This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: May 2015

Input back-off and output back-off –

• The input back-off is the difference in dB between the carrier input power at the operating point and saturation point which would be required for single-carrier operation
• Output back-off is the corresponding drop in output power. Output back off is about 5dB less than input back-off.

Input back-off and output back-off ratio relationship

Uplink rain-fade margin and downlink rain fade margin –

Uplink rain fade margin –

• Rainfall results in attenuation of the signal and an increase in noise temperature, degrading the $[\frac{C}{N_0} ]$ at the satellite in two ways. The increase in noise, however, is not usually a major factor for the uplink. This is so because the satellite antenna is pointed toward a “hot” earth, and this added to the satellite receiver noise temperature tends to mask any additional noise induced by rain attenuation
• What is important is that the uplink carrier power at the satellite must be held within close limits for certain modes of operation, and some form of uplink power control is necessary to compensate for rain fades
• The power output from the satellite may be monitored by a central control station or in some cases by each earth station, and the power output from any given earth station may be increased if required to compensate for fading. Thus the earth-station HPA must have sufficient reserve power to meet the fade margin requirement
• As an example, for Ottawa, the rain attenuation exceeds 1.9 dB for 0.1 percent of the time. This means that to meet the specified power requirements at the input to the satellite for 99.9 percent of the time, the earth station must be capable of providing a 1.9-dB margin over the clear-sky conditions.

Downlink rain-fade margin –

• Rainfall introduces attenuation by absorption and scattering of signal energy, and the absorptive attenuation introduces noise. Let [A] dB represent the rain attenuation caused by absorption. The corresponding power loss ratio is $A =10^{[A]/10}$, and substituting this for L gives the effective noise temperature of the rain as

$T_rain=T_a (1-\frac{1}{A})$

• Here $T_a$ is known as the apparent absorber temperature. It is a measured parameter which is a function of many factors including the physical temperature of the rain and the scattering effect of the rain cell on the thermal noise incident upon it
• Rainfall therefore degrades the received $[\frac{C}{N_0} ]$ in two ways: by attenuating the carrier wave and by increasing the sky-noise temperature.