數位電視傳輸技術 DVB-T簡介
林信標
台北科技大學 電腦與通訊研究所
2006.8
1
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
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Wireless Comm. Lab.
DVB-T Introduction
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
The Worldwide Digital TV (1/2)
‹ Europe:
European Telecommunications Standards Institute (ETSI) Î Digital
Video Broadcasting (DVB)
‹ America:
Advanced Television Systems Committee (ATSC) Î ATSC DTV
‹ Japan:
Association of Radio Industries and Business (ARIB). Î integrated
services digital broadcasting (ISDB).
‹ Korea:
Digital multimedia broadcasting (DMB)
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Wireless Comm. Lab.
The Worldwide Digital TV (2/2)
‹ DVB-T:
Europe、largest part of Asia、Australia、Africa
‹ ATSC:
United States、Canada、Mexico
‹ ISDB-T:
Japan
‹ DMB:
South Korea
‹ Unclear:
People’s Republic of China6 and Latin America Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
DVB-T Transmitter
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Wireless Comm. Lab.
DVB-T Receiver
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Wireless Comm. Lab.
Wireless Propagation Properties
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Wireless Comm. Lab.
Mobile Radio Environment
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Wireless Comm. Lab.
Wireless Channel Model
Fading process
‹Path loss
‹Shadowing
‹Fast fading(Doppler effect、Multi-path delay)
Transmit
Antenna
Path
Loss
Shadowing
Fast
Fading
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fading process
Receive
Antenna
Additive
Noise
Wireless Comm. Lab.
Wireless Channel Noise
Wireless channel noise
‹Multipliable noise (Rayleigh、Rician fading)
‹additive noise (Gaussian noise)
Wireless channel
Transmitter
Receiver
Multipliable
noise
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additive
noise
Wireless Comm. Lab.
Narrowband vs. Wideband
Narrowband:
Multipath fading comes about as a result of small path length
differences between rays coming from scatters in the near
vicinity of the mobile
These differences , lead to significant phase differences.
The rays all arrive at essentially the same time.
Wideband :
The time differences may be significant .
The relative delays >> the basic unit of information transmitted
on the channel ( a symbol or a bit )
The signal will experience significant distortion , which varies
across the channel bandwidth .
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Wireless Comm. Lab.
Effect of Delay Spread
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Wireless Comm. Lab.
Effect on Error Rate
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Wireless Comm. Lab.
OFDM Concepts
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Wireless Comm. Lab.
OFDM Basics
Main idea: split data stream into N parallel
streams of reduced data rate and transmit each on
a separate subcarrier.
When the subcarriers have appropriate spacing to
satisfy orthogonality, their spectra will overlap.
OFDM modulation is equivalent to the IDFT:
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Wireless Comm. Lab.
Modulation techniques: monocarrier vs. multicarrier
N carriers
Channelization
Channel
Similar to
FDM technique
Guard bands
B
Pulse length ~1/B
– Data are transmitted over only one carrier
B
Pulse length ~ N/B
– Data are shared among several carriers
and simultaneously transmitted
Drawbacks
Advantages
– Selective Fading
Furthermore
– Flat Fading per carrier
– Very short pulses
– N long pulses
– ISI is compartively long
– ISI is comparatively short
– EQs are then very long
– N short EQs needed
– Poor spectral efficiency
because of band guards
– Poor spectral efficiency
because of band guards
– It is easy to exploit
frequency diversity
– It allows deployment of
2D coding techniques
– Dynamic signaling
To improve the spectral efficiency:
Eliminate band guards between carriers
To use orthogonal carriers (allowing spectrum overlapping)
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Wireless Comm. Lab.
OFDM Concept
Ch. 1
Ch. 2
Ch. 3
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Ch. 10
Conventional Multicarrier Technique
f
Saving of bandwidth
f
20
Orthogonal Multicarrier
Modulation Technique
Wireless Comm. Lab.
Orthogonal Frequency Division Multiplex (OFDM)
¾
Parallel data transmission on several orthogonal
subcarriers with lower
rate
c
f
k3
t
Maximum of one subcarrier frequency appears exactly at a frequency
where all other subcarriers equal zero
‰
superposition of frequencies in the same frequency range
Amplitude
subcarrier: sin(x)
SI function=
x
f
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Wireless Comm. Lab.
OFDM II
¾
Properties
‡
‡
‡
‡
‡
¾
Advantages
‡
‡
‡
¾
Lower data rate on each subcarrier Î 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)
no equalizer necessary
no expensive filters with sharp edges necessary
better spectral efficiency (compared to CDM)
Application
‡
802.11a, HiperLAN2, DAB, DVB, ADSL
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Wireless Comm. Lab.
