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Satellite Communication. Freeha Azmat. Configurations:. 1)star topology: local telephone network Failure at one point will cause a problem Redundancy is provided at central point 2)Mesh Network Each node is connected to every other node
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Satellite Communication FreehaAzmat
Configurations: • 1)star topology: • local telephone network • Failure at one point will cause a problem • Redundancy is provided at central point • 2)Mesh Network • Each node is connected to every other node • Encouraged for high capacity systems for security purposes • Complete connectivity with built in redundancy • Expensive approach • A hub must control the communication setup tear down process, but need not be involved in carrying traffic.
Components of Communication Satellite • Every satellite involves the transmission of information from • An originating ground station to the satellite(uplink) • Retransmission of information from the satellite back to ground(down link) • Satellite must have • Receiver and receive antenna • Transmitter and transmit antenna • Method for connecting the uplink to the down link for retransmission • Prime electrical power to run all of the electronics
Basic Elements Satellite or Space Segment: Besides satellite, the space segment includes the means on Earth necessary for launching and station keeping • 2-way as in the case of long distance call • Ground stations' uplink is down linked over a wide region’s in TV broadcast. Ground Station ,or Earth Segment The earth segment includes earth stations containing transmitters and receivers for transmission and reception of signals from satellites • Transmitters • Antennas
Transmitters: • The amount of power , which a satellite transmitter needs to send out ,depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. • Geo synchronous satellite …..altitude of 22300 miles while the low earth satellite is only a few hundred miles Antennas : • One of the biggest difference between a low earth satellite and geosynchronous satellite is in their antennas. • Difference in power can be brought by changing the antenna design • The easiest way is to make antenna larger.
Power Generation • The satellite must generate all of its own power on the spacecraft • Power is usually generated by large solar panels covered with solar cells (sunlight electricity) • When there s no sunlight, the satellites requires the batteries on board which can supply the required power for the necessary time and then recharge by the time of the nest period of eclipse
Basic Satellite systems Transponder • A satellite that merely relays the uplink carrier as a downlink is referred to as a relay satellite , or repeater satellite. • The satellite transmits the downlink by responding to the uplink. Crosslink • Communication between two earth stations not visible to the same satellite. • By spacing multiple satellites in proper orbits around the earth, worldwide communication between remote earth stations in different hemispheres can be performed.
Ground Relay: • Satellite relay system involving earth stations, near earth users( aircrafts, ships etc) and satellites • An earth station communicates to another earth station or to a user by transmitting to a relay satellite ,which relays the modulated carrier to the user.
Satellite Repeater A satellite repeater is an electronic assembly ,which ensures the following functions: 1)Amplification of signals: from an input power in the order of -100dbW to an output power of about 10dbW. 2)Frequency Gain: which avoids interference between the powerful transmitted signal and the weak incoming signal.
Communication Repeaters 1.Transparent Repeaters: Transparent repeaters in which the uplink signals are translated in frequency ,but otherwise unchanged in form. The transparent repeaters which are widely used in all types of communications spacecraft, are relatively simple to design and use and are well suited to many system applications
2.Processing repeaters : are those in which the uplink signals are changed in form by de modulation ,correlation ,or some other complex process. Processing repeaters are used to provide enhanced performance characteristics such as an increased immunity to interfering signals or increased communications capacity.
Up conversion and Down Conversion RF signal and IF signals: RF signal is coming from an antenna. So it contains electromagnetic waves. But when the signal is processed using electronics circuits, the range of frequency may not be sufficient. So that RF has to be convert to some other frequency acceptable by the circuit. And IF is the intermediate frequency used for tuning & other purpose.
Up conversion and Down Conversion Up conversion is the translation of intermediate frequency to RF frequency while Down conversion is translation of RF frequency to intermediate frequency (a)Up Conversion (b)Down Conversion
Low Noise Amplifier • Low noise amplifier is a non-linear device as its power gain characteristics are nonlinear. Two parameters used to describe the degree of non-linearity are • 1-db compression point • Third order intercept point
Noise Figure • LNA (Low noise Amplifier) is a special type of amplifier used in communication systems to amplify very weak signals captured by an antenna. • LNA is often located very close to an antenna • It is a key component which is placed at the front-end of a radio receiver circuit. Noise figure =(S/N)in –(S/R)out
High Power Amplifier • One of the key elements of any spacecraft transponder is the output transmitter, consisting of a high power amplifier(HPA) TWT Amplifier characteristics: • For maximum power efficiency ,it would be desirable to work at the peak of the curve(saturation),but for linearity it is necessary to operate at a lower point. • When only a single input carrier is involved ,this saturation causes no carrier distortion, but only a limitation to its output power.When the TWTA input corresponds to multiple carriers ,however as in FDMA format, this non-linearity of the saturation effect cause inter-modulation products(beats) among these carriers. • These beat terms produce Additional frequency components at the non-linear output that can interfere with the desired carriers.
