Digital Satellite Downlink Reception

[الدكتور عبد الكريم]

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Chapter 4 : Digital Satellite Downlink Reception
The AFRTS signal is a digitally compressed MPEG signal and as with any digital
signal there is perfect reception or nothing at all. Tuning to an MPEG
compressed digital signal, however, is a little different from tuning to a standard
analog signal. Weak signals appear to be random noise; the receiver will not
display any picture at all until sufficient signal is reaching the antenna. Then,
once the digital threshold of the receiver/decoder is exceeded, a perfect picture
will appear on the TV screen. MPEG digital reception is like a light switch; it’s on
or off. This is to say that a digital signal has two states, perfect (on) picture
quality and reception or nothing at all (off). Furthermore, if the installer moves
past the antenna’s peak performance position, the picture will “freeze frame” on
the last picture in its buffer memory. The IRD will not receive any further video
until the antenna is repositioned to receive a signal above minimum receiver
threshold. Peaking the signal improves the overhead above threshold and
ensures a good picture under poor weather conditions.
Typical Satellite TVRO Equipment Configuration
The typical equipment arrangements used to receive AFRTS services are
provided at Figure 3-2. Specific equipment requirements for receiving AFRTS
services are provided in the section titled Qualification of Satellite Terminals or
Digital Reception.
General Satellite Concepts
The concepts underlying satellite broadcasting are straightforward: signals
beamed into space by an “uplink” dish are received by an orbiting satellite,
electronically processed, re-broadcast or “down-linked” back to earth and then
detected by a dish and associated electronics. A receiving station can be situated
anywhere within the satellite’s “footprint” (see Chapter 3, satellite footprint maps).
The overwhelming strength of satellite broadcasting lies in its ability to reach an
unlimited number of sites regardless of their ******** without the need for any
physical connections.
Nearly all communication satellites designated for commercial use are positioned
or “parked” in the “Clarke Belt”, also known as the “geostationary” arc. The
Clarke belt lies in the equatorial plane 22,300 miles above the equator. This
circle around the earth is unique because in this orbit the velocity of a spacecraft
matches that of the surface of the earth below. Therefore each satellite appears
to remain in a fixed orbital slot in the sky above. This allows a stationary dish to
be permanently aimed towards a targeted geostationary satellite.
A satellite receives the up-linked signal, lowers its frequency and re-broadcasts it
to any chosen geographic area. Downlink transmit antennas can target over 40%
of the earth’s surface with “global” beams, can broadcast to selected countries or
continents via “zone” beams, or can pinpoint smaller areas with “spot” beams.
Many domestic C-band broadcast satellites direct one beam that blankets the
continental U.S. and a second more localized one to the Hawaiian Islands. KuDefense
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band satellites, operating in the higher frequency 12 GHz range, are configured
for spot beams and require smaller antennas to receive their signals.
The Receive Site
At the receive site a dish reflects and concentrates as much of the very weak
down-linked signal as possible to its focus where a feed channels the signals into
the first electronic component, the low noise block converter (LNB). The signal is
then cabled indoors to the satellite receiver and processed into a form that can
be deciphered by a television, stereo or computer.
Radio Waves and Communications
The transmission of extremely low power microwaves, a form of radio waves,
underlies the operation of radio, conventional television, satellite broadcasting
and other man-made communication devices. They are one form of more general
phenomena known as electromagnetic waves that travel at the speed of light,
equal to 186,000 miles per second. At this rate, a signal travels from the uplink,
to a satellite and back again to earth in about 4/10ths of a second.
Radio Waves
Radio waves are defined by their frequency, power and polarization. These
parameters are briefly discussed below.
Signal Frequency
The frequency of a radio wave is the number of vibrations that occur every
second. Just like the frequency of sound vibrations determines whether a musical
note is either a soprano or a bass, so the frequency of radio wave determines
whether they are used to transmit regular AM radio broadcasts or satellite
television broadcasts. Microwaves have frequencies in excess of one billion
cycles per second (known as one gigahertz and abbreviated 1 GHz) to as high
as 50 GHz. C and Ku-band satellite downlink signals fall in the 4 and 12 GHz
range, respectively.
Polarization
Radio waves can be polarized. Two standard formats commonly used in C and
Ku-band satellite communication links are linear and circular polarity.
Linearly polarized signals can have either vertical or horizontal polarity. In this
case, the electric and magnetic fields of the signal remain in the same planes in
which they were originally transmitted. Horizontally polarized waves vibrate in a
horizontal plane; vertically polarized waves vibrate in a vertical plane. Most Cband
signals broadcast to TVROs (television receive-only) are linearly polarized.
In circularly polarized signals the electrical and magnetic fields rotate in a circular
motion as they travel through space, somewhat analogous to a spiral. The
direction of the rotation determines the type of circular polarization. A signal
rotating in a right-hand direction is termed right-hand circular polarization (RHCP)
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and a signal rotating in the left-hand direction is termed left-hand circular
polarization (LHCP).
A principle advantage of
circular polarization is the
elimination of the need for
skew adjustment. A feed
designed to receive a linearly
polarized signal must be
correctly lined up with its plane
of polarization to allow
reception of the highest
possible power and therefore
clearest picture. It requires a
skew adjustment for finetuning.
However, a feed that
receives a RHCP or LHCP
signal can be attached at the
focal point of the dish in any
orientation.
There are three noteworthy components of a satellite receive antenna which
collectively capture and amplify the signal to a level large enough to break the
receiver reception threshold, normally around negative 45dB. These are the
reflective surface or parabolic curvature, the feedhorn and the amplifier section
“Low Noise Amplifier (LNA), Low Noise Block converter (LNB), Low Noise
Converter (LNC), and Low Noise Feedhorn (LNF). We will focus on these areas
