SATELLITE COMMUNICATIONS SYSTEM

In a geostationary satellite communication system, a message signal is transmitted from an earth station via an uplink to a satellite amplified in a transponder (i.e. electronic circuitry) on board the satellite, and then retransmitted from the satellite via a downlink to another earth station. The most popular frequency band for satellite communication is 6 GHZ (c-band) for the uplink and 4 GHZ for the downlink.

The uses of this frequency band others the following advantages.
Relatively in expensive microwave equipments.
Low attenuation due to rainfall, rainfall is the primary cause of signal degradation.
Insignificant sky background noise; the sky background noise due the random noise emissions from galactic, solar, and terrestrial sources) reaches at its lowest level between 1 to 10 GHZ.

However, radio interference limits the applications of communication satellite operating in the 6/4 GHZ band coincide with those used for terrestrial microwave systems. This problem is eliminated in the more powerful “second generation” communication satellites that operate in the 14/12 GHZ band (i.e. ku-band); moreover, the use of these higher frequencies makes it possible to build smaller and therefore less expensive antennas.

The basic components of a single transponder channel of a typical communication satellite. Specifically, the receiving antenna output of the uplink is applied to the cascade connection of the following components.

· Band pass filter, designed to separate the received signal from among the different radio channels.
· Low-noise amplifier.
· Frequency down converter, the purpose of which is to convert the received radio frequency (RF) signal to the desired downlink frequency.
· Traveling- wave Tube amplifier, which provides high gain over a wide band of frequencies.
· In traveling –wave Tube (TWT) an electromagnetic signal travels along a helix (i.e. a spring shaped coil of wire), while electrons in a high voltage beam travel through the helix at a speed close to that of the signal wave; the net result is the transfer power from the electrons to the wave, which grows rapidly as the signal wave travels down the helix.
· In a satellite, non linearity of the transponder is the primary cause of interference between users. To contain this serious problem, the traveling-wave tube amplifier in the transponder is purposely operated below capacity. Consequently we find that an in FDMA system the power efficiency of the system is reduced because of the necessary power back off of the traveling wave tube amplifier.