Study on feasibility of SATCOM for railway communication

Final Report
4. SATELLITE COMMUNICATION SYSTEM DESCRIPTION
4.1 BASELINE SATCOM SYSTEM DESCRIPTION
A satellite communications system (commonly referred to as SATCOM system) is a communication
system that makes use of an artificial satellite launched into space to provide global
telecommunications. A SATCOM system is mainly composed of three segments: space segment,
ground (or control) segment and user (or terminal) segment, as can be seen in Figure 11.
SATELLITE PAYLOADS
When talking about satellite communications, one important concept is the communication payload,
which refers to the RF equipment and a set of receive and transmit antennas placed on satellites. It is
important to distinguish between two main types of payload technologies: transparent and
regenerative (i.e. On-Board Processing - OBP). Among these types, there exist several hybrid options.
Transparent payload means that satellite is acting as a mirror in the space whereas regenerative
payload is recovering the signal at the space segment and regenerating it again, employing the same
or different waveform (e.g. typically DVB-RCS → DVS-S2) in order to be transmitted to the receiver
station. The regenerative payload is able to remove the impairments suffered in the uplink, and in this
way the link budget on the downlink is highly improved enabling the use of smaller stations/terminals.
In conclusion, regenerative payloads ease the deployment of mesh networks where the delay can be
reduced due to only single hop is necessary to connect two terminals.
SATELLITE CONSTELLATIONS
Several constellations are usually used for SATCOM systems depending on the services to be
offered and the required coverage. Most of these systems are based on geostationary orbits (i.e.
GEO), which provide important advantages with respect to non-geostationary orbits (i.e. LEO, MEO
and HEO), since ground antenna tracking is not required and with only up to 3 satellites it can provide
worldwide coverage (except for polar regions). However, due to the distance from the earth surface
(~36.000 Km), big delays are encountered and conversational voice services are not too much
satisfactory from user QoE point of view. Furthermore, lower orbits like LEO or MEO are achieving
worldwide coverage, including Polar Regions (e.g. Iridium, Globalstar, etc) and enabling the use of the
handhelds. On the contrary, these constellations are requiring complex ground antenna tracking and
in some cases inter-satellite links with regenerative payloads.
The main features of the most typical constellations are presented in the following table:
Features GEO MEO LEO
Height (Km) 36,000 6,000-12,000 200-3000
Time per Orbit (hr) 24 5-12 1,5
Speed (Km/hr) 11,000 19,000 27,000
Time in site of GW Always 2-4hrs