Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

3.5 SATELLITE RADIO

Arthur C. Clarke proposed the concept of communication satellites. A communication satellite is a relay in the sky. If the satellite is placed at a distance of about 36,000 km above the surface of the earth, then it appears stationary with respect to the earth because it has an orbital period of 24 hours. This orbit is called a geostationary orbit, and the satellites are called geostationary satellites. As shown in Figure 3.4, three geostationary communication satellites can cover the entire earth.

Figure 3.4: Three geostationary satellites covering the entire earth.

On Earth, we need satellite antennas (which are a part of the Earth stations) that point toward the satellite for communication. A pair of frequencies is used for communication with the satellite—the frequency used from Earth station to the satellite is called the uplink frequency, and the frequency from the satellite to the Earth station is called the downlink frequency. The signals transmitted by an Earth station to the satellite are amplified and then relayed back to the receiving Earth stations.

The main attraction of communication satellites is distance insensitivity. To provide communication facilities across the continents and also to rural and remote areas where laying cables is difficult, satellite communication will be very attractive. However, satellite communication has a disadvantage—delay. The propagation time for the signal to travel all the way to the satellite and back is nearly 240 msec. Also, because the signal has to travel long distances, there will be signal attenuation, and high-sensitivity receivers are required at both the satellite and the Earth stations.

To develop networks using satellite communications, there are two types of configurations—mesh and star, which are shown in Figure 3.5.

Figure 3.5: Satellite communication network configurations.

In mesh configuration, two Earth stations can communicate via the satellite. In this configuration, the size of the antennas at the Earth stations is large (starting from 4.5 meters diameter).

In star configuration, there will be a central station (called the hub) and a number of Earth stations each with a Very Small Aperture Terminal (VSAT). When a VSAT has to communicate with another VSAT, the signal will be sent to the satellite, the satellite will relay the signal to the hub, the hub will amplify the signal and resend it to the satellite, and then the satellite will relay it to the other VSAT. In this configuration, the roundtrip delay will be double that of the mesh configuration. However the advantage is that smaller Earth stations can be used. VSAT communication is now used extensively for communication networks because of the low cost.

Satellite communication systems work in two configurations: mesh and star. In mesh configuration, Earth stations communicate with each other directly; in star configuration, the Earth stations communicate via a central station.

ITU allocated specific frequency bands for satellite communications. Some bands used for satellite communications are 6/4GHz, 14/12GHz, and 17/12GHz bands. As the frequency goes up, the size of the antenna goes down, and use of higher frequencies results in lower cost of equipment. However, the effect of rain on signals of different frequencies varies. Rain attenuation is more in 14/ 12GHz bands as compared to 6/4GHz bands.

In addition to the geostationary satellites, low Earth orbiting satellites also are being deployed for providing communication facilities.

In Chapter 13, "Satellite Communcation Systems," we will discuss satellite communication systems in greater detail.