DVB-T gapfiller
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Transcript of DVB-T gapfiller
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In an AM signal, the idea is to modulate the
carrier waveform without changing phase - onlythe amplitude is supposed to change.
e(t)=A(1+m*cos(Wm*t))*cos(Wc*t) Wc is the carrier frequency,
Wm is the modulation frequency and m is themodulation index. A modulation index of 1 means thatthe modulation is just as big as the carrier, ie., 100%modulation.
The above expression can be broken into a
carrier part plus two modulation parts: e(t)=A*cosWc*t + [(m/2)*cos(Wc+Wm) + (m/2)*cos(Wc-Wm)]
Carrier modulation component 1 modulationcomponent 2
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In frequency modulation the amplitude is kept
constant and the frequency is modulated by theamplitude of the modulating signal.
FM signal can be represented as:
v = ac sin(wct + m sin wmt )
The modulation index for fm ism = maximum frequency deviation/modulatingfrequency.
The frequency spectrum can be found by rewritingthe above expression as a sum of components ofconstant frequency using the properties of the Bessel
Functions. This gives:-v = ac{Jo(m) sin(wct)+ J1(m)[sin(wc + wm)t - sin( wc - wm)t]+ J2(m)[sin(wc + 2wm)t + sin( wc - 2wm)t]+ J3(m)[sin(wc + 3wm)t - sin( wc - 3wm)t]+ ...
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QPSK -
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QAM
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Digital video has many advantages. It can be
compressed with predictable quality and, once
digitized, it does not degrade with storage and
transmission. Transmission includes many ways ofdelivering the image such as DVD, satellite, cable,
and over-the-air systems. To view digital video with
our analog human eyes, it must be reconstructed
into a specific signal then display as analog light.
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MPEG-2
MPEG-2 is one of most popular digital content compressionschemes. MPEG-2 became an international standard since1995
MPEG-2 systems layer provides us with a standardizedmethod of providing integrated video, audio and data services
Currently programs consist of primarily one video channel andpossible multiple audio channels
The data streams are used only to broadcast program relateddata, like close captioning
Multiple video streams for the same program (differentcamera shots of a football game!!)
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MPEG2TS
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DVB-T
The DVB-T Standard is published as EN
300 744, Framing structure, channel coding
and modulation for digital terrestrial television. ETSI TS 101 154, Specification for the use of
Video and Audio Coding in Broadcasting
Applications based on the MPEG-2 Transport
Stream
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OFDM -
Rather than carrying the data on a single radio
frequency (RF) carrier, OFDM works by splitting the
digital data stream into a large number of slower
digital streams, each of which digitally modulate a setof closely spaced adjacent carrier frequencies.
In the case of DVB-T, there are two choices for the
number of carriers known as 2K-mode or 8K-mode.
These are actually 1705 or 6817 carriers that areapproximately 4 kHz or 1 kHz apart.
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OFDM continued
In OFDM, the sub-carrier frequencies are
chosen so that the sub-carriers are orthogonal
to each other, meaning that cross-talkbetween the sub-channels is eliminated and
inter-carrier guard bands are not required.
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OFDM
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DVB-T
DVB-T as a digital transmission delivers data in a series ofdiscrete blocks at the symbol rate. DVB-T is a COFDMtransmission technique which includes the use of a GuardInterval. It allows the receiver to cope with strong multipath
situations. Within a geographical area, DVB-T also allowssingle-frequency network (SFN) operation, where two or moretransmitters carrying the same data operate on the samefrequency.
Doppler shift, deep fading, and shadowing are the
dominant factors that decrease the systemperformance in mobile environments.
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Guard Interval
The purpose of the guard interval is to introduce immunity topropagation delays, echoes and reflections; and avoid ISI(inter-symbol-interfernce)
Longer guard periods allow more distant echoes to betolerated. However, longer guard intervals reduce the channelefficiency.
In DVB-T, four guard intervals are available (given asfractions of a symbol period): 1/32 ; 1/16 ; 1/8 ; 1/4
Hence, 1/32 gives lowest protection and the highest data rate.1/4 results in the best protection but the lowest data rate.
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SFNSingle Frequency Network
A SFN is a broadcast network where several
transmitters simultaneously send the same
signal over the same frequency channel. A simplified form ofSFN can be achieved by a
low power co-channel repeater, booster or
broadcast translator.
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IMD Inter-Modulation Distortion
:
The third-order intermodulation products (IMPs)
occur where
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Shoulder
DVB-T transmitters include very linear AB amplifiers.
However, in reality, non-linearity causes
intermodulation products to form from the many
COFDM carriers. These intermodulation productsare recognizable as "white nose" along the edges.
These edges are called DVB-Tsignal shoulders.
To further reduce the shoulders, mask filters can be
used in a critical or uncritical mask.
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Shoulder (continued)
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MER
The Modulation Error Ratio (MER) is a measure of the sum ofall interference that affects a digital TV signal. It is typicallyexpressed in dB as a logarithmic relationship between theRMS value of the signal amplitude and the MER. A high MER
value indicates good signal quality. In practice, a good DVB-T transmitter has a MER in the range ofapproximately 35 dB.
When receiving DVB-T signals over a outdoor antenna with gain, aMER of 20 dB to 30 dB would be measurable at the antenna box.
Values between 13 dB and 20 dB are expected for portable receiverswith a indoor antenna.
The MER is the single most important quality parameter for aDVB-T system.
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MER (continued)
As a reference, the practical figures were
given: the MER values need to conserve to
demodulate
a QPSK modulation scheme, MER cannot be
lower than 5dB;
a 16QAM it is needed at least 11dB MER
and for a 64 QAM it is needed at least 19dB MER.
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Digital Transposer
Precise tuned receive antenna LinearPhase, very good return loss
Gain is 15dbi or higher
Very good front back ratio
Side lobe is optimized
Low Noise Amplifier with excellent linearity
4-cavity adjacent channel filter optimized for selectivity andgroup delay
Low IMD driver stage
AGC and over-drive protection
High power high linearity LDMOS power amplifier
Low loss combiner and power splitter
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DVB-T Spectral Limit Filter
For non-critical masking, 4-cavity filter is
necessary.
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Power Combiner
Low insertion low combiner which can be
easily expanded by cascading combiners