# Review and Evaluation of MAC Protocols for Satellite IoT Systems Using Nanoatellites
## 2 Overview of Space and Communications Systems
### Communication Satellite Systems  
One of the areas in which satellites have been relevant is in communication networks. Due to the innate capacity of these spacecraft to cover the whole terrestrial surface, satellite systems are able to provide connectivity to remote or isolated areas that by other means are almost impossible to connect. There are three main types of architecture used in satellite communication systems: store and forward, bent-pipe, and crosslink [8]. In the first, the satellite retrieves data from one point, stores it for some time, and then downloads it to the first ground station it establishes a connection with. In the second case, the satellite acts as a relay, collecting data and retransmitting it to another point on Earth. In the crosslink architecture, the data is transmitted immediately through a satellite network via inter-satellite links. Satellite communication systems can be deployed in different orbits, offering a different set of services according to the channel/network conditions derived from the characteristics of the orbit of deployment. The types of orbits are the following:  
* Geosynchronous equatorial orbit (GEO): This corresponds to an orbit whose rotation period is the same as the Earth's. Consequently, the satellite seems to \"stand still\" to an observer at one point on the planet. To achieve this effect, the satellites are placed at a distance of approximately 35,786 km from Earth. Given such a long distance, the communication delays are considerable, in the order of 120 ms, in the satellite-ground direction or vice versa, for the best scenario; also, the transmission power required to establish effective links is high. Nevertheless, these systems have an excellent and broad coverage, reaching a 30% of the Earth's surface. The placement process of a satellite into this orbit is an expensive task, and in order to remain at that position, the crew on the ground must perform orbital maneuvers from time to time. * Low Earth orbit (LEO): Most of the satellites in space today are placed in this type of orbit. Its height ranges from 300-2000 km and, therefore, the delay in communications is low, in the order of tens of milliseconds for the worst case. The transmission power required to establish the links from this orbit are as low as hundreds of milliwatts [16]. Satellites in this orbit have low  
Figure 2: Active Earth orbiting satellites, separated by country of origin and type of orbit, from a total of 1957 active satellites reported until 30 th November 2018. Data published by the Union of Concerned Scientists (UCS) in its annual report [15]. temporal and spatial coverage. Because of the speed--about 7.5 km per second for a satellite in a 500 km orbit--the Doppler effect has to be considered in these systems. * Medium Earth orbit (MEO): Heights are between the low and geostationary orbits--2000-35,786 km. One example system, the O3b network, is placed at a height of 8000 km and has a theoretical minimum delay of 26 ms satellite-ground, or vice versa. All global navigation satellite systems (GNSS) constellations are placed in this orbit. * Highly elliptical orbit (HEO): Orbits with a large apogee and a small perigee. The most famous of this kind is the Molinya orbit, which offers large coverage for high latitudes. Another example is the Tundra orbit. In Molinya, the apogee is greater than a geostationary--about 40,000 km. Satellites in this particular orbit have an approximate period of 12 h. The Soviet Union was the first country to use it to provide communication services throughout its territory and also to obtain meteorological images. The classical services provided by satellite communication systems are the following:
* Broadband communications: The commercial satellite networks providing this service offer connectivity with broadband data rates. For example, the new Iridium's NEXT constellation offers connectivity at 1.5 Mbps [17], whereas the Inmarsat's BGAN HDR offers connections at 800 kbps [18].