90
Wireless Comm. Lab. Wireless Comm. Lab. 1 數位電視傳輸技術 數位電視傳輸技術 DVB DVB - - T T 簡介 簡介 林信標 林信標 台北科技大學 台北科技大學 電腦與通訊研究所 電腦與通訊研究所 2006.8 2006.8

DVB T Overview

  • Upload
    prasse

  • View
    34

  • Download
    2

Embed Size (px)

Citation preview

DVB-T 2006.81Wireless Comm. Lab.

OutlineDVB-T Introduction Wireless Propagation Properties OFDM Concepts DVB-T System Parameters Hierarchical Modulation DVB-T Modulator and Transmitter Architecture DVB-T Receiver Architecture2Wireless Comm. Lab.

DVB-T Introduction

3

Wireless Comm. Lab.

DVB-T HistoryThe 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.

4

Wireless Comm. Lab.

The Worldwide Digital TV (1/2)Europe: European Telecommunications Standards Institute (ETSI) Video Broadcasting (DVB) America: Advanced Television Systems Committee (ATSC) Japan: Association of Radio Industries and Business (ARIB). services digital broadcasting (ISDB). Korea: Digital multimedia broadcasting (DMB)5Wireless Comm. Lab.

Digital

ATSC DTV integrated

The Worldwide Digital TV (2/2)DVB-T: Europelargest part of AsiaAustraliaAfrica ATSC: United StatesCanadaMexico ISDB-T: Japan DMB: South Korea Unclear:6 Peoples Republic of China and Latin America Wireless Comm. Lab.

DVB-T IntroductionEuropean 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.7Wireless Comm. Lab.

DVB-T Transmitter

8

Wireless Comm. Lab.

DVB-T Receiver

9

Wireless Comm. Lab.

Wireless Propagation Properties

10

Wireless Comm. Lab.

Mobile Radio Environment

11

Wireless Comm. Lab.

Wireless Channel ModelFading processPath loss Shadowing Fast fading(Doppler effectMulti-path delay)

Transmit Antenna

Path Loss

Shadowing

Fast Fading

Receive Antenna

Additive Noise

12 fading process

Wireless Comm. Lab.

Wireless Channel NoiseWireless channel noise Multipliable noise (RayleighRician fading) additive noise (Gaussian noise)Wireless channel

Transmitter

Receiver

Multipliable

noise13

additive noiseWireless Comm. Lab.

Narrowband vs. WidebandNarrowband: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 .14Wireless Comm. Lab.

Effect of Delay Spread

15

Wireless Comm. Lab.

Effect on Error Rate

16

Wireless Comm. Lab.

OFDM Concepts

17

Wireless Comm. Lab.

OFDM BasicsMain 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:

18

Wireless Comm. Lab.

Modulation techniques: monocarrier vs. multicarrierChannelization Channel Guard bands Similar to FDM technique B Pulse length ~ N/B Data are shared among several carriers and simultaneously transmitted Advantages Furthermore Flat Fading per carrier N long pulses ISI is comparatively short N short EQs needed Poor spectral efficiency because of band guards It is easy to exploit frequency diversity It allows deployment of 2D coding techniques Dynamic signaling N carriers

B Pulse length ~1/B Data are transmitted over only one carrier

Drawbacks Selective Fading Very short pulses ISI is compartively long EQs are then very long Poor spectral efficiency because of band guards

To improve the spectral efficiency: Eliminate band guards between carriers To use orthogonal carriers (allowing spectrum overlapping)

19

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

f20 Orthogonal Multicarrier Modulation Technique Comm. Lab. Wireless

Orthogonal Frequency Division Multiplex (OFDM)Parallel data transmission on several orthogonal subcarriers with lower rate ck3 f

t

Maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zerosuperposition of frequencies in the same frequency range Amplitudesubcarrier: sin(x) SI function= x

f21Wireless Comm. Lab.

OFDM IIPropertiesLower data rate on each subcarrier 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)

Advantagesno equalizer necessary no expensive filters with sharp edges necessary better spectral efficiency (compared to CDM)

Application802.11a, HiperLAN2, DAB, DVB, ADSL

22

Wireless Comm. Lab.

OFDM in Real environmentsISI of subsequent symbols due to multipath propagation Symbol has to be stable during analysis for at least Tdata Guard-Intervall (TG) prepends each symbnol (HIPERLAN/2: TG= 0.8 s; Tdata= 3.2 s; 52 subcarriers)impulse response fade out fade in OFDM symbol OFDM symbol OFDM symbol

OFDM symbol

OFDM symbolanalysis window

OFDM symbol t TGWireless Comm. Lab.

TG

Tdata

TG23

Tdata

OFDM System BlockX(m) Binary data Modulation mapping x(n) xGI(n)

S/P

Pilot insertion

IFFT

GI insertion

P/S

D/A

Channel Y(m)Binary received data

h(n) AWGN w(n)

y(n)

yGI(n)

Modulation de-mapping

P/S

Channel estimation base on pilot and signal correction

FFT

GI re moval

S/P

A/D

24

Wireless Comm. Lab.

Modulation & MappingThe 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)25Wireless Comm. Lab.

IDFT & DFTInverse DFT and DFT are critical in the implementation of an OFDM system.

