C H A P T E R 4
Microwave Link Engineering
Microwave link engineering is the branch of communication engineering that deals
with the analysis, design, implementation, and testing of radio paths in the atmosphere
or in space, operating in the range of about 1 to 100 GHz. Satellite and
other microwave radio links obey certain rather predictable laws of nature, allowing
engineers to design satellite networks on paper (or, more likely, on computer) with
good accuracy. An unobstructed line-of-sight path provides the most predictable
transmission link; however, various forms of signal absorption (attenuation),
blockage, and multipath propagation (caused by reflections) introduce time-varying
fading, which can best be estimated using statistical techniques. Nevertheless, the
service obtained from microwave links between the satellites and ground stations
can be very acceptable, even when compared to terrestrial cable and radio (wireless)
systems.
This chapter is intended for engineers and scientists new to the field and is
perhaps the most mathematical chapter in the book. Nontechnical readers may
wish to scan it rather than delve into the details. In general, microwave energy
travels in straight lines through space in the same manner as a light beam. That
should not be a surprise because both light and microwaves are forms of electromagnetic
energy, light being at much higher frequencies or, equivalently, shorter wavelengths.
Radio waves in general and microwaves in particular can propagate through
space like light and heat, spreading out as they move farther and farther from the
source. The waves travel through a vacuum at the universal speed of light, the
same for all forms of electromagnetic energy. Microwaves can be kept from propagating
freely by forcing them to travel in a closed metallic pipe called a waveguide.
The physical similarity to common household pipes has led to the use of the term
plumbing when referring to the waveguides found in Earth stations and satellites.
At the input end of the waveguide would be found the transmitter, the electronic
device that generates the microwave energy on the proper frequency, with the other
end connected to an antenna such as a parabolic ‘‘dish.’’ Waveguides are similarly
used on the receiving end.
4.1 The Decibel
Microwave power is measured in watts, the same unit we associate with electric
power used for lighting and motors. Link effects that either reduce or increase that
power almost always are expressed in terms of the decibel (dB), which is why it
109
110 Microwave Link Engineering
is important to introduce the concept early in the discussion. Converting a quantity
to decibels simply means taking the logarithm to the base 10, or common logarithm,
and multiplying the result by 10:
A (in dB) = 10 log10 (a)
where a is a real, nonnegative number. In actuality, a is a ratio, such as the power
gain of an amplifier, that is,
a = Pout /Pin
A sample of decibel values versus their equivalent ratios is provided in Table
4.1. Recall that adding logarithms is equivalent to multiplying ratios. So, for
example, adding 4.0 dB is equivalent to multiplying by a factor of 2.51. In the
table, relative power changes are computed to the closest tenth of a decibel (i.e.,
0.1-dB precision) because measurement accuracy on an actual satellite link is about
0.5 dB at best. Since the logarithm of the number 1 is equal to zero, a zero-dB
change simply means there has been no change in the power level of the signal.
You may recall that logarithms are exponents, and in the case of logarithms to
Table 4.1 Example of a Decibel Table,
Indicating the Ratio, Percentage
Increase, and Corresponding Decibel
Value
Power Ratio % Increase dB
1.0 0 0
1.12 12 0.5
1.26 26 1.0
1.41 41 1.5
1.59 58 2.0
1.78 78 2.5
2.00 100 3.0
2.24 124 3.5
2.51 151 4.0
2.82 182 4.5
3.16 216 5.0
3.55 255 5.5
3.98 298 6.0
4.47 347 6.5
5.01 401 7.0
5.62 462 7.5
6.31 531 8.0
7.08 608 8.5
7.94 694 9.0
8.91 791 9.5
10.0 900 10.0
20 13.0
50 17.0
100 20.0
200 23.0
500 27.0
1000 30.0
4.2 Propagation on the Earth-Space Link 111
the base 10, they are exponents of the number 10. The inverse of a decibel is
expressed as:
a = 10A/10
which again is a ratio.
A decrease would be represented by a negative decibel value. For example,
reducing a power to one-half its value (e.g., multiplying it by 0.5) is the same as
subtracting 3 dB (e.g., adding −3.0 dB). Another consequence is that a decrease of
1.0 dB (adding −1.0 dB) represents a decrease of 20%. Table 4.1 indicates that
adding 1.0 dB is an increase of 26%.
An inexpensive scientific calculator is the most convenient device for converting
back and forth between ratios of powers and decibels. Just be sure to use the
common logarithm (log) button and the 10X button, where appropriate. This is
safer than trying to memorize Table 4.1 (although such knowledge is the sign of
a ‘‘dB artist’’).
The following are a few simple examples of the decibel approach. A 3-dB
difference in power level is nearly equal to a factor of 2, while a factor of 10 is
exactly equal to 10 dB, that is, 10 log (10) = 10. In satellite communications, it is
not uncommon to encounter a factor of 10−20 decrease in signal power. That can
more conveniently be expressed in decibels as a 200-dB reduction, called a loss.
Gains are added, while losses are substracted. When dealing with factors in an
equation, it is convenient to first convert each factor to decibels. Then the equation
can be solved by summing the decibel values instead of having to resort to multiplication.
Likewise, division becomes simple subtraction, as in the example of the
200-dB loss. That is the basis for the link power balance calculation and the link
budget, reviewed later in this chapter.
4.2 Propagation on the Earth-Space Link
The process by which radio signals reach the receiving antenna from the transmitting
station is called radio wave propagation. Radio waves represent a part of the
electromagnetic spectrum, ecompassing radio, infrared, visible light, ultraviolet, and
X rays (given in increasing order of frequency or decreasing order of wavelength).
The part of the electromagnetic spectrum that is of interest in commercial
satellite communications lies between 1 and 60 GHz, as illustrated in Figure 4.1.
Figure 4.1 The microwave spectrum, indicating the approximate location of satellite bands.
112 Microwave Link Engineering
The relative merits of each band segment were covered in Chapter 1. Using the
popular letter designations for the various segments, the spectrum offers a total
bandwidth of approximately 60 GHz. The frequencies below 30 GHz are those
currently in use or under development; the frequencies above 30 GHz are a still
untapped resource. RF bandwidth can be used and reused many times by multiple
satellites and the radio beams that they can generate over portions of the Earth. The
following sections review the propagation, generation, and reception of microwave
signals, in preparation for studying the unique aspects of satellite communications
presented in Chapters 5, 6, 7, and 9.
4.2.1 Basic Microwave Propagation
Once it is several emitter dimensions away from its emitter, electromagnetic wave
energy propagates out radially. The spreading pattern decreases in intensity
inversely to the square of the distance. All radio waves behave in this manner in
free space, but various forms of matter produce interesting and potentially disruptive
results when placed in their path. That is because microwave energy can be
absorbed, scattered, bent, and reflected, as indicated in Table 4.2. As a result, there
is additional loss of signal power, in decibels, which must be accounted for in the
link design.
A question that comes to mind is, ‘‘How does such a wave, traveling at the
speed of light, come into existence in the first place?’’ Because space consists of
nothing, a radio wave does not propagate like the crest of a wave in the ocean.
Instead, it usually is the result of the high-frequency vibration of electrons in a
piece of wire or other conducting material of appropriate dimensions. Alternating
current from a transmitter causes the electrons to vibrate back and forth; by not
flowing continuously in one direction, the electrons lose most of their energy by
throwing it off into space. That is illustrated in the drawing sequence provided in
Figure 4.2.
A high transmitter power, say, 1,000W (1 kW), produces a correspondingly
high level of microwave radiation from the antenna at the point of exit into space.
That is the same principle behind a microwave oven, which uses a high-power
Table 4.2 Propagation Effects to Introduce Additional Attenuation of Microwave Signals as They
Propagate Between the Transmitting Station and Receiving Station
Propagation Mode Effect Source Medium
Absorption Conversion of microwave energy Air, water vapor, precipitation
into simple heat (rain and snow), nonmetallic
structures, and foliage
Scattering Multiple reflections and bending Water vapor, precipitation, dust,
due to minute particles ion plasma (the ionosphere)
Refraction Bending of microwaves due a Air, ion plasma
change in the refractive index
Reflection Total reflection of the microwave Temperature inversion layers
signal (ducting), metal structures, bodies
of water, buildings, and flat terrain
Diffraction Bending around physical obstacles Metal structures, buildings, and
partial terrain blockage
4.2 Propagation on the Earth-Space Link 113
Figure 4.2 The fundamental concept of electromagnetic wave propagation from an elemental
antenna. The packages of energy are separated by one wavelength in distance.
magnetron amplifier to generate an intense field for cooking purposes. The field
causes the molecules in the food to oscillate at the same frequency; that generates
the necessary heat to raise the temperature. Such intense radiation around a highpower
microwave transmitting station can ‘‘bake’’ you if you come too close. At
sufficient distance from a transmitting antenna, the microwave energy induces less
heating. There is a defined area around the antenna, called the near-field region,
where the energy has not quite coalesced into a clearly defined beam. Within the
near field, microwave energy varies in intensity, depending where one stands relative
to the antenna structure. At sufficient distance from the antenna, the radiation
field is formed into the type of beam associated with the particular antenna structure,
called the far-field region. A parabolic reflector antenna has a sharply defined
far field beam that is aimed at the satellite. The transition between the two regions
is roughly defined by the following equation:
Rf ≈ 2D2/_
where Rf is the approximate distance, D is the diameter of dish reflector in meters,
and _ is the wavelength in meters. This equation indicates something unexpected:
the distance to the transition increases with frequency rather than decreasing.
For example, proper testing of a Ku-band reflector antenna demands that the
transmitting (or receiving) source be farther away than if the same reflector were
to be used at C-band. From the beginning of the far field, the radiated signal keeps
on decreasing in intensity by the inverse of the square of the distance from the
source (the same property is visible light from a point source such as a star). This
particular concept is the basis for accurate link design.
