Ec 8094-Satellite Communication: BY R.Saktheeswari Ap/Ece

EC 8094-SATELLITE
COMMUNICATION
BY
R.SAKTHEESWARI AP/ECE
The Uplink
• The uplink of a satellite circuit is the one in
which the earth station is transmitting the
signal and the satellite is receiving it specifically
that the uplink is being considered.
• C/N=[EIRP]-[LOSSES]+[k]
• In this Eq the values to be used are the earth
station EIRP, the satellite receiver feeder losses,
and satellite receiver G/T. The free-space loss
and other losses which are frequency-
dependent are calculated for the uplink
frequency.
Input back-off
• Number of carriers are present simultaneously
in a TWTA, the operating point must be backed
off to a linear portion of the transfer
characteristic to reduce the effects of inter
modulation distortion.
• Such multiple carrier operation occurs with
frequency- division multiple access (FDMA. The
point to be made here is that back-off (BO)
must be allowed for in the link- budget
calculations.
Input back-off
• Suppose that the saturation flux density for
single-carrier operation is known. Input BO
will be specified for multiple-carrier operation,
referred to the single carrier saturation level.
The earth-station EIRP will have to be reduced
by the specified BO, resulting in an uplink
value of,
• [EIRP]u=[EIRPs]+[BO]i
The earth station HPA
• The earth station HPA has to supply the
radiated power plus the transmit feeder losses,
denoted here by TFL, or [TFL] dB. These include
waveguide, filter, and coupler losses between
the HPA output and the transmit antenna.
• The earth station itself may have to transmit
multiple carriers, and its output also will require
back off, denoted by [BO]HPA. The earth station
HPA must be rated for a saturation power
output given by, [PHPA,sat]=[PHPA]+[BO]HPA
Downlink
• The downlink of a satellite circuit is the one in
which the satellite is transmitting the signal
and the earth station is receiving it. Equation
can be applied to the downlink, but subscript
D will be used to denote specifically that the
downlink is being considered. Thus Eq.
becomes,
• C/N=[EIRP]-[LOSSES]+[k]
Downlink
• In Eq. the values to be used are the satellite
EIRP, the earth- station receiver feeder losses,
and the earth-station receiver G/T. The free
space and other losses are calculated for the
downlink frequency.
• The resulting carrier-to-noise density ratio
given by Eq. is that which appears at the
detector of the earth station receiver.
Output back-off
• Where input BO is employed as described in a
corresponding output BO must be allowed for
in the satellite EIRP. As the curve of Fig. shows,
output BO is not linearly related to input BO.
• A rule of thumb, frequently used, is to take the
output BO as the point on the curve which is 5
dB below the extrapolated linear portion.
• Since the linear portion gives a 1:1 change in
decibels, the relationship between input and
output BO is [BO]0 [BO]i 5 dB.
Output back-off
• For example, with an input BO of [BO]i 11 dB,
the corresponding output BO is [BO]o.
• Figure; Input and output back-off relationship
for the satellite traveling-wave-tube amplifier;
[BO]i [BO]o 5 dB.
Effects of Rain
• In the C band and, more especially, the Ku band,
rainfall is the most significant cause of signal fading.
Rainfall results in attenuation of radio waves by
scattering and by absorption of energy from the wave.
• Rain attenuation increases with increasing frequency
and is worse in the Ku band compared with the C band.
This produces a depolarization of the wave; in effect,
the wave becomes elliptically polarized.
• This is true for both linear and circular polarizations,
and the effect seems to be much worse for circular
polarization.
Effects of Rain
• The C/N0 ratio for the downlink alone, not
counting the PNU contribution, is PR/PND, and
the combined C/N0 ratio at the ground
receiver is; [(a) Combined uplink and
downlink; (b) power flow diagram]
Effects of Rain
• The reason for this reciprocal of the sum of
the reciprocals method is that a single signal
power is being transferred through the
system, while the various noise powers, which
are present are additive. Similar reasoning
applies to the carrier-to-noise ratio, C/N.
THANK YOU

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