because they are the components that we personally come in contact with and
have the greatest control over.
Antenna Reflector
The reflective surface in a perfect world would rely on the geometric properties of
its true parabolic curve to reflect the satellite signal to a very sharp focal point.
The focal point on a parabolic antenna is out in front and to the center of the
surface. This would be a well-defined area if a perfect parabolic curve were
defined, however this isn’t as defined as we would prefer. The focal point is not
as perfect as theory would dictate but is still within a small radius and is a
defining difference in a perfect or marginal signal reception. This you may say is
where the “rubber meets the road” and collection of the signal is critical in this
area.
Figure 4-1 Satellite dish parts
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Reflective surfaces come in several different shapes and sizes but are most
common in the parabolic or offset shape. Offset shaped antennas are nothing
more than a small section of the original parabolic antenna see figure 4-2. The
larger the reflective service the better defined the focal point becomes and
therefore more gain can be expected. The reflector sometimes mistakenly called
the antenna is the first step in a well-engineered system that will continue to
provide service under harsh
environments. If the size of
your dish is too small for the
signal you intend to capture,
nothing is going to compensate
for that. Working with an
analog signal you could get by
with a smaller dish but suffer
with a noisy picture. A digital
signal on the other hand is
perfect or nothing situation and
with a marginal or less
reflective surface you can
expect nothing.
Many of the small aperture Kuband
dishes sold these days
use an offset antenna, see
figure 4-2, a feedhorn design
which places the focal point
below the front and center of
the dish. This type of antenna,
as defined earlier is actually a
small oval subsection from a
much larger parabolic antenna design, is oval in shape with a minor axis (left to
right) that is narrower than its major axis (top to bottom). Because of its unique
geometry, the offset fed antenna requires a specially designed feedhorn, which
matches the antenna geometry precisely. For this reason, the offset fed antenna
and feedhorn are usually sold together as a single unit. This type of feed is called
a Low Noise Feed or LNF.
Amplifier “LNA/B/C/F”
The concentrated signal from the reflective surface is channeled to a low noise
amplifier that has a very low noise floor. The job for this section is to amplify the
signal to a level that is above the receiver’s threshold. The Low Noise Amplifier
(LNA) amplifies the signal at the output of the earth station’s antenna. The most
commonly used LNAs use gallium arsenide field effect transistors (GaAsFETs).
Typical noise temperatures of amplifiers produced today range from 15° K to 60°
K (LNB\C\F).
Figure 4-2 An offset satellite antenna
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The LNA is a weather sealed unit that provides enough gain to transport the
signal from the antenna to the receiver. It is located as close the feedhorn as
possible to minimize signal loss and thereby improving signal to noise ratio. The
problem with an LNA is that the signal is in several gigahertz frequency range
and requires expensive transmission lines to carry the signal from the antenna to
the receiver. A much more efficient way of doing this is to down-convert the
signal at the antenna to a lower frequency for transmission to the receiver. This is
accomplished with the newer LNB/C/F to lower the satellite normal GHz
frequencies to an L-band frequency between 940 MHz to 1450 MHz. For ease of
discussion, all Low Noise Amplifier types will be referred to as a LNB, form this
point forward
There is a basic tradeoff between LNB noise temperature and antenna size,
which is gain, expressed by the system figure of merit G/T. Smaller antennas
require a cooler LNB temperature for *****alent system performance. Whereas a
larger antenna allows use of an LNB with a higher noise temperature. This
should not be misunderstood and you should not be mislead that an amplifier
with a lower noise temperature will correct for any antenna size. G/T is a
measure of the ability of a receiving system to amplify very weak signals, such as
those of a satellite transmitter 22,300 miles away over the background noise. The
“G” is antenna gain and the “T” is its noise temperature. The job for the LNB is to
overcome this noise figure with a carrier to noise C/N separation of greater than
8dB, see Spectrum Analyzer plots. The average for reliable reception of the
AFRTS digital signal is 12dB of signal above the noise floor. It should be noted
also that a digital signal reacts to noise and interference differently than a analog
signal. Noise or interference introduced in a digital environment will cause
pixelization and even loss of signal reception. Whereas in the analog world,
received video will have noise or sparkles but in most instances would not suffer
total loss of signal. The advantage of the digital signal is, there is no change in
the signal quality until it deteriorates below the receiver reception threshold. But,
at that point the received video will go from perfect to total loss of signal; notice
there is no in between.
The noise figure or temperature, expressed in decibels or degrees Kelvin,
respectively, is a measure of the degree by which this amplifier degrades or
decreases the signal-to-noise ratio of the satellite signal as it passes through the
device. This scale is ****d on the fact that at a temperature known as absolute
zero, 0° K (equal to minus 273.16° C or minus 459.72° F), molecular motion
ceases and consequently all electronic noise disappears. The lower the noise
temperature or figure, the better amplifier performance. There are amplifiers on
the market today with noise temperatures as low as 15°. Getting below 15° K,
requires external cooling of the electronics and is a very expensive endeavor.
Gain is also very important in characterizing low noise amplifiers. The more
common LNB gains today usually range from 60 to 70 dB. LNBs must be
designed with sufficient gain to overcome cable losses as well as the effects of
noise contributed within this device and overall system noise temperature.
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The low noise block down converter, the LNB, detects the signal relayed from the
feed, converts it to an electrical current, amplifies it and down-converts or lowers
its frequency. LNBs in both analog and digital systems down-convert the signal to
a band in the 950 to 1450 MHz range. The “down-converted” signal is
subsequently relayed along cable to the indoor satellite receiver.
Signals reaching the input of an LNB from a typical 8-foot C-band dish have
powers of less than 10 –14 watts/m2. Therefore, an LNB must contribute very little
