# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 1 Introduction  
Non-orthogonal multiple access (NOMA)-based systems have been recently investigated by the 3 rd Generation Partnership Project (3 GPP) [1] as a promising set of emerging technologies able to provide a more efficient utilization of wireless resources for future 5G networks. Research activities have been accelerating in recent years around these technologies but are mainly confined to theoretical and simulation analysis for terrestrial wireless applications. Ten years before the 3 GPP 5G standardization effort started, NOMA technologies were pioneered for Internet of Things (IoT) applications to exploit, at best, the limited resources of satellite-based networks, starting from mobile satellite service (MSS) applications below 3 GHz. The requirements set forth for MSS systems were as follows:  
* Efficiently and reliably support a very large number of users, sporadically transmitting small to medium-sized packets, typical of satellite-based IoT applications;
* Capable of operating in systems with a limited channelization bandwidth per service area (e.g., from 0.2 to a few megahertz);
* Energy-efficient solution allowing unattended terminal operation for a long time;* Easy network scalability, overhead minimization and low-cost, easy to install terminals. These challenging requirements stimulated the search for an appropriate solution, and the result was the development of the first NOMA-based system using an Enhanced Spread Spectrum ALOHA (E-SSA) [2]. Random access (RA) scheme, featuring iterative successive interference cancellation (i-SIC), implemented, for the first time, in the S-band MSS frequency range [3]. The solution was also standardized by the European Telecommunications Standards Institute (ETSI) as S-band Mobile Interactive Multimedia (S-MIM) [4]. Once the solution developed for those relatively low frequency bands demonstrated its excellent efficiency and flexibility, it was decided to extend the use of this NOMA-based system to higher frequency bands. The S-MIM extension targeted the Ku and Ka bands (corresponding to 11-14 GHz and 20-30 GHz frequency ranges, respectively), where a few gigahertz of bandwidth per service area was available and commercially exploited by a large number of geostationary satellites. This development effort has materialized into the specification and implementation of the so-called F-SIM system [5] and the launch of the Eutelsat SmartLNB technology, today operationally deployed in four continents under the \"IOT FIRST\" brand, with SmartLNB terminals reaching their third generation [6]. For the first time, a satellite NOMA-based system has been conceived, developed, industrialized and put into operation, and hence several lessons can be derived from this experience and used to better guide future terrestrial wireless-related developments. This is opening up the possibility of developing and exploiting satellite- and terrestrial-based systems in a synergic and interoperable architecture. The renewed interest in 3 GPP in the integration between satellite-based and terrestrial-based 5G systems is a good precursor to the integration of a NOMA-based multiple access system, starting from a proposal for 3 GPP to be integrated in release 18 of the 5G standard. A proposal inspired by the long development and operational experience cumulated in the last 10 years, in the actual implementation of E-SSA-based systems, may represent a solid premise for an effective integration of satellite and terrestrial systems for massive machine-type communications (mMTC).