Description of the Technology
Communication satellites represent a first solution to enable broadband Internet access on board trains. The main advantages of such solutions are :
• The easy coverage of a large geographical area (one geostationary satellite can cover a quarter of the earth surface);
- • The well adapted broadband connectivity for connection and aggregation of the traffic of a large number of mobile terminals;
- • The resistance to high velocity;
- • The low CAPital EXpenditure (CAPEX) due to the absence of installation of a dedicated infrastructure on track.
Nevertheless, the use of satellites leads to several constraints on systems design, which have to be taken into account :
- • Use of satellite requires satellite in Line-Of-Sight (LOS) in order to obtain broadband connectivity. Any obstacle between the satellite and the receiving antenna (catenary, bridge, high buildings) generates fadings or total loss of signal;
- • Antennas require high antenna gain and a very thin beamwidth. It is then necessary to implement a precise tracking of the satellite. Moreover, train suffers of several movements, tracking solution of the satellite have to be even more precise in order to avoid interferences with other satellites;
- • NLOS areas, such as tunnels, urban areas or stations, can lead to signal cut-off of several minutes and require the combination with other technologies, so-called “gap-filler”. Two main solutions can be considered:
- - Satellite repeaters: one antenna is installed on the ground in order to recover the satellite signal and to redistribute it in the non-visibility areas. This kind of solution requires the deployment of an infrastructure along the track. Furthermore, specific authorizations have to be asked to railway infrastructure owner and to telecommunications regulator;
- - Vertical handover: allowing switching to other technologies, such as Wi-Fi, WiMAX or cellular networks (3G/4G).
- • Railway constraints have to be obviously taken into account: Electromagnetic Compatibility with existing systems, installation, maintenance and space to install the antennas. Furthermore, satellite solutions can provide high throughput by using large antennas. Railway constraints force the railway operators to limit the size of the antennas, limiting then the delivered throughput, especially in the case of double deck trains.
Different kinds of satellites exist (cf. Appendix B.4): Geostationary Earth Orbit (GEO) satellites, Medium Earth Orbit (MEO) satellites and Low Earth Orbit (LEO) satellites. GEO satellites are generally very attractive because they use a geosynchronous orbit located at 36,000 km from the surface of the Earth, at equator level, which allows them to be seen as a fixed point in the sky. Moreover, GEO satellites cover a large geographical area and they are the only ones capable of providing broadband connectivity for mobile users. Thus, they are largely used in several existing communication and broadcasting systems. Satellites may have still some drawbacks. The use of GEO satellites leads to important propagation delays (around 400 ms) compared to MEO or LEO ones. This propagation delay may become a problem in the case of highly interactive applications. Modifications and optimizations are then necessary to accelerate the TCP/IP flow. Furthermore, GEO satellites being at equator level, north latitudes are then at weak elevation angles. This conducts to a reduced availability of satellite in case of obstacles. Finally, bandwidth has high costs (more than 1.5 M euros in Europe for a 36MHz transponder per year).
Despite all these inconveniences, all connectivity solutions on board trains using satellite technology rely on GEO satellites. That can be explained by the fact that GEO satellites guarantee a large choice of products, constructors and satellite operators, together with a high capacity. Indeed, MEO and LEO are not able to provide broadband connectivity.
Another problem of satellite systems is a high OPerational EXpenditure (OPEX) due to the satellite capacity. Available throughput depends on satellite capacity; generated costs have to be taken into account in the business model. Nonetheless, clients desire more and more throughput, which arises bandwidth costs. An increase of the number of clients can generate an increase of incomes, but not an increase of throughput. Business model causes some big problems.
Satellites in Ka band can represent a solution to this problem because of their high capacity, which induces a reduction of bandwidth costs (3 to 5 less expensive than the Kuband). Moreover, satellites in Kaband operate at higher frequencies, which allows reducing the size of the antenna. The use of these satellites causes some problems yet. First of all, equipments in Ku band are not compatible with Ka band, which requires the development of new equipment fitting railway constraints. Moreover, signals in Ka band suffer of high attenuation in the case of bad atmospheric conditions (fog, snow, rain). These attenuations can reach 15 dB in worst cases. Finally, existing satellites in Ka band have a coverage area of about 250-500 km in order to allow a geographical reuse of frequency bands (and then optimize satellite capacity). A dynamic frequency allocation and a horizontal handover have to be implemented to assure connectivity of train from a cell to another. Global system will then be more complex. A complete study on Ka band still have to be performed, such as investigation on mobility effects and cell changes. These issues will be seen in the Chap. 3.
It is also important to notice that there have been recent developments regarding billing of bandwidth. Only bandwidth actually used is now charged, a “billing per us”. Furthermore, the future is to use flat antennas that can be much more easily installed on trains.