DIRECT BROADCAST
SATELLITE TV
INTRODUCTION :
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Satellite communication means communicating through satellites.
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Now Satellite services have shifted away from telephone, toward
video and data delivery, with television broadcasting directly to the
home .
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GEO satellites carry the majority of services, because the use of the
high gain fixed antennas at earth stations maximize the capacity of
the satellite .
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We previously used undersea cables for voice communication, but it’s
limited bandwidth prevented their use for video signals. Satellites are
very effective way to distribute wide band signals.
EARTH STATION:
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An earth station complex is
equipped to uplink analog and
digital video signals.
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A 9m C band cassegrain antenna
used to transmit analog video FM.
TV C band signals to a domestic
satellite.
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A 5.5 m ku band cassegrian
uplink antenna and to transmit
multiple digital compressed video
signals.
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A Simulasat antenna which has
parabolic torus antenna reflector
aligned with the GEO are with
seven feeds.
Direct Broadcast Satellite:
• Direct Broadcast Satellite is a communication link that transmits
directly from the transmitter to the user.
• The end user receives data from a high frequency satellite in
geosynchronous orbit (GEO).
• The high power of transmission from this satellite allows the
user's receiving dish to be the size very small.
• By using video compression and encoding in the transmission,
hundreds of channels can be broadcast over the bandwidth
available.
Digital DBS-TV:
• The bandwidth allocated for DBS-TV is 12.2-12.7 GHz. This
band is exclusively used for DBS-TV satellite in GEO.
• Mainly C-band and Ku band used in the transponders in the
satellites.
• For C band, one transponder used for each video signal, but
Ku band satellite and compressed digital video signal made it
possible to transmit several video signal through one
transponder.
• Typical transponder output levels are 100 to 240 w with flux
density as the earth’s surface up to - 105 dB W/M².
DBS TV Satellite:
Echostar 5 DBS-TV Satellite. (Photo Courtesy Of Echostar Communications Corporation)
• This is a large GEO three axis stabilized DBS TV
Satellite.
• The satellites can carry up to 32 transponders, giving a
total transmitted RF power up to 3.2 kW, higher than
for any other commercial satellite.
• The flux density at the earth surface produced by
medium and high power transponders used on DBS
satellites is in the range - 105 to -115 dB W/M², which
allows small receiving antennas (dishes) to be used for
DBS-TV reception, with diameters in the range 0.45 0.9m.
DBS TV Receiving Antennas:
www.directv.com
www.dishnetwork.com
• Single feed allow reception from only one satellite.
• Two feeds to allow reception from 2 satellites at separate
locations in the geostationary orbit can be viewed by
different people at same time.
DirectV:
• The first entrant into the high power DBS-TV field is DirectV.
DirectV was four satellites in two pairs spaced half a degree
apart at a nominal orbital location of 100 degrees west.
• The satellites at each orbit location transmit in opposite hands
of circular polarization (CP). Signals with opposite hands of
circular polarization are orthogonal, and suitably designed earth
station antenna can separate two signals with opposite hands
of circular polarization.
• LHCP and RHCP polarizer are placed behind the receiving
antenna feed.
• Low noise block converter unit receives the signal by changing
voltage supply.
• 7v supply volt cause the antenna to receive one polarization
and reject other.
• Increasing the voltage above 14 volt causes the antenna to
switch polarization.
• The polarizer converts the circularly received polarization signal
to a linearly polarized signal in a section of waveguide and a
linear probe in the wave guide converts the signals to currents
that drive the LNA input.
DBS TV Receiver:
• The LNB minimize the loss of signal and maintain lowest possible
system noise temperature.
• The entire 12.2-12.7 GHz band is down converted to 900-1400
MHz by LNB, here the cable losses are lower than at Ku band.
• The down converter consists of a dielectric resonator local
oscillator and mixer, followed by an IF amplifier and band pass
filter.
• The set top box accept the entire 500 MHz band and separate
out the individual trans ponder frequencies.
• The user enter a desired channel number into the set top box
using an IR remote control, which is converted via a stored look
up table in the receiver to an RF channel frequency and
polarization .
