COMMUNICATION SATELLITE LINK DESIGN
WEEK 5
Information
Recommended Study Material:
Book-1: Electronic Communication Systems, By George
Kennedy Book-2: Satellite Communications, By D.C Agarwal
Lecture Notes
COMMUNICATION SATELLITE LINK DESIGN
Topics:
Introduction
General Link Design Equations
System Noise temperature, C/N and G/T
Ratios
Design of the Satellite Link:
The satellite link is probably the most basic in
microwave communications since a line-of-sight path typically exists between the Earth and space.
This means that an imaginary line extending
between the transmitting or receiving Earth station and the satellite antenna passes only through the atmosphere and not ground
obstacles.
Such a link is governed by free-space
propagation with only limited variation with respect to time due to various constituents of the atmosphere.
Design of the Satellite Link
Free-space attenuation is determined by
the inverse square law, which states that the power received is inversely proportional to the square of the distance.
The same law applies to the amount of light that
reaches our eyes from a distant point source such as an automobile headlight or star.
There are, however, a number of additional
effects that produce a significant amount of degradation and time variation.
These include rain, terrain effects such as
absorption by trees and walls, and some less-obvious impairment produced by unstable
Design of the Satellite Link
It is the job of the communication
engineer to identify all of the significant
contributions to performance and make
sure that they are properly taken into
account.
The required factors include the
Design of the Satellite Link
Also important is the efficient transfer of
user information across the relevant
interfaces at the Earth stations, involving
such issues as the precise nature of this
information, data protocol, timing, and the
telecommunications interface standards
that apply to the service.
A proper engineering methodology
Design of the Satellite Link
Baseband
The basic direct output signal in an intermediate frequency based obtained directly from a television camera, satellite television receiver, or video tape recorder. Baseband
signals can be viewed only on studio monitors. To display the baseband signal on a conventional television set a "modulator" is required to convert the baseband signal to one of the VHF or UHF television channels which the
television set can be tuned to receive.
Carrier
The basic radio, television, or telephony center of
Design of the Satellite Link
The RF carrier in any microwave communications
link begins at the transmitting electronics and propagates from the transmitting antenna
through the medium of free space and absorptive atmosphere to the receiving antenna, where it is recovered by the receiving electronics.
The carrier is modulated by a baseband signal
that transfers information for the particular application.
The first step in designing the microwave link is to
identify the overall requirements and the critical components that determine performance.
For this purpose, we use the basic arrangement of
Design of the Satellite Link
Design of the Satellite Link
LNB (LOW NOISE BLOCK DOWN CONVERTER)
A device mounted in the dish, designed to amplify the satellite
signals and convert them from a high frequency to a lower
frequency. LNB can be controlled to receive signals with different polarization. The television signals can then be carried by a double-shielded aerial cable to the satellite receiver while retaining their high quality. A universal LNB is the present standard version, which can handle the entire frequency range from 10.7 to 12.75 GHz and receive signals with both vertical and horizontal polarization.
Demodulator
A satellite receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier.
Decoder
A box which, normally together with a viewing card, makes it
Design of the Satellite Link
Modulation
The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or
data signal.
Modulator
A device which modulates a carrier. Modulators are
found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which
enable the recorded video information to be played
Design of the Satellite Link
The example shows a large hub type
Earth station in the uplink and a small
VSAT in the downlink; the satellite is
represented by a simple frequency
translating type repeater (e.g., a bent
pipe).
Most geostationary satellites employ
bent-pipe repeaters since these allow the
widest range of services and
Design of the Satellite Link
Bidirectional (duplex) communication
occurs with a separate transmission from
each Earth station.
Due to the analog nature of the radio
frequency link, each element contributes
a gain or loss to the link and may add
Design of the Satellite Link
Carrier to Noise Ratio (C/N)
The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the
better the received picture.
G/T
A figure of merit of an antenna and low noise
amplifier combination expressed in dB. "G" is the net gain of the system and "T" is the noise
Design of the Satellite Link
The result in the overall performance is presented
in terms of the ratio of carrier power to noise (the carrier-to-noise ratio, C/N) and, ultimately,
information quality (bit error rate, video impairment, or audio fidelity).
Done properly, this analysis can predict if the link
will work with satisfactory quality based on the specifications of the ground and space
components.
Any uncertainty can be covered by providing an
appropriate amount of link margin, which is over and above the C/N needed to deal with
Design of the Satellite Link
The result in the overall performance is presented
in terms of the ratio of carrier power to noise (the carrier-to-noise ratio, C/N) and, ultimately,
information quality (bit error rate, video impairment, or audio fidelity).
Done properly, this analysis can predict if the link
will work with satisfactory quality based on the specifications of the ground and space
components.
Any uncertainty can be covered by providing an
appropriate amount of link margin, which is over and above the C/N needed to deal with
The Received Power
Calculation
General
Link Design
Equations
Geometry of a Radio Link
The Received Power
Calculation
General
Link Design
Equations
Geometry of a Radio Link
The power density over area A0 is PT/Ao
If AR the effective area of the receiving antenna then poser incident upon is
Directivity GT of the receiving antenna
2.1
The Received Power
Calculation
General
Link Design
Equations
Combining equation 2.1 and 2.2
Gain of the receiving antenna GR in relation to effective area
AR
Substituting AR of equation 2.4 in 2.3
First Transmission Equation=
The Power attenuation in decibel is, where GT and GR are the gain of
transmitting and receiving antennas in decibel.
2.3
2.4
2.5
The Received Power
Calculation
General
Link Design
Equations
The product PTGT is called effective isotropic radiated power.
From eq. 2.7 it is clear that path loss increase with frequency
But can be compensated by increasing antenna gain.
Uplink path loss at 6Ghz =199dB Downlink path loss at 4Ghz =196dB
2.7
The Received Power
Calculation
General
Link Design
Equations
Thus eq. 2.8 cab be written as
2.9
System Noise temperature, C/N
and G/T Ratios
Noise Power due to thermal noise is given by
Where
K is Boltzmann constant
Ts is system noise temperature B is the bandwidth of the system
If G is the overall Gain
The Noise Power at the demodulator input
2.11
System Noise temperature, C/N
and G/T Ratios
If Pr is the power reaching at the input of the receiver.
Then the signal power reaching at the input of the modulator is PrG.
The carrier to noise ratio at the demodulator input isA Satellite Receiver• RF amplifier
• Down converter mixer • IF amplifier
And their respective noise temperature is Trf , Tm and Tif
Noise model of an RF receiver
System Noise temperature, C/N
and G/T Ratios
The total noise power at the output of IF amplifier is
Let Ts be the Noise temperature of the total receiver
2.14
System Noise temperature, C/N
and G/T Ratios
Equating eq.2.14 with 2.15, we get
Let Ts be the Noise temperature of the total receiver
2.16
System Noise temperature, C/N
and G/T Ratios
Again using eq. 2.5 and 2.8, eq 2.13 can be written as
The noise power spectral density No =N/B in Watt/Hz
The carrier Power to Noise Power spectral density ratio is
2.17
System Noise temperature, C/N
and G/T Ratios
Again using eq. 2.5 and 2.8, eq 2.13 can be written as
The noise power spectral density No =N/B in Watt/Hz
The carrier Power to Noise Power spectral density ratio is
2.17