4.2.2 Isotropic Radiator
The most fundamental type of radio antenna is the isotropic source, which is
analogous to the lightbulb illustrated in Figure 4.3. At a fixed radius from an
114 Microwave Link Engineering
Figure 4.3 An isotropic source can be represented by a lightbulb radiating energy in all directions
with equal intensity.
isotropic source, which defines a sphere, the energy intensity is constant regardless
of direction. The area of the sphere of uniform received energy is equal to the
constant _ (3.14159...) multipled by 4 multiplied by the square of the radius, that
is, A = 4_R2. It is common practice to measure the signal intensity at the particular
radius in units of watts per square meter, calculated by dividing the power of the
isotropic source by the area of the sphere in square meters:
P/A = Pt /4_R2, W/m2
The denominator, 4_R2, is sometimes called the spreading factor.
RF power driving the isotropic source produces a constant power density at a
fixed distance. That density decreases by 1/R2 as the point of reception moves
farther away from the source. Ignoring losses, it is theoretically possible to capture
all the transmitted power by collecting it with a closed surface around the source
(the physical size of the collector is immaterial as long as it closes on itself).
Receiving antennae, therefore, work by way of the area that they expose to
the RF energy radiating from the source. This is illustrated in Figure 4.4 by an
isotropic source that radiates energy equally in all directions, shown as equally
spaced rays. There are two receiving antenna surfaces of equal area: one at distance
R2 is farther away from the source than the other at distance R1. Notice how the
closer antenna intercepts considerably more power than the more distant area.
That illustrates how a radio signal becomes weaker as the receiver is moved farther
from the transmitter. It also demonstrates the important concept of capture area,
that is, the relationship between the effective area of an antenna and the strength
of the signal received by it. The challenge of antenna design is to maximize the
fraction of the energy that the antenna actually delivers from the reflector surface
to the receiver. The key parameter is the efficiency of the antenna, defined as the
ratio of the effective area (i.e., the area that would perfectly capture the same
amount of energy) to its physical area. Typical values of dish antenna efficiency
4.2 Propagation on the Earth-Space Link 115
Figure 4.4 The radio energy captured by a fixed antenna area decreases as the distance to the
source increases.
are between 55% and 70%. Antennas that do not use reflectors but employ an
active receiving area to the incoming wave can achieve up to 90% efficiency.
Examples include horns and arrays of elemental antennas.
4.2.3 Directional Properties of Antennas
The isotropic antenna is neither practical (its ideal characteristic cannot be achieved
with a simple physical structure) nor particularly useful for satellites (due to its
low gain and inability to discriminate directions). What we rely on for receiving
and transmitting signals is the concept of directivity, which simply means that the
antenna has the ability to focus the energy in specific directions. Energy that would
have been radiated in unnecessary directions around the sphere is concentrated by
the structure of the antenna and redirected to increase the intensity in the desired
direction.
4.2.3.1 Directivity and Gain
The directivity in a given direction can be expressed as the ratio of the measured
signal to the maximum signal in the peak direction. The gain, on the other hand,
is an absolute measure, obtained by comparing the signal from the antenna to that
of an isotropic radiator. Our absolute measure of maximum gain could be as little
as 10 or greater than a factor of 1 million. When expressed in decibels, these
particular values would be 10 dBi and 60 dBi, respectively, where the ‘‘i’’ indicates
that the gain is with respect to our isotropic antenna. That implies that an isotropic
antenna has a gain of 0 dBi. There must be a fixed offset in decibels between the
absolute value of gain and the corresponding relative value of directivity, because
our definition of directivity is that it starts at a value of 0 dB at the peak and
follows the pattern in a negative-going sense for all other angles.
Another important property of an antenna is called reciprocity, which means
that the gain and the directivity are the same at a given frequency whether it is
used to receive or to transmit. That allows the antenna to receive with precisely
the same directional characteristics as it transmits. Figure 4.5 illustrates a simplified
antenna gain pattern for transmission or reception with a main beam (the region
116 Microwave Link Engineering
Figure 4.5 Gain of a high-gain antenna expressed as a power ratio and presented as a 360-degree
polar plot.
of maximum directivity) oriented toward the right. For comparison, the uniform
pattern of an isotropic source is superimposed to scale on the directional antenna
pattern. In addition to a main beam, every real antenna operates in undesired
directions, shown in the figure as a pair of sidelobes and a backlobe. The maximum
gain, also called the peak gain, is indicated at the center, and the backward direction
is indicated at }180 degrees.
4.2.3.2 Beamwidth and Sidelobes
There are a number of other useful definitions of antenna performance besides the
peak gain. The half-power beamwidth (often called simply the beamwidth) is the
angular width of the main beam measured between the points where the power
intensity is one-half that of the peak. An equally accurate name that is often used
is the 3-dB beamwidth, since the half power point is where the directivity is 3 dB
down. Assuming that the microwave link can still function with a 3-dB decrease
in signal strength, the half-power beamwidth defines the range of antenna pointing
(alignment angle) over which the antenna or satellite can move. It is a common
practice, however, to allow only a 0.5-dB (12%) drop in signal power, which
demands either tighter antenna pointing accuracy or satellite position control.
When installed at an Earth station, the antenna is attached to a mount, which
may allow the beam to be repointed. Simple RO antennas used for direct reception
usually are fixed to their mounts and need only be pointed toward the satellite
when initially installed. Movable antennas usually have motorized mounts to allow
remote repointing; that could also be part of an automatic tracking system to
initially locate (acquire) a satellite and to keep the beam aligned with (track) the
satellite. It is normal to refer to the elevation angle of the direction of pointing,
where zero is for pointing at the horizon and 90 degrees corresponds to directly
overhead (zenith). The mount also points the antenna in azimuth, that is, along
the directions of the compass. Most mounts work directly in terms of elevation
and azimuth, but some are polar and behave like an astronomer’s telescope. The
advantage of a polar mount is that it can repoint along the equatorial orbit plane
by adjusting only one axis.
Presentations of antenna performance are called antenna patterns and are
illustrated in Figure 4.6 (a hypothetical example) and Figure 4.7 (a real 10-m
antenna) in terms of gain and directivity, respectively. Almost every antenna has
a backlobe in the opposite direction from the main beam. However, as shown in
Figure 4.6, the gain of the backlobe can be made to be less than unity, in this case,
producing a negative gain of −2 dBi or lower. Sidelobes and backlogs are important
4.2 Propagation on the Earth-Space Link 117
Figure 4.6 Gain of a directional antenna in decibels for all angles with respect to the peak of the
main beam indicating the worst-case envelope of sidelobes.
Figure 4.7 A comparison of the radiation (directivity) pattern of a real 10m C-band antenna with
the ITU sidelobe envelope.
characteristics of Earth station antennas when one is considering transmit or receive
interference. Figure 4.7 includes a smooth curve, called a sidelobe envelope, that
defines a specification of maximum sidelobe gain. Two such envelopes are shown,
the lower being a tighter specification that applies between 1° and 20° off the peak
of the main beam.
4.2.3.3 Isolation
The directive property of an antenna dertermines how effective it will be for getting
signal power from the source to the receiver. However, any link can be degraded
by signals on the same frequency that enter the receiving antenna from a direction
118 Microwave Link Engineering
other than along the main beam. Likewise, a transmitting station can cause degradation
to other systems by sidelobe radiation. Any undesired signal that can potentially
degrade reception is RFI. There is a whole field of engineering study that focuses
on the identification of sources of RFI, the establishment of criteria for acceptable
operation in the presence of RFI, and the development of techniques for countering
its effects. Techniques such as beam shaping, cancelation, and shielding are effective
in that regard.
The presence of RFI is a consequence of the fact that all radio communications
systems reuse frequencies, that is, there will be more than one radio station operating
on any particular frequency at any particular time. What keeps the RFI within
acceptable limits is isolation that is either natural (e.g., geographical or angular/
orbital separation) or artificial (e.g., measures such as RFI shielding or beam
cancellation). Satisfactory operation of independent microwave users on the same
frequency often requires cooperation (called frequency coordination) because ‘‘one
user’s radio link is another user’s RFI.’’ Several satellites can operate in the geostationary
arc in the same frequency band because each directional ground antenna
can focus on one particular satellite, suppressing the RFI produced by adjacent
satellites (see Figure 1.26).
For example, if we assume the receiving user’s antenna has the characteristic
given in Figure 4.6 and the interfering satellite is located 45 degrees away from
the satellite that we wish to receive (the desired satellite), then the RFI is suppressed
by at least 20 dB. Actual ground antennas used in FSS and BSS links provide 40
to 60 dB of peak gain with suppression at 45 degrees of 50 to 70 dB. At closerin
angles, the isolation may amount to only 20 to 30 dB, which is adequate to
allow satellite spacing as small as 2 degrees. The first sidelobe, in particular, is
normally the strongest, being typically 15 to 20 dB down from and within 1 degree
(more or less) of the peak gain. The control of those sidelobes demands careful
design and installation of such antennas. A useful specification in that regard is
the sidelobe envelope (shown in Figure 4.7), which defines the worstcase potential
for RFI. A standardized formula, which was adopted by the ITU as Recommendation
S.465-5, provides a common standard for the larger ground antennas used at
C and Ku bands. In it, gain at a particular off-axis angle is specified in the direction
of a potentially interfering (or interfered-with) satellite by:
G(_) ≤ 29 − 25 log10 _, dBi
where _ is the offset angle between the direction of the main beam and that toward
the interfering or interfered-with satellite, for angles between }1 degree and
}37 degrees. Furthermore:
G(_) ≤ 8 dBi, for angles between }7 degrees and }9.2 degrees
For angles between }9.2 degrees and 48 degrees:
G(_) ≤ 32 − 25 log10 (_), dBi
Lastly, G(_) ≤ −10 dBi for _ > 48°.
