Satellite Link Design: EIRP, Transmission Losses, Free-space transmission, System noise temperature and G/T ratio, Noise figure, Design of downlinks, Design of uplink, Design of specified C/N: combining C/N and C/I values in satellite links, Overall C/No, Link design procedure.

1. Satellite Communication
Md. Humayun Kabir
Adjunct Lecturer
Dept. of Electronic & Telecommunication Engineering
International Islamic University Chittagong

2. Satellite Link Design

3. Introduction:
✓ A satellite link consists of an uplink (transmit earth station to satellite) and a downlink (satellite to the receive
earth station).
✓ Signal quality over the uplink depends on how strong the signal is when it leaves the source earth station and how
the satellite receives it.
✓ Also, on the downlink side, the signal quality depends on how strongly the satellite can retransmit the signal and
how the receiving earth station receives the signal.
✓ Satellite link design involves a mathematical approach to the selection of link subsystem variables in such a way
that the overall system performance criteria are met.
The Earth station design consists of:
✓ The Transmission Link Design or the Link Budget.
✓ The Transmission System Design.
✓ The Link Budget establishes the resources needed for a given service to achieve the performance objectives.

4. Satellite Link Parameters:
Important parameters that influence the design of a satellite communication link include the following:
1. Choice of operating frequency
2. Propagation considerations
3. Noise considerations
4. Interference-related problems

5. Choice of Operating Frequency:
✓ The choice of frequency band from those allocated by the International Telecommunications Union (ITU) for
satellite communication services such as the fixed satellite service (FSS), the broadcast satellite service (BSS) and
the mobile satellite service (MSS) is mostly governed by factors like propagation considerations, coexistence with
other services, interference-related issues, technology status, economic considerations and so on.
✓ While it may be more economic to use lower frequency bands, there would be interference-related problems as a
large number of terrestrial microwave links use frequencies within these bands.
✓ Also, lower frequency bands would offer lower bandwidths and hence a reduced transmission capacity.
✓ Higher frequency bands offer higher bandwidths but suffer from the disadvantage of severe rain induced
attenuation, particularly above 10 GHz.
✓ Also, above 10 GHz, rain can have the effect of reducing isolation between orthogonally polarized signals in a
frequency re-use system.
✓ It may be mentioned here that for frequencies less than 10 GHz and elevation angles greater than 5◦,
atmospheric attenuation is more or less insignificant.

6. Propagation Considerations:
✓ The nature of propagation of electromagnetic waves or signals through the atmospheric portion of an Earth
station--satellite link has a significant bearing on the link design.
✓ From the viewpoint of a transmitted or received signal, it is mainly the operating frequency and to a lesser
extent the polarization that would decide how severe the effect of atmosphere is going to be.
✓ From the viewpoint of atmosphere, it is the first few tens of kilometres constituting the troposphere and then
the ionosphere extending from about 80 km to 1000 km that do the damage.
✓ The effect of atmosphere on the signal is mainly in the form of attenuation caused by atmospheric scattering
and scintillation and depolarization caused by rain in the troposphere and Faraday rotation in the ionosphere.
✓
✓ While rain-induced attenuation is very severe for frequencies above 10 GHz, polarization changes due to
Faraday rotation are severe at lower frequencies and are almost insignificant beyond 10 GHz.
✓ In fact, atmospheric attenuation is the least in the 3 to 10 GHz window. That is why it is the preferred and
most widely used one for satellite communications.

7. Noise Considerations:
✓ In both analogue and digital satellite communication systems, the quality of signal received at the Earth station
is strongly dependent on the carrier-to-noise ratio of the satellite link.
✓ The satellite link comprises an uplink, the satellite channel and a downlink.
✓ The quality of the signal received on the uplink therefore depends upon how strong the signal is, as it leaves
the originating Earth station and how the satellite receives it.
✓ On the downlink, it depends upon how strongly the satellite can retransmit the signal and then how the
destination Earth station receives it.
✓ Because of the large distances involved, the signals received by the satellite over the uplink and received by the
Earth station over the downlink are very weak.
✓ Satellite communication systems, more so the geostationary satellite communication systems are therefore
particularly susceptible to noise because of their inherent low received power levels.
✓ In fact, neither the absolute value of the signal nor that of the noise should be seen in isolation for gauging the
effectiveness of the satellite communication link.
✓ If the received signal is sufficiently weak as compared to the noise level, it may become impossible to detect
the signal.
✓ Even if the signal is detectable, steps should be taken within the system to reduce the noise to an acceptable
level lest it impairs the quality of the signal received.

