Transcript DVB Digital Video Broadcasting

DVB Digital Video Broadcasting
• DVB systems distribute data using a variety approaches,
including by satellite (DVB-S, DVB-S2), cable (DVB-C),
terrestrial television (DVB-T) and terrestrial television for
handhelds (DVB-H).
• These standards define the physical layer and data link
layer of the distribution system.
• Devices interact with the physical layer via a
synchronous parallel interface (SPI), synchronous serial
interface (SSI), or asynchronous serial interface (ASI).
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• DVB-T stands for Digital Video Broadcasting – Terrestrial
and it is the DVB European consortium standard for the
broadcast transmission of digital terrestrial television.
• This system transmits a compressed digital audio/video
stream, using OFDM modulation with concatenated
channel coding (i.e. COFDM).
• The adopted source coding methods are MPEG-2 and,
more recently, H.264.
• Figure 1 gives a functional block diagram of the system.
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Splitter
Source coding
and MPEG-2
multiplexing
MUX adaptation,
energy
dispersal
External
encoder
External
interleaver
Internal
encoder
MUX adaptation,
energy
dispersal
External
encoder
External
interleaver
Internal
encoder
Guard
interval
insertion
OFDM
Frame
adaptation
DAC
and
Front End
AERIAL
Figure 1. Functional block diagram of the DVB-T system.
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interleaver
Mapper
TPS and
pilot signal
3
Source encoding and MPEG-2 multiplexing.
• Compressed video, audio and data streams are
multiplexed into Programme Streams (PS).
• One or more PSs are joined together into an MPEG-2
Transport Stream (MPEG-2 TS), this is the basic digital
stream which is being transmitted and received by home
Set Top Boxes (STB).
• Allowed bitrates for the transported data depend on
number of coding and modulation parameters, it can
range from about 5 Mbits/sec to about 32 Mbits/sec.
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•
•
•
•
Splitter
Two different TSs can be transmitted at the same time,
using a technique called Hierarchical Transmission.
It may be used to transmit, for example, a standard
definition SDTV signal and a high definition HDTV signal
on the same carrier.
Generally, the SDTV signal is protected better than the
HDTV one.
At the receiver, depending on the quality of the received
signal, the STB may be able to decode the HDTV
stream, or, if signal strength lacks, it can switch to the
SDTV one.
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• In this way, all receivers that are in the proximity of the
transmission site can lock the HDTV signal, whereas all
the other ones, even the farthest, may still be able to
receive and decode a SDTV signal.
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• MUX adaptation and energy dispersal
• The MPEG-2 TS is identified as a sequence of data
packets, of fixed length (188 bytes).
• With a technique called energy dispersal, the byte
sequence is decorrelated.
• This randomization ensures adequate binary transitions.
• The process is accomplished with the Pseudo Random
Binary Sequence (PRBS) generator.
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External encoder
• A first level of protection is applied to the transmitted
data, using a nonbinary block code, a Reed-Solomon
RS(204, 188) code, allowing the correlation of up to
maximum of 8 wrong bytes for each 188-byte packet.
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External interleaver
• Convolutional interleaving is used to rearrange the
transmitted data sequence, such way it becomes more
rugged to long sequences of errors, Figure 2.
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SYNC
1 byte
MPEG-2 transport MUX data
187 bytes
MPEG-2 transport MUX packet
8 Transport MUX packets
PRBS period=1503 bytes
SYNC 1
Randomized Data
187 bytes
SYNC2
Randomized Data
187 bytes
…
SYNC8
Randomized Data
187 bytes
SYNC 1
Randomized Data
187 bytes
…
Randomized transport packets: SYNC bytes and Randomized Data bytes
Figure 2a. Steps in the process of adaptation, energy dispersal, outer coding and interleaving.
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204 bytes
SYNC 1
SYNCn
or
187 bytes Randomized Data
16 Parity bytes
Reed-Solomon RS(204, 188, 8) error protected packets
SYNC 1
SYNCn
or
203 bytes
SYNC 1
SYNCn
or
203 bytes
SYNC 1
SYNCn
or
Data structure after outer interleaving; interleaving depth I=12 bytes
SYNC1 : Non randomized complemented sync byte
SYNCn: Non randomized sync byte, n=2, 3, …, 8
Figure 2b. Steps in the process of adaptation, energy dispersal, outer coding and interleaving.