noise power or received satellite signals will be drowned out in the roar of
amplifier internal thermal noise. This feat is made possible by advances in
transistor technology. Without such progress, satellite broadcasting would not
exist as we know it today.
LNB Performance
There are three specifications that affect the performance of the LNB and have a
direct effect on the ability of a system to satisfactorily capture a satellite signal. In
order of importance for digital reception is, the noise temperature, Local
Oscillator stability (L.O.), and its gain expressed in dB. The noise temperature of
the amplifier must be low enough to overcome the noise floor of the antenna to a
minimum of 8dB above the signal to noise floor.
Feedhorn Assembly
Feedhorns, as with the reflective
surface also come in several
different forms with the most
common ****g the scalar feedhorn.
The scalar feedhorn has a large
circular plate with a series of circular
rings attached to its surface, see
figure 4-3. These rings collect the
signal at the antennas focal point
and conduct the incoming signal to
the waveguide attached between the
rings and the LNB. The effect of the
scalar rings is to concentrate the
signal in an effort to correct the
imperfections of the parabolic shape.
Therefore the effect of the feedhorn
to focus or channel the incoming signal is critical in signal reception. Adjustment
of the feedhorn will be discussed later but is a must to take advantage of the
systems overall gain and therefore reducing the overall system noise floor.
The scalar feedhorn primarily sees or is illuminated by the inner portion of the
antenna’s surface area, while attenuating the signal contribution from the outer
portion of the dish by 8 to 22 dB, depending on whether the dish is deep or
shallow in its construction. Molecular motion within the Earth itself generates
random noise, which permeates the entire electromagnetic Spectrum used for
Figure 4-3 Feedhorn assembly
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the transmission of satellite signals. This random noise is many times stronger
than the satellite signals reaching any ********. The attenuation or illumination
taper provided by the feed sharply reduces the reception of the Earth noise which
lies just beyond the antenna’s rim. The outer area of the antenna’s surface
therefore acts more as an Earth shield for the feedhorn than as a contributor to
the overall signal gain of the receiving antenna.
Feedhorn Adjustments
Focal length between the center of the
antenna surface hub and bottom of the
feedhorn assembly facing the antenna
surface should be initially set to the distance
recommended by the antenna manufacture,
see figure 4-4. Adjustments of 1/8 inch or
more in or out from the recommended
distance should be made while using a signal
meter or Spectrum analyzer to determine the
precise position required for maximum signal
acquisition. This is particularly important for
antennas composed of individual segments,
especially those composed of mesh panels
as antenna surface irregularities due to
careless antenna assembly can actually shift
the optimum position of the focal point from
the value recommended by the antenna
manufacturer.
When adjusting the feedhorn in or out, be sure that the waveguide opening
remains precisely centered over the dish at all times. You can check this by
measuring from the antenna’s rim to the outer ring of the waveguide opening
from four equidistant positions around the rim. All of these measurements should
be equal.
There is an important difference in the process of aiming an analog and a digital
dish. When even a faint signal is received a hint of a television picture appears
with a conventional TVRO. Then fine adjustments can be made to improve
reception. A digital system either acquires the signal or nothing. Therefore the
aiming angles should be set as accurately as possible before powering on. Once
the signal has been acquired, then the signal strength can be monitored for finetuning.
One saving grace with small dish systems is that the beam ***** is so
wide that aiming errors of even a degree or more will not have a major impact.
While fine-tuning the digital dish monitoring the signal strength is a good
indication of raw RF, but as a word of caution, don’t sacrifice BER for signal
strength.
Figure 4-4 Focal length
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Polarization
There are four polarities most common to communications satellites in orbit
today. These are horizontal, vertical, left and right hand polarization and your
system pickup probe must be aligned accordingly for best reception. There are
several different types of feeds: some will need to be manually polarized and
some will not depending on the type of feedhorn used. This adjustment is best
accomplished while monitoring the satellite signal on the display of a spectrum
analyzer. If a spectrum analyzer isn’t available, make this adjustment and
maximize the BER of the receiver. Rotate the feedhorn until you begin to see the
other polarization. Turn your receiver on and look at the BER. You will notice that
it gets worse as the other polarity begins to increase. The idea is to minimize the
other polarization and at the same time maximize the BER or signal quality of
your receiver. If you notice that rotating the feedhorn in a 360o rotation makes no
difference to the BER/Signal quality. This indicates that your feedhorn is not
adjustable and is factory set to the polarization of the satellite transponder and no
further adjustments are necessary.
Qualification of Satellite Terminals for Digital Reception
The following three subsections include lists of equipment needed to receive the
AFRTS signal. The boxes cover equipment for SATNET C-band, SATNET Kuband
and Television-Direct to Sailor (TV-DTS) C-band digital reception.
Equipment needed for SATNET C-band reception
1. Dish Size: 4.5 meter (minimum size)
2. Mid-band Gain: 43.6 dBi
3. Feedhorn
3.1. For Domestic Region (IntelSat Americas-5) C-band Linear
Vertical Polarization (V)
3.2. For Atlantic Ocean Region: C-band Right Hand Circular
Polarization (RHCP)
3.3. For Pacific Ocean Region: C-band Left Hand Circular
Polarization (LHCP)
4. Low Noise Block (LNB)
4.1. Noise Temperature: 25° K (+ -) 5° K
4.2. LO Stability: 1,000 kHz (+ -) 100 kHz
4.3. Recommend using a NORSAT Model 8525F
5. Cable: RG-6 or RG-11
6. L-band Splitter: Caution terminate all unused ports
6.1. Must be diode steerable, power passing on all legs
6.2. Recommend using a Channel Master 1x4 Model. 24141FD
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7. L-band in Line Amplifier
7.1. 20dB gain from .9 ∼ 1.75 (GHz)
7.2. Recommend using a DX Antenna Model ES-25
8. R.F. Connectors
8.1. For RG-6, recommend using Anixter P/N 144017
8.2. For RG-11, recommend using Anixter P/N 095178
Equipment needed for SATNET Ku-band reception
1. Dish Size: 80 centimeters to 1.5 meter (For the size needed in your ********,
