# Enabling Trustworthy Multicast Wireless Services through D2D Communications in 5G Networks
## 3 Background  
LTE-A is the solution proposed by 3 GPP to bring broadband on mobile radio systems [22]. It offers a bandwidth of up to 100 MHz by exploiting the following additional bands compared to LTE: 450-470 MHz, 698-862 MHz, 790-862 MHz, 2.3-2.4 GHz, 3.4-4.2 GHz, and 4.4-4.99 GHz. At the physical layer, Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) are used, respectively, for the downlink and the uplink directions. In OFDMA, the Resource Block (RB) is the smallest unit of resources that can be allocated to a user. The RB is 180 kHz wide in frequency and 1 slot long in time. In frequency, resource blocks are either 12 \(\times\) 15 kHz subcarriers or 24 \(\times\) 7.5 kHz subcarriers wide. The number of subcarriers used per RB for most channels and signals is 12 subcarriers. The number of RBs to allocate to each user varies depending on the available bandwidth. For example, if a bandwidth of 20 MHz is available, 100 RBs can be allocated. Every millisecond, the eNodeB decides how many RBs to assign to each user based on the Channel Quality Indicator (CQI) that has been communicated to it. CQI also determines which Modulation and Coding Scheme (MCS) that the user can support. The CQI to MCS mapping foreseen in LTE-A networks is reported in Table 1[5].  
The eMBMS architecture, depicted in Figure 1, has been standardized to support multicast and broadcast services over LTE-A networks [3]. As explained in [16], the eMBMS architecture defines nodes belonging both to the radio access network (E-UTRAN) and to the evolved packet core (EPC). The eNB and the multicell/multicast coordination entity (MCE) belong to the E-UTRAN. The former is the evolved network node responsible for the direct interaction with users, and the latter has to manage the coordination of the various eNBs. It allocates resources to each eNB and coordinates admission control and reporting necessary to the MBMS session. The EPC is composed of the broadcast multicast-service center (BM-SC) and the MBMS-gateway (MBMS-GW). The first is responsible for the initialization of the MBMS session and for the management of some security functions (e.g., the authorizations for the MBMS subscribers), the second is in charge of forwarding the MBMS packets to the eNBs involved in the delivery service. These nodes need to be enhanced in order to support multicasting over future 5G networks.  
In order to efficiently manage the multicast and broadcast services over the forthcoming 5G network, two main issues have to be faced: designing efficient radio resource allocation algorithms and providing trustworthy communications. With respect to the resource allocation problem, as previously discussed, the CMS approach [4] has been traditionally utilized for delivering multicast content because of its native short-term fairness capability. The possibility to improve the performance of CMS by exploiting D2D communication has been proposed in [5]. However, since D2D communications are managed directly by the users, serious security problems arise.