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•
•
•
•
•
Internal encoder
A second level of protection is given by a punctured
convolutional code, which is often denoted in STBs
menus as FEC (Forward Error Correction).
There are five valid coding rates: 1/2 (unpunctured), 2/3,
3/4, 5/6, and 7/8.
Puncturing is a technique used to make a m/n rate code
from a basic rate 1/2 code.
It is reached by deletion of some bits in the encoder
output.
Bits are deleted according to puncturing matrix, Figure 3.
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code rate puncturing matrix
1
1/2
1
10
2/3
11
101
3/4
110
10101
5/6
11010
1000101
7/8
1111010
Figure 3. A frequently used puncturing matrices.
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• For example, if we want to make a code with rate 2/3
using the appropriate matrix from the table, we should
take a basic encoder output and transmit every second
bit from the first branch and every bit from the second
one.
• The specific order of transmission is defined by the
respective standard.
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•
•
•
•
Internal interleaver
Data sequence is rearranged again, aiming to reduce the
influence of burst errors.
This time, a block interleaving technique is adopted, with
a pseudo-random assignment scheme (this is really
done by two separate interleaving processes, one
operating on bits and another one operating on groups of
bits).
The input (up to two bit streams) to the internal
interleaver is demultiplexed into n sub-streams, where
n=2 for QPSK, n=4 for 16-QAM, and n=6 for 64-QAM.
In non-hierarchical mode, the single input stream is
demultiplexed into n sub-streams.
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• Each sub-stream from the demultiplexer is processed by
a separate bit interleaver.
• There are therefore up to six interleavers depending on
n, labelled I0 to I5.
• I0 and I1 are used for QPSK, I0 to I3 for 16-QAM and I0
to I5 for 64-QAM.
• Bit interleaving is performed only on the useful data.
• The block size is the same for each interleaver, but the
interleaving sequence is different in each case.
• The bit interleaving block size is 126 bits.
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• The block interleaving process is therefore repeated
exactly twelve times per OFDM symbol of useful data in
the 2K mode (12*126=1512 bits) and forty-eight times
per symbol in the 8K mode (48*126=6048 bits).
• The outputs of the n interleavers are grouped to form the
digital data symbols, such that each symbol of n bits will
consist of exactly one bit from each of the n interleavers.
• Hence, the output from the bit-wise interleaver is a n bit
word.
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• The purpose of the symbol interleaver is to map n bit
words onto the 1512 (2K mode) or 6048 (8K mode)
active carriers per OFDM symbol.
• The symbol interleaver acts on blocks of 1512 (2K
mode) or 6048 (8K mode) data symbols.
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•
•
•
•
Mapper
The digital bit sequence is mapped into a base band
modulated sequence of complex symbols.
The system uses Orthogonal Frequency Division
Multiplex (OFDM) transmission.
All data carriers in one OFDM frame are modulated
using either QPSK, 16-QAM, 64-QAM, non-uniform 16QAM or non-uniform 64-QAM constellations.
The exact proportions of the constellations depend on a
parameter α, which can take the three values 1, 2 or 4.
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• α is the minimum distance separating two constellation
points carrying different HP-bit values divided by the
minimum distance separating any two constellation
points, Figure 4.
• Non-hierarchical transmission uses the same uniform
constellation as the case with α=1.
• The exact values of the constellation points are z∈{n+jm}
with values of n, m given below for the various
constellations:
QPSK
n∈{-1, 1}, m∈{-1, 1}
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d
4d
Figure 4. Non-uniform, hierarchical 64-QAM with α=4.
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16-QAM (non-hierarchical and hierarchical with α=1)
n∈{-3, -1, 1, 3}, m∈{-3, -1, 1, 3}
Non-uniform 16-QAM with α=2
n∈{-4, -2, 2, 4}, m∈{-4, -2, 2, 4}
Non-uniform 16-QAM with α=4
n∈{-6, -4, 4, 6}, m∈{-6, -4, 4, 6}
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64-QAM (non-hierarchical and hierarchical with α=1)
n∈{-7, -5, -3, -1, 1, 3, 5, 7}, m∈{-7, -5, -3, -1, 1, 3, 5, 7}
Non-uniform 64-QAM with α=2
n∈{-8, -6, -4, -2, 2, 4, 6, 8}, m∈{-8, -6, -4, -2, 2, 4, 6, 8}
Non-uniform 64-QAM with α=4
n∈{-10, -8, -6, -4, 4, 6, 8, 10}, m∈{-10, -8, -6, -4, 4, 6, 8,
10}
• Some examples are in Figure 5.
