digital video broadcasting tmyn1 dvb digital video broadcasting dvb systems distribute data using a...

Digital Video Broadcasting

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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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DACandFront End

Source codingand MPEG-2multiplexing

Splitter

MUX adaptation,energydispersal


Externalencoder

Externalencoder

Externalinterleaver

Externalinterleaver

Internalinterleaver

Internalencoder

Internalencoder

AERIAL

MapperFrameadaptation

TPS andpilot signal

OFDMGuardintervalinsertion

Figure 1. Functional block diagram of the DVB-T system.


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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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Randomized Data187 bytes

SYNC2Randomized Data

187 bytes1SYNC SYNC8Randomized Data

187 bytesRandomized Data

187 bytes1SYNC

SYNC1 byte

MPEG-2 transport MUX data187 bytes

……

PRBS period=1503 bytes

8 Transport MUX packets

MPEG-2 transport MUX packet

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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Figure 2b. Steps in the process of adaptation, energy dispersal, outer coding and interleaving.

203 bytes 203 bytesorSYNCn

SYNC 1or

SYNCn

SYNC 1or

SYNCn

SYNC 1

187 bytes Randomized Data 16 Parity bytesorSYNCn

SYNC 1

204 bytes

Reed-Solomon RS(204, 188, 8) error protected packets

Data structure after outer interleaving; interleaving depth I=12 bytes

1SYNC: Non randomized complemented sync byte

SYNCn: Non randomized sync byte, n=2, 3, …, 8


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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

1/211

2/31 01 1

3/41 0 11 1 0

5/61 0 1 0 11 1 0 1 0

7/81 0 0 0 1 0 11 1 1 1 0 1 0

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 16-QAM 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:

QPSKn∈{-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=2n∈{-4, -2, 2, 4}, m∈{-4, -2, 2, 4}

Non-uniform 16-QAM with α=4n∈{-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=2n∈{-8, -6, -4, -2, 2, 4, 6, 8}, m∈{-8, -6, -4, -2, 2, 4, 6, 8}

Non-uniform 64-QAM with α=4n∈{-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

10 01

001

-1

-1 1

Figure 5a. The QPSK mapping and the corresponding bit patterns,Non-hierarchical, and hierarchical with α=1.


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-3 -1 31

-3

-1

1

3

1100

10101000 0010 0000

0001001110111001

1101 1111 0111 0101

010001101110

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

100001 100011 101011 101001

101101101111100111100101

100100 100110 101110 101100

001000 001010 000010 000000

000001000011001011001001

001101 001111 000111 000101

000100000110001110001100

110100 110110 111110 111100 011100 011110 010110 010100

110101 110111 111111 111101 011101 011111 010111 010101

110001 110011 111011 111001 011001 011011 010011 010001

110000 110010 111010 111000 011000 011010 010010 010000

Figure 5c. The 64-QAM mapping and the corresponding bit patterns,Non-hierarchical, and hierarchical with α=1.

1

3

5

7

-1

-3

-5

-7


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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 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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ModeGuard intervalΔ/TU

1/4 1/8 1/16 1/32 1/4 1/8 1/16 1/32

Duration of symbolpart TU

Duration of guardinterval Δ

2048*T224 μs

1024*T112 μs

512*T56 μs

256*T28 μs

512*T56 μs

256*T28 μs

128*T14 μs

64*T7 μs

Symbol durationTS=Δ+TU

10240*T1120 μs

9216*T1008 μs

8704*T952 μs

8448*T924 μs

2560*T280 μs

2304*T252 μs

2176*T238 μs

2112*T231 μs

8K mode 2K mode

8192*T896 μs

2048*T224 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 pulse-shaping 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 8K mode 2K modeNumber of carriers K 6817 1705Value of carrier number Kmin 0 0

Value of carrier number Kmax 6816 1704

Duration TU 896 μs 224 μs

Carrier spacing 1/TU 1116 Hz 4464 Hz

Spacing between carriers Kmin and Kmax 7.61 MHz 7.61 MHz

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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1/4 1/8 1/16 1/321/2 4.98 5.53 5.85 6.032/3 6.64 7.37 7.81 8.043/4 7.46 8.29 8.78 9.055/6 8.29 9.22 9.76 10.057/8 8.71 9.68 10.25 10.561/2 9.95 11.06 11.71 12.062/3 13.27 14.75 15.61 16.093/4 14.93 16.59 17.56 18.105/6 16.59 18.43 19.52 20.117/8 17.42 19.35 20.49 21.111/2 14.93 16.59 17.56 18.102/3 19.91 22.12 23.42 24.133/4 22.39 24.88 26.35 27.145/6 24.88 27.65 29.27 30.167/8 26.13 29.03 30.74 31.67

16-QAM

64-QAM

Guard intervalCoding rateModulation

QPSK

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 codingand MPEG-2multiplexing

Differentialencoding


Channelencoder

InterleaverByte/m-tupleconversion

QAMmapper

Base-bandshaping

DACandFront End

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 256-

QAM, 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 bitrate16-QAM 25.6432-QAM 32.0564-QAM 38.47128-QAM 44.88256-QAM 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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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]

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

DVB-C DVB-C2


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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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