DVB-T Introduction

Wireless Comm. Lab.Wireless Comm. Lab.1

DVBDVB--TT

2006.82006.8

2 Wireless Comm. Lab.Wireless Comm. Lab.

Outline DVB-T Introduction Wireless Propagation Properties OFDM Concepts DVB-T System Parameters Hierarchical Modulation DVB-T Modulator and Transmitter Architecture DVB-T Receiver Architecture


DVB-T Introduction


DVB-T History

The commercial requirements for the development of a digital video broadcasting (DVB) system for terrestrial broadcasting

date back to early 1994.

The main objective at that time was to support the stationary reception of terrestrial signals by means of rooftop antennas.


The Worldwide Digital TV (1/2) Europe:

European Telecommunications Standards Institute (ETSI) Digital Video Broadcasting (DVB)

America:Advanced Television Systems Committee (ATSC) ATSC DTV

Japan:Association of Radio Industries and Business (ARIB). integrated services digital broadcasting (ISDB).

Korea:Digital multimedia broadcasting (DMB)


The Worldwide Digital TV (2/2)

DVB-T:Europelargest part of AsiaAustraliaAfrica

ATSC:United StatesCanadaMexico

ISDB-T:Japan

DMB:South Korea

Unclear:Peoples Republic of China and Latin America


DVB-T Introduction

European standard for transmission of digital TV via satellite,cable or terrestrial

DVB-S (satellite)QPSK quadrature phase-shift keying

DVB-T (terrestrial)COFDM coded orthogonal frequency division multiplexing

MPEG-2 compression and transport stream Support for multiple, encrypted program stream.


DVB-T Transmitter


DVB-T Receiver


Wireless Propagation Properties


Mobile Radio Environment


Wireless Channel Model

Fading processPath lossShadowingFast fading(Doppler effectMulti-path delay)

FastFading

TransmitAntenna

PathLoss

Shadowing ReceiveAntenna

AdditiveNoise

fading process


Wireless Channel Noise

Wireless channel noiseWireless channel noise

Multipliable noise (RayleighRician fading)additive noise (Gaussian noise)

Transmitter Receiver

additive noise

Multipliablenoise

Wireless channel


Narrowband vs. WidebandNarrowband vs. WidebandNarrowbandNarrowband::

MultipathMultipath fading comes about as a result of small path length fading comes about as a result of small path length differences between rays coming from scatters in the near differences between rays coming from scatters in the near vicinity of the mobilevicinity of the mobile

These differences , lead to significant phase differences.These differences , lead to significant phase differences.

The rays all arrive at essentially the same time.The rays all arrive at essentially the same time.

Wideband : Wideband : The time differences may be significant .The time differences may be significant .

The relative delays >> the basic unit of information transmittedThe relative delays >> the basic unit of information transmittedon the channel ( a symbol or a bit )on the channel ( a symbol or a bit )

The signal will experience significant distortion , which variesThe signal will experience significant distortion , which variesacross the channel bandwidthacross the channel bandwidth ..


Effect of Delay SpreadEffect of Delay Spread


Effect on Error RateEffect on Error Rate


OFDM Concepts


OFDM BasicsOFDM Basics

Main idea: split data stream into N parallel Main idea: split data stream into N parallel streams of reduced data rate and transmit each on streams of reduced data rate and transmit each on a separate a separate subcarriersubcarrier..

When the When the subcarrierssubcarriers have appropriate spacing to have appropriate spacing to satisfy satisfy orthogonalityorthogonality, their spectra will overlap. , their spectra will overlap. OFDM modulation is equivalent to the IDFT:OFDM modulation is equivalent to the IDFT:


Modulation techniques: monocarrier vs. multicarrier

To improve the spectral efficiency:

To use orthogonal carriers (allowing spectrum overlapping)Eliminate band guards between carriers

