Direct Broadcast Satellite TvPresentation Transcript
DIRECT BROADCAST SATELLITE TV
Satellite communication means communicating through satellites.
Now Satellite services have shifted away from telephone, toward video and data delivery, with television broadcasting directly to the home .
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 .
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.
An earth station complex is equipped to uplink analog and digital video signals.
A 9m C band cassegrain antenna used to transmit analog video FM. TV C band signals to a domestic satellite.
A 5.5 m ku band cassegrian uplink antenna and to transmit multiple digital compressed video signals.
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.
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:
This is a large GEO three axis stabilized DBS TV Satellite.
Echostar 5 DBS-TV Satellite. (Photo Courtesy Of Echostar Communications Corporation)
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:
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.
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 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
Mixing prerecorded and live material in single transponder helps even out the bursty nature of live material.
The compressed digital video signal bit stream is split into blocks of bits and encoded with Reed-Solomon Linear block code.
The coded bit stream is then interleaved and encoded again with a convolutional code.
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.
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 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 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.
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.
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Theodore S. Rappaport,”Wireless communications”, Prentice Hall, 1996.