4.2 Propagation on the Earth-Space Link 119
4.2.4 Polarization (Linear and Circular)
Up to this point, we have dealt with radio propagation as if it were pure energy.
However, there is a property of an electromagnetic wave called polarization, that
depends on the orientation (or angle of rotation) of the transmitting antenna. You
may be familiar with polarization as it relates to light. For example, true threedimensional
(3-D) movies utilize vertically and horizontally polarized light to simultaneously
project appropriate left and right images on the screen. The images are
separated by invisible polarizing grids imbedded in the lenses of the viewer’s glasses,
where one lens is aligned vertically while the other is aligned at a 90-degree angle
(horizontally). Vertical and horizontal polarization, therefore, can ‘‘reuse’’ (i.e.,
use twice) a transmission path, such as the projection of a movie on a screen or,
as described below, a radio path.
4.2.4.1 Linear Polarization
The concept of polarization discrimation in radio communications is illustrated in
Figure 4.8. Shown at the top of the figure is a type of simple wire or rod antenna
called a dipole. Electrical current from the transmitter flows along the rod first
upward and then downward, oscillating at the frequency of transmission. At C-band
downlink frequencies, the rate of oscillation is 4 billion times per second (i.e.,
4 GHz). The alternating current in the rods produces an electromagnetic wave that
propagates off into space (Figure 4.2). A dipole is not a true isotropic source
because there is no radiation in the direction the rods point. Instead, what is formed
is a doughnut-shaped pattern that is aligned horizontally. The electrical currents
in the rods cause the electromagnetic wave to have its electric component to be
lined up in the same direction, which is vertical (in the direction of the two arrows).
This type of polarization is called linear polarization (LP) because the electric
component has a fixed orientation. Horizontal LP is obtained when the dipole is
rotated 90 degrees, so that the direction of the electrical current also is horizontal.
Reception occurs when the electric component of the incoming wave produces a
Figure 4.8 Properties of linear polarization as radiated and received by dipole rod antennas.
120 Microwave Link Engineering
current in the receiving antenna, which cannot occur if the conductors of the
receiving antenna are perpendicular with the incoming polarization. In the lower
half of Figure 4.8, horizontally polarized transmitting and receiving antennas provide
for a maximum amount of power to be carried (coupled) between them. A
vertically polarized receive antenna, which is perpendicular to and therefore crosspolarized
with the transmitter, minimizes the amount of coupled energy.
Microwave antennas usually make use of waveguide structures and solid
reflecting surfaces because they are much more efficient and predictable than wires
and rods. For example, if one encloses the dipole antenna in a metal box, it can
feed a waveguide. Rectangular waveguide, illustrated behind the horn in Figure
4.9, is simple in design and efficient in the transmission of microwave energy and
hence is very popular in satellite and Earth station design. The electric component
is vertical for the orientation shown, extending between the upper and lower side
walls (dimension b). The magnetic component lies in the plane of the top and
bottom walls and is perpendicular to the electric component at every point along
the waveguide. The horizontal dimension of these walls (dimension a) must be
greater than one-half wavelength at the operating frequency. At any lower frequency,
the wave cannot propagate because its wavelength literally cannot fit. This
is termed waveguide beyond cutoff. The narrower dimension, b, is not as critical
and is adjusted like a transformer to match the characteristic impedance of the
waveguide to the antenna.
The relationship between horn antenna orientation and wave polarization is
shown in Figure 4.9, which basically is a slightly flared piece of rectangular waveguide.
Instead of electrical currents, the waveguide carries a bound electromagnetic
wave with the electric components extending between the parallel walls. Creation
of the guided waves in the first place is accomplished with the small monopole
element inserted into the waveguide at the input end. Note how the electric components
vary in intensity along the width of the horn, being maximum at the center
(closely spaced arrows) and dropping to zero at the edges. That particular natural
Figure 4.9 Linear polarization of microwave radiation from vertically and horizontally polarized
feed horns.
4.2 Propagation on the Earth-Space Link 121
property of a simple horn provides a radiation taper used in the design of hornfed
reflector antennas. In any case, the radiated electromagnetic wave is linearly
polarized, just like that from a vertically oriented dipole. The polarization can be
changed to horizontal simply by rotating the horn and waveguide by 90 degrees.
As with wire antennas, maximum coupling occurs when the transmitter and
receiving horns are co-polarized. If one horn is rotated 90 degrees with respect to
the other, minimum energy transfer (coupling) results and they are said to be crosspolarized.
The performance of LP at intermediate points follows a simple law: The
relative energy coupled is equal to the square of the cosine of the angle. That
characteristic is plotted at the left of Figure 4.10 for the copolarized case, that is,
for the level of signal received as the receiving antenna is rotated from maximum
coupling to minimum coupling. An ideal LP wave and antenna are assumed, for
which the coupling goes from a maximum of 1 to a minimum of zero. Maximum
coupling always occurs at some angle, and minimum coupling is as close to zero as
physically possible. Properly designed and installed antennas can deliver minimum
coupling values of 0.0001, or −40 dB. Larger structures that are highly symmetrical
in the direction of radiation can even exceed that value. However, due to slight
misalignments and other imperfections, values in the range of 30 to 40 dB are
more typical.
Probably the most important application of polarization is in frequency reuse,
where two cross-polarized signals are transmitted at the same time on the same
frequency. The right curve in Figure 4.10 shows how the level of the cross-polarized
signal increases as the receiving antenna is rotated from 0 to 90 degrees. Notice
how at 45 degrees both signals are at the same level, which results in no isolation
between the signals on opposite polarizations. Figure 4.11 plots coupling in decibels,
termed polarization isolation, between the desired and undesired polarizations.
Maximum isolation occurs at zero offset angle, that is, where the receiving antenna
is aligned in polarization with the transmitting antenna and the undesired polarization
is nulled out (minimized). Alignment for maximum coupling is not particularly
Figure 4.10 Relative receive power as a true ratio for the copolarized and cross-polarized signals
in a linearly polarized link as the polarization angle of the receiving antenna is rotated.
122 Microwave Link Engineering
Figure 4.11 Polarization isolation in decibels for a linearly polarized RF signal as a function of the
offset angle from maximum isolation.
critical (}5 degrees of error introduces very little loss of signal); however, the crosspolarization
isolation is extremely critical, and precise alignment is a necessity. The
maximum value of isolation of 40 dB in Figure 4.11 corresponds to the typical
value described in the preceding paragraph. All FSS and BSS satellite communications
systems use polarization isolation to increase the capacity of the particular
orbit position. Therefore, users must employ antennas on the ground that meet
polarization requirements; linear polarizations should be adjusted from time to
time by rotating the feedhorn for minimum RFI. (The only way around the latter
requirement is to use circular polarization, discussed in the next section.) Isolation
can subsequently degrade from propagation effects, particularly Faraday rotation,
which is a twist of linear polarization caused by the ionosphere. If the ionosphere
caused a change of 6°, then the cross-polarization isolation would be approximately
20 dB. Such a low value generally is considered to be unacceptable.
4.2.4.2 Circular Polarization
Another type of polarization that is used in international, DTH broadcasting and
MSS satellites is circular polarization (CP). Being circular in polarization, the
receiving antenna is not aligned for the transmitting antenna’s polarization. Rather,
it is the inherent polarization of the signal, either right-hand CP (RCP) or lefthand
CP (LCP), that must be supported by both ends of the link. An example of
RCP is provided in Figure 4.12.
To understand the composition of CP, first observe that LP can be represented
in the form of a vector, where the direction of the vector is in line with the electric
component and its magnitude is proportional to the square root of the power of
the signal. This vector is always perpendicular to the direction of propagation. CP
is a particular combination of two equal LP waves that are cross-polarized with
respect to each other (horizontal and vertical are two of the four vector states in
Figure 4.12). If the two polarizations represent the same signal on the same frequency
and they are precisely in phase with each other, the resultant vector lies at
4.2 Propagation on the Earth-Space Link 123
Figure 4.12 Rotation of the electric component during the propagation of a circularly polarized
RF signal.
a 45-degree angle between the two polarizations with an amplitude that is √2
times either amplitude. In CP, however, the same two vectors are out of phase
with each other by 90 degrees. That is accomplished by first splitting the transmit
signal in two at the source antenna and then delaying one component by a quarter
period before radiating the components through a dual-polarized antenna element.
What is produced is a resultant vector that rotates like a corkscrew (Figure 4.12)
as it propagates through space. The sense of polarization is either rotating to the
right (RCP) or to the left (LCP), looking in the direction of propagation. That is
determined by which linear component is delayed with respect to the other. Frequency
reuse is possible because LCP and RCP waves are oppositely polarized with
respect to each other, just like vertical and horizontal LP waves.
The advantage of CP is that the receiving antenna does not need to be aligned
in polarization since the electric component rotates 360 degrees at a rate equal to
the frequency of transmission. A receiving antenna could consist of combined
vertical and horizontal LP elements, allowing the components to be coupled and
recombined with the proper phase relationship. The sense of combining determines
whether RCP or LCP will be recovered. Because most waveguides are linearly
polarized, a device called a polarizer is used to make the conversion to CP. Also,
the waveguide must be square or round to prevent cutoff at the operating microwave
frequency. The most common approach is to generate two orthogonal linear polarizations,
delay one of them by 90 degrees (one-quarter wavelength), and then add
them. In general, the amount of polarization isolation thus obtained is not as high
as that achieved by LP because of the imbalance between the two components that
often results in practice. This imbalance produces an elliptical pattern measured
in terms of the axial ratio. (CP has an axial ratio of 1, or 0 dB.)
