Blog: Satellite Link Design Considerations

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Satellite links are used to deliver long distance telecommunications voice/data services, broadband world wide web services, television broadcast and communications with remote and hard to reach areas. Satellite link consists of an uplink and a downlink. Uplink connects transmit earth station to satellite and downlink connects satellite to the receive earth station. Signal quality within the uplink depends on how strong the signal is transmitted from the source earth station and how the satellite receives it. Around the downlink side, signal quality depends on how strong the satellite retransmits the signal and how the earth station receives it.

 Earth stations are categorized by the size from the earth station antenna. Large earth station has antenna with ten to 30 meters in diameter. Small earth station has antenna with diameter between 1 to 10 meters. VSAT (Very Small Aperture Terminal) earth station has antenna with 0.3 to 1 meter in diameter. You can find three earth station system parameters need to be considered when designing satellite links. The first 1 is transmitter EIRP (Effective Isotropic Radiated Power) which is equal to Pt (transmitter output energy) Gt (transmit antenna gain). EIRP measures the signal energy sends out from the transmit earth station. The second parameter may be the earth station Figure of Merit which is equal to G (receive antenna gain) T (system noise temperature). G/T measures the sensitivity on the receiving system as well as the quality of the received signal. The third parameter will be the earth station system noise temperature which measures the amount of noise power generated by the receive earth station.

 The main objective of satellite link design is to maximize link data rate while minimizing the BER (Bit Error Rate) at the receiver subjects to the constraint of received power and channel bandwidth. The BER of a satellite link is a function of Eb/No (energy per bit noise density ratio) within the information channel. A satellite communications system designer have to attempt to ensure a minimum Eb/No within the receiver channels to satisfy the link BER criteria which also meet the constraints on satellite transmit energy and RF bandwidth. In digital transmission, Eb/No within a baseband channel depends on C/N (carrier to noise ratio) from the receiver, the type of modulation used within the baseband channel and the channel bandwidth. Hence, the main concern in design and analysis of satellite link is to calculate and evaluate the link C/N in which C is the received carrier power inside the earth station/satellite and N may be the noise plus interference (caused by earth stations of adjacent satellite) energy inside the receiver.

 Loss due to rain is the most important impairment to the transmission of satellite signal. Rain attenuation is really a function of rain rate at earth station location and satellite link carrier frequency. Average worth of rain rate (mm/hr) of a country might be obtained from the department of Meteorology of that country. Rain effects become severe at wavelength approach the rain drop size. If the satellite link is to maintain during rainfall, then extra transmit power is necessary to overcome the maximum attenuation induced by the rain. Hence accurate assessment of expected rain loss needs to be made when evaluating link parameters.

 The earth station antenna look angle (azimuth & elevation) may be calculated using the longitude of the geosynchronous satellite plus the latitude and longitude of the earth station. The deviation from the earth station antenna pointing direction from the antenna electric axis is termed the antenna pointing error. This error reduces the gain on the earth station antenna since the antenna gain is maximum when measured at its electric axis. Hence, antenna point loss needs to be evaluated in satellite link design and it applies to each transmit and receive antennas.