TECHNICAL CHARACTERISTICS OF DIGITAL SYSTEMS

Stanko PERPAR

Chairman ITU-R TG 6/8

ITU DTTB model

Standards for Digital Terrestrial Television Broadcasting

• Digital system A: ATSC • Digital system B: DVB -T• Digital system C: ISDB – T

• System A: Single carrier modulation

(8-VSB)• Systems B and C: Multicarrier modulation

(OFDM)

Coded Orthogonal Frequency Division Multiplex

Number of carriers per symbol:

• 2k - 1705

• 8k – 6817

Modulation:

• QPSK

• 16 QAM (uniform or non-uniform)

• 64 QAM (uniform or non-uniform)

OFDM Carriers

Guard interval

Ts = Tu + Tg

Tg = ¼, 1/8, 1/16 or 1/32 Tg

• Ts = symbol duration• Tu = usefull symbol duration• Tg = guard intervalSome carriers are pilots used for synchronisation,

transmission of parameters and signal recovery purposes

Forward Error Correction

• Outer code: Reed Solomon (204, 188, 16)

• Interleaving

• Inner code: convolutional with Viterbi soft decision decoding.– Coding rates: ½, 2/3, ¾, 5/6, 7/8

Multipath propagation

Reflected path

Direct path

Reception modes

• Fixed reception

• Class A portable reception (outdoor)

• Class B portable reception (ground floor indoor)

• Mobile reception (moving with such speed that Doppler effect appears)

Coverage

• Location (0,5 x 0,5)m covered for 99% of the time

• Small area (100 x 100) m– Good coverage for > 95% of locations

– Acceptable for > 70% of locations

• Coverage area – sum of individual small areas

Small area field strength distribution

Distance [m]

[m]

Percentage of locations50%

50%

50%

Minimum field strength for 50% locations

E

distance (m)

Percentage of locations70%

Percentage of locations50% to 70%

Limit value for planning

• Analogue: S/N = 30 dB (unweighted)

• Digital: BER 2x10-4 (Quasi Error Free)

• - 3 dB difference means ½ grade on quality scale in analogue picture, but

• no picture in digital

Cut-off characteristics

E (dB)

Excellent

analogue

digital

threshold

midopinion

Unusable

Transmission Channels

• Gaussian Channel – direct sight, no multipath (dB

• Rice Channel (1 < < 3 dB) – stationary reception using directional antenna

• Rayleigh Channel ( > 3 dB) – portable reception using omnidirectional antenna

Ricean Channel

Rayleigh Channel

Required C/N

Planning parameters

• Minimum field strengths

• Protection ratios– Co-channel – Adjacent channel

Minimum field strengths Fixed reception - 64 QAM 2/3 Rice channel

BAND III IV V

Analogue 55 65 70

Digital

70% locations 39 44 48

95% locations 45 50 54

Minimum field strengths Portable outside reception 64 QAM 2/3

Rayleigh channel

BAND III IV V

Analogue 55 65 70

. Digital

Locations 70% 59 65 69

Locations 95% 64 71 75

Minimum field strengths Portable inside reception 64 QAM 2/3

Rayleigh channel

BAND III IV V

Analogue 55 65 70

Digital

Locations 70% 66 73 77

Locations 95% 73 83 87

Minimum field strengths Portable inside reception 16 QAM 1/2

Rayleigh channel

BAND III IV V

Analogue 55 65 70

Digital

Locations 70% 59 66 70

Locations 95% 68 76 80

Mobile reception - Typical urban 16 QAM 1/2 non diversity

BAND III IV V

Analogue 55 65 70

Max.speed 254 102 64

Locations 70% 58 65 69

Locations 95% 64 71 77

Locations 99% 68 75 79

Mobile reception - Typical urbanantenna diversity

BAND III IV V

Analogue 55 65 70

Max.speed 508 203 127

Locations 70% 52 59 63

Locations 95% 58 65 69

Locations 99% 63 69 73

CO - CHANNEL INTERFERENCE

ANALOGUEOffset 8/12

Norm.offset500 Hz

Prec. offset1 Hz

Tropospheric 30 22

Continiuous 40 27

DIGITAL Rice Rayleigh

64 QAM 2/3 20 23

16 QAM 1/2 11 13

ATSC 15 (19) 15 (19)

ADJACENT CHANNEL

ANALOGUE

Adj.channel N - 1 N +1

Tropospheric -9 -12

Continuous 1 -2

DIGITAL

DVB -T -30 -30

ATSC -27 -27

Direct path

Multipath

Multipath analogue

Advantages of DVB-T

• Flexible approach ( cca 120 possibilities)

• SFN

• Mobile reception

• Possible use of TABO channels

• Robust to multipath effects

• Similarity with DVB-S, DVB-C

• Set-top box

UPGRADE FROM ANALOGUE TO DIGITAL TV

TRANSMITTER

Analogue TV Transmittercommom mode amplification

Analogueexciter

Poweramplifier

Lowpassfilter

Video input

Audio input

RF output

DVB-T exciterwith COFDMmodulator

Poweramplifier

Bandpassfilter

ASI input

MPEG-2transport stream

RF output

ANALOGUE TV TRANSMITTER

DIGITAL TV TRANSMITTER

Analogueexciter

Poweramplifier

Lowpassfilter

Video input

Audio input

RF output

DVB-T exciterwith COFDMmodulator

Poweramplifier

Bandpassfilter

ASI input

MPEG-2transport stream

RF output

ANALOGUE TV TRANSMITTER

DIGITAL TV TRANSMITTER

Analogue TV Transmittersplit amplification

RFoutputVideo input

Audio input

DVB-T exciterwith COFDMmodulator

Poweramplifier

Bandpassfilter

ASI input

MPEG-2transport stream

RF output

Notchfilter

Analogueexciter

Lowpassfilter

Videopower

amplifier

Audiopower

amplifier

Video/audio

combiner

ANALOGUE TV TRANSMITTER

DIGITAL TV TRANSMITTER

RF video

RF audio

RFoutputVideo input

Audio input

DVB-T exciterwith COFDMmodulator

Poweramplifier

Bandpassfilter

ASI input

MPEG-2transport stream

RF output

Notchfilter

Analogueexciter

Lowpassfilter

Videopower

amplifier

Audiopower

amplifier

Video/audio

combiner

ANALOGUE TV TRANSMITTER

DIGITAL TV TRANSMITTER

RF video

RF audio