Block diagram of earth station transmitter and receiver

A satellite communication channel provides broad-area coverage in a continental as well as intercontinental sense. Moreover, access to remote areas not covered by conventional cable or fiber communications is also a distinct feature of satellites. The basic elements of satellite communication system are earth stations, terres­trial system and users. The basic structure of a satellite communication system is shown in Fig. 24.7.

Satellite communication system consist of many earth stations on the ground and these are linked with a satellite in space. The user is connected to the earth station through a terrestrial network and this terrestrial network may be a telephone switch or dedicated link to a earth station. The user generates a baseband signal that is processed through a terrestrial network and transmitted to a satellite at the earth station. The satellite consists of a large number of repeaters in space, receives the modulated RF carrier in its uplink frequency spectrum from all the earth stations in the network, amplifies these carriers and retransmits them back to the earth stations in the downlink frequency spectrum. To avoid the interference downlink frequency spectrum should be different from uplink frequency spectrum. The signal at the receiving earth stations is processed to get back the baseband signal, it is sent to the user through a terrestrial network.

Commercial satellite communication system use a frequency band of 500 MHz bandwidth near 6 GHz for uplink transmission and another 500 MHz bandwidth near 4 GHz for downlink transmission. An uplink of 5.725 to 7.075 GHz and a downlink of 3.4 to 4.8 GHz is used. Since 6/4 GHz band is also used in many countries for microwave links (terrestrial communications), so the problem of mutual interference may occurs. The 500 MHz allocation of frequency band is usually divided into 12 channels of approximately 40 MHz each. This allows each of 12 transponders to carry one TV channel or 1500 analog FM voice circuits.

Modern satellite communication system also employ frequency reuse to increase the number of transponders in the allotted bandwidth. 6/4 GHz band is most popular because of less propagation problem. Rain attenuation and sky noise is low at 4 GHz so it is possible to build a receiving system. 14/12 GHz band is also used in commercial communication satellites because of over crowding of geostationary satellites at 6/4 GHz band. Here uplink is of 12.75 to 14.8 GHz and downlink of 10.7 to 12.3 GHz. This band may be extensively used in future. A third band is the 20/30 GHz band where 2.5 GHz bandwidth has been allocated with a uplink of 27.5 to 31 GHz and downlink of 18.1 to 21.2 GHz.

Some of the advantages and disadvantages of 6/4 GHz are given below:

Advantages of 6/4 GHz Band:

• In this band there is no absorption by the rain.
• It has the fewest propagation problems. Attenuation is low below 10 GHz at an elevation angle of 5° or more.
• There is no change of polarization when the waves pass through ionosphere.
• RF components for these band were being used for terrestrial relay links also and hence were available easily.
• Sky noise is low at 4 GHz and therefore signal-to-noise ratio at the receiving antenna is not much deteriorated.
• Broad beam width allowed large area coverage for receiving.
• It is easier to generate high power at 4 GHz at the satellite than at 11 GHz.

Disadvantages of 6/4 GHz Band:

• Bandwidth is limited to 500 MHz which can be extended to 1,000 MHz at the most by using orthogonal polarization scheme.
• Interference from other users is more. The band is rather congested.
• Power cannot be concentrated in very small area on the earth. Higher frequencies can produce narrower beams.
• Direct reception in home TVs is not easily possible because of the need of big sized parabolic dishes. Higher bands are required for the same.

Reasons for the Downlink Frequency to be Lower than the Uplink Frequency

Frequency of downlink is smaller than that of the uplink, several factors are there such as

• Output power amplifier in transponder : It is the most important factor because the final power amplifier in the transponder generates more power at lower frequencies than at higher frequencies.
• Effective area of the receiving antenna : The effective capture area of the downlink antenna should be more so as to receive more energy at the earth’s receiving antenna. Effective area of an antenna is directly proportional to the square of the frequency. Thus, the frequency should be kept lower.
• Path loss : It is less at lower frequencies than at higher frequencies.
• Beamwidth : It should be wider as a satellite would send energy to a large number of earth stations. Lower the frequency wider is the beamwidth.

The basic block diagram of an earth station transmitter is shown in Fig. 24.8.

The baseband signal from the terrestrial network is proc­essed through encoder and modulator, then it is converted to uplink frequency. Finally it is amplified by high power ampli­fier and directed towards the appropriate part of antenna. The block diagram of an earth station receiver is shown in Fig. 24.9.

The signal received from the satellite is processed through low noise amplifier and then it is down converted, demodulated by demodulator and decoded by de­coder. Thus the original baseband signal is obtained. In case of failure to maintain uninterruptable power some critical components will he installed with automatic switch over. The isolation of high power amplifier and low noise am­ plifier is of much concern in the design considerations of an earth station. Some mutual interference effects may be there and these includes interference caused by side lobes if antenna in­terferes with an adjacent satellite or an earth station equipment. Other sources of interference are microwave relay links, sun tran­sit and intermodulation produces generated in the transponder.

Before 1983 the spacing between the two geostationary satellites was 4° of an equatorial arc and the smallest earth station antenna diameter was 5 m. Nowadays the spacing allowed between two adjacent satellite in space is 2° along the equatorial arc. The closer spacing has allowed twice as many satellite to occupy the same orbital arc, so nowadays every earth station antenna is designed accordingly.

Advantages and Disadvantages of Satellite Communication System:

Communication through satellite has several advantages and disadvantages. Until the evolution of satellite communication system, long distance communication through space could be done by using cascaded radio relays, very low frequency (below 30 kHz) and high frequency or short waves radio (3-30 MHz). But the cascaded radio relays were limited to overland spans. So the satellite has given a major contribution in the field of communication, capable of transmitting high capacity over long distances either overland or water. Also because of its unique geometry, it is inherently a broadcast medium with a natural ability to transmit simultaneously from one point to an arbitrary number of other points within its converge area.

Advantages:

1. Point to multipoint communication is possible whereas terrestrial relays are point-to-point. This is because satellite relays are wide area broadcast.
2. Circuits for the satellite can be installed rapidly. Once the satellite is in position, earth stations can be installed and communication may be established within some days or even hours.
3. During critical conditions each earth stations may be removed relatively quickly from a location and reinstalled somewhere else.
4. Mobile communications can be easily achieved by the satellite communication system because of its flexibility in interconnecting mobile vehicles.
5. Satellite communication system has the economical advantage that the satellite costs are independent of the distance.
6. As compared to fibre optic cable, the satellite communication system has the advantage that the quality of transmitted signals and the location of stations, sending and receiving information is independent of distance.

Disadvantages:

1. With the satellite in position the communication path between the terrestrial transmitter and the receiver is approximately 75,000 km long (twice the distance of geostationary orbit from earth).
2. There is a delay of 1/4 second between the transmission and reception of a signal because the velocity of electromagnetic wave is 3 x 10 5 km/second. Thus between talks there is an elapse of 1/2 second and one may feel it annoying. This delay produces echo which is actually caused by an imperfect impedance matching.
3. The time delay reduces the efficiency of satellite in data transmission and long file transfer which carried out over the satellites.
4. Overcrowding of available bandwidths due to low antenna gains.
5. High atmospheric losses above 30 GHz limit carrier frequencies.

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