Consider the general structure of a Satellite Communication System,
This consists of a satellite in space that links many earth stations on the ground. The user is connected the earth station through terrestrial network. This terrestrial network may be a telephone switch or a dedicated link to the earth station.
The user generates the base-band signal that is processed and transmitted to the satellite at the earth station. Thus, the satellite may be thought of as a large repeater in space that receives the modulated RF carriers in its up-link (earth to space) frequency spectrum from all the earth stations in the network, amplifies these carriers and re-transmits them back to the earth in the down-link (space to earth) frequency spectrum which is different from the up-link frequency spectrum in order to avoid the interference.
The signal at the receiving earth station is processed to get back the baseband signal which is then sent to the user through a terrestrial network.
Had there been no difference in the up-link and down-link frequencies, the satellite transmitted signals would have blocked up the up-link received signals and so there would have been no isolation between the transmitter output and the receiver input.
On the guidelines of WARC-1979, commercial communication satellite use a frequency band of 500MHz bandwidth near 6GHz for up-link transmission and another 500MHz bandwidth near 4GHz for down-link transmission (i.e. 6/4 GHz band). In fact an up-link of 5.725 to 7.075GHz and a down-link of 3.4 to 4.8GHZ is used.
The 500MHz allocation is usually divided into 12 channels of approximately 40MHz each and the transmit power per channel is typically of the order of 5 to 10W. This allows each of up to 12 transponders to carry one TV channel or 1500 analog FM voice circuits.
This 6/4 band have been the most popular because they offer the fewest propagation problems and also RF components for these bands have been readily available.
Rain attenuation is also not much serious at these bands. Sky noise is also low at 4GHz and so it is possible to build receiving system with low noise at 4GHz.
With the overcrowding of GEO satellites at 6/4 GHz band, 14/12 GHz band is also being used in commercial communication satellites.
A third band where extremely high capacities are potentially available is the 30/20 GHz band.
Consider the basic block diagram of an earth station,
The baseband signal from the terrestrial network enters the earth station at the transmitter after having processed by the baseband equipment.
After encoding and modulating the baseband signal, it is converted to the uplink frequency.
Then it is amplified and directed to the appropriate polarization port of the antenna feed.
The signal received from the satellite is amplified in a low noise amplifier first and is then down converted to the downlink frequency. It is then demodulated and decoded and then the original baseband signal is obtained.
The isolation of low noise receiver from the high power transmitter is of much concern in the design consideration of earth station. There may also be satellite/earth terminal mutual interference effects. Other sources of interference include ground microwaves relay links, sun transit effects and inter-modulation products generated in the transponder or earth terminal.
Before 1983, the spacing between two GEO satellites was established at 40 of the equatorial arc and the smallest earth station antenna for the simultaneous transmit-receive operations was 5m in diameter. Now the spacing allowed between two adjacent satellites in space is 20 along the equatorial arc. The close spacing has allowed twice as many satellites to occupy the same orbital arc.
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