refer to the satellite footprint maps in chapter 3, figures 3-6 for Japan and Korea
or figure 3-7 for Europe.)
2. MidBand Gain: 80 CM 37.6 dBi
MidBand Gain: 1 meter 39.5 dBi
MidBand Gain: 1.2 meter 41.7 dBi
MidBand Gain: 1.8 meter 44.5 dBi
3. Feedhorn Ku-band Linear Vertical Polarization (H)
4. Low Noise Block (LNB)
4.1. Noise Temperature: 0.6 to 0.8° dB
4.2. LO Stability: 750 kHz (+ -) 100 kHz
4.3. Recommend using a NORSAT Model 4708C
5. Cable: RG-6 or RG-11
6. L-band Splitter: CAUTION TERMINATE ALL UNUSED PORTS
6.1. Must be diode steerable, power passing on all legs
6.2. Recommend using a Channel Master 1x4 Model 24141FD
7. L-band in Line Amplifier
7.1. 20dB gain from .9 ∼ 1.75 (GHz)
7.2. Recommend using a DX Antenna Model ES-25
8. R.F. Connectors
8.1. For RG-6, recommend using Anixter P/N 144017
8.2. For RG-11, recommend using Anixter P/N 095178
Equipment needed for Direct to Sailor (DTS) C-band reception
1. Dish size: 1.2 meter
2. MidBand Gain: 43.6 dBi
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3. Feedhorn C-band Left Hand Circular Polarization (LHC)
4. Low Noise Block (LNB)
4.1. Noise Temperature: 20° K (+ -) 5° K
4.2. LO Stability: 500 kHz (+ -) 100 kHz
4.3. Recommend using a NORSAT Model 8520C or California Amplifier
Model 140194.
5. Cable: RG-6 or RG-11
6. L-band Splitter: CAUTION TERMINATE ALL UNUSED PORTS
6.1. Must be diode steerable, power passing on all legs
6.2. Recommend using a Channel Master 1x4 Model. 24141FD
7. L-band in Line Amplifier
7.1. 20dB gain from .9 ∼ 1.75 (GHz)
7.2. Recommend using a DX Antenna Model ES-25
8. R.F. Connectors
8.1. For RG-6, recommend using Anixter P/N 144017
8.2. For RG-11, recommend using Anixter P/N 095178
Some New Terms You Should Know and Understand
Moving into the new digital age will require a basic understanding of a few new
terms that make up this new technology. The following is a brief explanation of
some of the new digital acronyms and ******** that you will come across and
need to understand.
(1) Receiver/Decoder Threshold: Unlike traditional analog
Receiver/Decoder, where the unit continues to deliver a picture even when
it is operating below the receiver/decoder threshold, digital systems will
not operate below their minimum threshold. The difference ****g, in the
analog world the picture quality will deteriorate from crystal clear, to noisy
(sparkles) without total loss of picture. The digital receiver will not show
signs of weakened signals and it will have a digital cliff where the signal is
no longer processed and is discarded. Therefore, you cannot rate the
quality of the signal by comparing it with how good the video is, it’s always
the same above the threshold.
(2) Bit Rate: This is the amount of data information ****g transmitted in one
second of time. The total stream passing through a single satellite
transponder consists of as many as ten TV services and associated audio,
auxiliary audio services, conditional access data, and auxiliary data
services such as tele****. The informational bit rate for this transmission
may be as high as 49 mega (million) bits per second (Mb/s) over a 36
MHz satellite transponder. Single video signals within this bit stream will
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have a lower bit rate. For example, a VHS quality movie can be
transmitted at a bit rate of 1.544 Mb/s (T-1); general entertainment
program at 3.0 Mb/s; live sports with a lot of motion at 4. or studio quality
at a rate of more than 8 Mb/s.
(3) Bit Error Rate (BER): Measured in exponential notation, the BER
expresses the performance level of the digital receiver. For example, a
lower BER of 0.0 E-6 is superior to a BER of 1.0 E-3. The lower the BER,
the greater the receiver/decoder’s ability to perform well during marginal
reception conditions, such as during a heavy rainfall or wind gusts.
Depending on which model of Scientific Atlanta Integrated Receiver
Decoder (IRD) ****g used, the quality of the received signal is
represented in BER or a signal quality scale of 1-10; 10 ****g the best.
The 9223 will represent signal quality in BER and the 9234 set-top
measures quality on a scale of 1 to 10.
Sun Outages
A sun outage is similar in behavior to a rain fade. The high energy level and
broadband nature of the sun's energy can overpower a satellites downlink signal
and effectively wash out a receive signal with noise. This problem is technically
impossible to overcome at this time.
Due to the angle of the sun in relationship to the satellite, a sun outage is actually
a mixture of degraded receive performance with the possibility of a circuit outage.
A circuit outage might be typically 20% of the total predicted sun outage duration
period. Many factors influence how robust a receive circuit may be, therefore it is
extremely difficult to predict exactly how long an outage might possibly be. The
digital nature of the AFRTS signal means that you’ll either have very good signal
or none at all with very short periods of degraded “pixilated” signal.
At certain times of year, approximately one month either side of the spring and
autumn equinoxes, there may be a conjunction of the sun and satellite positions.
Depending upon the size of the earth station antenna, such events can lead to a
serious impairment of the space-earth link.
The outages typically last only a few minutes at a time once a day with a normal
worse case outage of about ten to fifteen minutes. Outages will affect each link in
multi-hop circuits. For example viewers in Europe or the Indian Ocean area
would be affected by an outage of first, the Atlantic satellite and then secondly, of
the actual satellite feeding their antenna.
Antennas should not be adjusted or re-pointed at these lost-of-signal times. The
viewer should wait out the outage until the sun finishes passing directly behind
the satellite.
RF Interference in Digital Networks
The transmission of digitally compressed video over satellite allows many high
quality video signals to be transmitted in a satellite transponder, which formerly
could accommodate only a single high quality video signal. The “compression” of
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these services into a narrow band***** causes some inevitable trade-offs in the
complexity of both the transmit and receive earth stations. Transmit earth
stations must be equipped with tremendously complex video “encoders” which
digitize and compress the large amounts of video and audio information into a
much smaller band*****. Receive earth stations must be compatible with the
reception of a wide band digital carrier. While most Television / Receive-Only
(TVRO) earth stations are compatible with the digital video technology, some will
be susceptible to Radio Frequency Interference (RFI), sources which were not
significant with analog video transmissions.
In the traditional analog world, interference was spread across a much broader
information **** where individual elements of information were less critical. With
