ND SatCom SKYWAN IDU 1070, Manual

Page: 85 / 154

2010-10-26 Network Design and Engineering Guide 81
Outdoor Unit and Satellite Link Design
Fundamentals of Link Budget Calculation
Figure 3-6 Attenuation under maximum Rain Fade Condition
1 X
T
and X
S
represent the required EIRP for the transmitting earth station and the satellite transponder without
rain fade.
2 X
R
is the minimum Eb/No level for the given carrier modulation and coding.
If we are operating the earth stations with constant power level, the transmitting earth station
would still use the same power even after the rain fade has disappeared, refer to figure 3-7.
Figure 3-7 Power Conditions with constant Power Level
At clear sky condition the satellite transponder would operate at a power level which is 5 dB
higher than under maximum rain fade and actually 9 dB higher than necessary for clear sky
conditions.
That leads to an unnecessary high power requirement for the satellite transponder which may
even exceed the available power for that transponder. A more optimal approach is to dynami-
cally adjust the transmission power on the uplink, so that the power requirement for the satellite
remains constant, irrespective of the rain condition on the uplink. At clear sky condition this will
result in the power levels as described in figure 3-8.