, it is fairly evident that these technologies have evolved over the years to meet requirements more and more challenging that nowadays are very close (if not the same) to those of the future 5G systems. Multicast transmission is an effective means for delivering group-oriented services since users can be fed through a single point-to-multipoint (PtM) transmission by exploiting the broadcast nature of the radio channel. In order to handle multicast and broadcast services over cellular networks, the 3 rd Generation Partnership Project (3 GPP) has standardized the evolved Multimedia Broadcast Multicast Service (eMBMS) [3]. Although it represents the current standard to support group-oriented services over mobile networks, significant work must be done in order to effectively support multicast/broadcast traffic in 5G networks. The approach for delivering multicast traffic in cellular networks, the so-called Conventional Multicast Scheme (CMS) [4], is known to suffer from poor spectral efficiency. According to it, all users belonging to the multicast group are served with the same data rate imposed by the user with the worst channel conditions. Despite this approach guaranteeing fairness because all users are always served and receive the same treatment, it suffers from poor performances because users with good channel conditions are constrained to the low data rates that cell-edge users can sustain. In our earlier work in [5], we proposed a D2D-aided radio resource management policy for eMBMS, called D2D-enhanced CMS with Single Frequency (D2D-SF), with the aim to increase the aggregate data rate of CMS while maintaining the CMS short-term fairness. This protocol includes a first step, in which users with the best channel conditions are served via CMS directly by the eNodeB. Then, the served nodes forward the received data to the excluded users over D2D links. The peculiarity of this approach is that the forwarding devices send data simultaneously on the same frequency to their D2D receivers.