digital compression, much more information is transmitted in a compressed
format, which increase the importance of each “Information packet”. Digital
compression signals react differently to problems caused by RF Interference in
the RF (Radio Frequency) path as compared with traditional analog video
signals. Where RF Interference caused either a white line, sparkle or “hum” bar
in the Analog video realm, in the digital domain it can result in digital artifacts
such as “blocking”’ and/or a “black screen” or “freeze frames” depending upon
the magnitude and duration of the interference and the concealment algorithms
used.
TVRO sites experiencing RFI do not always experience any observable effects.
A typical transponder operating with a compressed digital video signal may
contain up to 8 television programs. Although one might expect each of these
signals to be 8 times as susceptible to RFI as a traditional analog signal; in
practice the signals are of a higher quality (for a given antenna size) than
traditional analog transmission due to the sophisticated error correction and
concealment algorithms employed.
Much has been learned about the cause and mechanics of many external
interfering sources that enter through the antenna and associated subsystems.
This paper will help identity potential origins of RF Interference in addition to
providing methods of reducing the effects of interference on the satellite carrier.
While it is impossible to eliminate RFI, there are ways in which to both reduce the
level of interference and conceal the event so that it has the least amount of
perceived effect on the video.
We will address two major interference scenarios, which may be caused by a
number of ground-****d sources. These sources and their method of interaction
with a typical receive terminal are explained. Several methods of reducing the
interference and its effects are also explored.
The two types of RFI encountered are Destructive Interference (DI) and Out of
Band Interference (OBI). Destructive interference is encountered when the
desired receive signal is completely overwhelmed, or disrupted, by an interfering
signal (or noise source) in the channel of the desired signal, and at a level equal
to or greater than the desired signal. Out of Band Interference is defined as a
signal (or noise source) which does not interact directly with the desired signal,
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but interacts with other components of the receive system such that the desired
signal is impaired or destroyed. Both DI and OBI may originate from the same
sources. An interfering carrier from a terrestrial microwave system may act as DI
on a carrier at one frequency, and an OBI on carrier at another frequency at the
same TVRO site.
Current Technology
Digital video compression receivers differs from traditional FM video receivers in
that they receive video and audio signals that are digitized, compressed and
modulated using Quadrature Phase Shift Keyed (QPSK) digital modulation. This
technique allows the transmission and reception of several high quality video
channels and associated audio in a 36MHz transponder. In comparison,
traditional analog FM modulation provides only one video and its associated
audio signals to be transmitted per transponder.
Error Correction
Because of the increased capacity attained using digital compression and
transmission, special error protection is used to either correct errors or provide
concealment when the error rate exceeds the capability for the decoder to
provide complete correction. To detect and correct errors caused by thermal
noise, a technique called soft decision convolutional decoding is used. The IRD
and associated up-link equipment use a convolutional encoder to provide error
correction to thermal noise down to about 7 dB C/N. Also, to protect against burst
noise interference, a special data interleave and Reed Solomon block decoder
are used. The com***ation provides error correction to burst interference outages
that can be caused by engine ignition noise, industrial microwave oven
interference, and adjacent band interference from such sources as aircraft radar
altimeters.
Because there may be instances when the error rate is high enough so that not
all errors can be corrected, the IRD contains sophisticated software algorithms
that provide image concealment for small-uncorrected errors, and either freeze
frames or black-frame substitution for larger uncorrected errors.
The FM Analog *****alent to digital errors is the well-known “white line” or
“sparkles that appears on the TV screen when the received signal level drops
below the FM threshold of about 10dB C/N. Unlike analog transmission where
the “white lines” or “sparkles” are superimposed on the video, uncorrected digital
errors can create a loss of digital synchronization resulting in outages that can
last longer than the actual duration of the interference. It is during these
instances that image concealment is important Typically, instead of a single
“white fine” or “sparkle”, a digital error can result in the generation of artifacts
ranging from ”no perceptible error” to “multiple block errors” that look like FM
threshold sparkles to “freeze frames” or “black screens” for really significant
errors.
Defense Media Center Satellite Handbook V.3.12
4-14
Reacquisition
Improvements in technology against terrestrial interference focus on two primary
areas, reacquisition of the carrier, and concealment. Reacquisition deals with the
time it takes to reacquire the carrier, decode and restore video after an RFI “hit”
takes place. Reduction of the reacquisition time to its lowest value is the
objective in any design consideration.
Concealment
Concealment deals with the methods employed in the IRD as it relates to video
presented to the viewing audience during the reacquisition period. Various
approaches can be employed, use of a “black screen”, displaying digital artifacts,
or freezing the video frame are all methods that can be used to display video
during the reacquisition sequence.
Sources of Interference
There are a variety of sources of interference, which can affect a digital
compression path. Identification of the interfering source is an important step in
the goal of reducing the effects of RF interference on the desired signal.
Interference can have two effects on a digital carrier:
1) Compression or saturation of the RF receiving equipment including LNA’s,
LNB’s, line amplifiers, and RF Tuner inside the IRD.
2) Direct corruption of the digital carrier.
There are three areas, which need to be addressed in protecting the digital
carrier against interfering sources:
1. Protection from saturation or compression in the RF path
2. Error correction and reacquisition of the digital carrier
3. Concealment with regard to the source material displayed to the
viewing audience.
The following section details the potential sources of RF Interference.
Terrestrial Microwave Interference