Selective Fading

Very short pulses

ISI is compartively long

EQs are then very long

Poor spectral efficiencybecause of band guards

Drawbacks

It is easy to exploitfrequency diversity

Flat Fading per carrier

N long pulses

ISI is comparatively short

N short EQs needed

Poor spectral efficiencybecause of band guards

AdvantagesFurthermore

It allows deployment of2D coding techniques

Dynamic signaling

N carriers

BPulse length ~ N/B

Similar toFDM technique

Data are shared among several carriersand simultaneously transmitted

BPulse length ~1/B

Data are transmitted over only one carrier

Channel

Guard bands

Channelization


OFDM ConceptOFDM Concept

Ch. 2 Ch. 3 Ch. 4 Ch. 5 Ch. 6 Ch. 7 Ch. 8 Ch. 9 Ch. 10Ch. 1

Saving of bandwidth

f

f

Conventional Multicarrier Technique

Orthogonal Multicarrier Modulation Technique


Orthogonal Frequency Division Multiplex (OFDM)Orthogonal Frequency Division Multiplex (OFDM) Parallel data transmission on several orthogonal

subcarriers with lower rate

Maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero superposition of frequencies in the same frequency range

k3f

t

c

Amplitude

f

subcarrier: SI function=

sin(x)x


OFDM II Properties

Lower data rate on each subcarrier less ISI interference on one frequency results in interference of one subcarrier only no guard space necessary orthogonality allows for signal separation via inverse FFT on receiver side precise synchronization necessary (sender/receiver)

Advantages no equalizer necessary no expensive filters with sharp edges necessary better spectral efficiency (compared to CDM)

Application 802.11a, HiperLAN2, DAB, DVB, ADSL


OFDM in Real environments ISI of subsequent symbols due to multipath propagation Symbol has to be stable during analysis for at least Tdata Guard-Intervall (TG) prepends each symbnol (HIPERLAN/2: TG= 0.8 s; Tdata= 3.2 s; 52 subcarriers)

OFDM symbolfade out

OFDM symbolfade in

impulse response

OFDM symbol OFDM symbol OFDM symboltanalysis window

Tdata

OFDM symbol

TGTG TGTdata


OFDM System Block

Modulation mapping S/P Pilot

insertionIFFT GI

insertionP/S D/A

Channel

AWGN

A/DS/PGI removal

FFT

Channel estimation

base on pilot and

signal correction

P/SModulation de-mapping

Binary data

X(m) x(n) xGI(n)

h(n)

w(n)

yGI(n)y(n)Y(m)

Binary received data


Modulation & Mapping The process of mapping the information bits onto the signal

constellation plays a fundamental role in determining the

properties of the modulation.

An OFDM signal consists of a sum of sub-carriers, each ofwhich contains M-ary phase shift keyed (PSK) or quadrature

amplitude modulated (QAM) signals.

Modulation types over OFDM systemsPhase shift keying (PSK)

Quadrature amplitude modulation (QAM)


IDFT & DFT

Inverse DFT and DFT are critical in the implementation of anOFDM system.


Orthogonal


DVB-T System Parameters


Two Mode Characteristic

A DVB-T channel have a bandwidth of 8,7 or 6MHz. There are two different operating modes : the 2k and 8k mode .

In DVB-T, It was decided to use symbols with a length of about 250 us (2k mode) or 1ms (8k mode).

The 2K mode has greater subcarrier spacing of about 4KHz but the symbol period is much shorter. Compared with the 8K

mode with a subcarrier spacing of about 1KHz.


Two Mode Purpose

2k mode is much less susceptible to spreading in the frequency domain caused by Doppler effects due to mobile reception and

multiple echoes but much more susceptible to greater echo delay.

In single frequency networks, for example, the 8k mode will always be selected because of the greater transmitter spacing

possible.


Modulation Select

Apart from the symbol length, which is a result of the use of 2k or 8k mode, the guard interval can also be adjusted within a range

of 1/4 to 1/32 of the symbol length.

It is possible to select the type of modulation (QPSK,16-QAM or 64-QAM).

The DVB-T transmission can be adapted to the respective requirement with regard to robustness or net data rates by

adjusting the code rate(1/2.7/8).


Carriers Type

DVB-T contains the following types of carrier :

Payload carriers with fixed position. Inactive carriers with fixed position. Continual pilots with fixed position. Scattered pilots with changing position in the spectrum. TPS carriers with fixed position.


Payload & Inactive Carriers

The meaning of the words 'payload carrier' is clear: these aresimply the carriers used for the actual data transmission.

The edge carriers at the upper and lower channel edge are set to zero, i.e. they are inactive and carry no modulation at all,

i.e. their amplitudes are zero.


Continual Pilots

The continual pilots are located on the real axis, i.e. the I (in-phase) axis, either at 0 degrees or at 180 degrees and have a

defined amplitude.

