Satellite Communication -----General Principle

Satellite Communication:

Now what is basically satellite communication?
But before that you  must know what is communication satellite?

Communication satellite is nothing but a satellite placed in an orbit around the earth the earth that carries aboard some communication equipments ,thus enabling communication to be established between different points on earth

Basic Satellite system

The basic satellite system consists of satellites and earth stations(ES). The user is  connected with the earth station through a terrestrial or dedicated link. The base band signals from the user are processed at the earth station. The earth station modulates signal on the carrier frequency (in GHz range) and this modulated signal is amplified in a High Power Amlpifier (HPA) and radiated into space using narrow beamwidth antennas (like paraboloid reflector or Cassegrain ).
                                                                                                                The satellite receives the modulated signals through a high gain antenna.The received signal is amplified using a Low Noise Amplifier(LNA) and retransmitted to the destination earth station using the same antenna.
The path from ES to satellite is called uplink. And the path from satellite to ES is called downlink.

Now one might wonder that if the satellite is receiving and transmitting through the same antenna ,then is the satellite using same frequency for uplink and downlink?


The answer is NO. The reason why uplink and downlink frequency are not same is related to satellite trans-receivers.

• Trying to receive and retransmit an amplified version of the same uplink waveform at same satellite will cause unwanted feedback or ring around from the downlink waveform back into receiver.
• The satellite generates a signal that would jam its own receive.

Morever the uplink frequency is kept higher than downlink frequency.
There are basic two reasons for this:

• At high frequency attenuation is more and so power required will also be more .And this calls for the use of HPA with excellent ratings and good heat sinks.This will definitely increase the weight of equipment. For earth station an increase in weight will not make any significant difference but for an ES it will surely do!!
• ES must direct the beam towards satellite with high directionality and little beam spillover(losses) . Since beamwidth is inversely proportional to frequency.Hence if uplink frequency is kept high we would get high directive antennas

FREQUENCY BANDS used in Satellite Communication:

1. C-Band : Uplink (5.925-6.425 GHz), Downlink (3.7-4.2 GHz), Bandwidth (0.5 GHz).                It is basically used for television broadcast.       
2. Ku- Band: Uplink (14-14.5 GHz), Downlink (11.7-12.2 GHz), Bandwidth (0.5 GHz). It is used for television broadcast or fixed point services. Also used for non military applications.
3. Ka-Band: Uplink (27-30 GHz) or (30-31 GHz), Downlink (17-20 GHz) or (20 or 21GHz), Bandwidth (3 or 1 GHz). It is mostly uded for commercial broadcasting or military use.