8. Interference-related Problems:
Major sources of interference include interference between satellite links and terrestrial microwave links sharing the same
operational frequency band, interference between two satellites sharing the same frequency band, interference between two Earth
stations accessing different satellites operating in the same frequency band, interference arising out of cross polarization in
frequency re-use systems, adjacent channel interference inherent to FDMA systems and interference due to intermodulation
phenomenon.
Interference between satellite links and terrestrial links could further be of two types: first where terrestrial link transmission
interferes with reception at an Earth station and the second where transmission from an Earth station interferes with terrestrial link
reception.
The level of inter-satellite and inter-Earth station interference is mainly governed by factors like the pointing accuracy of antennas,
the width of transmit and receive beams, intersatellite spacing in the orbit of two co-located satellites, and so on.
Cross-polarization interference is caused by coupling of energy from one polarization state to another polarization state when a
frequency re-use system employs orthogonal linear polarizations (horizontal and vertical polarization) or orthogonal circular
polarization (left-hand and right-hand circular polarization).
This coupling of energy occurs due to a finite value of cross-polarization discrimination of the Earth station and satellite antennas
and also to depolarization caused by rain.
Adjacent channel interference arises out of overlapping amplitude characteristics of channel filters.
Intermodulation interference is caused by the intermodulation products produced in the satellite transponder when multiple carriers
are amplified in the high power amplifier that has both amplitude as well as phase nonlinearity.

9. 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.
✓ 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.
✓ 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 conditions of the
air and ionosphere.

10. ✓ 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 performance of the satellite itself,
✓ The configuration and performance of the uplink and downlink Earth stations, and
✓ The impact of the propagation medium in the frequency band of interest.
✓ 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 base band 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 the link shown in Figure.
✓ 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 noise and interference as well.
✓ The result in the overall performance is presented in terms of the ratio of carrier power to noise and, ultimately,
information quality
✓ 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 propagation effects and nonlinearity in the Earth stations and satellite repeater.

12. EIRP – (Equivalent Isotropic Radiated Power)
Equivalent Isotropic Radiated Power (EIRP) is a measurement of radiated output power from an ideal isotropic antenna in a single direction.
An isotropic antenna is meant to distribute power equally in all directions – When we channel that power into a single direction and calculate the
power it is known as EIRP. It will be the maximum power emitted by the antenna in the direction with highest antenna gain. When calculating EIRP
we must take in to account the losses in the transmission line and the loss of power due to the connectors.
The IEEE definition for effective radiated power (ERP), which is similar to EIRP, is used to measure RF frequency sources, such as transmitters, and
indicate the power of the main lobe of the antenna that the system would radiate transmitting over a half-wave dipole antenna. The ERP
measurement is equivalent to the input power of the antenna multiplied by the antenna gain. EIRP and ERP are different, in that EIRP is based on a
hypothetical isotropic antenna, for which a half-wave dipole antenna has an antenna gain 1.64 times, or 2.15 decibels, that of an isotropic antenna.
EIRP can be used to compare any two emitters regardless of type, size or form. Its unit is dBi.

13. Power Level Units (Convention)
The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical
quantity. Power ratios of 2, 10 and 100 correspond to 3 dB, 10 dB and 20 dB respectively. It is
typically used to express the gain or attenuation of a system or circuit.
The dBm is a measure of the signal level relative to 1 milliWatt expressed in decibels.
The dBW is a measure of the signal level relative to 1Watt expressed in decibels.
Antennas
Antennas are normally passive devices. Though they have gain, they do not add any energy to the
signal. Instead they concentrate the available transmitted or received signal energy into a
preferred direction. See the diagrams below which show the radiation patterns of a different
antennas.
The Equivalent Isotropic Radiated Power (EIRP) of an antenna is equal to the product of
the Input Power applied to the terminals of the antenna and the Antenna Gain.
Example: A typical ground station communications transmitter with an output power of 100
watts, (20 dBW) feeding through an antenna with a gain of 60 dB will have an equivalent radiated
power (EIRP) in the direction of the antenna main beam of 80 dBW or 100,000,000 Watts.
An Isotropic radiator is an omnidirectional antenna which radiates equally in all spherical
directions.

