DVB-T gapfiller

8/6/2019 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

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DVB-T gapfiller