The continual pilots are boosted by 3 dB compared with the average signal power and are used in the receiver as phase

reference and for automatic frequency control (AFC), i.e. for

locking the receive frequency to the transmit frequency.


Scattered Pilots

Within each symbol, there is a scattered pilot every 12th carrier. Each scattered pilot jumps forward by three carrier positions

in the next symbol.

The scattered pilots are also on the I axis at 0 degrees and 180 degrees and have the same amplitude as the continual pilots.


Carriers Position


TPS Carriers (1/2) The TPS carriers are located at fixed frequency positions. TPS stands for Transmission Parameter Signaling. These carriers

represent virtually a fast information channel via which the

transmitter informs the receiver about the current transmission

parameters.


TPS Carriers (2/2)

All the TPS carriers in one symbol carry the same information, i.e. they are all either at 0 degrees or all at 180 degrees on the I

axis.

The complete TPS information is broadcast over 68 symbols and comprises 68 bits.


DBPSK

DBPSK Modulated TPS Carriers


TPS Purpose & Content


TPS Carry Information

Thus, the TPS carriers keep the receiver informed about:

The mode (2k, 8k). The length of the guard interval (1/4, 1/8, 1/16, 1/32). The type of modulation (QPSK, 16QAM, 64QAM). The code rate (1/2, 2/3, 3/4, 5/6, 7/8). The use of hierarchical coding.


DVB-T Constellation Diagram(1/2)

Continual Pilots, Scattered Pilots and TPS Carriers in the DVB-T Constellation Diagram


DVB-T Constellation Diagram(2/2)

DVB-T Constellation Diagrams for QPSK,16-QAM and 64-QAM


IFFT

In DVB-T, an IFFT with 2048 or 8192 points is used. In theory, 2048 or 8192 carriers would then be available for

the Data transmission. However, not all of these carriers are

used as Payload carriers.

In the 8k mode, there are 6048 payload carriers and in the 2k mode there are 1512.


Carrier Type Value


Hierarchical Modulation


High Priority & Low Priority(1/2) If hierarchical modulation is used, the DVB-T modulator has two

Transport stream inputs and two FEC blocks.

One transport stream with a low data rate is fed into the so-calledHigh priority path (HP) and provided with a large amount of error

protection, e.g. by selecting the code rate 1/2.

A second transport stream with a higher data rate is supplied In parallel to the low priority (LP) and is provided with less error

protection, e.g. with the code rate 3/4


High Priority & Low Priority(2/2) In principle, both HP and LP transport streams can contain the

same programs but at different data rates, i.e. with different

amounts of compression.

On the high priority path, QPSK is used which is a particularlyrobust type of modulation.

On the low priority path, a higher level of modulation isneeded due to the higher data rate.


Modulation Type

In DVB-T, the individual payload carriers are not modulated with different types of modulation.

Instead, each payload carrier transmits portions both of LP and of HP. The high priority path is transmitted as so-called

embedded QPSK in 16QAM or 64QAM.


Constellation Diagram

Embedded QPSK in 64-QAM with Hierarchical Modulation


Embedded QPSK in 64-QAM A 64-QAM modulation enables 6 bits per symbol to be

transmitted. However, since the quadrant information, as

QPSK, diverts 2 bits per symbol for the HP stream, only 4 bits

per symbol remain for the transmission of the LP stream.

The gross data rates for LP and HP thus have a fixed ratio of 4:2 to one another.


Embedded QPSK in 16-QAM QPSK embedded in 16QAM is also possible. The ratio

between the gross data rates of LP and HP is then 2:2.

To make the QPSK of the high priority path more robust, i.e. less susceptible to interference, the constellation diagram can

be spread at the I axis and the Q axis.

A factor of 2 or 4 increases the distance between the individual quadrants of the 16QAM or 64QAM diagrams.


Factor

is the minimum distance separating two constellation points carrying different HP-bit values divided by the minimum distance separating any two constellation points.


TPS Carriers

The information about the presence or absence of hierarchical modulation and the factor and the code rates for LP and HP

are transmitted in the TPS carriers.

This information is evaluated in the receiver which automatically adjusts its demapper accordingly.


DVB-T Modulator and Transmitter


DVB-T Modulator and Transmitter

A DVB-T modulator can have one or two transport stream inputs followed by forward error correction (FEC) and this

only depends on whether this modulator supports hierarchical

modulation or not.

If hierarchical modulation is used, both FEC stages are completely independent of one another but are completely

identical as far as their configuration is concerned.