Much of the world’s populated areas are utilizing terrestrial microwave signals.
These signals range from typically 2 GHz to 15 GHz with a major concentration
in the 3.1 GHz to 4.99 GHz band. Terrestrial microwave transmitter/antennas will
be located at or near places of commerce, metropolitan areas, near airports, or
large industrial facilities. Microwave repeaters may be found at intermediate
points in the path throughout populated and often times unpopulated regions.
Most terrestrial microwave interference manifests itself as a single modulated or
unmodulated carrier, and is readily observable in the C-band pass band of the
system with a Spectrum analyzer. A site survey should be performed prior to final
******** of the earth station to ensure that terrestrial microwave carriers will not
be a problem. Microwave interference may require re******** of the satelliteDefense
Media Center Satellite Handbook V.3.12
4-15
receiving antenna into a “clear” path. Should the presence of these carriers be
detected prior to site ********, they can be treated as part of the satellite link
analysis to evaluate their affect on performance.
Impulse and Ignition Noise
A digitally compressed video signal can be susceptible to interference from
impulse generators. Some typical sources of impulse noise are power equipment
(power generators) or ignition noise from engines (vehicles, motorcycles,
mopeds, lawn mowers, power blowers). Spark emissions cover a wide band of
RF frequencies including C-band and can enter through the satellite dish and
LNB. These emissions can originate from engines where broken, intermittent or
“arcing” spark plug cables are used. Ignition wires are typically resistive wires
that dampen RF radiation, however a broken or intermittent ignition wires can arc
and emit excessive radio interference. Ignition “burst noise” can last in excess of
1 millisecond, exceeding the interleave depth of the error correction system
designed into the IRD and can have a power level 40 dB higher than the satellite
carrier. The repetition rates greater than once every 70 millisecond have been
detected.
When planning an earth station you should site the station well away from
sources of ignition interference such as busy roads, highways, intersections, or
car parks. You may want to restrict the use of gasoline-powered lawn mowers
and other combustion engines during peak usage hours.
Because ignition noise represents broadband interference an operator
experiencing ignition noise should address both the issue of saturation as well as
attempt to reduce the magnitude of the interfering source. To address saturation,
attenuators should be utilized both at C-band (if used) and L-band. An interfering
carrier from a automobile ignition can be more than 40 dB higher than the
receiving signal and saturate LNB’s, line amplifiers and the RF tuner in the
satellite receiver. Severe ignition noise problems can be addressed by re********
of the receiving antenna, use of an “earth berms”, or installation of an RFI
grounded fence between the interfering sources and the earth station antenna.
Aircraft Radar Altimeters/Airport Ground Radar
If your downlink antenna is located near an airport or flight path your system can
pick up interfering carriers from aircraft radar altimeters. The radar altimeter
Spectrum is 4.200 to 4.400 GHz. This corresponds to 750 to 950 MHz at the Lband
output of the LNB. These carriers have been measured in excess of
+40dBc relative to the desired satellite carrier. This kind of interference often
results in the saturation of any line amplifiers to the extent that the amplitude of
the desired Spectrum is reduced below a measurable level. The effects of this
interference may last several seconds until the aircraft passes out of the earth
station antenna beam. The interference appears as a chirp or energy spread
over the indicated Spectrum. It is first observed as a low level signal and
gradually builds to its maximum level before gradually diminishing.
Defense Media Center Satellite Handbook V.3.12
4-16
These interfering carriers are usually out-of-band and can be dealt with by
installing a C-band block filter that can be specifically manufactured for greater
protection at the aircraft radar frequency.
Other potential sources of interference from airports are ground looking radar
that can saturate LNA/LNB’s. Frequency coordination in some countries allow for
adjacent bands to be utilized where they can cause out-of-band interference.
Once again, C-band band pass or block filters remain an effective means of
controlling the interfering carrier.
Ship-board Radar
Another potential source of interference in coastal areas is shipboard naval
radar. Usually, this on-board radar is not supposed to be utilized within a radius
of the shore; however, there are ********ed cases where this radar has been
“turned on” with deleterious effects to the local coastal viewing audience.
Commercial Microwave Ovens
Commercial microwave ovens operating in fast-food chains and earth station
lunchrooms are potential sources of interference. Emissions levels allowed by a
microwave oven can be as much as 20 dB higher than a C-band satellite carrier;
however, microwave oven manufactures are normally required to replace units
that are known to interfere with commercial broadcast systems. A typical
operating frequency for a microwave oven is 2250 MHz with a considerable
amount of wide band noise generated in the 3900 MHz to 4500 MHz range. This
noise can become more apparent over the life of the magnetron and can be
prevalent near the end of its useful life.
Walkie-Talkies
Walkie-talkies have been observed to interfere with the operation of IRDs.
Operating a walkie-talkie in the vicinity of the IRD can interfere with the operation
of the IRD. Restricted use of walkie-talkies is recommended in the vicinity of a
downlink earth station.
Cell Phones
Cell (Cellular) Telephones operate in the 900 Mhz range and can directly
interfere with the down converted (IF) signal from the LNB to the IRD. The
activation of a cell phone unit near the IRD may generate unacceptable
destructive or out of band interference which may enter the IRD through poorly
shielded cabling or improperly terminated dividers and connectors.
Random RFI (Fluorescent and Sodium Vapor Lamps, Lightning)
Particularly on start-up, fluorescent lamps can flicker causing an interfering
source to an earth station antenna nearby. Another potential source is sodium
vapor lamps when in a “failed” condition. Lightning is another known source of
RFI that can effectively wipeout both digital and analog carriers. Though these
sources are not a common occurrence, they should be mentioned in the
investigation of a RFI occurrence.
Defense Media Center Satellite Handbook V.3.12
4-17
Protection from Interference
Selecting a site
Site selection is the most important pro-active stop an earth station operator can