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10
1 00
1
-1
-1
10
01
Figure 5a. The QPSK mapping and the corresponding bit patterns,
Non-hierarchical, and hierarchical with α=1.
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1000
1010
0010
3
0000
1001
1011
0001
-3
-1
0011
1
1
1101
1111
0111
-1
0101
1100
1110
0110
-3
0100
3
Figure 5b. The 16-QAM mapping and the corresponding bit patterns,
Non-hierarchical, and hierarchical with α=1.
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100000 100010
101010
101000
001000
7
001010
000010
000000
100001 100011
101011
101001 001001
5
001011
000011
000001
100101 100111
101111
101101 001101
3
001111
000111
000101
100100 100110
101110
101100
001100
1
001110
000110
000100
110100 110110
111110 111100 011100
-1
011110
010110
010100
110101 110111
111111
111101 011101
-3
011111
010111
010101
110001 110011
111011
111001
011001
-5
011011
010011
010001
110000 110010
111010
111000
011000
-7
011010
010010
010000
Figure 5c. The 64-QAM mapping and the corresponding bit patterns,
Non-hierarchical, and hierarchical with α=1.
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• Frame adaptation
• The transmitted signal is organized in frames.
• Each frame has a duration of TF and consists of 68
OFDM symbols.
• Four frames constitute one super-frame.
• Each symbol is constituted by a set of K=6817 carriers in
the 8K mode and K=1705 carriers in the 2K mode and
transmitted with a duration TS.
• It is composed of two parts: a useful part with duration TU
and a guard interval with a duration Δ.
• The guard interval consists in a cyclic continuation of the
useful part TU and is inserted before it.
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• Four values of guard intervals may be used, Figure 6.
• The symbols in an OFDM frame are numbered from 0 to
67.
• All symbols contain data and reference information.
• Since the OFDM signal comprises many separatelymodulated carriers, each symbol can in turn be
considered to be divided into cells, each corresponding
to the modulation carried on one carrier during one
symbol.
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Mode
Guard interval
Δ/TU
Duration of symbol
part TU
Duration of guard
interval Δ
Symbol duration
TS=Δ+TU
8K mode
1/4
2048*T
224 μs
10240*T
1120 μs
1/8
2K mode
1/16
8192*T
896 μs
1024*T
512*T
112 μs
56 μs
9216*T
8704*T
1008 μs 952 μs
1/32
256*T
28 μs
8448*T
924 μs
1/4
512*T
56 μs
2560*T
280 μs
1/8
1/16
1/32
2048*T
224 μs
256*T
128*T
28 μs
14 μs
2304*T
2176*T
252 μs
238 μs
64*T
7 μs
2112*T
231 μs
Figure 6. Duration of symbol part for the allowed guard intervals for 8 MHz channels.
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Pilot and TPS signals
• In order to simplify the reception of the signal being
transmitted on the terrestrial radio channel, additional
signals are inserted in each block.
• Pilot signals (scattered pilot cells, continual pilot carriers)
can be used for frame synchronization, frequency
synchronization, time synchronization, channel
estimation, transmission mode identification and also to
follow the phase noise.
• Transmission Parameters Signalling (TPS) signals are
used to send the parameters of the transmitted signal
and to unequivocally identify the transmission cell.
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• It should be noted that the receiver must be able to
synchronize, equalize and decode the signal to gain
access to the information held by the TPS pilots.
• Thus, the receiver must know this information
beforehand, and the TPS data is only used in special
cases, such as changes in the parameters,
resynchronizations, etc.
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OFDM Modulation
• The sequence of blocks is modulated according to the
OFDM technique, using 2048, 4096, or 8192 carriers
(2K, 4K, 8K mode, respectively).