4.2.5 Propagation Losses
The free-space path between transmitter and receiver may pass through a combination
of ground obstructions and atmospheric phenomena. They can be divided into
124 Microwave Link Engineering
those that are constant (and usually predictable) and those that are both time
variant and random (which may be estimated statistically). Taken together, these
losses reduce the strength of the received signal and can cause its level to vary over
time even to the point where the signal fades away. This is illustrated in Figure
4.13, which shows received signal power (signal level) as a function of time during
a period of rapid variation (fading). The mean signal level results from the constant
and predictable losses on the link; the random losses cause the level to decrease
and occasionally drop below the acceptable threshold. When the signal dips below
that threshold, it could become unusable for communication purposes because of
excessive noise and, in the case of digital links, an undesirable high error rate and
ultimately a loss of synchronization as well. The difference in decibels between the
mean level and the threshold is called the link margin, which is discussed at the
end of this chapter.
Table 4.3 summarizes the primary losses that must be determined and accounted
for in any microwave link design. The table compares typical downlink values for
L-, C-, Ku-, and Ka-bands. Certain losses are critical in all satellite systems, while
others are of concern only in specific cases.
4.2.5.1 Free-Space Loss
We already have covered in some detail the mechanism for free-space loss, which
diminishes signal strength due to distance alone. For 1 m2 of effective antenna
area, the captured energy is simply
Pr = Pt /4_R2
where Pt = the radiated power in watts and R = the distance in meters.
The typical distance from a point on the Earth to a satellite in GEO is 36,000
km (22,300 miles).
This relationship, which should be recognized from the previous discussion of
isotropic radiation, does not depend on frequency. It is a common practice to
introduce frequency into the equation by splitting apart the propagation loss from
the gain of the receiving antenna (here assumed to have an effective area of 1m2).
Figure 4.13 Variation of the received carrier power on a microwave link during a period of severe
fluctuation (fading).
4.2 Propagation on the Earth-Space Link 125
Table 4.3 Comparison of Significant Propagation and Fading Modes and Resulting Losses That Affect
Microwave Links over GEO Paths (Values Are Rough Order of Magnitude, in Decibels)
Propagation Relative Importance L-Band C-Band Ku-Band Ka-Band
Mode on Satellite Links (1.6/1.5 GHz) (6/4 GHz) (14/12 GHz) (30/20 GHz)
Free space Dominating factor 187 196 205 210
downlink
Atmospheric Relatively small and 0.1 0.2 0.3 0.5
nearly constant at
high elevation angles
Rain attenuation Severe as frequency 0.1 0.5 2 6
and elevation angle
increases
Refraction Significant at times at 6 3 2 1
low elevation angles
Scattering Produces local RFI
Diffraction Considered for 6 to 12
mobile links without
line of sight
Ionospheric Occasional wide 3 to 6 1 to 3 <1
scintillation signal variation
(multipath)
Doppler Frequency shift for
moving vehicles or
satellites
The free-space path loss, which would be in the denominator of the preceding
equation, is
a0 = (4_R/_ )2
where _ = the wavelength in meters and R = the path length in meters.
Substituting frequency for wavelength (e.g., f = c /_ ) and converting to decibels,
we obtain the following basic relationship for free space loss:
A0 = 183.5 + 20 log( f ) + 20 log(R/35,788)
where f is the frequency in gigahertz and R is the path length in kilometers. (The
multiplier of 20 for the log accounts for the squaring of the terms within the
brackets.)
This formula is plotted at three downlink frequencies as a function of path
length in Figure 4.14. The figure illustrates the basic free-space loss that must be
countered as a function of the particular orbital configuration. In particular, we
see a fundamental difference of 30 dB between GEO and LEO satellite systems.
4.2.5.2 Absorption
RF energy is absorbed to some extent (and converted into heat) as it passes through
clear air, water vapor, and smog. The impact of frequency of operation is shown
in Figure 4.15 for an Earth station at sea level and transmitting or receiving at an
126 Microwave Link Engineering
Figure 4.14 Free-space loss, A0, as a function of orbit altitude and frequency band.
Figure 4.15 Typical relationship between atmospheric absorption and the microwave frequency
of transmission, expressed in decibels.
elevation angle toward the zenith (_ = 90°). The absorption essentially is constant
and will be under 1 dB at C- and Ku-bands. The only place on the ground where
_ = 90° is at the subsatellite point, on the equator directly below the satellite. At
other locations the elevation angle takes values down to perhaps a practical low
of 5°. At a particular frequency shown along the X-axis, the atmospheric absorption
for elevation angles less than 90° would increase due to the greater thickness of
atmosphere. Below 10°, absorption is highly sensitive to multipath due to ground
reflections and atmospheric ducting. The following formula indicates how atmospheric
absorption changes for elevation angles greater than 10°:
Aa ≈
8aO + 2aW
sin _
4.2 Propagation on the Earth-Space Link 127
The two parameters, aO and aW, represent constituent absorptions due to
oxygen and water vapor, respectively. We can view the numerator as being the
total attenuation indicated on the Y-axis of Figure 4.15.
As frequency increases above 15 GHz, the constituents of the atmosphere reach
individual points of resonance, and absorption can become very high, even total.
The bands of fequencies around 22 and 66 GHz correspond to resonances for
water vapor and oxygen, respectively, and are not employed for either uplinks
or downlinks. Direct links between satellites, called intersatellite links (ISLs) or,
alternatively, cross-links, bypass the atmosphere and hence may utilize the absorptive
bands.
4.2.5.3 Rain Attenuation
After free-space loss, the most detrimental effect on commercial satellite links
above C-band is rain attenuation, which results from absorption and scattering of
microwave energy by rain drops. That loss, which increases with frequency, was
discussed in Chapter 1 in the comparison of frequency bands. Rain attenuation is
not predictable on an instantaneous basis, but statistical estimates can be made
that allow links to be designed. Obviously, dry seasons and regions of the world
with low rainfall would not suffer greatly from this phenomenon. However, links in
regions with heavy thunderstorm activity—and hence rainfall—should be provided
with greater link margin, or service might not be maintained with sufficient availability
to satisfy commercial requirements.
Intense rain is contained in rain cells, which have somewhat limited geographic
size. The statistical relationship between local rainfall and the resulting attenuation
caused at a particular microwave frequency is a complex combination of several
factors. The dimensions of the rain cell vary based on the rain rate, measured
typically in millimeters per hour. At a particular rain rate and cell size, the attenuation
increases with path length through the cell. Because a cell is shaped like an
oblate spheroid (i.e., flat like a hamburger), the path length is inversely related to
the elevation angle. Thus, rain attenuation varies in much the same manner as does
atmospheric attention in the absence of rain. Other factors include gaseous water
vapor, clouds, and ice particles. The work of combining all of these factors is
largely done and available to us in the form of rain propagation models. The
ITU has adopted the DAH model, which was developed by the research team of
Dissanayake, Allnut, and Haidara. Figure 4.16 provides in summary form some
rain attenuation data as a function of frequency and for a temperate climate such
as that of Western Europe or the northeastern United States. Comparable data for
a tropical region would be two to three times greater in decibels. The best approach
is to use a software tool like SatMaster (discussed later in this chapter) that uses
the DAH model to calculate rain attenuation for a specific location.
As indicated in the figure, the lower the elevation angle from the ground, the
greater is the amount of attenuation from a given amount of rainfall. Heavy rainfall
also alters the polarization of the signal because atmospheric drag causes raindrops
to flatten and not be perfectly spherical. That reduces cross-polarization isolation
between linear polarized transmissions. For example, the maximum isolation in
Figure 4.11 decreases at Ku-band in heavy rain from 40 dB to approximately
128 Microwave Link Engineering
Figure 4.16 Rain attenuation versus frequency and elevation angle for a temperate climate.
25 dB. That would not be detrimental to most transmissions if appropriate adjustments
are made in the link design. Depolarization in rain is not particularly harmful
at C-band and the lower parts of Ku-band, but it can greatly reduce isolation at
frequencies above 15 GHz. It also turns out that depolarization of CP transmissions
is somewhat more severe than for LP.
The first step in using a rain model is to identify the particular rain region
where the service must be provided. The map in Figure 4.17 shows different regions,
each characterized by a type of climate and associated rain properties. Tropical
regions are the worst, due to a higher incidence of thunderstorms, which contain
the most intense rain cells. The shading and numbers designate the relative impor-
Figure 4.17 Regions corresponding to the five rain-climatic zones, indicating the rain rate in
mm/hour to be exceeded for 0.01% of the time [2].
4.2 Propagation on the Earth-Space Link 129
tance of rain in the service design. This presentation is not detailed enough to
indicate higher rain rates in the most intense areas of Brazil, Indonesia, and central
Africa, where they reach 145 mm/hr for 0.01% of the time. Note that the attenuation
is approximately proportional to rain rate.
4.2.5.4 Refraction
The lower portion of the atmosphere, called the troposphere, decreases in density
upward from the Earth’s surface. As illustrated in Figure 4.18, electromagnetic
waves are bent as they pass through the medium; as a result, the satellite has a
virtual position slightly above that of its true position. That characteristic is taken
into account in terrestrial microwave system design by increasing the Earth’s radius
by a factor of 4/3 in the profile map used to plot the line-of-sight path. In satellite
links, the bending is less significant because of the higher angle toward the satellite.