take in prevention of terrestrial interference. Busy roads and highways, parking
lots, power generators, and power equipment near the receiving antenna are all
potential sources of interference. Sites located near airports may need special
consideration due to aircraft radar altimeters.
Saturation and Compression
Many traditional earth station operators in the analog environment are concerned
with obtaining the highest signal level possible for their analog receiving
equipment High signal levels in the digital environment can be problematic where
terrestrial interference is present
Ignition noise is a common problem where saturation can occur in the RF path.
Interfering carriers can potentially be 40 dB higher than the satellite carrier
resulting in compression of the RF subsystems.
Optimizing signal levels through the use of C-band and L-band attenuator pads
to increase the “headroom” of the system where RFI is found can dramatically
improve performance of the receiving equipment. Installation of 6dB and 10dB
pads in front of line amplifiers, block down converters, and video
receiver/decoders can provide the additional “headroom” needed to prevent
saturation during a RFI hit. Operating IRD’s in a “low gain” mode is another
useful way to add additional “headroom” for RFI “hits”.
Many earth station operators utilize line amplifiers in traditional analog systems,
which can aggravate the effect of RFI and compression. Signals that are spiked
due to RFI in com***ation with a high gain line amplifier can saturate
downstream block down converters and RF tuners inside the IRD. Optimization
of the RF path, including line amplifiers is necessary when combating RFI.
Out-of-band Filtering
For sites experiencing aircraft radar or out-of-band interference, C-band filtering
in front of the LNA/LNB is an effective way to protect from interfering carriers.
Special notch filters have been made for aircraft radar that are effective in those
specific ********s near airports or aircraft approaches.
RFI (Radio Frequency Interference) Fencing
Special RFI fencing can often reduce the source of interfering carriers or ignition
noise where it is present Wire fences of the proper diameter, located between the
interfering source and the earth station antenna can be an effective way of
dealing with terrestrial interference. Fences that can be utilized for RFI protection
can be as simple as fine wire mesh of galvanized steel, property grounded that
roughly meets the desired dimensions of 1/10 wavelength beyond cutoff of the Cband
carrier. It is important to install the fence at the proper ****** and distance
Defense Media Center Satellite Handbook V.3.12
4-18
from the earth station antenna, with special attention ****g paid to the
construction, (galvanized steel is preferred).
A wire mesh fence, property constructed, will scatter-back and absorb the energy
and appear to the interfering signal much like a solid sheet of ****l. The
optimum dimension for the mesh fencing is a mesh size smaller than 1.27cm,
(1/2 inch), which offers adequate protection at C-band.
To block ignition impulse noise from a busy street or parking lot, a galvanized
steel fence with a mesh size smaller than 1.27cm (1/2 inch), should be grounded
with copper grounding rods or chemical ground system. The wire fence in
com***ation with the ground system should accommodate a wide variation of RF
emissions generated from engine ignition systems. Effective fences that have
also been utilized in the past are fine wire mesh and solid thin sheet ****l
barriers.
Earth Berms
A more drastic but very effective manner to protect from terrestrial interference is
the use of earth berms. Placing the antenna below ground level, while more
costly and not always practical, it still provides an excellent manner in which to
protect the integrity of the receiving signal from RFI.
When constructing a “earth berms” careful considerations should be given to the
side lobes of the antenna since the noise temperature of the earth is much higher
than that of the dark sky. The surrounding earth in the earth berms may cause a
noise figure degradation if it is not significantly outside of the antenna side lobe.
Summary
Digital Video Compression systems will continue to be the choice for future
satellite video broadcasting because of the band***** efficiency and
unsurpassed video quality. The traditional FM analog approach to earth
station operation will enter a new era with the advent of video
compression. Many video earth station operators are learning the same
sensitivities to RFI as the traditional digital common carriers (IDR)
networks used in the telecommunications industry. Through education of
earth station operators, adaptation to the environment, and advances in
technology, digital compression systems will become the standard in
satellite video broadcast delivery throughout the world. Education and
understanding of the effects of terrestrial interference, and its prevention,
are the most important steps in achieving the high standard of service
demanded by subscribers in the worldwide marketplace.
Table 4-1 Spectrum Analyzer Setup
1. Connect the input of the spectrum analyzer with a T-connector between the LNB and
the receiver. Caution: This will put 13-19 volts DC on the input of the spectrum
analyzer and could damage it. To prevent this from happening use a DC blocker on
Defense Media Center Satellite Handbook V.3.12
4-19
the input of the analyzer while still feeding the LNB with the required receiver DC
voltage. This will allow you see spectrum plot for the signal you intend to capture.
2. Set the frequency to satellite L-band frequency between 950 MHz and 1450 MHz.
3. Span to 100 MHz.
4. Amplitude to –45 dB
5. Vertical scale to 1 dB per scale. If signal is out of range adjust accordingly
Table 4-2 Typical Satellite Receiver Setup
9234 9223
a. Freq. Mode
b. Frequency
c. Polarization
d. FEC Rate
e. Symbol Rate
f. L.O. Freq
g. Video Standard (NTSC)
a. Band
b. L-band Freq
c. Polarization
d. FEC Rate
e. Symbol Rate
f. L.O. Freq
g. Video Standard (NTSC)
Defense Media Center Satellite Handbook V.3.12
4-20
Table 4-3 Bit Error Rate (BER) to Threshold Margin Table
Bit Error Rate Reading SatNet FEC ¾ DTS FEC 2/3
2.00E-02 -- 0.22
1.00E-02 0.36 1.44
5.00-03 1.36 2.36
2.00-03 2.38 3.36
1.00E-03 3.12 4.10
5.00E-04 3.78 4.76
2.00E-04 4.56 5.54
1.00E-04 5.08 6.10
5.00E-05 5.58 6.60
2.00E-05 6.14 7.12
1.00E-05 6.50 7.48
5.00E-06 6.78 7.78
2.00E-06 7.18 8.18
1.00E-06 7.42 8.46
Note: The information shown is the amount of margin, in dB, over the DVB specification threshold
for a given BER display. For example, a BER reading of 5.00E – 04 on a SATNET decoder
provides 3.78 dB of margin over the Eb/No threshold of 5.5 dB or a total Eb/No of 9.28 dB. At the
same BER, DTS provides 4.76 dB of margin over the Eb/No threshold of 5.0 dB for a total Eb/No
of 9.76 dB.
Scientific Atlanta developed the table from actual testing of decoders over a range of symbol
rates. The standard deviation is 0.2 dB.