Coding Diagram


Synchronize InvertedIt uses for this the sync byte which has a constant value of

47HEX at intervals of 188 bytes. Every eighth sync byte is

then inverted and becomes B8HEX.


Reed Solomon Encoder

Following this, initial error control is performed in the Reed

Solomon encoder. The TS packets are now expanded by 16

bytes error protection.


Interleave & Convolutional

After this block coding, the data stream is interleaved in order to be able to break up error bursts during the deinterleaving at

the receiver end.

In the convolutional encoder, additional error protection is added which can be reduced again in the puncturing stage.


Modulator Diagram


Bit Interleaver

The error-controlled data of the HP and LP paths, or the data of the one TS path in the case of non-hierarchical modulation, then

pass into the demultiplexer where they are then divided into 2,4

or 6 outgoing data streams depending on the type of modulation

(2 paths for QPSK, 4 for 16QAM and 6 for 64QAM).


Symbol Interleaver

In the symbol interleaver following, the blocks are then again mixed block by block and the error-controlled data stream is

distributed uniformly over the entire channel.

Together, this is then COFDM Coded Orthogonal Frequency Division Multiplex.


Mapper & Frame Adaptation

After that, all the payload carriers are then mapped depending on whether hierarchical or nonhierarchical modulation is used,

and on the factor a being = 1, 2 or 4.

This results in two tables, namely that for the real part Re(f) and that for the imaginary part Im(f). However, they also

contain gaps into which the pilots and the TPS earners are

then inserted by the frame adaptation block.


IFFT The complete tables, comprising 2048 and 8192 values,

respectively, are then fed into the heart of the DVB-T

modulator, the IFFT block.

After that, the OFDM signal is available separated into real and imaginary part in the time domain. The 2048 and 8192

values, respectively stored in buffers organized along

the lines of the pipeline principle.


Guard Interval Insert (1/2) they are alternately written into one buffer whilst the other one

is being read out. During read-out, the end of the buffer is read

out first as a result of which the guard interval is formed.


Guard Interval Insert (2/2)

The signal is either digital/analog converted separately for I and Q at the I/Q level and then supplied to an analog I/Q modulator

which allows direct mixing to RF in accordance with the

principle of direct modulation, a principle commonly used at

present.


FIR Filter The signal is then usually digitally filtered at the temporal I/Q

level (FIR filter) to provide for better attenuation of the shoulders.

At the same time it is clipped in order to limit the DVB-T signal with respect to its crest factor since otherwise the output stages

could be destroyed because of the very high crest factor of the

OFDM signal due to its very high and very low amplitudes.


DVB-T Receiver


Tuner & SAW Filter The first module of the DVB-T receiver is the tuner. It is used

for converting the RF of the DVB-T channel down to IF.

The tuner is followed by the DVB-T channel at 36 MHz band center.

At intermediate frequency, the signal is band pass filtered to a bandwidth of 8, 7 or 6 MHz, using surface acoustic wave

(SAW) filters.


Mixing & LPF

In the next step, the DVB-T signal is converted down to a lower, second IF at approx. 5 MHz. This is frequently an IF of

32/7 MHz = 4.571429MHz.

After this mixing stage, all signal components above half the sampling frequency are then suppressed with the aid of a low-

pass filter in order to avoid aliasing effects.


A/D Converter

This is followed by analog/digital conversion. The A/D converter is usually clocked at exactly four times the

second IF , i.e. at 4 * 32/7 = 18.285714 MHz.

Following the A/D converter, the data stream, which is now available with a data rate of about 20 Megawords/s , is

supplied to the time synchronization stage.


Time Synchronization

In this stage , autocorrelation is used to derive synchronization information. Using autocorrelation , signal components are

detected which exist in the signal several times and in the

same way.

The autocorrelation function will supply an identification signal in the area of the guard intervals and in the area of the

symbols.


Changeover Switch The autocorrelation function is then used to position the FFT

sampling window into the area of guard interval plus symbol

free of inter-symbol interference and this positioning control

signal is fed into the FFT processor in the DVB-T receiver.

In parallel with the time synchronization, the data stream coming from the A/D converter is split into two data streams

by a changeover switch. e.g., the odd-numbered samples pass

into the upper branch and the even-numbered ones pass into

the lower branch.


FIR & Delay However, these streams are offset from one another by half a

sampling clock cycle. To eliminate this offset, the intermediate

values are interpolated by means of an FIR filter.