• The signal from the required transponder is then selected by the
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receiver by setting the correct polarization at the antenna and
tuning the set top local oscillator to the appropriate IF channel
frequency.
The QPSK signal is then demodulated.
The result is a multiplexed bit stream, typically at a bit rate up to
40 Mbps, which contains the bits for channel number and several
other video signals.
The bit stream is encrypted and contains error control coding bits
an data bits.
The bit stream is processed to correct and detect errors, de
interleaved and decrypted.
A digital demultiplexer then extracts the bits for the wanted
channel, sends them to a MPEG 2 decoder and finally generates
analog audio and video signals with D/A converters to drive the
TV set.
Error Control In Digital DBS TV:
•When prerecorded material such as movies is digitized and
processed through MPEG 2 compression, the data rate can be
reduced to 1.6Mbps. Live program material with a lot of motion in
the picture can cause the bit rate of an MPEG 2 coded signal to
increase above the average value of 6.2 Mbps.
Block Diagram Of Coding & Decoding Operation in a DBS-TV Signal
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Mixing prerecorded and live material in single transponder helps
even out the bursty nature of live material.
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The compressed digital video signal bit stream is split into blocks
of bits and encoded with Reed-Solomon Linear block code.
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The coded bit stream is then interleaved and encoded again with a
convolutional code.
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The purpose of interleaving is to spread out the errors that
occurred in a burst and thus to make it easier for an error
correction system to recover the original data.
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At the receiver, the recovered bit stream is first decoded with a
viterbi decoding algorithm to remove the convolutional coding and
a limited number of error in the bit steam is the de-interleaved
and a Reed-Solomon decoding algorithm is applied.
Other Cause of Error:
• The signals are in the form of bits and a group of bits form a
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word.
Bit errors in a digital video or audio signal result in wrong
voltage occurring.
If the system knows that a particular word is in error, that word
can be flagged and the error can be removed by interpolation of
the analog wave form. When a word is know to contain a bit
error, it is replaced by a new word which is calculated to have a
value midway between two adjacent words in the bit stream.
The result is an interpolated value in the analog voltage
waveform output by the DAC.
The interleaving process ensures that most bit errors are single
errors, increasing the probability that words contain only a single
error and that the error will be detected.
DBS TV UPLINK EARTHSTATION:
• Many signals are broadcast by
the satellites, collected at a
master control station and
uplinked to the satellites by
group of large antennas.
• The video and audio signals
www.dishnetwork.com
that which are uplinked to the
DBS TV satellites 24 hour a
day, 365 days a year.
• The uplink station have
hundreds of tape and video
disc players, all under
computer control, which
supply the video and audio
signals for each channel.
DBS TV Uplink Earth Station:
• One uplink antenna will typically transmit up to 16RF channels
to one DBS-TV satellite. Each RF signal is a QPSK modulated ku
band carrier with a symbol rate of up to 20 Msps, occupying a
bandwidth up to 27 MHz.
• The encoded, compressed and multiplexed bit stream drives a
video exciter which generates QPSK modulation of an
intermediate frequency carrier, typically at 70 MHz . The 70
MHz signal is unconverted to the transmitter, which contains a
traveling wave tube high power amplifier.
• The signals together in microwave combiners and sent to the
antenna feed for transmission to the satellite.
CONCLUSION :
• DBS seems most appealing to persons who either are
disenchanted with cable television or who live in areas that are
not served by cable.
• DBS-TV systems operate with small antennas and low cost
receiving systems, and offer a very large number of video and
audio channels, making them attractive to customers.
• Delivery of bit stream through a direct broadcast satellite can
be adapted to serve Internet users who require the download
of large blocks of data.
• Both DirectV and Dishnetwork offer internet access terminals
with uplink capability.
REFERENCES :
• Timothy Pratt, Charles Bostian and Jeremy Allnut,”Satellite
Communications", John Wiley & Sons,Inc,Second
Edition,2006.
• Leon W. Couch," Modern Communication Systems:
Principles and Applications", Prentice Hall,1995.
• Tiberiu Artzi,”Direct Satellite Communication", US Patent
No.6553009,Issued on April 22,2003.
• Theodore S. Rappaport,”Wireless communications”,
Prentice Hall, 1996.
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