14. Transmission Losses:
The difference between the power sent at one end and received at the receiving station is known as Transmission
losses. The losses can be categorized into 2 types.
✓ Constant losses
✓ Variable losses
The losses which are constant such as feeder losses are known as constant losses. No matter what precautions
we might have taken, still these losses are bound to occur. (Feeder losses occur in the several components
between the receiving antenna and the receiver device, such as filters, couplers and waveguides.)
Another type of loses are variable loss. The sky and weather condition is an example of this type of loss. Means if
the sky is not clear signal will not reach effectively to the satellite or vice versa.
Therefore, our procedure includes the calculation of losses due to clear weather or clear sky condition as 1st
because these losses are constant. They will not change with time. Then in 2nd step, we can calculate the losses
due to foul weather condition.
Losses occur along the way, some of which are constant.
Losses for clear weather conditions = losses which don’t vary significantly with time and losses which are calculated
statistically
➢Free-space loss
➢Antenna misalignment losses
➢Fixed atmospheric and ionospheric losses
Losses which are weather related which fluctuate with time , allowed for by introducing fade margins into the
transmission equation.

15. ✓ 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.
✓ 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.

16. Free-space-transmission:
The transmission of radio or optical signals in free space, i.e., space devoid of physical obstructions that might
hinder signal propagation. In this context, the term physical obstruction suggests trees, buildings, hills,
mountains, and other significant material objects. The term does not suggest atomic, molecular, or particulate
matter that commonly is present in the atmosphere. Neither does it suggest water vapor, rain, snow, sleet, or hail.
Free space transmission does not include radio or optical transmission through waveguides. See also free space,
FSO, transmission medium, and waveguide.

19. 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. The polarization
misalignment losses are usually small, and it will be assumed that the antenna misalignment losses, denoted by [AML], include both
pointing and polarization losses resulting from antenna misalignment. It should be noted.

20. Fixed Atmospheric and Ionospheric Losses

22. Noise Considerations:
✓ Satellite communication systems are particularly susceptible to noise because of their inherent low received
power levels, as the signals received by the satellite over the uplink and received by the Earth station over the
downlink are very weak due to involvement of large distances.
✓ Sources of noise include natural and man-made sources and also the noise generated inside the Earth station
and satellite equipment.
✓ From the viewpoint of satellite communications, the natural and man-made sources of noise can either be
taken care of or are negligible.
✓ It is mainly the noise generated in the equipment where attention primarily needs to be paid. In the
paragraphs to follow, various parameters that can be used to describe the noise performance of various
building blocks individually and also as a system, which is a cascaded arrangement of those building blocks,
will be briefly discussed.

24. Noise Figure
✓ Noise figure is a number by which the noise performance of an amplifier or a radio receiver can be specified.
The lower the value of the noise figure, the better the performance.
✓ Essentially the noise figure defines the amount of noise an element adds to the overall system.
✓ It may be a pre-amplifier, mixer, or a complete receiver. Often the noise figure may be used to define the
performance of a receiver and in this way it can be used instead of the signal to noise ratio.
✓ In view of its widespread applicability, noise figure is a particularly important parameter for a wide variety of
radio communications systems from fixed or mobile radio communications systems, two way radio
communications systems, and satellite radio communications systems.