26

Wireless Comm. Lab.

Orthogonal

27

Wireless Comm. Lab.

DVB-T System Parameters

28

Wireless Comm. Lab.

Two Mode CharacteristicA 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.

29

Wireless Comm. Lab.

Two Mode Purpose2k 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.

30

Wireless Comm. Lab.

Modulation SelectApart 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).31Wireless Comm. Lab.

Carriers TypeDVB-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.

32

Wireless Comm. Lab.

Payload & Inactive CarriersThe 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.

33

Wireless Comm. Lab.

Continual PilotsThe 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.34Wireless Comm. Lab.

Scattered PilotsWithin 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.

35

Wireless Comm. Lab.

Carriers Position

36

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.

37

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.

38

Wireless Comm. Lab.

DBPSK

DBPSK Modulated TPS Carriers39Wireless Comm. Lab.

TPS Purpose & Content

40

Wireless Comm. Lab.

TPS Carry InformationThus, 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.

41

Wireless Comm. Lab.

DVB-T Constellation Diagram(1/2)

Continual Pilots, Scattered Pilots and TPS Carriers in the DVB-T Constellation Diagram42Wireless Comm. Lab.

DVB-T Constellation Diagram(2/2)

DVB-T Constellation Diagrams for QPSK,16-QAM and 64-QAM43Wireless Comm. Lab.

IFFTIn 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.

44

Wireless Comm. Lab.

Carrier Type Value

45

Wireless Comm. Lab.

Hierarchical Modulation

46

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 3/447Wireless 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.

48

Wireless Comm. Lab.

Modulation TypeIn 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.

49

Wireless Comm. Lab.

Constellation Diagram

Embedded QPSK in 64-QAM with Hierarchical Modulation50Wireless Comm. Lab.

Embedded QPSK in 64-QAMA 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.

51

Wireless Comm. Lab.

Embedded QPSK in 16-QAMQPSK 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 of 2 or 4 increases the distance between the individual quadrants of the 16QAM or 64QAM diagrams.

52

Wireless Comm. Lab.

Factor

is the minimum distance separating two constellation points carrying different HP-bit values divided by the minimum distance separating any two constellation points.53Wireless Comm. Lab.

TPS CarriersThe information about the presence or absence of hierarchical modulation and the 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.

54

Wireless Comm. Lab.

DVB-T Modulator and Transmitter

55

Wireless Comm. Lab.

DVB-T Modulator and TransmitterA 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.

56

Wireless Comm. Lab.

Coding Diagram

57

Wireless Comm. Lab.

Synchronize InvertedIt 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.

58

Wireless Comm. Lab.

Reed Solomon EncoderFollowing this, initial error control is performed in the Reed Solomon encoder. The TS packets are now expanded by 16 bytes error protection.

59

Wireless Comm. Lab.

Interleave & ConvolutionalAfter 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.

60

Wireless Comm. Lab.

Modulator Diagram

61

Wireless Comm. Lab.

Bit InterleaverThe 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).

62

Wireless Comm. Lab.

Symbol InterleaverIn 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.

63

Wireless Comm. Lab.

Mapper & Frame AdaptationAfter that, all the payload carriers are then mapped depending on whether hierarchical or nonhierarchical modulation is used, and on the factor a being = 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.64Wireless Comm. Lab.

IFFTThe 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.

65

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.

66

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.

67

Wireless Comm. Lab.

FIR FilterThe 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 its 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.

68

Wireless Comm. Lab.

DVB-T Receiver

69

Wireless Comm. Lab.

DVB-T Receiver

70

Wireless Comm. Lab.

Tuner & SAW FilterThe 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.

71

Wireless Comm. Lab.

Mixing & LPFIn 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.

72

Wireless Comm. Lab.

A/D ConverterThis 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.

73

Wireless Comm. Lab.

Time SynchronizationIn 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.

74

Wireless Comm. Lab.

Changeover SwitchThe 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.75Wireless Comm. Lab.

FIR & DelayHowever, 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).

76

Wireless Comm. Lab.

AFCThis 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.77Wireless Comm. Lab.

NCOIt 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.

78

Wireless Comm. Lab.

FFTThe 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.

79

Wireless Comm. Lab.

Continual & Scattered PilotsHowever, 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.

80

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.

81

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.

82

Wireless Comm. Lab.

Channel Decoder

DVB-T Receiver Channel decoder83Wireless Comm. Lab.

Deinterleaver & PunctureThe 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.

84

Wireless Comm. Lab.

RS Decoder & Energy DispersalThe 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.

85

Wireless Comm. Lab.

Synchronize Inverter RemoveThis 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.

86

Wireless Comm. Lab.

Set-Top BoxThe 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.

87

Wireless Comm. Lab.

Comparison

Convolutional code

88 Fig. DTV system comparison

Wireless Comm. Lab.

Reference1. 2. Digital Television Walter Fischer ROHDE&SCHWARZ 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. 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. Reimers, U.H., DVBThe Family of International Standards for Digital Video Broadcasting, Proceedings of the IEEE, Vol. 94, Issue. 1, pp. 173-182, Jan. 2006.

3.

4.

89

Wireless Comm. Lab.

The End

90

Wireless Comm. Lab.