| Satellite links are used to deliver long distance | | | | bit noise density ratio) in the information channel. A |
| telecommunications voice/data services, broadband | | | | satellite communications system designer must |
| internet services, television broadcast and | | | | attempt to ensure a minimum Eb/No in the receiver |
| communications with remote and hard to reach | | | | channels to satisfy the link BER criteria which also |
| locations. Satellite link consists of an uplink and a | | | | meet the constraints on satellite transmit power and |
| downlink. Uplink connects transmit earth station to | | | | RF bandwidth. In digital transmission, Eb/No in a |
| satellite and downlink connects satellite to the receive | | | | baseband channel depends on C/N (carrier to noise |
| earth station. Signal quality in the uplink depends on | | | | ratio) of the receiver, the type of modulation used in |
| how strong the signal is transmitted from the source | | | | the baseband channel and the channel bandwidth. |
| earth station and how the satellite receives it. On the | | | | Hence, the main concern in design and analysis of |
| downlink side, signal quality depends on how strong | | | | satellite link is to calculate and evaluate the link C/N in |
| the satellite retransmits the signal and how the earth | | | | which C is the received carrier power in the earth |
| station receives it. | | | | station/satellite and N is the noise plus interference |
| Earth stations are categorized by the size of the | | | | (caused by earth stations of adjacent satellite) |
| earth station antenna. Large earth station has | | | | power in the receiver. |
| antenna with 10 to 30 meters in diameter. Small earth | | | | Loss due to rain is the most important impairment to |
| station has antenna with diameter between 1 to 10 | | | | the transmission of satellite signal. Rain attenuation is |
| meters. VSAT (Very Small Aperture Terminal) earth | | | | a function of rain rate at earth station location and |
| station has antenna with 0.3 to 1 meter in diameter. | | | | satellite link carrier frequency. Average value of rain |
| There are three earth station system parameters | | | | rate (mm/hr) of a country can be obtained from the |
| need to be considered when designing satellite links. | | | | department of Meteorology of that country. Rain |
| The first one is transmitter EIRP (Effective Isotropic | | | | effects become severe at wavelength approach the |
| Radiated Power) which is equal to Pt (transmitter | | | | rain drop size. If the satellite link is to maintain during |
| output power) × Gt (transmit antenna gain). EIRP | | | | rainfall, then extra transmit power is needed to |
| measures the signal power sends out from the | | | | overcome the maximum attenuation induced by the |
| transmit earth station. The second parameter is the | | | | rain. Hence accurate assessment of expected rain |
| earth station Figure of Merit which is equal to G | | | | loss needs to be made when evaluating link |
| (receive antenna gain) ÷ T (system noise | | | | parameters. |
| temperature). G/T measures the sensitivity of the | | | | The earth station antenna look angle (azimuth & |
| receiving system and the quality of the received | | | | elevation) can be calculated using the longitude of the |
| signal. The third parameter is the earth station | | | | geosynchronous satellite plus the latitude and |
| system noise temperature which measures the | | | | longitude of the earth station. The deviation of the |
| amount of noise power generated by the receive | | | | earth station antenna pointing direction from the |
| earth station. | | | | antenna electric axis is termed the antenna pointing |
| The main objective of satellite link design is to | | | | error. This error reduces the gain of the earth station |
| maximize link data rate while minimizing the BER (Bit | | | | antenna because the antenna gain is maximum when |
| Error Rate) at the receiver subjects to the constraint | | | | measured at its electric axis. Hence, antenna point |
| of received power and channel bandwidth. The BER | | | | loss needs to be evaluated in satellite link design and |
| of a satellite link is a function of Eb/No (energy per | | | | it applies to both transmit and receive antennas. |