25. Noise temperature is one way of expressing the level of available noise power introduced by a
component or source.

26. Satellite Link Design
The four factors related to satellite system design:
1. The weight of satellite
2. The choice frequency band
3. Atmospheric propagation effects
4. Multiple access technique
✓The major frequency bands are 6/4 GHz, 14/11 GHz and 30/20 GHz
(Uplink/Downlink)
✓At geostationary orbit there is already satellites using both 6/4 and 14/11
GHz every 2˚(minimum space to avoid interference from uplink earth
stations)

27. Link Budget:
A link budget is the term used that accounts for the power received at the receiver.
This accounts for all of the gain and losses from the transmitter to the point at
which it is received by the receiver. It includes losses from cables/fibers and other
components in the Tx/Rx chain, gains from the antenna, amplifiers etc. and
propagation loss when travelling through air or another medium.
Received power (dBm) = Transmitted power (dBm) + gains (dB) - losses (dB)
✓ A link budget makes a log by keeping all entries of losses and gains in signal
propagation.
✓ A wave is attenuated via amplifiers and antennas to increase the gain product
and eliminate noise.
✓ Similarly, data can be lost during propagation of a signal between the
transmitter and receiver within one device or between two or more devices.
✓ Keeping track of such losses and gains is important to calculate the reliability
and efficiency of a link (through which the transmitter and receiver
communicate).
✓ Moreover, methods are implemented to reduce data loss by increasing antenna
diversity and bandwidth of the medium.
✓ A link budget is usually made for radio and satellite services where noise and
losses are generally high.

28. LINK BUDGET
The link budget determines the antenna size to deploy,
➢ Power requirements,
➢ Link availability,
➢ Bit error rate,
➢ Overall customer satisfaction with the satellite service.
✓ A link budget is a tabular method for evaluating the power received and the noise ratio in a radio link .
✓ It simplifies C/N ratio calculations
✓ The link budget must be calculated for an individual transponder, and must be recalculated for each of the individual links
The satellite link is composed primarily of three segments:
I. The transmitting Earth station and the uplink media;
II. The satellite; and
III. The downlink media and the receiving Earth station.
The carrier level received at the end of the link is a straightforward addition of the losses and gains in the path between
transmitting and receiving Earth stations.
✓ C/N ratio calculation is simplified by the use of link budgets
✓ Evaluation of the received power and noise power in radio link
✓ The link budget must be calculated for individual transponder and for each link
✓ When a bent pipe transponder is used the uplink and down link C/N ratios must be combined to give an overall C/N

29. Link Budget Example:
✓ Satellite application engineers need to assess and allocate performance for each source of gain and loss.
✓ The link budget is the most effective means since it can address and display all of the components of the power
balance equation, expressed in decibels.
✓ In the past, each engineer was free to create a personalized methodology and format for their own link budgets.
✓ This worked adequately as long as the same person continued to do the work.
✓ Problems arose, however, when link budgets were exchanged between engineers, as formats and assumptions
can vary.
✓ A standardized link budget software tool should be used that performs all of the relevant calculations and
presents the results in a clear and complete manner.
✓ We will now evaluate a specific example using a simplified link budget containing the primary contributors.
✓ This will provide a typical format and some guidelines for a practical approach.
✓ Separate uplink and downlink budgets are provided; our evaluation of the total end-to-end link presumes the use
of a bent-pipe repeater.
✓ This is one that transfers both carrier and noise from the uplink to the downlink, with only a frequency translation
and amplification.

30. Link Budget Example
✓ The three constituents are often shown in a single table, but dividing them should make the development
of the process clearer for readers.
✓ The detailed engineering comes into play with the development of each entry of the table.
✓ Several of the entries are calculated using straightforward mathematical equations; others must be
obtained through actual measurements or at least estimates thereof.
✓ This particular example is for a C-band digital video link at 40 Mbps, which is capable of transmitting 8 to
12 TV channels using the Motion Picture Experts Group 2 (MPEG 2) standard.

