DVB Digital Video Broadcasting

1
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).

2

• 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.

3
Source coding and MPEG-2 multiplexing
Splitter
MUX adaptation, energy dispersal
External interleaver
Internal interleaver
Internal encoder
External encoder
MUX adaptation, energy dispersal
External interleaver
Internal encoder
External encoder
DAC and Front End
Guard interval insertion
Frame adaptation
Mapper
OFDM
AERIAL
TPS and pilot signal
Figure 1. Functional block diagram of the DVB-T
system.
4

• 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.

5

• 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.

7

• 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.

8

• 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.

9

• 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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MPEG-2 transport MUX packet
8 Transport MUX packets
PRBS period1503 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
Reed-Solomon RS(204, 188, 8) error protected
packets
Data structure after outer interleaving
interleaving depth I12 bytes
Non randomized complemented sync byte
SYNCn Non randomized sync byte, n2, 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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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 n2 for QPSK, n4 for 16-QAM, and n6 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 (121261512 bits) and
  forty-eight times per symbol in the 8K mode
  (481266048 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
  16-QAM or non-uniform 64-QAM constellations.
• The exact proportions of the constellations
  depend on a parameter a, which can take the three
  values 1, 2 or 4.

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• a 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 a1.
• The exact values of the constellation points are
  z?njm 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
a4.
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• 16-QAM (non-hierarchical and hierarchical with
  a1)
• n?-3, -1, 1, 3, m?-3, -1, 1, 3
• Non-uniform 16-QAM with a2
• n?-4, -2, 2, 4, m?-4, -2, 2, 4
• Non-uniform 16-QAM with a4
• n?-6, -4, 4, 6, m?-6, -4, 4, 6

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• 64-QAM (non-hierarchical and hierarchical with
  a1)
• n?-7, -5, -3, -1, 1, 3, 5, 7, m?-7, -5, -3,
  -1, 1, 3, 5, 7
• Non-uniform 64-QAM with a2
• n?-8, -6, -4, -2, 2, 4, 6, 8, m?-8, -6, -4,
  -2, 2, 4, 6, 8
• Non-uniform 64-QAM with a4
• 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
00
1
-1
1
-1
10
01
Figure 5a. The QPSK mapping and the corresponding
bit patterns, Non-hierarchical, and hierarchical
with a1.
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1000
0010
1010
0000
3
0001
0011
1011
1001
1
-3
-1
3
1
0111
1101
1111
0101
-1
1100
0100
0110
1110
-3
Figure 5b. The 16-QAM mapping and the
corresponding bit patterns, Non-hierarchical, and
hierarchical with a1.
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100000
100010
101010
101000
001000
001010
000010
000000
7
101011
000011
001011
001001
100001
100011
101001
000001
5
101111
100111
100101
001101
001111
000111
000101
101101
3
100110
101100
100100
101110
000100
000110
001110
001100
1
111110
111100
011100
010100
110100
110110
011110
010110
-1
110101
111111
011101
011111
010111
110111
111101
010101
-3
110001
110011
111011
111001
011001
011011
010011
010001
-5
110000
110010
111010
111000
011000
011010
010010
010000
-7
Figure 5c. The 64-QAM mapping and the
corresponding bit patterns, Non-hierarchical, and
hierarchical with a1.
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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 K6817
  carriers in the 8K mode and K1705 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
  ?.
• 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
  separately-modulated 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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Figure 6. Duration of symbol part for the
allowed guard intervals for 8 MHz channels.
30

• 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
  pulse-shaping filter.
• The carriers are indexed by k?Kmin Kmax and
  determined by Kmin0 and Kmax1704 in 2K mode and
  Kmax6816 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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Figure 7. Numerical values for the OFDM
parameters for the 8K and 2K modes for 8 MHz
channels.
36

• 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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Figure 8. Available bitrates for a DVB-T system
in 8 MHz channels.
40

• 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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DAC and Front End
QAM mapper
Base-band shaping
RF Cable Channel
Figure 9. Functional block diagram of the DVB-C
system.
42

• 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
  256-QAM, m4, 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.

46

• 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.

47
Figure 10. Available bitrates for DVB-C system in
an 8 MHz channel.
48

• 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
DVB-C2
Input Interface Single Transport Stream (TS) Multiple Transport Stream and Generic Stream Encapsulation (GSE)
Modes Constant Coding Modulation Variable Coding Modulation and Adaptive Coding Modulation
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
The final DVB-C2 specification was approved by
the DVB Steering Board in April 2009. 2 Modes
and features of DVB-C2 in comparison to DVB-C 2
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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.