# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 2 The S-MIM System  
### Historic Background  
A few years after the turn of the millennium, the European Commission (EC) accepted the satellite industry proposal to bring to use some spectrum in the S band, the mobile satellite service (MSS) band, ranging between 1980-2010 and 2170-2200 MHz [7]. The frequency allocation also allowed the deployment of iso-frequency terrestrial gap fillers to ensure high-quality service provision in urban and suburban areas. The satellite band allocation was conveniently adjacent to the terrestrial third-generation Universal Mobile Telecommunication System (UMTS) allocated band, thus making it possible to exploit synergies between satellite and terrestrial UMTS services. The main difficulty was that, in order to support mobility, the user antenna had to be very small--and this implied the need for a large deployable antenna reflector on the satellite, which was at the limit of what industry could provide.  
The EC decided to split the available bandwidth into two slots (each comprising 15 MHz for the downlink and 15 MHz for the uplink) to be operated over the whole European Union by two different entities. After a competitive selection process, Solaris Mobile Limited and Inmarsat Ventures Limited were assigned the S-band MSS spectrum.  
The main use case scenarios identified for the MSS were as follows:  
* Broadcasting multimedia content to handheld user terminals, in the spirit of the XMRadio/Sirius experience in the US, but extended to encompass video content broadcasting;
* Mobile data acquisition services, such as collecting data from mobile sensors (e.g., vehicles), toll payments and environmental monitoring--today considered as part of the \"Machine to Machine\" (M2M) or \"Internet of Things\" (IoT) services.  
The two use cases were perfectly complementary, as each used only one direction of the satellite path (forward link only for the multimedia broadcasting, and almost exclusively the return link for mobile data acquisition/IoT). IoT applications have a great market potential, as the number of \"connected objects\" is expected to have an exponential growth in the coming years. Consequently, the satellite, complemented by the ancillary terrestrial gap fillers network, could capture this new market quickly, providing pan-European coverage. At the same time, terrestrial networks were not yet prepared to support mMTC, and their planned coverage was concentrated in populated areas. The satellite had the advantage of full global coverage, including unpopulated areas where mobile users or objects should be served (e.g., for environmental monitoring).  
Some stringent requirements on the selected communication protocol had to be satisfied in order to serve the mobile IoT market from the satellite:  
* Reliable performance when operating in typical land mobile satellite (LMS) channels;
* Massive scalability, i.e., capability to handle a very large number of objects (order of millions);
* High spectral efficiency, as the available spectrum was limited. It was necessary to accommodate all the capacity requests of a spot beam (roughly the size of a European country such as Italy or Germany) in the 5 MHz allocated per beam, and to provide a cost per bit appropriate for the IoT market;
* Low-cost technology for the objects: as the typical sensor costs just a few dollars, the communication part should be of the same order of cost. This implies, in particular, a limited transmit power, simple algorithms and loose requirements on clock synchronization;
* Optimized for small transactions, typical of objects communication, minimizing overheads such as IP headers or bandwidth assignment demands.  
As the search for an optimal protocol for the satellite broadcast application had already been successfully concluded by means of the standardized DVB-SH protocol [8; 9; 10], the effort towards the satellite IoT protocol was concentrated on the return link only, and the future ETSI S-MIM standard would be a combination of DVB-SH and the newly defined E-SSA return link protocol.