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1. Satellite uplink and downlink Analysis and Design:
1.1 Introduction
This chapter describes how the link-power budget calculations are made.
These calculations basically relate two quantities, the transmit power and the
receive power, and show in detail how the difference between these two powers
is accounted for.
Link-budget calculations are usually made using decibel or decilog
quantities. These are explained in App. G. In this text [square] brackets are used
to denote decibel quantities using the basic power definition.
Where no ambiguity arises regarding the units, the abbreviation dB is used.
For example, Boltzmann’s constant is given as 228.6 dB, although, strictly
speaking, this should be given as 228.6 deci logs relative to 1 J/K.
1.2 Equivalent Isotropic Radiated Power
A key parameter in link-budget calculations is the equivalent isotropic radiated
power, conventionally denoted as EIRP. From Eqs, the maximum power flux
density at some distance r from a transmitting antenna of gain G i
An isotropic radiator with an input power equal to GPS would produce the same
flux density. Hence, this product is referred to as the EIRP, or EIRP is often
expressed in decibels relative to 1 W, or dBW. Let PS be in watts; then [EIRP] =
[PS] x [G] dB ,where [PS] is also in dBW and [G] is in dB.
1.3 Transmission Losses
The [EIRP] may be thought of as the power input to one end of the transmission
link, and the problem is to find the power received at the other end. Losses will
occur along the way, some of which are constant.
Other losses can only be estimated from statistical data, and some of these are
dependent on weather conditions, especially on rainfall.
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The first step in the calculations is to determine the losses for clear- weather or
clear-sky conditions. These calculations take into account the losses, including
those calculated on a statistical basis, which do not vary significantly with time.
Losses which are weather-related, and other losses which fluctuate with time, are
then allowed for by introducing appropriate fade margins into the transmission
equation.
Free-space transmission:
As a first step in the loss calculations, the power loss resulting from the spreading
of the signal in space must be determined.
Feeder losses:
Losses will occur in the connection between the receive antenna and the receiver
proper. Such losses will occur in the connecting waveguides, filters, and
couplers.Thesewill be denotedby RFL,or[RFL]dB, forreceiver feeder losses.
Antenna misalignment losses:
When a satellite link is established, the ideal situation is to have the earth station
and satellite antennas aligned for maximum gain, as shown in Fig. There are two
possible sources of off-axis loss, one at the satellite and one at the earth station,
as shown in Fig.
The off-axis loss at the satellite is taken into account by designing the link for
operation on the actual satellite antenna contour; this is described in more detail
in later sections. The off-axis loss at the earth station is referred to as the antenna
pointing loss. Antenna pointing losses are usually only a few tenths of a decibel;
In addition to pointing losses, losses may result at the antenna from misalignment
of the polarization direction (these are in addition to the polarization losses The
polarization misalign- ment losses are usually small, and it will be assumed that
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the antenna misalignment losses, denoted by [AML], include both pointing and
polar- ization losses resulting from antenna misalignment. It should be noted
Figure 2.15 (a) Satellite and earth-station antennas aligned for maximum gain; (b) earth station situated on a
given satellite “footprint,” and earth-station antenna misaligned.
2. The Link-Power Budget Equation:
Now that the losses for the link have been identified, the power at the receiver,
which is the power output of the link, may be calculated simply as [EIRP]
[LOSSES] [GR], where the last quantity is the receiver antenna gain.
Note carefully that decibel addition must be used. The major source of loss in
any ground-satellite link is the free-space spreading loss [FSL], the basic link-
power budget equation taking into account this loss only. However, the other
losses also must be taken into account, and these are simply added to [FSL]. The
losses for clear-sky conditions are
[LOSSES] = [FSL] + [RFL] + [AML] + [AA] - [PL] equation for the
received power is then
[PR] = [EIRP] x [GR] - [ LOSSES ]
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where [PR] received power, dBW
[EIRP] equivalent isotropic radiated power, dBW [FSL] free-space
spreading loss, dB
[RFL] receiver feeder loss, dB
[AML] antenna misalignment loss, dB
[AA] atmospheric absorption loss, dB [PL] polarization mismatch loss, dB
3 Amplifier noise temperature
Consider first the noise representation of the antenna and the low
noise amplifier (LNA) shown in Fig. 2.15. The available power gain of the
amplifier is denoted as G, and the noise power output, as Pno.
Figure LNA Amplifier gain
Figure Source of Dennis Roddy –Satellite Communication ,4th Edition
For the moment we will work with the noise power per unit bandwidth, which is
simply noise energy in joules as shown by Eq. The input noise energy coming
from the antenna is N0,ant = kTant
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