• Orthogonal Frequency-Division Multiplexing – essentially
identical to Coded OFDM – is a digital multi-carrier
modulation scheme, which uses a large number of
closely-spaced orthogonal sub-carriers.
• Each sub-carrier is modulated with a conventional
modulation scheme (such as quadrature amplitude
modulation) at a low symbol rate, maintaining data rates
similar to conventional single-carrier modulation
schemes in the same bandwidth.
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• In practice, OFDM signals are generated using the Fast
Fourier transform algorithm.
• The primary advantage of OFDM over single-carrier
schemes is its ability to cope with severe channel
conditions – for example, multipath and narrowband
interference – without complex equalization filters.
• Channel equalization is simplified because OFDM may
be viewed as using many slowly-modulated narrowband
signals rather than one rapidly-modulated wideband
signal.
• The orthogonality of the sub-carriers results in zero
cross-talk, even though they are so close that their
spectra overlap.
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• Low symbol rate helps manage time-domain spreading
of the signal (such as multipath propagation) by allowing
the use of a guard interval between symbols.
• The guard interval also eliminates the need for a pulseshaping filter.
• The carriers are indexed by k∈[Kmin; Kmax] and
determined by Kmin=0 and Kmax=1704 in 2K mode and
Kmax=6816 in 8K mode respectively.
• The spacing between adjacent carriers is 1/TU while the
spacing between carriers Kmin and Kmax are determined
by (K-1)/TU.
• The numerical values for the OFDM parameters are
given in Figure 7.
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Parameter
Number of carriers K
Value of carrier number Kmin
8K mode 2K mode
6817
1705
0
0
Value of carrier number Kmax
6816
1704
896 μs
224 μs
Carrier spacing 1/TU
1116 Hz
4464 Hz
Spacing between carriers Kmin and Kmax
7.61 MHz 7.61 MHz
Duration TU
Figure 7. Numerical values for the OFDM parameters for the 8K and 2K modes for 8 MHz channels.
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• The values for the various time-related parameters are
given in multiples of the elementary period T and in
microseconds.
• The elementary period T is 7/64 μs for 8 MHz channels.
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Guard interval insertion
• To decrease receiver complexity, every OFDM block is
extended, copying in front of it its own end (cyclic prefix).
• The width of such guard interval can be 1/32, 1/16, 1/8,
or 1/4 that of the original block length, Figure 6.
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• DAC and front-end
• The digital signal is transformed into an analog signal,
with a digital-to-analog converter (DAC), and then
modulated to radio frequency (UHF) by the RF front-end.
• The occupied bandwidth is designed to accommodate
each single DVB-T signal into 8 MHz wide channels.
• Available bitrates for DVB-T system in 8 MHz channels
are presented in Figure 8. All decimal values are in
Mbit/s.
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Modulation Coding rate
QPSK
16-QAM
64-QAM
1/2
2/3
3/4
5/6
7/8
1/2
2/3
3/4
5/6
7/8
1/2
2/3
3/4
5/6
7/8
1/4
4.98
6.64
7.46
8.29
8.71
9.95
13.27
14.93
16.59
17.42
14.93
19.91
22.39
24.88
26.13
Guard interval
1/8
1/16
5.53
5.85
7.37
7.81
8.29
8.78
9.22
9.76
9.68
10.25
11.06
11.71
14.75
15.61
16.59
17.56
18.43
19.52
19.35
20.49
16.59
17.56
22.12
23.42
24.88
26.35
27.65
29.27
29.03
30.74
1/32
6.03
8.04
9.05
10.05
10.56
12.06
16.09
18.10
20.11
21.11
18.10
24.13
27.14
30.16
31.67
Figure 8. Available bitrates for a DVB-T system in 8 MHz channels.
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• DVB-C stands for Digital Video Broadcasting – Cable
and it is the DVB European consortium standard for the
broadcast transmission of digital television over cable.
• This system transmits an MPEG-2 family digital
audio/video stream, using a QAM modulation with
channel coding.
• Figure 9 gives a functional block diagram of the system.
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Source coding
and MPEG-2
multiplexing
MUX adaptation,
energy
dispersal
Channel
encoder
Interleaver
Byte/m-tuple
conversion
DAC
and
Front End
Base-band
shaping
QAM
mapper
Differential
encoding
RF Cable Channel
Figure 9. Functional block diagram of the DVB-C system.