Unstable atmospheric conditions like temperature inversions, clouds, and fog
produce discontinuities and fluctuations in what otherwise would be a uniform
air density distribution. The consequent random, temporary bending can cause
significant signal fading akin to rain attenuation. The effect is more pronounced
for paths that are nearly parallel to the Earth (line-of-sight microwave and satellite
links at low elevation angles). That is also a worry for making precise gain measurements
of an Earth station antenna when the reference transmitting source is on a
ground-based tower. Generally, refraction by itself does not impair typical satellite
links because the amount of bending is small relative to the beamwidth of the
satellite and the Earth station antennas.
The ionosphere, the layer of ionized particles at around 150 km, completely
reflects frequencies between 0.1 and 30 MHz under certain conditions, producing
the ‘‘sky wave’’ effect, which allows short-wave transmissions to cover long dis-
Figure 4.18 Refraction is the bending of a microwave signal as it passes through the layers of the
atmosphere.
130 Microwave Link Engineering
tances. In addition to refraction, the ionosphere rotates the linear polarization of
microwave transmissions (Faraday effect), although that can be compensated for
at frequencies above 3 GHz by appropriate adjustment of feed-horn angle at the
receiving Earth station. This is complicated by the fact that the angle of rotation
varies from day to night by as much as 6°. Below 3 GHz, Faraday effect produces
one or more full rotations, which is why CP is preferred. Faraday rotation increases
during periods of high sunspot activity, according to the 11- and 22-year solar
cycles. Faraday effect decreases with frequency and can be ignored at Ku-band and
above.
4.2.5.5 Scattering, Diffraction, and Shadowing
It was mentioned previously that water droplets scatter microwave signals. That
would reduce the direct-path power level, spraying some of it back toward the
source. At an Earth station site, the occasional scatter can place RFI in the direction
of terrestrial microwave receivers that otherwise would have been adequately
protected.
Diffraction, on the other hand, occurs when microwaves encounter and bend
over a physical obstacle such as a building or mountain. The principle behind
diffraction is illustrated in Figure 4.19, which shows how an Earth station might
be shielded from local microwaves by an intervening hill or mountain. The heights,
H1 and H2, introduce shielding but also provide the possibly diffraction path for
microwave signals in both directions. If the hilltop just happens to touch the lineof-
sight path between the Earth station and local microwave antennas (e.g., H1
and H2 are zero), a diffraction loss of precisely 6 dB is introduced. As the obstacle
begins to block the path, the amount of diffraction loss increases until there essentially
is total blockage.
The line-of-sight nature of microwave transmission means that blockage may
reduce signal strength below the threshold level. For frequencies above 3 GHz,
such blockage must be assumed whenever there is an obstacle that results in a
situation like that illustrated in Figure 4.19. The situation at lower frequencies
such as L- and S-bands might be somewhat more favorable because of the significantly
larger wavelength involved. Depending on the type of ground antenna in
use, it might be possible to receive adequate signal strength from reflections off of
buildings and hills. It has been proposed that partially blocked and reflected signals
Figure 4.19 The use of terrain shielding to block RFI between a terrestrial microwave link and an
Earth station.
4.2 Propagation on the Earth-Space Link 131
be used to directly communicate or to augment communications for mobile users
either with handheld satellite phones or in vehicles. This is certainly done in terrestrial
cellular systems, but they have the advantage of much higher link margins
from closely spaced base stations. Using a GEO or LEO satellite, it is not feasible
to obtain such high margin and so one cannot expect reliable communications under
shadowing conditions. The one area where some shadowing could be tolerated is
where it is produced by trees that are relatively far apart. Even here, heavy tree
trunks will block signals much the same as a building.
4.2.5.6 Multipath
Multipath is the result of the same RF signal taking both a direct path and a slightly
longer refracted path, the latter arriving at the receiving antenna delayed in time
from the former. It is illustrated in Figure 4.20 for refraction caused by discontinuities
in the ionosphere. Multipath also occurs under shadowing conditions. It will
be shown that both signal cancellation and enhancement can occur. The twinkling
of a star is a multipath phenomenon, where two light rays, one direct and the
other bent back in the troposphere, reach the eye and combine to produce a
variation in light intensity. The twinkling occurs when the variability in the air
causes the refracted path to change over time.
Due to the fact that rays are wave phenomena, the direct and refracted paths
combine using vector addition, as illustrated in Figure 4.21. Shown at the right
are the two signal paths reaching the same antenna, wherein they combine to form
a resultant signal (that is because both paths contain the same signal, although the
refracted path, being longer, introduces a delay and consequent phase shift). The
two drawings at the left of Figure 4.21 represent the extremes: enhancement at the
top and cancellation at the bottom. The direct path is shown as the vertical signal
vector of constant length. Added to that on a vector basis is the refracted signal
(the dotted vector), which can arrive at any random relative phase angle, as indicated
by the circle about the tip of the direct path vector. The resultant received signal
vector, shown as a heavy arrow, is only slightly affected by the weak refracted
Figure 4.20 Multipath propagation caused by ionospheric scintillation.
132 Microwave Link Engineering
Figure 4.21 Multipath signals produce enhancement or cancellation of the direct path signal when
received through a common antenna, as illustrated by vector diagrams.
signal. An increase or a decrease in strength occurs, depending on the relative phase
angles.
If the refracted signal is comparable in strength to the direct path, cancellation
occurs when the relative phase is 180 degrees. There also can be significant amplification
(up to 6 dB) when the two vectors instantaneously combine in phase.
Fortunately, the unstable conditions that produce this in the ionosphere only occur
during two periods annually, around the equinoxes. Earth stations near the geomagnetic
equator experience the most severe scintillation fades during those periods
of activity. Ionospheric scintillation is not constant, with the result that the received
signal both decreases and increases rapidly. The fact that the combining is frequency
dependent produces selective fading, in which a spectrum of signals fluctuates
widely. That is in contrast to shadowing due to terrain blockage, termed flat fading
because it is not frequency selective.
Finally, multipath is an influence on L- and S-band mobile links because the
user antenna is generally not very directional so can receive signals from several
directions at the same time. Generally, the direct path dominates, but the reflected
path or paths can produce pronounced fading. Terrestrial mobile radio services
do not depend on the line-of-sight path and actually utilize multipath to provide
coverage in areas of heavy terrain blockage. There, as much as 30 dB of margin
is afforded by the close range between mobile user and lease station.
Satellite links, on the other hand, do not normally generate 30 dB of margin
and so cannot depend heavily on multipath as a positive factor. Rather, signals
reflected off buildings and local terrain features more likely will cause fading. The
basic problem is illustrated in Figure 4.22 for a mobile receiver traveling along a
highway. Two paths reach the mobile user: the direct line-of-sight path and a path
that is reflected off the highway itself. They produce either fading or enhancement,
4.3 Microwave Transmitters and Receivers 133
Figure 4.22 Generation of mobile multipath on flat, smooth terrain.
which is nearly constant as long as the satellite remains in the same position relative
to the vehicle. The amount of loss depends on the reflection coefficient experienced
by the reflected signal. Alternatively, reflection off a fixed feature, such as a hill
or a building, produces time-varying fading as the vehicle moves relative to the
reflection point. The rapidity of the fading depends on the speed of the vehicle.
The design of MSS links is, therefore, the most challenging, because they are subject
to greater variation and uncertainty.
4.3 Microwave Transmitters and Receivers
The basic elements in an end-to-end satellite communication link are illustrated in
Figure 4.23. In the figure are the transmitting Earth station, which establishes the
uplink path, a simplified satellite and its microwave repeater, the downlink path,
and a receiving Earth station.
The entry and exit points to the propagation medium are provided by the
transmitting and receiving antennas. As previously discussed, antennas convert
electrical energy at microwave frequencies into electromagnetic waves, and vice
versa. A transmitting Earth station consists of equipment that impresses (modulates)
the information to be sent on an RF signal called the carrier, translates it to the
appropriate frequency and amplifies it to a high-enough power level to provide an
adequate uplink. A receiving Earth station works in the exact opposite direction.
Deferred to Chapters 6 and 9 are detailed descriptions of the electronics of the
satellite and the Earth station, respectively.
4.3.1 Transmitting Station
Figure 4.24 portrays a single transmitting chain of a generic microwave station,
with the signal input at the left and the RF output from the transmitting antenna
at the right. The signal to be transmitted consists of information in electrical form,
such as one or more voice channels for telephone service, digital data in the form
134 Microwave Link Engineering
Figure 4.23 The main elements of a satellite link on an end-to-end basis.
4.3 Microwave Transmitters and Receivers 135
Figure 4.24 Simplified block diagram of a microwave transmitting station capable of baseband
signal processing, modulation, frequency translation, and high power amplification.
of a high-speed bit stream, or a composite video signal such as that delivered from
a video tape recorder. In modern satellite systems, analog information forms like
voice and TV are first digitized and then compressed to reduce the required bandwidth.
From that point, the link is digital in nature.
4.3.1.1 Encoding and Modulation
In the encoding element of the station (the first element to be encountered), the
digital information is prepared for transmission through the link. Encoding consists
of one or more numerical processes that better match the data to the specific
characteristics of the satellite link. Table 4.4 summarizes the most common forms
Table 4.4 Typical Forms of Encoding Applied to Microwave Links over Communications
Satellites
Impact on Data
Encoding Technique Application Benefits Throughput
Forward error Error rate reduction Lower error rate, or Increases the output
correction reduced power bit rate to
requirement (Eb /No ) accommodate
redundant bits
Compression Reduces the total bit Less data to send or Increases throughput;
count, either lossless store—better may reduce quality
or lossy utilization of the link and introduce delay
and system
Encryption Information security Makes data private Generally, no increase
and difficult to in data rate; involves
corrupt complex management
and could introduce
delay
Protocol adaptation Data communications Improves the user May increase actual
networks, typically experience by throughput;
using the Internet countering satellite introduces
Protocol suite link impairments, complexity; tied to the
(TCP/IP) such as bit errors and type of information
delay transfer or application
136 Microwave Link Engineering
of encoding along with their benefits and characteristics. Forward error correction
(FEC) is one type of encoding that is always applied as it can only improve the
quality of transmission in terms of the bit error rate (BER). A reduction in BER
can be taken either as a quality enhancement or to reduce the amount of transmitted
power (alternatively, the antenna size). Modern FEC techniques use combinations
of individual error correcting codes through the process of concatenation.