The two data streams are then fed to a complex mixer which is supplied with carriers by a numerically controlled oscillator

(NCO).


AFC

This mixer and the NCO are then used for correcting the frequency of the DVB-T signal but because the oscillators

lack accuracy, the receiver must also be locked to the transmitted

frequency by means of automatic frequency control (AFC).

If the receiver frequency differs from the transmitted frequency, all the constellation diagrams will rotate more or less quickly

clockwise or anticlockwise.


NCO It is then only necessary to measure the position of the

continual pilots in the constellation diagram.

The phase difference is a direct controlled variable for the AFC, i.e. the NCO frequency is changed until the phase

difference becomes zero.


FFT

The FFT signal processing block, the sampling window of which is controlled by the time synchronization.

Since the FFT sampling window is not placed precisely over the actual symbol, there exists a phase shift in all OFDM

subcarriers, i.e. all constellation diagrams are twisted.


Continual & Scattered Pilots

However, the DVB-T signal carries a large quantity of pilot signals which can be used as measuring signal for channel

estimation and channel correction in the receiver.

Measuring the amplitudes and phase distortion of the continual and scattered pilots enables the correction function for the

channel to be calculated, rotating the constellation diagrams

back to their nominal position.


TPS Signal (1/2)

In parallel with the channel correction, the TPS carriers are decoded in the uncorrected channel.

The transmission parameter signalling carriers do not require channel correction since they are differentially coded.

Each symbol contains a large number of TPS carriers andeach carrier carries the same information.


TPS Signal (2/2)

The TPS information is needed by the demapper following the channel correction, and also by the channel decoder.

The demapper is then correspondingly set to the correct type of modulation, i.e. the correct demapping table is loaded.


Channel Decoder

DVB-T Receiver Channel decoder


Deinterleaver & Puncture

The demapped data pass from the demapper into the symbol and bit deinterleaver where they are resorted and fed into the

Viterbi decoder.

At the locations where bits have been punctured, dummy bits are inserted again.


RS Decoder & Energy Dispersal The Reed Solomon decoder corrects up to 8 bit errors per

packet with the aid of the 16 error control bytes.

If there are more than 8 errors per packet, the 'transport error indicator' is set to one and then this transport stream packet cannot

be processed further in the MPEG-2 decoder and error masking

must be carried out. As well, the energy dispersal must then be

undone.


Synchronize Inverter Remove

This stage is synchronized by the inverted sync bytes and this sync byte inversion must also be undone, after which the

MPEG-2 transport stream is available again.

These are followed by a DVB-T demodulator chip which contains all modules of the DVB demodulator after the A/D

converter.


Set-Top BoxThe transport stream coming out of the DVB-T demodulator is

fed into the MPEG-2 decoder where it is decoded back into

video and audio.

All these modules are controlled by a microprocessor via an I2C bus.


Comparison Comparison

Fig. DTV system comparison

Convolutional code


ReferenceReference

1. Digital Television Walter Fischer ROHDE&SCHWARZ

2.2. Digital video broadcasting (DVB); Framing structure, channel codDigital video broadcasting (DVB); Framing structure, channel coding ing and modulation for terrestrial television, European Standard (ENand modulation for terrestrial television, European Standard (EN) 300 ) 300 744 V1.5.1, European Telecommunications Standards Institute (ETS744 V1.5.1, European Telecommunications Standards Institute (ETSI), I), Nov. 2004.Nov. 2004.

3.3. LadebuschLadebusch, U., U. LissLiss, C.A , , C.A , Terrestrial DVB (DVBTerrestrial DVB (DVB--T): a broadcast T): a broadcast technology for stationary portable and mobile usetechnology for stationary portable and mobile use, Proceedings of the , Proceedings of the IEEE, Vol. 94,IEEE, Vol. 94, Issue 1, pp. 183Issue 1, pp. 183--193, Jan. 2006.193, Jan. 2006.

4.4. ReimersReimers, U.H., , U.H., DVBDVBThe Family of International Standards for The Family of International Standards for Digital Video BroadcastingDigital Video Broadcasting, Proceedings of the IEEE, Vol. 94, Issue. 1, , Proceedings of the IEEE, Vol. 94, Issue. 1, pp. 173pp. 173--182, Jan. 2006.182, Jan. 2006.


The EndThe End

Documents

DVB-T Introduction