OFDM in Real environments
ISI of subsequent symbols due to multipath propagation
¾ Symbol has to be stable during analysis for at least Tdata
¾ Guard-Intervall (TG) prepends each symbnol
¾ (HIPERLAN/2: TG= 0.8 µs;
Tdata= 3.2 µs;
52 subcarriers)
¾
impulse response
OFDM symbol
fade out
OFDM symbol
fade in
OFDM symbol
OFDM symbol
OFDM symbol
OFDM symbol
t
analysis window
TG
Tdata
TG
Tdata
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TG
Wireless Comm. Lab.
OFDM System Block
X(m)
Binary
data
Modulation
mapping
S/P
Pilot
insertion
x(n)
IFFT
xGI(n)
GI
insertion
P/S
D/A
Channel
Y(m)
Binary
received
data
Modulation
de-mapping
P/S
Channel
estimation
base on
pilot and
signal
correction
y(n)
FFT
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yGI(n)
GI
re moval
h(n)
AWGN
w(n)
S/P
A/D
Wireless Comm. Lab.
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 of
which contains M-ary phase shift keyed (PSK) or quadrature
amplitude modulated (QAM) signals.
‹ Modulation types over OFDM systems
Phase shift keying (PSK)
Quadrature amplitude modulation (QAM)
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Wireless Comm. Lab.
IDFT & DFT
‹ Inverse DFT and DFT are critical in the implementation of an
OFDM system.
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Wireless Comm. Lab.
Orthogonal
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DVB-T System Parameters
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Wireless Comm. Lab.
μ
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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).
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Payload & Inactive Carriers
‹ The meaning of the words 'payload carrier' is clear: these are
simply 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.
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Wireless Comm. Lab.
Continual Pilots
‹ The continual pilots are located on the real axis, i.e. the I (inphase) 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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Carriers Position
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
DBPSK
DBPSK Modulated TPS Carriers
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Wireless Comm. Lab.
TPS Purpose & Content
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
DVB-T Constellation Diagram(1/2)
Continual Pilots, Scattered Pilots and TPS Carriers in the
DVB-T Constellation Diagram
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Wireless Comm. Lab.
DVB-T Constellation Diagram(2/2)
DVB-T Constellation Diagrams for QPSK,16-QAM and
64-QAM
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Carrier Type Value
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Wireless Comm. Lab.
Hierarchical Modulation
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Wireless Comm. Lab.
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-called
High 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
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Wireless Comm. Lab.
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 particularly
robust type of modulation.
‹ On the low priority path, a higher level of modulation is
needed due to the higher data rate.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Constellation Diagram
Embedded QPSK in 64-QAM with
Hierarchical Modulation
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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 α of 2 or 4 increases the distance between the
individual quadrants of the 16QAM or 64QAM diagrams.
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Wireless Comm. Lab.
α Factor
α is the minimum distance separating two constellation
points carrying different HP-bit values divided by the
minimum distance separating any two constellation points.
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Wireless Comm. Lab.
TPS Carriers
‹ The information about the presence or absence of hierarchical
modulation and the α 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.
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Wireless Comm. Lab.
DVB-T Modulator and
Transmitter
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Coding Diagram
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Wireless Comm. Lab.
Synchronize Inverted
It 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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Modulator Diagram
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Wireless Comm. Lab.
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).
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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 α = 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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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 it’s 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.
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Wireless Comm. Lab.
DVB-T Receiver
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Wireless Comm. Lab.
DVB-T Receiver
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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 lowpass filter in order to avoid aliasing effects.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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).
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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 and
each carrier carries the same information.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Channel Decoder
DVB-T Receiver Channel decoder
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
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.
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Wireless Comm. Lab.
Set-Top Box
‹ The 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.
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Wireless Comm. Lab.
Comparison
Convolutional code
88 system comparison
Fig. DTV
Wireless Comm. Lab.
Reference
1.
“Digital Television” Walter Fischer ROHDE&SCHWARZ
2.
Digital video broadcasting (DVB); Framing structure, channel coding
and modulation for terrestrial television, European Standard (EN) 300
744 V1.5.1, European Telecommunications Standards Institute (ETSI),
Nov. 2004.
3.
Ladebusch, U. Liss, C.A , “Terrestrial DVB (DVB-T): a broadcast
technology for stationary portable and mobile use”, Proceedings of the
IEEE, Vol. 94, Issue 1, pp. 183-193, Jan. 2006.
4.
Reimers, U.H., “DVB—The Family of International Standards for
Digital Video Broadcasting”, Proceedings of the IEEE, Vol. 94, Issue. 1,
pp. 173-182, Jan. 2006.
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Wireless Comm. Lab.
The End
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