# Enabling Trustworthy Multicast Wireless Services through D2D Communications in 5G Networks
## 4 The Proposed eCMS-sD2D Protocol  
The reference scenario of this work is composed of a set of devices interested in downloading data over the 5G network. This generic scenario is suitable to several types of applications, from software update to video downloading, and to both human-oriented and machine-oriented communications. Data transmission is accomplished over a multicast transmission by the eNB. The considered architecture is composed of all the nodes foreseen in the eMBMS standard architecture, but procedures must be improved to make multicast transmission more suitable for future 5G networks. Our proposed solution called enhanced CMS with secure D2D communications (eCMS-sD2D) aims to enhance performance and security of a multicast CMS transmission. All steps of the proposed eCMS-sD2D solution are depicted in Figure 2.  
Our proposal is tailored to cellular 5G networks where the eNodeB acts as a central coordinator of both D2D and traditional communications. In fact, we focus on network-assisted D2D communications in a licensed spectrum, which is the preferred solution because of the expected gains it offers [1]. In such a scenario, it is a natural choice to also make the eNodeB work a trusted third party for the implementation of the security mechanisms. Differently, in a distributed scenario (that is not the case analyzed in this paper and that is not the preferred option for D2D communications), the selection of the node that will act as a trusted third party is a tricky choice that needs to be carefully investigated. _Step 1--Multicast service delivery notification_  
Through this first step, the eNB invites users interested in the service to form the multicast group (MG) by registering with the network. _Step 2--CQI collection_  
Users belonging to the MG send their CQI values to the eNB. Each user equipment (UE) has to communicate not only the cellular CQI value for its link towards the eNB, but also those for the D2D links to its neighbors. The eNB stores the received D2D CQI values in a matrix. _Step 3--Multicast and D2D configuration selection_  
Thanks to the collected information, the eNB can establish: (i) the set of registered UE to serve through the multicast transmission; (ii) the MCS to use for the multicast transmission in the CMS coverage area; (iii) the users served in a multicast that can act as relay nodes by forwarding data received directly by the eNB towards the cell-edge users; (iv) the D2D pair to establish; and (v) the MCSs to use in each D2D communication. The sD2D-eCMS approach foresees the formation of some D2D clusters. The cluster formation and resource allocation are important factors for the D2D-based multicast performance. This is true in general and not only for the eCMS-sD2D solution specifically presented in the current paper. However, in our previous research on the topic, we have investigated these aspects in detail. Since in this paper these aspects are not in focus, we rely on the results of our previous research. In particular, for the eCMS-sD2D proposal, we implement the best performing solution obtained in Reference [5]. The resulting configuration of each cluster is a cluster composed of the selected RN and the associated users that it has to serve through D2D communications. Users inside the cluster receive the multicast  
Figure 2: eCMS-sD2D procedures. content by the RN by means of unicast D2D communications, i.e., the RN will transmit the same content to each D2D receiver belonging to its cluster by setting the appropriate MCS to each D2D link. The multicast and D2D configuration selection is accomplished through an iterative procedure. First of all, the eNB orders the received cellular CQI values from the lowest to the highest. For every CQI value, it determines the subset of UE that can decode data transmitted with the correspondent MCS and the subset of UE, which, on the contrary, are in the worst channel conditions and must be served through D2D communications.