# Energy Efficient Communications for Reliable IoT Multicast 5G/Satellite Services
## 1 Introduction  
The new fifth generation (5G) cellular system has been recently defined as the network of networks because it should consist of multiple seamlessly-integrated radio access technologies (RATs). In addition, the 5G system has been addressed as a densification of networks for its ability to dynamically integrate heterogeneous networks, from Wi-Fi access points, machine-to-machine (M2M) communications, and Device to Device (D2D) communications up to small cells also operating in the millimeter wave bands [1; 2]. Finally, 5G could be considered a dense network for its challenge to connect a massive number of things, i.e., the billions of objects and devices expected in 2022 to be on the Internet of Things (IoT) [3; 4; 5]. Recently, terrestrial low-power, wide area network (LPWAN) technologies have been proposed to interconnect massive IoT devices on licensed/unlicensed frequencies, including proprietary and open standard options, for instance the proprietary, unlicensed: (1) SigFox ultra-narrowband technology, operating over a public network in the 868 MHz or 902 MHz bands; (2) LoRa spread-spectrum technology transmitting in several sub-gigahertz frequencies of Industrial, Scientific and Medical (ISM) band; and (3) the open standards of the 3 rd Generation Partnership Project (3 GPP), i.e., (i) the Narrowband-IoT (NB-IoT); and (ii) LTE-M operating on existing licensed bands of the Global System for Mobile communications (GSM) and Long Term Evolution (LTE), respectively. In this context, satellite technology plays a key role to provide anywhere global IoT connectivity, especially for mission critical and industrial applications. LPWAN terrestrial solutions can support massive IoT devices, while a satellite link, usually via a concentrator (gateway) can provide connectivity among LPWAN networks in remote areas such as desolate tundra or oil platforms on the sea. For thesescenarios, the latency requirements are often relaxed for many IoT applications, e.g., for continuous monitoring of air pollution or wildlife positions, and the low data rate transmission allows the use of low bandwidth satellite infrastructure [6]. The IoT platform provider Stream Technologies and the LoRa Alliance already support reliable delivering of near real-time communications for a global connectivity over Iridium mesh architecture, which comprises 66 Low Earth Orbit (LEO) satellites. In addition, Vodafone and Inmarsat provide satellite backhaul for IoT devices to remotely track animals in wide geographical rural areas. Additionally, the support of direct access of IoT devices to satellite is the challenge taken by ORBCOMM even if the requirements of low maintenance costs and extremely long battery duration are quite demanding. In any case, the satellite segment must be part of the mix of 5G access technologies, as considered in 3 GPP Release 16, for its major advantage to cover global areas and for providing the increasing dominant IoT connectivity guaranteed by the advent of 5G era. Another key point of satellite technology is the ability to _multicast_ a message to a group of IoT devices over a large area, hence driving down cost. This is particularly relevant in several M2M/IoT applications, where smart devices could be grouped according to their services requirements and/or for the information that they need to receive. In this context, reliable satellite multicast communications can reduce the control signals overhead and the overall energy consumption. For example, the forward link communication could be used to provide alerts or commands to a group of actuators deployed in a large and inaccessible area or in emergency situations to support communications among rescuers for updating periodically their position in disaster area and/or for sending some updates including warning, alarms and security messages [7].