Satellite Link Design Considerations

Satellite links are used to deliver long distance telecommunications voice/data services, broadband internet services, television broadcast and communications with remote and hard to reach locations. 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 in the uplink depends on how strong the signal is transmitted from the source station and how the satellite receives it. On the downlink side, signal quality depends on how strong the satellite retransmits the signal and how the station receives it.

Earth stations are categorized by the size of its antenna. Large station has antenna with 10 to 30 meters in diameter. Small station has antenna with diameter between 1 to 10 meters. VSAT (Very Small Aperture Terminal) has antenna with 0.3 to 1 meter in diameter. There are three earth station system parameters need to be considered when designing satellite links. The first one is transmitter EIRP (Effective Isotropic Radiated Power) which is equal to Pt (transmitter output power) multiple by Gt (transmit antenna gain). EIRP measures the signal power sends out from the transmit earth station. The second parameter is the Figure of Merit which is equal to G (receive antenna gain) divided by T (system noise temperature). G/T measures the sensitivity of the receiving system and the quality of the received signal. The third parameter is the 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) in the information channel. A satellite communications system designer must attempt to ensure a minimum Eb/No in the receiver channels to satisfy the link BER criteria which also meet the constraints on satellite transmit power and RF bandwidth. In digital transmission, Eb/No in a baseband channel depends on C/N (carrier to noise ratio) of the receiver, the type of modulation used in 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 in the earth station/satellite and N is the noise plus interference (caused by earth stations of adjacent satellite) power in the receiver.

Loss due to rain is the most important impairment to the transmission of satellite signal. Rain attenuation is a function of rain rate at earth station location and satellite link carrier frequency. Average value of rain rate (mm/hr) of a country can 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 needed 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) can be calculated using the longitude of the geosynchronous satellite plus the latitude and longitude of the earth station. The deviation of the antenna pointing direction from the antenna electric axis is termed the antenna pointing error. This error reduces the gain of the earth station antenna because 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 both transmit and receive antennas.