31. Link Budget Example: Downlink Budget
The following Table 2.3 presents the downlink budget in a manner that
identifies
✓ The characteristics of the satellite transmitter and antenna,
✓ The path,
✓ The receiving antenna,
✓ And the expected performance of the Earth station receiver.
✓ It contains the elements that select the desired radio signal (i.e., the
carrier) and demodulates the useful information (i.e., the digital baseband
containing the MPEG 2 “transport” bit stream).
✓ Once converted back to baseband, the transmission can be applied to
other processes, such as de-multiplexing, decryption, and digital-to-analog
conversion (D/A conversion).
✓ Each of the link parameters relates to a specific piece of hardware or some
property of the microwave path between space and ground.
✓ A good way to develop the link budget is to prepare it with a spreadsheet
program.
✓ This permits the designer to include the various formulas directly in the
budget, thus avoiding the problem of external calculation or the potential
for arithmetic error
✓ Commercial link budget software, such as SatMaster Pro from Arrowe
Technical Services, does the same job but in a standardized fashion.

33. Satellite Link Design -Uplink
• Uplink design is easier than the down link in many cases
✓Earth station could use higher power transmitters
• Earth station transmitter power is set by the power level
required at the input to the transporter, either
✓A specific flux density is required at the satellite
✓A specific power level is required at the input to the
transporter
• Analysis of the uplink requires calculation of the power
level at the input to the transponder so that uplink C/N
ratio can be found
• With small-diameter earth stations, a higher power earth
station transmitter is required to achieve a similar satellite
EIRP.
✓Interference to other satellites rises due to wider
beam of small antenna
• Uplink power control can be used against uplink rain
attenuation

36. Carrier to Noise Ratios
• C/N: carrier/noise power in RX BW (dB)
• Allows simple calculation of margin if:
• Receiver bandwidth is known
• Required C/N is known for desired signal type
• C/No: carrier/noise p.s.d. (dbHz)
• Allows simple calculation of allowable RX bandwidth if required C/N is
known for desired signal type
• Critical for calculations involving carrier recovery loop performance
calculations
[C/N0]D = [EIRP]D + [G/T]D - [LOSSES]D – [k]
[C/N0]U = [EIRP]U + [G/T]U - [LOSSES]U – [k]

37. Link Budget Example: Overall Link Budget
• The last step in link budgeting for a bent-pipe repeater is to combine the two link performances and
compare the result against a minimum requirement—also called the threshold.
• Table 2.5 presents a detailed evaluation of the overall link under the conditions of line-of-sight
propagation in clear sky.
• We have included an allocation for interference coming from sources such as a cross-polarized
transponder and adjacent satellites.
• This type of entry is necessary because all operating satellite networks are exposed to one or more
sources of interference.
• The bottom line represents the margin that is available to counter rain attenuation and any other
losses that were not included in the link budgets.
• Alternatively, rain margin can be allocated separately to the uplink and downlink, with the combined
availability value being the arithmetic product of the two as a decimal value (e.g., if the uplink and
downlink were each 99.9%, then the combined availability is 0.999 × 0.999 = 0.998 or 99.8%).

38. Link Budget Example: Overall Link Budget

40. SATELLITE LINK DESIGN METHODOLOGY
The design methodology for a one-way satellite communication link can be summarized into the following steps.
The return link follows the same procedure:
Step 1: Frequency band determination.
Step 2: Satellite communication parameters determination. Make informed guesses for unknown values.
Step 3: Earth station parameter determination; both uplink and downlink.
Step 4: Establish uplink budget and a transponder noise power budget to find (C/N)up in the transponder
Step 5: Determine transponder output power from its gain or output backoff.
Step 6: Establish a downlink power and noise budget for the receiving earth station
Step 7: Calculate (C/N)down and (C/N)u for a station at the outermost contour of the satellite footprint.
Step 8: Calculate SNR/BER in the baseband channel.
Step 9: Determine the link margin.
Step 10: Do a comparative analysis of the result vis-à-vis the specification requirements.
Step 11: Tweak system parameters to obtain acceptable (C/N)0 /SNR/BER values.
Step 12: Propagation condition determination.
Step 13: Uplink and downlink unavailability estimation.
Step 14: Redesign system by changing some parameters if the link margins are inadequate.
Step 15: Are gotten parameters reasonable? Is design financially feasible?
Step 16: If YES on both counts in step 15, then satellite link design is successful – Stop.
Step 17: If NO on either (or both) counts in step 15, then satellite link design is unsuccessful – Go to step 1.