[mohammed66]

بارك الله فيك أخي الكريم

[ANDER TAKER]

مشكووووووووووووووووووووووووووور

حبيبب قلبى

يعطيك العافية

[الدكتور عبد الكريم]

مشكورين على المرور

[mayzoo]

بسال نفسى واقول ليه الحال مش معدول-- -معدش فى قريب ولا حبيب ارمى عليه الحمول

مالجرح انا بتاخد بس عليه بقيت واخد ياما كتير داريت واداريت وفى الاخير كله واحد

ايام وبقضيها وحجات وبعديها ياما سامحت ناس وبقول خلاص وياريت بيطمر فيها

هزعل على مين ولا مين هو الجرح سايب بسال نفسى واقول للحال مش معدول معدش فى قريب ولا حبيب ارمى عليه الحمول

[mayzoo]

زي غيرها صوتها لونها شوق عيونها الكدابين زي غيرها هان ضميرها
ضيعت عشرة سنين
زي غيرها صوتها لونها شوق عيونها الكدابين زي غيرها هان ضميرها
ضيعت عشرة سنين
زي غيرها ماعدش باين اللي صادق من اللي خاين حب عشرة كل هاين
والهوى إحساس حزين
زي غيرها صوتها لونها شوق عيونها الكدابين
قلبي اللي هان حبك عليه في عنيك تاهت سكتـو ولاعمري قلبك حسني والغلطة كانت غلطته
بكرة الليالي عليك تفوت وجراحه تاخد وقتها غلطة ومش ممكن تعود والدرس قلبي تعلمه
زي غيرها ماعدش باين اللي صادق من اللي خاين حب عشرة كل هاين
والهوى إحساس حزين
آ آ آ آ آ آ آ آ آ آ آ آ آ ه

[ahmed aliraqi]

مشكوررررررررررررر

[ABOADNAN]

مشكووووووووووور على المجهود الرائع

[ABOADNAN]

مشكووووووووووور على المجهود الرائع.

[ABOADNAN]

مشكووووووووووور على المجهود الرائع

[agiiba_eg]

على الطلاق منا فاهم حاجه الكلمه الوحيدة اللى اعرفها hello وبس وكل دة انجليزى

[ahm1st]

Thanks alot

[murssi_p]

السلام عليكم ورحمة الله وبركاته انا حبيت ارد الاول لاني لم ارى البرنامج وعلى كلا اشكر كل من يساهم في هذا المنتدى الغالي وشكرا

[waad_2009]

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[akimcolor]

is it possible to have firms alic here,please?