In many cases, FEC is incorporated into the modulation and demodulation
function, which are performed by the modem. Compression techniques fall into
two categories: lossless compression, which only removes useless bits that convey
no information, and lossy compression, where information is removed but is
intended to provide sufficient quality to satisfy the user and application. Information
that starts out in analog form, such as voice, image, and video, can employ lossy
compression as users are accustomed to some amount of impairment. However,
the amount of compression deserves careful review by the intended audience to
assure that the service is not compromised as to acceptability. Encryption is highly
desirable on satellite links as it is a relatively simple matter to intercept satellite
transmissions and potentially to introduce either bogus data or to otherwise disrupt
information transfer. Protocol adaptation was discussed in Chapter 3 and is very
effective for reducing the perceived delay of Internet services over GEO satellites.
The output of the encoding stage is referred to as the baseband. It is the purpose
of the modulator to take the baseband and apply it to an RF carrier. The type of
modulator used in a microwave station often handles a wideband baseband input
such as that obtained from a high-capacity data stream measured in megabits per
second. In SCPC service, the output of the modem is kept on to allow a continuous
stream of data to be uplinked to the satellite. TDMA operation, on the other hand,
requires that the modem transmit in noncontinuous bursts. That is because the RF
channel is being shared by multiple Earth stations that transmit in different time
slots.
The opposite of modulation is simply demodulation, which is the process
whereby the baseband is removed from the carrier. The demodulator intially must
acquire the incoming carrier, demodulate the bit stream, and then produce the
baseband in a form that can be used by the decoder. The process is repeated
for each received transmission, particularly for TDMA operation. The decoding
circuitry corrects a majority of the errors produced by noise and interference on
the uplink and downlink.
4.3.1.2 Frequency Conversion and RF Amplication
The RF carrier coming from the modulator typically is not at microwave frequencies
but rather is centered within a standard frequency channel, the intermediate frequency
(IF). Most transmitting and receiving stations use 70MHzas the IF, allowing
modulators and demodulators to be conveniently interchanged and interconnected
by patch cords and coaxial switches. In low-cost consumer equipment, the IF is
internal to the unit and hence need not follow any particular standard. Another
point is that there are cases where the RF bandwidth is larger than 140 MHz,
making it unfeasible to use 70 MHz as the IF (since the bandwidth would extend
below zero frequency). This can be overcome by directly modulating a carrier at
4.3 Microwave Transmitters and Receivers 137
the microwave frequency of transmission or, more likely, by using an adequately
high IF, such as 140 MHz.
For digital modulation, the bandwidth is directly proportional to the input
data rate:
B = am _ R
where R is the data rate in bits per second and am is a constant determined by the
particular type of digital modulation and FEC. For example, QPSK is a popular
form of digital modulation used on satellites, in which case am is nominally equal
to 0.6. The proportionality constant, am, also depends on the order of the modulation.
For example, going from QPSK to 16 QAM cuts am in half, to 0.3. Employing
FEC with a code rate of 3/4 increases am to 0.8 and 0.4 for QPSK and 16 QAM,
respectively.
The function of the upconverter is to translate the carrier without modification
from IF to the desired microwave frequency of transmission. Within the upconverter
are a microwave mixer and a local oscillator (LO). Translation is governed by a
simple mathematical relationship that states that the output frequency equals the
sum of the input IF and the frequency of the LO. For example, if the IF is at
70 MHz and the LO is at 6,030 MHz, the output RF is 6,100 MHz. (Although
in theory the difference between the LO and IF could be employed, this is not done
in practice due to more complex filtering.) From a practical standpoint, the RF
usually is assigned by someone else and the IF is fixed. Therefore, the LO frequency
must be selected properly to put the RF carrier in the right place (e.g., on the
assigned frequency, which is 6,100 MHz in the example). Modern upconverters
are frequency agile, which means that the LO can be tuned for a different RF
channel from the front panel in much the same way as a TV set or car radio. The
particular type of LO is called a frequency synthesizer because the frequencies are
generated digitally using computational techniques. A nonsynthesized type of agile
LO could cover the frequency range but would not be adequately stable over time
and temperature. The desired frequency would be selected either from the front
panel using an input keypad or remotely by computer over a data line.
The design of the IF aspect of Earth stations has evolved to a more flexible
concept using a wideband IF nominally within the L-band within the broad range
of 0.5 to 2.5 GHz. This is a continuous spectrum that is not directly radiated and
so does not interfere with MSS services within this range. Using low-cost coaxial
cable (coax), this L-band IF carries 500 MHz of bandwidth between the outdoor
antenna system and indoor electronics used to select the specific transponder and
frequency, and for carrier demodulation and baseband processing. The appropriate
receiving device at the antenna is called a low-noise block converter (LNB), which
contains a low-noise amplifier (discussed later in this chapter) and a block downconverter.
On the transmit side, there would need to be an upconverter and highpower
amplifier; if the transmit power required is less than about 10W, then it is
possible to obtain both functions within what is called a block-upconverter (BUC).
4.3.1.3 High-Power Amplification
The last active element of the transmitting station is the high-power amplifier
(HPA). Because all processing and frequency translation have been accomplished
138 Microwave Link Engineering
in prior stages, the only function of the HPA is to increase the power of the
microwave carrier from the low output of the upconverter to the power level needed
to achieve satisfactory uplink operation. The HPA must have sufficient bandwidth
to operate at the assigned microwave frequency and cover the active RF bandwidth
of any anticipated carrier type. Examples of typical microwave HPA devices and
their respective power capabilities are shown in Figure 4.25. Vacuum-tube HPAs,
which do not require water cooling, offer the highest power. They include the
klystron power amplifier (KPA) and the traveling wave tube amplifier (TWTA).
Low to moderate power is affected by solid state power amplifiers (SSPAs), which
are designed for a variety of power levels.
The KPA has a capability measured in kilowatts and is popular for video
uplinks. Within the klystron microwave tube there is a resonant waveguide cavity
that is tuned to the specfic frequency of operation. The operating bandwidth of a
KPA is in the range of 50 to 100 MHz, making it necessary to retune the internal
structure to change transponders. That difficulty is overcome with the TWTA,
another class of microwave amplifier that happens to be very common on satellites
as well (see Chapter 6). TWTAs have a higher-percentage bandwidth, which is the
ratio of usable bandwidth to the center operating frequency, than KPAs and so
can transmit over a total bandwidth of 500 MHz at C-band and as much as 2,000
MHz at Ku- and Ka-bands. Practical TWTAs can be found with power outputs
of from 50W to as much as 800W, although 10-kW water-cooled TWTAs were
used in early INTELSAT Earth stations. Being vacuum tubes, the TWTA and the
KPA require sophisticated high-voltage power supplies, and both employ heated
cathodes to emit electrons for use in the process of amplification. In the case of
HPAs used in Earth stations, it is unavoidable that both types of high-power tubes
will wear out and need to be replaced after a few years of operation. TWTAs used
in satellites must have longer lifetimes, lower mass, and greater dc-to-RF efficiencies
Figure 4.25 RF output capability at C-, Ku-, and Ka-bands of standard uplink microwave power
amplifiers.
4.3 Microwave Transmitters and Receivers 139
and so have to be designed differently. That also results in lower power outputs,
generally under 300W at the time of this writing.
In low-power applications, such as the VSATs mentioned above, SSPAs can
be employed for stable, long-term operation without maintenance. A detailed discussion
of SSPAs can be found in Chapter 6. Figure 4.25 presents SSPA power
levels available in 2008: up to 800W at C-band, 250W at Ku-band, and 20W at
Ka-band. The basic building block of the SSPA is the gallium arsenide field effect
transistor (GaAsFET), and the overall power capability is determined by the number
of individual transistor stages that can be effectively paralleled. Bandwidth of SSPAs
is somewhere between those of the KPA and the TWTA, based on the biasing and
tuning of the transistor stages. That generally cannot be altered after the SSPA is
manufactured.
4.3.2 Receiving Station
The reverse process found in the receiving station is illustrated in Figure 4.26.
Because the microwave signal collected by the receiving antenna is weak, it first
is necessary to raise the power to a level that can be accommodated by the processing
elements. That is performed by the low-noise amplifier (LNA), whose gain must
meet the requirements described above. However, the internal noise contribution
of the LNA must be held small enough, or the weak signal input literally can be
buried in noise. The rest of the elements perform functions that are inverse to those
of the transmitting station. If an integrated downconverter is appended to the LNA,
the resulting unit is called a low-noise block converter (LNB).
4.3.2.1 Low-Noise Amplifiers
It is possible to express noise in terms of an equivalent noise temperature in Kelvin
(K). The Kelvin scale has gradations the same size as degrees C (centigrade) but
begins at the noiseless state of absolute zero and measures the average random
energy of motion of electrons within the receiver electronics. According to the
theory, the random energy of the electrons is proportional to the noise power that
overlays the desired signals within the passband of the amplifier. A device cooled
to 0K (an impossibility) would not contribute noise to the system.
Figure 4.26 Simplified block diagram of a microwave receiving station capable of low-noise amplification,
frequency conversion, demodulation, and baseband signal processing.
