# Energy Efficient Communications for Reliable IoT Multicast 5G/Satellite Services
## 4 Multicast Nc Satellite Approach  
According to our system model, different group-based M2M/IoT devices can be envisaged, according to their service requirements, as presented in Figure 2. In each group (or cluster), actuators/smart devices need to receive the same forward control signals and/or data packets typically from a ground control center (CC). It is worth noticing that clusters are located in the same spotbeam. The forward link communication is shown in Figure 2 only to highlight the multicast scenario. A direct access can be provided for sensors / devices to communicate with a satellite in the case of M2M via satellite proprietary protocol; however, they could be based on the DVB-S2 standard for the forward link (the link from the CC on the ground to the actuators/devices) and on DVB-RCS2 standard for the return link (the link from the on-Earth sensors/devices to the CC), or even on the protocols selected by the 3 GPP long-term evolution (LTE) via satellite [7]. Figure 2 also highlights: (1) the multicast communications from satellite to a cluster of sensors and actuators; and (2) the indirect access mode where each sensor communicates with the satellite through gateway (GW). Since satellite has generally limited computation capability, for the sake of reducing complexity, the NC operations are carried out at the CC and a transparent Geostationary Earth Orbit (GEO) satellite is considered. In the following, we focus on the dissemination of the data flow \(\mathcal{N}\) to all the on-Earth nodes. Each content is composed of \(N\) packets, which have been processed by the CC station according to the RLNC principle [18; 21; 27], i.e., by transmitting \(K>N\) linear combinations of the \(N\) original data packets with random coefficients belonging to \(\mathrm{GF}(q)\), i.e., a Galois field of size \(q\). To this purpose, it is required that the overall outage probability is lower than a predefined value \(\theta\), where \((1-\theta)\) represents the probability that all the on-Earth devices correctly received the content. Due to satellite channel errors and the consequent packet erasures, the NC redundancy, i.e., \(K-N\), remarkably increases. To mitigate this effect and to satisfy the outage probability, since no acknowledgement is provided by the intended destinations, we adopt OFDM signal where at the source \(m\) replicas of the data symbols are transmitted over multiple subcarriers. Since in the multicast group sensors/devices may experience heterogeneous propagation conditions, we assume that the subcarriers used are selected at random among all the available ones. This increasing of the data reliability impacts on the overall energy consumption, which is directly proportional to \(m\). The redundancy (and, hence, the energetic cost) jointly introduced by the NC scheme (\(K\)) and the symbols repetition factor (\(m\)) needs to be traded-off with reliability of the delivery with respect to different application scenarios. Therefore, an optimization policy is proposed in the following section to identify the minimum value of \(m\) that obtains a lower overall energy consumption to complete the delivery to all the on-Earth nodes, while matching a target outage probability. This policy considers an adaptive transmission strategy based on the signal-to-noise ratio (SNR), which affects packets erasure. According to most modern standards (for both cellular and satellite systems), different modulations (M-PSK and M-QAM) can be considered according to the channel impairments. Further, different services can be characterized according to the different Quality of Services (QoS) requirements. Usually, sending control commands to a group of actuators requires low data rate and, consequently, a QPSK modulation with a low spectral efficiency can be adopted; otherwise, high-order modulations can be used, e.g., for content distribution services, but this is obviously limited by the channel propagation conditions.