Page 179 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 11: Wireless WAN

Notes
Figure 11.6: Geosynchronous Orbit

Source: http://k-12.pisd.edu/currinst/network/11_806A_4-2_SG.pdf
Because geosynchronous orbit satellites are so far away from the Earth, it takes a lot of power to
transmit a signal that will reach them. This makes them less suitable for use by small wireless
networks or mobile users. Another problem with geosynchronous satellites is latency. It takes a
relatively long time for a signal to reach a satellite, get processed, and then travel back to Earth.
This delay is called latency and can create problems for data transmissions. A third problem
arises because the signals are more susceptible to interference since they are transmitted over
such a long distance.
One way to overcome the power, latency, and interference problems is to use satellites closer
to Earth. But satellites in lower orbits do not maintain their position relative to the ground. If a
low orbit satellite were relaying data, the transmission would be interrupted when the satellite
moved out of range. The only way to use lower orbit satellites is to put enough of them in orbit
so that when one satellite moves out of range, another satellite has already moved into range and
can pick up the transmission. Think of this system as a cellular system in which the base stations
are moving as well, and hand-offs happen as the base stations and the mobile users move.
Figure 11.7: Low Earth Orbit (LEO) Satellite Network

Source: http://k-12.pisd.edu/currinst/network/11_806A_4-2_SG.pdf
A system of low Earth orbit (LEO) satellites (figure 11.7) requires so many satellites that the price
has been prohibitive until very recently. With the explosion of wireless users, consortiums of
companies have invested huge sums of money to create LEO satellite networks.
The Iridium system uses 66 satellites in LEO (750 kilometers). The satellites act as the base stations
for more than 1,600 cells that cover the entire surface of the globe. Each cell has 174 channels; the
system can handle over 280,000 calls simultaneously.
Another system called Teledesic will have 288 LEO satellites by 2002. The goal is to have systems
that can transmit (downlink) at 64 Mbps to the users and can receive (uplink) at 2 Mbps.

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