140 Microwave Link Engineering
All LNAs and LNBs found in satellite communications Earth stations today
use low-noise transistors such as gallium arsenide field effect transistors (GaAs-
FETs). These are also referred to as high electron mobility transistors (HEMTs),
which are more efficient and produce less noise than their silicon bipolar transistor
counterparts (although silicon transistors are more rugged and static discharge
resistant). Most transistors used in large-scale integrated circuits are FETs as are
microwave amplifiers of various types. Indium phosphide is another material used
in HEMTs and is gaining in popularity due to its improved performance over
GaAsFETs. Typical values of LNA or LNB noise temperature at C-, Ku-, and
Ka-bands are 30K, 60K, and 90K, respectively. Another important property of the
LNA and LNB is its gain, which can be selected in the range of 50 to 70 dB. This
involves one or two low-noise HEMT stages followed by one or more mediumpower
amplification stages that can be either HEMT or silicon. An LNB also
incorporates the block downconverter stage that employs a wideband mixer and
a local oscillator. The latter has a significant impact on the performance of the
Earth station, particularly for reception of narrowband signals that cannot tolerate
frequency instability or the oscillator impairment called phase noise.
It is common practice in Earth station design to place the LNA as close as
possible to the feed and thereby minimize the input loss. That has two benefits:
(1) it improves the effective gain of the receiving system because any loss on the
input must be subtracted from that of the antenna, and (2) reducing this input loss
also decreases the effective noise of the receiving system. The latter effect is discussed
in later chapters.
4.3.2.2 IF-to-Baseband Chain
As discussed in Chapter 1, the satellite’s downlink frequency usually is offset from
its uplink frequency to prevent self-interference on the satellite (the exception is
for single-band systems like Iridium that sequentially transmit and receive packets
on the same frequency). To receive the carrier, the frequency of the LO of the
downconverter would differ from that of the upconverter by the fixed frequency
offset (2,225 MHz for standard C-band) introduced by the satellite repeater. To
complete a previous example, the downconverter LO should be set to the (6,100
− 2,225) − the IF.
The demodulator is tuned to the IF to produce the baseband from the received
carrier and sometimes provides FEC decoding. Along with the information are
versions of noise and interference picked up over the link. Specifically, the received
digital carrier on the downlink is demodulated to deliver a digital bit stream with
the digitized version of the desired information. The demodulated performance is
measured in terms of bit error rate. The bits are processed further to reduce the
error rate, decompress the underlying information, and, if appropriate, convert the
data back into the original analog form. Greater detail on digital modulation
characteristics is contained in Chapter 5.
4.3.3 Definition of a Transponder
The satellite communication industry has long used the term transponder in reference
to a defined RF channel of communication within the communication payload.
4.3 Microwave Transmitters and Receivers 141
The term itself is a contraction of transmitter-responder, originally referring to a
single-frequency repeating device found on aircraft. The purpose of the aircraft
transponder is to add the identification of the aircraft and actively enhance the
power to be reflected back to the radar transmitter. A satellite transponder is
entirely different because it is more of a transparent microwave relay channel, also
taking into account the need to translate the frequency from the uplink range to
the downlink range.
We can better define a transponder by examining the two different payload
configurations shown in Figure 4.27. The single-channel repeater (shown in A)
does just what its name implies: provide a single channel of transmission within
the satellite. As shown at the top of the figure, the entire uplink band is translated
in the downconverter and applied to a single power amplifier. As a consequence, this
amplifier must accommodate the entire uplink bandwidth, which would amount to
500 MHz at C- or Ku-band. That tends to limit the power-handling capability,
particularly if multiple carriers are amplified by the common output stage. The
first generation of LEO and GEO satellites like Telstar and Syncom, respectively,
had repeaters of this type, capable of carrying one TV channel each.
The transponderized design (shown in B in Figure 4.27) breaks up the downlink
range into individual frequency channels. Figure 4.28 suggests how eight transponders
would be divided from the uplink bandwidth, indicating the usable transponder
bandwidth and a guard band. Note that the uplink section still carries the entire
bandwidth of 500 MHz and is shared by all transponders. After the downconverter,
there is a bank of RF filters, called the input multiplexer (IMUX), that separates
the transponder channels on a frequency basis. The output of each IMUX filter
contains one transponder bandwidth, to be amplified in an individual power amplifier
(PA), either a TWTA or SSPA, depending on the frequency and power required.
On the output side, the RF power of the PAs is summed in a passive power combiner
called the output multiplexer (OMUX); it is composed of specially designed low-
Figure 4.27 General arrangement of (a) a single-channel repeater and (b) a channelized repeater
with multiple transponders.
142 Microwave Link Engineering
Figure 4.28 Hypothetical frequency plan illustrated for eight transponders in one polarization.
loss waveguide filters. A single waveguide transfers all the power from the OMUX
to the antenna system.
The transponder, then, is a combination of elements within the payload. On
the input side, it represents a share of the common uplink and receive equipment
within the repeater. We are able to identify specific equipment for each transponder
on the downlink side, consisting of the input filter, power amplifier, and output
filter. Not shown in the figure are the necessary spare active elements (redundancy)
to ensure continuity of service in the event of amplifier or receiver failure. Also
not shown at the input to each PA is a level controlling channel amplifier or
linearizer often found on modern satellites.
The fact that a transponder can be assigned to a particular user application
network has caused them to be rented or sold like condominium flats. In actuality,
it is the microwave channel of communication bandwidth that the transponder
lessor or purchaser acquires.
4.4 Overall Link Quality
One of the more complex problems in microwave link engineering is knowing
precisely how a particular signal will be affected by the noise, which is random in
nature, and by interference from other radio carriers. This section reviews some
of the factors that determine the actual quality and threshold for digital receivers.
A clear understanding of the threshold of the link allows the engineer to predict
how much margin will be available to overcome fading from the various sources
discussed at the beginning of this chapter. It is presumed that the circuitry involved
is properly impedanced matched, so that signal reflections do not unduly distort
the signals.
4.4.1 How Noise and Interference Affect a Microwave Link
Both noise and interference will degrade service quality and if not controlled will
at times render the link unusable. Interference, as explained previously, is often
4.4 Overall Link Quality 143
due to the RF transmission of someone else. On the other hand, noise is totally
random in nature because it results from the random motion of electrons or other
elemental particles in the environment or the receiving equipment. Pure white noise
is most common on satellite links and produces random voltage fluctuations whose
probability follows a normal (Gaussian) distribution. The term white refers to the
fact that the frequency spectrum does not have discrete components at specific
frequencies but rather is a continuum of frequencies, like white light. Ideal white
noise, running from zero to infinite frequency, is physically impossible since it
would represent infinite power. Noise is sufficiently white if it is constant over the
bandwidth of the signal in question.
The resulting density of the noise power (noise power density) in watts per
hertz is proportional to the equivalent noise temperature in Kelvins, that is,
N0 = k _ T
where k is Boltzmann’s constant (i.e., 1.3803 _ 10−23 W-sec/K or −228.6 dBW-sec/K
when expressed in decibels).
This simple relationship demonstrates why the noise performance of an LNA
of a receiving Earth station is rated in terms of an equivalent noise temperature.
The actual noise power that affects a given signal is that which lies with the signal’s
bandwidth, B, that is,
N = k _ T _ B
The effect of white noise on a modulated carrier containing binary data can
be shown using time waveforms, presented in Figure 4.29. The noise-free case is
shown in Figure 4.29(a) in the form of digital information impressed on a carrier.
The transition from binary 1 to 0 occurs where the phase of the sinewave reverses.
That type of modulation is called phase shift keying (PSK) because the shifting
(flipping) of phase by 180 degrees is the means by which information is transmitted.
It is the job of the demodulator to detect the phase reversal to convey to the user
Figure 4.29 (a–c) A digital phase-shift keyed signal combines with white noise to produce a
distorted signal waveform that must be detected by the receiver.
144 Microwave Link Engineering
the proper bit sense. The PSK signal is sent through the link and enters the receiver
along with white noise inherent in the electronics. That noise is depicted in Figure
4.29(b), where the mean voltage is zero and the standard deviation of the noise is
approximately one-half the carrier amplitude. Because power is proportional to
the square of the voltage, the true ratio of carrier power to noise power is 4, or
equivalently 6 dB. The rather high relative noise (low C/N) level yields the sum of
carrier plus noise shown in Figure 4.29(c), where the carrier appears to be somewhat
obliterated by noise spikes.
A well-designed demodulator can detect the transition from the 1 to 0 state in
most cases. The noise can occasionally reach a voltage greater than the signal
amplitude; if it has the opposite sense to the transmitted signal at the precise
moment of testing, it can cause the demodulator to make the wrong choice. The
assumed binary digit will then be incorrect (i.e., a 1 instead of a 0 or vice versa,
depending on what was sent). It should now be possible to visualize that the
strength of the signal relative to the noise will determine the rate at which errors
in detection are made. This is a significant area of engineering study in the communications
field. Some of the most important applications of digital signal processing
(DSP) allow low-cost sending and receiving equipment to identify errors and reverse
them prior to delivery to the user. Special codes that provide such forward error
correction (FEC) accomplish this in practice.
Wideband interfering signals can be treated as white noise and often are.
However, if the interference is narrower in bandwidth than the desired signal, more
sophisticated analysis techniques or even direct measurement must be employed.
As that type of investigation is beyond the scope of this book, we will assume that
the interference can be converted into an equivalent amount of white noise and
added on a power basis to the other noise within the bandwidth of the desired
signal.
4.4.2 Carrier-to-Noise Ratio
The strength of the carrier relative to the noise and not the absolute level of what
comes out is what determines the quality of transmission. This can be understood
by realizing that the power level of the baseband signal from the receiving station’s
signal processor can be raised by simple power amplification. However, the noise
that is present also will be amplified. Only by suppressing the noise at the input
to the receiver can the quality of transmission be improved. That is why the true
performance of the link is measured by the ratio of RF carrier power to noise
power. A variant used in digital transmission is the ratio of the energy per bit to
the noise power density, Eb /N0 . It will be shown that there is a simple constant
adjustment factor between C/N and Eb /N0 , that is,
C
N =
Eb
N0
_
Rb
B
where Rb is the information bit rate, and B is the carrier bandwidth.