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Source coding and MPEG-2 multiplexing
• Basically the same as with DVB-T
MUX adaptation and energy dispersal
• Basically the same as with DVB-T
Channel encoder
• Basically the same as with DVB-T External encoder
Interleaver
• Basically same as with DVB-T External interleaver
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Byte/m-tuple conversion
• This unit shall perform a conversion of the bytes
generated by the interleaver into QAM symbols.
• Depending on if there is 16-QAM, 32-QAM … , or 256QAM, m=4, 5, 6, 7, or 8.
Differential encoding
• In order to get a rotation-invariant constellation, this unit
shall apply a differential encoding of the two most
significant bits of each symbol.
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QAM Mapper
• The bit sequence is mapped into a base-band digital
sequence of complex symbols.
• The modulation of the system is quadrature amplitude
modulation with 16, 32, 64, 128, or 256 points in the
constellation diagram.
• Notice! The mapping is not identical with the
correspondent mapping of DVB-T.
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Base-band shaping
• The QAM signal is filtered with a raised-cosine shaped
filter, in order to remove mutual signal interference at the
receiving side.
DAC and front-end
• The digital signal is transformed into an analog signal,
with a digital-to-analog converter, and then modulated to
radio frequency by the RF front-end.
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• The standard paper says: “With a roll-off factor of 0.15,
the theoretical maximum symbol rate in an 8 MHz
channel is about 6.96 MBaud”.
• This piece of information gives rise to the following
figure, Figure 10. All decimal numbers are in Mbit/s.
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Modulation
16-QAM
32-QAM
64-QAM
128-QAM
256-QAM
bitrate
25.64
32.05
38.47
44.88
51.29
Figure 10. Available bitrates for DVB-C system in an 8 MHz channel.
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• The latest DVB-C specification is DVB-C2.
• Modes and features of DVB-C2 in comparison to DVB-C:
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DVB-C
Input Interface
DVB-C2
Single Transport
Stream (TS)
Multiple Transport Stream and Generic
Stream Encapsulation (GSE)
Modes
Constant Coding &
Variable Coding & Modulation and Adaptive
The final DVB-C2 specification
was approved by the DVB Steering Board in April 2009.
Modulation
Coding & Modulation
Modes and features of DVB-C2 in comparison to DVB-C:
[2]
[2]
FEC
Reed Solomon (RS)
LDPC + BCH
Interleaving
Bit-Interleaving
Bit- Time- and Frequency-Interleaving
Modulation
Single Carrier QAM
COFDM
Pilots
Not Applicable
Scattered and Continual Pilots
Guard Interval
Not Applicable
1/64 or 1/128
Modulation
Schemes
16- to 256-QAM
16- to 4096-QAM
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• Main features of the DVB-S2:
• Source may be one or more MPEG-2 TS (MPEG-2
Transport Stream). Packet streams other than MPEG-2
are also valid (MPEG-4 AVC/H.264).
• MPEG-2 TS are supported using a compatibility mode,
whereas the native stream format for DVB-S2 is called
Generic Stream (GS).
• Adaptative mode: this block is heavily dependent on the
application that generates the data. This means:
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• CRC-8 encoding; used by a DVB-S2 for error correction;
• merging full stream and subdivisions in blocks for error
correction encoding (DF, Data Fields).
• Backward compatibility to DVB-S, intended for end
users, and DVB-DSNG (DVB-Digital Satellite News
Gathering), used for backhauls and electronic news
gathering.
• Adaptive coding and modulation to optimize the use of
satellite transponders.
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• Four modulation modes:
• QPSK and 8PSK are proposed for broadcast
applications and they can be used in non-linear
transponders driven near to saturation
• 16APSK and 32APSK are used mainly for professional,
semi-linear applications, they can be also used for
broadcasting but they require a higher level of available
C/N and an adoption of advanced pre-distortion methods
in the uplink station in order to minimize the effect of
transponder linearity.
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• For forward error correction (FEC), DVB-S2 uses a
system based on the concatenation of the BCH code
with an inner LDPC code.
• Interleaving uses 8PSK, 16APSK, or 32APSK
modulation.
• Performance can be configured to be within 0.7 dB of the
Shannon limit.
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