# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 7 Conclusions  
NOMA technologies were pioneered about ten years ago in the satellite domain to exploit, at best, the growing demand for IoT applications, characterized by very large populations of users, sporadically transmitting small to medium-size packets with low-cost, easy to install terminals.  
The key features of the developed E-SSA NOMA system are as follows:  
* The achievable very high spectral efficiency while operating in pure random access mode. Networks today in operation in some operational configurations are reaching close to 2 bits/s/Hz efficiency;
* The easy network scalability and the support in the same band of multiple configurations to match the different application needs;
* The very low-cost and low-power two-way satellite terminals developed and industrialized crossing, for the first time, the USD 100 cost threshold, pushing the satellite connectivity market towards the consumer market.  
The proposed NOMA system also allows the inclusion, on the same technical platform, of a new class of integrated digital modem and smart antenna systems capable of simplifying the installation and exploitation of satellite-based systems to a level comparable to terrestrial communication systems, i.e., not requiring any manual pointing support.  
The development of a common IoT specification at the disposal of competing terminal manufacturers has, for the first time, allowed the development of a satellite-based two-way system able to operate with multiple suppliers and interoperable terminals.  
An end-to-end software-defined radio architectural approach has been selected from the very beginning for both the user terminal and, most importantly, the gateway/hub. This new paradigm allows the whole satellite IoT system to evolve in time, both in terms of software upgrades and overall system management.  
In recent years, a number of adaptations and enhancements to the existing specifications have been proposed in order to further enhance the spectral efficiency of the E-SSA-based scheme, and to adapt it to different utilization scenarios. In particular, the main requirements addressed are the support of both GEO and LEO scenarios, the possibility of operating a massive number of ultra-low cost IoT terminals and the support of both fixed and mobile IoT applications.  
NOMA technology is capable of exploiting, at best, the requirements stemming from IoT applications and, at the same time, adapting well to the specific characteristics of terrestrial and multi-layer satellite networks (GSO and non-GSO). The long development and operational experience cumulated in the last ten years, in the actual implementation of E-SSA-based IoT satellite systems, provides a solid ground for an effective integration of terrestrial and satellite networks for mMTC services in 5G and beyond. The challenge is to combine the NOMA high performance with affordable complexity and an extremely low cost for both the user terminal and the associated infrastructure. The objective is to make mMTC a reliable, affordable and scalable infrastructure for billions of devices to be interconnected.  
R.D.G. conceptualization. A.A. mainly contributed to Sections 4 and 5. D.F. mainly contributed to Section 2, Section 3, Section 4, Section 5. R.D.G. mainly contributed to Sections 1, 2 and 6. O.d.R.-H. mainly contributed to the abstract and Sections 2 and 6. S.C. mainly contributed to Sections 5 and 6. M.A. and R.A. mainly contributed to Sections 3-5. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.