A receiving system showing a carrier, white noise, and a single source of
interference is presented in Figure 4.30. The frequency spectrum is similar to the
4.4 Overall Link Quality 145
Figure 4.30 Frequency spectrum showing the desired carrier and the interfering carrier and white
noise, as seen on a spectrum analyzer.
display on a microwave spectrum analyzer, a useful piece of test equipment. Spectrum
analysis is vital to microwave communications because it provides one of the
few windows for viewing the reception process. The figure shows a constant spectrum
of white noise, providing what is referred to as a noise floor. The vertical
scale is linear, measuring power density in terms of watts per hertz. Piercing through
the floor are the desired carrier occupying bandwidth B and centered at frequency
F2, along with an interfering carrier at frequency F1. The total noise power, N,
over the bandwidth of the carrier is the product N0B. An actual spectrum analyzer
would have a vertical scale measured in decibels to simplify the measurement of
C/N. In that case, the C/N in decibels is equal to the difference between the
measured decibel levels of total carrier power and noise power within the bandwidth
B. While bandwidth can be taken into account by subtracting the quantity
10 log B from the spectrum analyzer reading, it is also possible to present the
spectrum analyzer’s measurement bandwidth to eliminate the need for manual
correction.
Another correction to the measurement of C/N on a spectrum analyzer results
from the fact that this instrument actually displays the sum of carrier power and
noise power, since the noise is present within the carrier bandwidth. This correction
has the following form:
(C + N)
N =
C
N +
N
N
=
C
N + 1
∴
C
N =
(C + N)
N − 1
146 Microwave Link Engineering
This formula is performed on true ratios of powers, and not in decibels. To
perform this calculation, take the reading in decibels, convert to a ratio, subtract
1, and then convert back to decibels. For example, if (C + N)/N equals 8 dB (a
factor of 6.3), then the actual C/N equals 5.3 as a ratio or 7.2 dB.
The type of interference shown in Figure 4.30 is not noiselike and therefore
cannot be analyzed in a straightforward manner. One consideration is that only
the upper corner of the interference actually affects the carrier. The ratio of carrier
power to interference power can be taken from the spectrum analyzer display, but
its effect on the reception process also depends on the frequency offset (F2 − F1)
between the two carriers and their relative bandwidths. If due to a malfunction,
the interference may rove through the bandwidth for the carrier, affecting it as
well as other carriers on adjacent frequencies.
4.4.3 Link Budget Analysis
The key station elements and propagation phenomena having been described, it is
now possible to review the analysis and prediction of link performance. Figure
4.31 shows a simplified microwave link with a key parameter (in dB) indicated
for each element. The transmitter can be characterized by the HPA power output,
with the transmitting waveguide introducing some loss as it carries the power to
the transmitting antenna. The electromagnetic wave propagates outward from the
antenna into the medium, where it is subjected to various losses, the free-space
loss being dominant. The small amount of signal power gathered by the receive
antenna is carried through waveguide to the LNA of the receiver. It is possible to
characterize the receiver by the minimum acceptable threshold, which takes into
account the RF noise as well as the quality desired by the user.
4.4.3.1 Power Balance Equation
The simple mathematical relationship shown at the bottom of Figure 4.31 is the
power balance equation. The parameters of the link are actually factors: however,
it is a common practice to express all of them in decibels because that reduces the
Figure 4.31 The relationship between the critical elements of a microwave link and the power
balance equation.
4.4 Overall Link Quality 147
analysis to addition and subtraction. The power balance in decibels can be stated
simply: The power received equals the power transmitted plus all gains and minus
all losses. Transmitter power is expressed in decibels relative to 1W (dBW). For
example, 1W is by definition 0 dBW; 2W is 3 dBW; 10W is 10 dBW; 100W is
20 dBW; and so on. To preserve proper dimensions, there can be only one dBW
term on each side of the equation (the rest are dimensionless dB terms). The power
balance equation usually is arranged in a table called a link budget, in which each
paramter of the link is provided with its own explanation and quantity. One
approach to the problem is to set up for each case to be analyzed, using a spreadsheet
program like Microsoft Excel. There are specialized link budget programs on the
market such as SatMaster that simplify and standardize the process [3].
4.4.3.2 Typical Link Budget (Ku-Band)
A typical link budget for a Ku-band satellite downlink is presented in Table 4.4.
The first entry is the RF power output of the transmitter, expressed first in watts
and then converted into dBW. Transmit waveguide loss of 1 dB (26% power
reduction) is subtracted, while the gain of the spacecraft antenna in the direction
of the receiving Earth station is added. A detailed discussion of spacecraft antenna
design and performance is given in Chapter 7. It is customary to show a subtotal
at this point called the effective isotropic radiated power (EIRP), indicating how
the satellite is performing compared to an isotropic source with 1W of RF drive
power (i.e., 0 dBW). The value of 53 dBW shown in the table is typical for current
high-power Ku-band satellites in the BSS. For a large country such as the United
States, China, Indonesia, or Australia, this can be delivered by a 250W TWT.
With the satellite in GEO at an altitude of 36,000 km, the link budget contains
a single entry of −205.6 dB of free-space loss. In contrast to spreading loss (e.g.,
4_R2 ), path loss includes _ as a component, having the arithmetic form:
A = (4_R2)(4_ /_2 )
= (4_R)2/_2
converting to dB, A = 92.5 + 20 log F + 20 log R where F is the frequency in GHz
and R is the range in km.
Other propagation losses, such as rain attenuation, that are random in nature
are evaluated separately and compared against the overall link margin. Since this
is a clear sky link budget, we include an allocation of 0.1 dB for atmospheric loss.
The next two items relate to the receiving Earth station: the peak gain of an assumed
45-cm antenna and 0.5 dB of waveguide loss. The combined power balance yields
a received power of −120.5 dBW at the input to the Earth station LNA and receive
electronics. For that hypothetical link, assume we have a receive system noise
temperature of 140K and a signal bandwidth of 27 MHz. For the corresponding
received noise power of −132.8 dBW (e.g., kTB), the downlink C/N result is
12.3 dB.
Uplink noise within the spacecraft receiver contributes a lesser amount to the
total link noise and is ignored in the present example. However, the standard
148 Microwave Link Engineering
approach is to prepare a separate link budget for the uplink, following approximately
the same format as Table 4.5. Then the overall link C/N is determined
using the following formula:
C/Nt = (Nd /C + Nu /C)−1
calculated as ratios and not decibels. The final step is to convert back to decibels.
Above, the subscripts t, d and u indicate total, down- and up-link components,
respectively.
The next three lines allow us to translate the expected clear-sky C/N into the
corresponding value of Eb /No . The process for doing this is to first take the ratio
of the carrier bit rate, here assumed to be 30 Mbps, and the carrier bandwidth.
That turns out to be 1.111, which corresponds to 0.5 dB of adjustment. This
is subtracted from C/N to obtain the Eb /No for the downlink under clear sky
conditions.
4.4.4 Link Margin
The final and perhaps key concept in microwave link engineering is that of link
margin, which is nothing more than the excess power in the carrier relative to the
threshold value. It is possible to measure the threshold in the laboratory under
controlled conditions. First, the link performance without fading can be determined
using the power balance equation and spectrum analysis. The margin, then, is the
difference in decibels between the minimum value and the threshold value. For
example, if the minimum acceptable (threshold) Eb /N0 is 8 dB, the link margin is
11.8 − 8 = 3.8 dB. After the link is operational, the margin can be verified by
manually decreasing the transmit power to the point where the received baseband
Table 4.5 Typical Ku-Band Link Budget for the Downlink to a Small DTH Receiver
Relative Value
(Expressed
Parameter as a Gain) Absolute Value Units
Transmit power (250W) 24.0 dBW
Transmit waveguide loss −1.0 dB
Transmit antenna gain 30.0 dBi
EIRP 53.0 dBW
Free-space loss −205.6 dB
Atmospheric loss −0.1 dB
Receive antenna gain (45-cm diameter) 32.7 dBi
Receive waveguide loss −0.5 dB
Received power (clear sky) −120.5 dBW
Bandwidth (27 MHz) 74.3 dB(Hz)
System noise temperature (140K) 21.5 dB(K)
Boltzmann’s constant −228.6 dB(Hz*K)−1
Received noise power −132.8 dBW
C/Nd 12.3 dB
Bit rate (30.0 Mbps)
Ratio of bit rate to bandwidth (1.111) 0.5 dB
Eb /No (downlink) 11.8 dB
4.4 Overall Link Quality 149
signal is barely acceptable. The recorded change in transmit power in decibels is
essentially the link margin.
Adequate link margin allows the link to deliver exceptionally good quality
(low bit error rate) under mean signal, clear weather conditions. However, when
either the uplink or the downlink is experiencing heavy rain or mulipath fading,
the available link margin determines how often and for how long the link will
drop below threshold. The criterion often used is the availability, that is, the
percentage of time the link is above threshold. Typical satellite links operate in the
range of 99% to 99.95%, demonstrating the high reliability of line-of-sight paths
between the satellite and its associated Earth stations.
References
[1] Final Acts of WRC-77, 2. Radio Propagation Factors, International Telecommunication
Union, Geneva, 1997.
[2] ITU Radio Regulations, Vol. 2, Appendix S7, Edition of 2008.
[3] Sat Master software, available fromArrowe Technical Services (http://www.satmaster.com/).
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