## 2 Related Work and Background  
It is now a fact that multicast has significant potential to push the limits of next generation communication systems [9]. However, still several challenges have to be addressed. Among these, the design of effective radio resource management (RRM) policies is a critical point. In Orthogonal Frequency Division Multiple Access (OFDMA) systems, the Adaptive Modulation and Coding (AMC) mechanism is commonly used; it appropriately chooses the transmission parameters in accordance to the channel conditions perceived on each link towards the base station. In particular, the Modulation and Coding Scheme (MCS) is set in order to cope with the user's channel status (i.e., Channel Quality Indicator, CQI). In case of multicast services, transmission parameters must be selected by the base station on a per-group basis rather than on a per-user basis. It is highly probable that multicast group members measure different radio link qualities and, thus, require different MCSs. While poor links need robust MCSs to face hostile propagation conditions, users in favorable channel conditions could receive data at higher bit rates. The RRM procedures designed for data multicasting can be classified in three macro-categories: conservative, opportunistic, and subgroup-based. The main example of conservative approach is the Conventional Multicast Scheme (CMS) [10], where all users receive the desired multicast content with the MCS level required for the correct reception of the user in the worst channel conditions. Therefore, users with a good channel quality suffer from low data rate. To overcome this inefficiency, the Opportunistic Multicast Scheme (OMS) [11] transmits the content to only the users with the best channel every Transmission Time Interval (TTI) in order to maximize the overall system throughput. Although OMS better utilizes the spectrum, it suffers from poor short-term fairness. In order to ensure a trade-off between throughput and fairness, the Multicast Subgrouping (MS) is proposed in [12]. The main idea behind MS is to group multicast users with similar channel qualities and perform a per-subgroup resource allocation. D2D communications represent an innovative solution to improve the performance of users with low channel quality. Indeed, the D2D technology would offer the 5G system its capability of improving spectrum efficiency and throughput, reducing power consumption, enhancing cell coverage, and offloading the network. As we proposed in [13], high-performing D2D links allow some devices to forward a given video content, previously received by the cellular network, to cell-edge devices, thus improving their perceived quality. In particular, a D2D-enhanced CMS with Single Frequency (D2D-SF) is proposed with the aim to combine the use of CMS and D2D links for increasing the system aggregate data rate while preserving the short-term fairness, typically guaranteed by CMS. Besides increasing the overall throughput, D2D communications are essential for reducing the energy consumption. Currently, energy consumption has become a major concern in the design of 5G wireless network. In the literature, several works offer solutions for reducing the devices' energy consumption, with a consequent increase in the energy efficiency of the whole network. The energy efficiency improvement is achieved by following different approaches. In [14] a D2D-based solution is proposed that guarantees a trade-off between energy and spectral efficiency, with the aim to avoid co-channel interference between D2D and cellular links and to increase the devices' battery life. The authors model the radio resource problem as a non-cooperative game and implement a resource allocation algorithm that satisfies spectral efficiency and transmission power constraints.