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Digital Video Broadcasting tMyn1 DVB Digital Video Broadcasting DVB systems distribute data using a variety approaches, including by satellite (DVB-S,

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Presentation on theme: "Digital Video Broadcasting tMyn1 DVB Digital Video Broadcasting DVB systems distribute data using a variety approaches, including by satellite (DVB-S,"— Presentation transcript:

1 Digital Video Broadcasting tMyn1 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 Digital Video Broadcasting tMyn2 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 Digital Video Broadcasting tMyn3 DAC and Front End Source coding and MPEG-2 multiplexing Splitter MUX adaptation, energy dispersal MUX adaptation, energy dispersal External encoder External encoder External interleaver External interleaver Internal interleaver Internal encoder Internal encoder AERIAL Mapper Frame adaptation TPS and pilot signal OFDM Guard interval insertion Figure 1. Functional block diagram of the DVB-T system.

4 Digital Video Broadcasting tMyn4 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 Digital Video Broadcasting tMyn5 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.

6 Digital Video Broadcasting tMyn6 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 Digital Video Broadcasting tMyn7 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 Digital Video Broadcasting tMyn8 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 Digital Video Broadcasting tMyn9 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.

10 Digital Video Broadcasting tMyn10 …… 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.

11 Digital Video Broadcasting tMyn11 Figure 2b. Steps in the process of adaptation, energy dispersal, outer coding and interleaving. 204 bytes Reed-Solomon RS(204, 188, 8) error protected packets Data structure after outer interleaving; interleaving depth I=12 bytes : Non randomized complemented sync byte SYNCn: Non randomized sync byte, n=2, 3, …, 8

12 Digital Video Broadcasting tMyn12 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.

13 Digital Video Broadcasting tMyn13 Figure 3. A frequently used puncturing matrices.

14 Digital Video Broadcasting tMyn14 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.

15 Digital Video Broadcasting tMyn15 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.

16 Digital Video Broadcasting tMyn16 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.

17 Digital Video Broadcasting tMyn17 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.

18 Digital Video Broadcasting tMyn18 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.

19 Digital Video Broadcasting tMyn19 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.

20 Digital Video Broadcasting tMyn20 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}

21 Digital Video Broadcasting tMyn21 d 4d Figure 4. Non-uniform, hierarchical 64-QAM with α=4.

22 Digital Video Broadcasting tMyn22 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}

23 Digital Video Broadcasting tMyn23 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.

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

25 Digital Video Broadcasting tMyn Figure 5b. The 16-QAM mapping and the corresponding bit patterns, Non-hierarchical, and hierarchical with α=1.

26 Digital Video Broadcasting tMyn Figure 5c. The 64-QAM mapping and the corresponding bit patterns, Non-hierarchical, and hierarchical with α=

27 Digital Video Broadcasting tMyn27 Frame adaptation The transmitted signal is organized in frames. Each frame has a duration of T F 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 T S. It is composed of two parts: a useful part with duration T U and a guard interval with a duration Δ. The guard interval consists in a cyclic continuation of the useful part T U and is inserted before it.

28 Digital Video Broadcasting tMyn28 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.

29 Digital Video Broadcasting tMyn29 Figure 6. Duration of symbol part for the allowed guard intervals for 8 MHz channels.

30 Digital Video Broadcasting tMyn30 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.

31 Digital Video Broadcasting tMyn31 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.

32 Digital Video Broadcasting tMyn32 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.

33 Digital Video Broadcasting tMyn33 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.

34 Digital Video Broadcasting tMyn34 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 ∈[K min ; K max ] and determined by K min =0 and K max =1704 in 2K mode and K max =6816 in 8K mode respectively. The spacing between adjacent carriers is 1/T U while the spacing between carriers K min and K max are determined by (K-1)/T U. The numerical values for the OFDM parameters are given in Figure 7.

35 Digital Video Broadcasting tMyn35 Figure 7. Numerical values for the OFDM parameters for the 8K and 2K modes for 8 MHz channels.

36 Digital Video Broadcasting tMyn36 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.

37 Digital Video Broadcasting tMyn37 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.

38 Digital Video Broadcasting tMyn38 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.

39 Digital Video Broadcasting tMyn39 Figure 8. Available bitrates for a DVB-T system in 8 MHz channels.

40 Digital Video Broadcasting tMyn40 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.

41 Digital Video Broadcasting tMyn41 Source coding and MPEG-2 multiplexing Differential encoding MUX adaptation, energy dispersal Channel encoder Interleaver Byte/m-tuple conversion QAM mapper Base-band shaping DAC and Front End RF Cable Channel Figure 9. Functional block diagram of the DVB-C system.

42 Digital Video Broadcasting tMyn42 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

43 Digital Video Broadcasting tMyn43 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.

44 Digital Video Broadcasting tMyn44 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.

45 Digital Video Broadcasting tMyn45 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 Digital Video Broadcasting tMyn46 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 Digital Video Broadcasting tMyn47 Figure 10. Available bitrates for DVB-C system in an 8 MHz channel.

48 Digital Video Broadcasting tMyn48 The latest DVB-C specification is DVB-C2. Modes and features of DVB-C2 in comparison to DVB-C:

49 Digital Video Broadcasting tMyn49 The final DVB-C2 specification was approved by the DVB Steering Board in April [2] [2] Modes and features of DVB-C2 in comparison to DVB-C: [2] [2] Input InterfaceSingle Transport Stream (TS) Multiple Transport Stream and Generic Stream Encapsulation (GSE) ModesConstant Coding & Modulation Variable Coding & Modulation and Adaptive Coding & Modulation FECReed Solomon (RS)LDPC + BCH InterleavingBit-InterleavingBit- Time- and Frequency-Interleaving ModulationSingle Carrier QAMCOFDM PilotsNot ApplicableScattered and Continual Pilots Guard IntervalNot Applicable1/64 or 1/128 Modulation Schemes 16- to 256-QAM16- to 4096-QAM DVB-C DVB-C2

50 Digital Video Broadcasting tMyn50 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:

51 Digital Video Broadcasting tMyn51 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.

52 Digital Video Broadcasting tMyn52 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.

53 Digital Video Broadcasting tMyn53 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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